Plasma Physics

Laser-induced plasma has been used for different diagnostic and technological applications as detection, thin film deposition, and elemental identification. Laser-Induced Plasma Spectroscopy ( LIPS), or sometimes called Laser-Induced Breakdown (LIBS) or Laser Spark Spectroscopy (LSS) is a powerful tool for rapid in-situ analyses of solid, liquid or gaseous samples. Some examples include on-line monitoring in the steel industry, control of materials and leakages in power plants, environmental analyses (soils, waters), analyses of objects related to Cultural Heritage, explosive identification in the protection against terrorism and planetary exploration (surface characterization).

Pulsed laser deposition is a thin film deposition (specifically a PVD) technique, where a high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the deposit material. Conceptually and experimentally, pulsed laser ablation is an extremely simple technique; probably the simplest among all thin-film growth techniques.

With progress in short-pulse laser techniques, newly generated plasma applications appear. Extreme ultraviolet (EUV) sources based on laser-produced plasmas (LPP) emitting at a wavelength of tens of nm are currently being developed with tremendous effort for the next generation of semiconductor microlithography. Besides high-power sources for high volume manufacturing, compact EUV sources of lower average power are also needed for metrology purposes, for example, for mask inspection, actinic material testing, or optics and sensor characterization. Moreover, tabletop LPP sources are also employed to generate soft x-ray (SXR) radiation for microscopy or absorption spectroscopy in the water window spectral range (2.2–4.4 nm).

The availability of synchrotron x-ray and laser-plasma x-ray sources have revolutionized the capabilities of time-resolved x-ray diffraction investigations, the development of streak camera x-ray detectors, static and streak mode charge-coupled device (CCD) x-ray area detector techniques, and specialized scintillation detection schemes.

Publications

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7 articles, 7 selected
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High Energy Laser Systems – Unique laser systems for extreme applications
APL2100 series – high energy picosecond amplifiers
Nanosecond Lasers
NL300 series – compact flash-lamp pumped Q-switched Nd:YAG lasers
Picosecond Lasers
PL2250 series – flash-lamp pumped picosecond Nd:YAG lasers
SL230 series – SBS compressed picosecond DPSS Nd:YAG lasers

Effects of pressure and substrate temperature on the growth of Al-doped ZnO films by pulsed laser deposition

Related products:  NL300 series

Authors:  Reeson Kek, Kwan-Chu Tan, Chen Hon Nee, Seong Ling Yap, Song Foo Koh, Abdul Kariem Bin Hj Mohd Arof, Teck Yong Tou, Seong Shan Yap

Al-doped ZnO (AZO) thin films were deposited on p-Si (100) by pulsed laser deposition from a composite ceramic target (ZnO:Al2O3) by using 355 nm laser at different O2 background pressure and substrate temperature. Upon ablation at laser fluence of 2 Jcm−2, plasma plume consists of Zn neutrals and ions, Al neutrals and O neutral are formed. As the O2 background pressure increases from 3 Pa to 26 Pa, the energy of the plasma species are moderated. The results show that the ions density and velocity reduced significantly above 13 Pa. The velocity of the ions reduced from 14 kms−1 to 11 kms−1 at 13 Pa, while the ions energy reduced from 63 eV to 42 eV respectively. Below 13 Pa, crystalline and homogeneous AZO nanostructured films were formed. Above 13 Pa, the process results in low crystallinity films with higher porosity. The resistivity of the films also increases from 0.1 ohmcm to 24 ohmcm as the pressure increased. At fixed O2 background pressure of 3 Pa, the adatom mobility of atoms on the substrates is altered by substrate heating. The resistivity of the films decreased to 10–3 ohmcm when the substrates are heated to 100 °C–300 °C during deposition. The films with highest carrier density of 1020 cm−3 and carrier mobility of 13 cmV−1 s−1 are achieved at 200 °C.

Published: 2020.   Source: Mater. Res. Express 7 016414

Soft x-ray emission from laser-produced strontium ions

Related products:  PL2250 series SL230 series

Authors:  Takanori Miyazaki, Gerry O'Sullivan, Padraig Dunne

Soft x-ray spectra, in the range from 2 nm to 9 nm, were recorded from strontium plasmas formed by pulses from 20 ps, 170 ps and 5.5 ns Nd:YAG lasers operating at the fundamental wavelength of 1064 nm. Features due to 3d–4p and 3d–4f transitions were identified by comparison with spectra from adjacent ions and atomic structure calculations with both the Cowan code and the Flexible Atomic Code. As in the spectra of ions of other elements in the fifth row of the periodic table, resonant lines 3dn–3dn−14p1, 3dn–3dn−14f1 and satellite lines 3dn−14s1–3dn−24s14p1, 3dn−14s1–3dn−24s14f1 of Δn = 1 were observed over the 3.0–8.5 nm region, emitted by 10+ to 19+ ions. These Δn = 1 transitions provide a range of narrow band emission features which may match to specific multi layer combinations for reflective optics in the extreme ultraviolet region of the spectrum.

Published: 2019.   Source: J. Phys. B: At. Mol. Opt. Phys. 53 025001

Optical coherence tomography (OCT) with 2 nm axial resolution using a compact laser plasma soft X-ray source

Related products:  NL300 series

Authors:  Przemysław Wachulak, Andrzej Bartnik, Henryk Fiedorowicz

We present optical coherence tomography (OCT) with 2 nm axial resolution using broadband soft X-ray radiation (SXR) from a compact laser plasma light source. The laser plasma was formed by the interaction of nanosecond laser pulses with a gaseous target in a double stream gas puff target approach. The source was optimized for efficient SXR emission from the krypton/helium gas puff target in the 2 to 5 nm spectral range, encompassing the entire “water-window” spectral range from 2.3 nm to 4.4 nm wavelength. The coherence parameters of the SXR radiation allowed for the OCT measurements of a bulk multilayer structure with 10 nm period and 40% bottom layer thickness to period ratio, with an axial resolution of about 2 nm and detect multilayer interfaces up to a depth of about 100 nm. The experimental data are in agreement with OCT simulations performed on ideal multilayer structure. In the paper, detailed information about the source, its optimization, the optical system, OCT measurements and the results are presented and discussed.

Published: 2018.   Source: Scientific Reports, vol. 8, art. 8494 (2018)

XUV spectra of 2nd transition row elements: identification of 3d–4p and 3d–4f transition arrays

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Authors:  Ragava Lokasani, Elaine Long, Oisin Maguire, Paul Sheridan, Patrick Hayden, Fergal O'Reilly, Padraig Dunne, Emma Sokell, Akira Endo, Jiri Limpouch and Gerry O'Sullivan

The use of laser produced plasmas (LPPs) in extreme ultraviolet/soft x-ray lithography and metrology at 13.5 nm has been widely reported and recent research efforts have focused on developing next generation sources for lithography, surface morphology, patterning and microscopy at shorter wavelengths. In this paper, the spectra emitted from LPPs of the 2nd transition row elements from yttrium (Z = 39) to palladium (Z = 46), with the exception of zirconium (Z = 40) and technetium (Z = 43), produced by two Nd:YAG lasers which delivered up to 600 mJ in 7 ns and 230 mJ in 170 ps, respectively, are reported. Intense emission was observed in the 2–8 nm spectral region resulting from unresolved transition arrays (UTAs) due to 3d–4p, 3d–4f and 3p–3d transitions. These transitions in a number of ion stages of yttrium, niobium, ruthenium and rhodium were identified by comparison with results from Cowan code calculations and previous studies. The theoretical data were parameterized using the UTA formalism and the mean wavelength and widths were calculated and compared with experimental results.

Published: 2015.   Source: J. Phys. B: At. Mol. Opt. Phys. 48 245009 (2015 )

Initiation of vacuum insulator surface high-voltage flashover with electrons produced by laser illumination

Related products:  NL300 series

Authors:  Ya. E. Krasik, J. G. Leopold

In this paper, experiments are described in which cylindrical vacuum insulator samples and samples inclined at 45° relative to the cathode were stressed by microsecond timescale high-voltage pulses and illuminated by focused UV laser beam pulses. In these experiments, we were able to distinguish between flashover initiated by the laser producing only photo-electrons and when plasma is formed. It was shown that flashover is predominantly initiated near the cathode triple junction. Even dense plasma formed near the anode triple junction does not necessarily lead to vacuum surface flashover. The experimental results directly confirm our conjecture that insulator surface breakdown can be avoided by preventing its initiation [J. G. Leopold et al., Phys. Rev. ST Accel. Beams 10, 060401 (2007)] and complement our previous experimental results [J. Z. Gleizer et al., IEEE Trans. Dielectr. Electr. Insul. 21, 2394 (2014) and J. Z. Gleizer et al., J. Appl. Phys. 117, 073301 (2015)].

Published: 2015.   Source: Physics of Plasmas 22, 083109 (2015)

Peculiarity of convergence of shock wave generated by underwater electrical explosion of ring-shaped wire

Related products:  NL300 series

Authors:  D. Shafer, G. R. Toker, V. Tz. Gurovich, S. Gleizer, Ya. E. Krasik

Nanosecond timescale underwater electrical wire explosions of ring-shaped Cu wires were investigated using a pulsed generator with a current amplitude up to 50 kA. It was shown that this type of wire explosion results in the generation of a toroidal shock wave (SW). Time- and space-resolved optical diagnostics were used to determine azimuthal uniformity of the shock wave front and its velocity. It was found that the shock wave preserves its circular front shape in the range of radii 50 μm<r<5 mm. At r≤15μm, azimuthal irregularities of the SW front were obtained indicating the appearance of azimuthal instability. A surprising finding is that the shock wave propagates with a constant velocity of vsw=1.2M, where M is the Mach number. The dynamics of the leading part of the shock wave, based on the oblique shock wave theory, is presented, explaining the constant velocity of the shock wave.

Published: 2013.   Source: Physics of Plasmas 20, 052702 (2013)

Emission properties of ns and ps laser-induced soft x-ray sources using pulsed gas jets

Related products:  SL230 series APL2100 series

Authors:  M. Müller, F.-Ch. Kühl, P. Großmann, P. Vrba, K. Mann

The influcence of the pulse duration on the emission characteristics of nearly debris-free laser-induced plasmas in the soft x-ray region (λ ≈1-5 nm) was investigated, using six different target gases from a pulsed jet. Compared to ns pulses of the same energy, a ps laser generates a smaller, more strongly ionized plasma, being about 10 times brighter than the ns laser plasma. Moreover, the spectra are considerably shifted towards shorter wavelengths. Electron temperatures and densities of the plasma are obtained by comparing the spectra with model calculations using a magneto-hydrodynamic code.

Published: 2013.   Source: Opt. Express 21, 12831-12842 (2013)
Photolysis

Photodissociation, photolysis, or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons. It is defined as the interaction of one or more photons with one target molecule. Photolysis plays an important role in photosynthesis, during which it produces energy by splitting water molecules into gaseous oxygen and hydrogen ions. This part of photosynthesis occurs in the granum of a chloroplast where light is absorbed by chlorophyll. This reacts with water and splits the oxygen and hydrogen molecules apart.

Laser Flash Photolysis is a technique for studying transient chemical and biological species generated by the short intense light pulse from a nanosecond/picosecond/femtosecond pulsed laser source (pump pulse). This intense light pulse creates short lived photo-excited intermediates such as excited states, radicals and ions. All these intermediates are generated in concentrations large enough for chemical and physical interaction to occur and for direct observation of the associated temporally changing absorption characteristics. Typically the absorption of light by the sample is recorded within short time intervals (by a so-called test or probe pulses) to monitor relaxation or reaction processes initiated by the pump pulse. Usage of Optical Parametric Oscillators (OPO) opens new possibilities in spectroscopic experiments.

Publications

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Tunable Wavelength Devices – standalone and integrated OPO sytems
NT230 series – high energy broadly tunable DPSS lasers
NT242 series – broadly tunable kHz pulsed DPSS lasers
NT270 series – tunable wavelength NIR-IR range DPSS lasers
NT342 series – high energy broadly tunable lasers
NT370 series – high energy IR range tunable lasers

Electronic spectroscopy and nanocalorimetry of hydrated magnesium ions [Mg(H2O)n]+, n = 20–70: spontaneous formation of a hydrated electron?

Related products:  NT230 series NT242 series NT342 series

Authors:  T. Taxer, M. Ončák, E. Barwa, Ch. van der Lindea, M. K. Beyer

Hydrated singly charged magnesium ions [Mg(H2O)n]+ are thought to consist of an Mg2+ ion and a hydrated electron for n > 15. This idea is based on mass spectra, which exhibit a transition from [MgOH(H2O)n−1]+ to [Mg(H2O)n]+ around n = 15–22, black-body infrared radiative dissociation, and quantum chemical calculations. Here, we present photodissociation spectra of size-selected [Mg(H2O)n]+ in the range of n = 20–70 measured for photon energies of 1.0–5.0 eV. The spectra exhibit a broad absorption from 1.4 to 3.2 eV, with two local maxima around 1.7–1.8 eV and 2.1–2.5 eV, depending on cluster size. The spectra shift slowly from n = 20 to n = 50, but no significant change is observed for n = 50–70. Quantum chemical modeling of the spectra yields several candidates for the observed absorptions, including five- and six-fold coordinated Mg2+ with a hydrated electron in its immediate vicinity, as well as a solvent-separated Mg2+/e pair. The photochemical behavior resembles that of the hydrated electron, with barrierless interconversion into the ground state following the excitation.

Published: 2018.   Source: Faraday Discuss., 2019, Advance Article

Photochemistry and spectroscopy of small hydrated magnesium clusters Mg+(H2O)n, n = 1–5

Related products:  NT230 series NT242 series NT342 series

Authors:  M. Ončák, T. Taxer, E. Barwa, Ch. van der Linde, M. K. Beyer

Hydrated singly charged magnesium ions Mg+(H2O)n, n ≤ 5, in the gas phase are ideal model systems to study photochemical hydrogen evolution since atomic hydrogen is formed over a wide range of wavelengths, with a strong cluster size dependence. Mass selected clusters are stored in the cell of an Fourier transform ion cyclotron resonance mass spectrometer at a temperature of 130 K for several seconds, which allows thermal equilibration via blackbody radiation. Tunable laser light is used for photodissociation. Strong transitions to D1–3 states (correlating with the 3s-3px,y,z transitions of Mg+) are observed for all cluster sizes, as well as a second absorption band at 4–5 eV for n = 3-5. Due to the lifted degeneracy of the 3px,y,z energy levels of Mg+, the absorptions are broad and red shifted with increasing coordination number of the Mg+ center, from 4.5 eV for n = 1 to 1.8 eV for n = 5. In all cases, H atom formation is the dominant photochemical reaction channel. Quantum chemical calculations using the full range of methods for excited state calculations reproduce the experimental spectra and explain all observed features. In particular, they show that H atom formation occurs in excited states, where the potential energy surface becomes repulsive along the O⋯H coordinate at relatively small distances. The loss of H2O, although thermochemically favorable, is a minor channel because, at least for the clusters n = 1-3, the conical intersection through which the system could relax to the electronic ground state is too high in energy. In some absorption bands, sequential absorption of multiple photons is required for photodissociation. For n = 1, these multiphoton spectra can be modeled on the basis of quantum chemical calculations.

Published: 2018.   Source: J. Chem. Phys. 149, 044309 (2018)

Photodissociation of Sodium Iodide Clusters Doped with Small Hydrocarbons

Related products:  NT230 series NT242 series NT270 series NT342 series NT370 series

Authors:  N. K. Bersenkowitsch, Dr. M. Ončák, J. Heller, Dr. Ch. van der Linde, Prof. Dr. M. K. Beyer

Marine aerosols consist of a variety of compounds and play an important role in many atmospheric processes. In the present study, sodium iodide clusters with their simple isotope pattern serve as model systems for laboratory studies to investigate the role of iodide in the photochemical processing of sea‐salt aerosols. Salt clusters doped with camphor, formate and pyruvate are studied in a Fourier transform ion cyclotron resonance mass spectrometer (FT‐ICR MS) coupled to a tunable laser system in both UV and IR range. The analysis is supported by ab initio calculations of absorption spectra and energetics of dissociative channels. We provide quantitative analysis of IRMPD measurements by reconstructing one‐photon spectra and comparing them with the calculated ones. While neutral camphor is adsorbed on the cluster surface, the formate and pyruvate ions replace an iodide ion. The photodissociation spectra revealed several wavelength‐specific fragmentation pathways, including the carbon dioxide radical anion formed by photolysis of pyruvate. Camphor and pyruvate doped clusters absorb in the spectral region above 290 nm, which is relevant for tropospheric photochemistry, leading to internal conversion followed by intramolecular vibrational redistribution, which leads to decomposition of the cluster. Potential photodissociation products of pyruvate in the actinic region may be formed with a cross section of <2×10−20 cm2, determined by the experimental noise level.

Published: 2018.   Source: Chem. Eur.J. 2018, 24,12433 –12443

Infrared spectroscopy of O˙− and OH− in water clusters: evidence for fast interconversion between O˙− and OH˙OH−

Related products:  NT270 series NT370 series

Authors:  J. Lengyel, M. Ončák, A. Herburger, Ch. van der Lindea, M. K. Beyer

We present infrared multiple photon dissociation (IRMPD) spectra of (H2O)n and (H2O)nOH cluster ensembles for [n with combining macron] ≈ 8 and 47 in the range of 2400–4000 cm−1. Both hydrated ions exhibit the same spectral features, in good agreement with theoretical calculations. Decomposition of the calculated spectra shows that bands originating from H2O⋯O˙ and H2O⋯OH interactions span almost the whole spectral region of interest. Experimentally, evaporation of OH˙ is observed to a small extent, which requires interconversion of (H2O)n into (H2O)n–1OH˙OH, with subsequent H2O evaporation preferred over OH˙ evaporation. The modeling shows that (H2O)n and (H2O)n–1OH˙OH cannot be distinguished by IRMPD spectroscopy.

Published: 2017.   Source: Phys. Chem. Chem. Phys., 2017,19, 25346-25351

Photodissociation spectroscopy of protonated leucine enkephalin

Related products:  NT230 series NT242 series NT342 series

Authors:  A. Herburger, Ch. van der Linde, M. K. Beyer

Protonated leucine enkephalin (YGGFL) was studied by ultraviolet photodissociation (UVPD) from 225 to 300 nm utilizing an optical parametric oscillator tunable wavelength laser system (OPO). Fragments were identified by absolute mass measurement in a 9.4 T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). Bond cleavage was preferred in the vicinity of the two aromatic residues, resulting in high ion abundances for a4, a1, b3, y2 and y1 fragments. a, b and y ions dominated the mass spectrum, and full sequence coverage was achieved for those types. Photodissociation was most effective at the short wavelength end of the studied range, which is assigned to the onset of the La π–π* transition of the tyrosine chromophore, but worked well also at the Lb π–π* chromophore absorption maxima in the 35 000–39 000 cm−1 region. Several side-chain and internal fragments were observed. H atom loss is observed only above 41 000 cm−1, consistent with the requirement of a curve crossing to a repulsive 1πσ* state. It is suggested that the photochemically generated mobile H atom plays a role in further backbone cleavages, similar to the mechanism for electron capture dissociation. The b4 fragment is most intense at the Lb chromophore absorptions, undergoing additional fragmentation at higher photon energies. The high resolution of the FT-ICR MS revealed that out of all x and z-type fragments only x3 and x4 were formed, with low intensity. Other previously reported x- and z-fragments were re-assigned to internal fragments, based on exact mass measurement.

Published: 2017.   Source: Phys. Chem. Chem. Phys., 2017,19, 10786-10795
SFG Spectroscopy
Principle of Sum Frequency Generation Vibrational Spectroscopy

Sum frequency generation vibrational spectroscopy (SFG-VS) is used to characterize vibrational bonds of molecules at surfaces or interfaces. SFG spectroscopy is particularly attractive because of molecular specificity and intrinsic interfacial sensitivity. Surface sensitivity of the technique arises from the fact that within the electric dipole approximation the nonlinear generation of the sum-frequency (SF) signal from the overlapped visible and infrared beams is forbidden in the media of randomly oriented molecules or in the centrosymmetric media but is allowed at the interface where inversion symmetry is broken. Molecular specificity emerges from the ability to record vibrational spectrum.

In SFG-VS measurements, carried out by employing SFG spectromer, a pulsed tunable infrared IR (ωIR) laser beam is mixed with a visible VIS (ωVIS) beam to produce an output at the sum frequency (ωSFG = ωIR + ωVIS). SFG signal is generated in visible spectral range, so it can be efficiently measured using sensitive detectors (PMT or CCD).

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All Products
Picosecond Lasers
PL2230 series – diode pumped high energy picosecond Nd:YAG lasers
PL2250 series – flash-lamp pumped picosecond Nd:YAG lasers
Tunable Wavelength Devices – standalone and integrated OPO sytems
PGx01 series – high energy broadly tunable picosecond OPA
PGx03 series – picosecond kHz repetition rate tunable OPA
PGx11 series – transform limited broadly tunable picosecond OPA
Laser Spectroscopy
SFG Spectrometer – Sum frequency generation vibrational spectrometer

Structure Determination of Hen Egg-White Lysozyme Aggregates Adsorbed to Lipid/Water and Air/Water Interfaces

Related products:  SFG Spectrometer PL2230 series PGx01 series PGx03 series PGx11 series

Authors:  S. Strazdaite, E. Navakauskas, J. Kirschner, T. Sneideris, G. Niaura

We use vibrational sum-frequency generation (VSFG) spectroscopy to study the structure of hen egg-white lysozyme (HEWL) aggregates adsorbed to DOPG/D2O and air/D2O interfaces. We find that aggregates with a parallel and antiparallel β-sheet structure together with smaller unordered aggregates and a denaturated protein are adsorbed to both interfaces. We demonstrate that to retrieve this information, fitting of the VSFG spectra is essential. The number of bands contributing to the VSFG spectrum might be misinterpreted, due to interference between peaks with opposite orientation and a nonresonant background. Our study identified hydrophobicity as the main driving force for adsorption to the air/D2O interface. Adsorption to the DOPG/D2O interface is also influenced by hydrophobic interaction; however, electrostatic interaction between the charged protein’s groups and the lipid’s headgroups has the most significant effect on the adsorption. We find that the intensity of the VSFG spectrum at the DOPG/D2O interface is strongly enhanced by varying the pH of the solution. We show that this change is not due to a change of lysozyme’s and its aggregates’ charge but due to dipole reorientation at the DOPG/D2O interface. This finding suggests that extra care must be taken when interpreting the VSFG spectrum of proteins adsorbed at the lipid/water interface.

Published: 2020.   Source: Langmuir 2020, 36, 17, 4766-4775

Hydrogen bonding interactions of H2O and SiOH on a boroaluminosilicate glass corroded in aqueous solution

Related products:  SFG Spectrometer

Authors:  D. Ngo, H. Liu, Z. Chen, H. Kaya, T.J. Zimudzi, S. Gin, T. Mahadevan, J. Du, S.H. Kim

Hydrogen bonding interactions play an important role in many chemical and physical processes occurring in bulk liquids and at interfaces. In this study, hydrous species (H2O and Si-OH) on nano-porous alteration layers (gels) formed on a boroaluminosilicate glass called International Simple Glass corroded in aqueous solutions at pH 7 and pH 9, and initially saturated with soluble silicon-containing species were analyzed using linear and non-linear vibrational spectroscopy in combination with molecular dynamics simulations. The simulation results revealed various possible types of hydrogen bonds among these hydrous species in nanoconfinement environments with their populations depending on pore-size distribution. The nano-porous gels formed on corroded glass surfaces enhance hydrogen bond strength between hydrous species as revealed by attenuated total reflectance infrared spectroscopy. Sum frequency generation spectroscopy showed some significant differences in hydrogen bonding interactions on alteration layers formed at pH 7 and pH 9. The glass dissolution under the leaching conditions used in this study has been known to be ten times faster at pH 7 in comparison to that at pH 9 due to unknown reasons. The simulation and experimental results obtained in this study indicate that the water mobility in the gel formed at pH 9 could be slower than that in the gel formed at pH 7, and as a result, the leaching rate at pH 9 is slower than that at pH 7.

Published: 2020.   Source: npj Mater Degrad 4, 1 (2020)

Vibrational Relaxation Lifetime of a Physisorbed Molecule at a Metal Surface

Related products:  SFG Spectrometer PL2230 series PGx11 series

Authors:  S. Kumar, H. Jiang, M. Schwarzer, A. Kandratsenka, D. Schwarzer, A.M. Wodtke

Previous measurements of vibrational relaxation lifetimes for molecules adsorbed at metal surfaces yielded values of 1–3 ps; however, only chemisorbed molecules have been studied. We report the first measurements of the vibrational relaxation lifetime of a molecule physisorbed to a metal surface. For CO(v=1) adsorbed on Au(111) at 35 K the vibrational lifetime of the excited stretching mode is 49±3  ps. The long lifetime seen here is likely to be a general feature of physisorption, which involves weaker electronic coupling between the adsorbate and the solid due to bonding at larger distances.

Published: 2019.   Source: Phys. Rev. Lett. 123, 156101

Reconfiguration of interfacial energy band structure for high-performance inverted structure perovskite solar cells

Related products:  SFG Spectrometer

Authors:  Moyao Zhang, Qi Chen, Rongming Xue, Yu Zhan, Cheng Wang, Junqi Lai, Jin Yang, Hongzhen Lin, Jianlin Yao, Yaowen Li, Liwei Chen, Yongfang Li

Charged defects at the surface of the organic–inorganic perovskite active layer are detrimental to solar cells due to exacerbated charge carrier recombination. Here we show that charged surface defects can be benign after passivation and further exploited for reconfiguration of interfacial energy band structure. Based on the electrostatic interaction between oppositely charged ions, Lewis-acid-featured fullerene skeleton after iodide ionization (PCBB-3N-3I) not only efficiently passivates positively charged surface defects but also assembles on top of the perovskite active layer with preferred orientation. Consequently, PCBB-3N-3I with a strong molecular electric dipole forms a dipole interlayer to reconfigure interfacial energy band structure, leading to enhanced built-in potential and charge collection. As a result, inverted structure planar heterojunction perovskite solar cells exhibit the promising power conversion efficiency of 21.1% and robust ambient stability. This work opens up a new window to boost perovskite solar cells via rational exploitation of charged defects beyond passivation.

Published: 2019.   Source: Nature Communications, volume 10, article number: 4593 (2019)

Heavy Anionic Complex Creates a Unique Water Structure at a Soft Charged Interface

Related products:  SFG Spectrometer PL2230 series PGx01 series

Authors:  W. Rock, B. Qiao, T. Zhou, A. E. Clark, A. Uysal

Ion hydration and interfacial water play crucial roles in numerous phenomena ranging from biological to industrial systems. Although biologically relevant (and mostly smaller) ions have been studied extensively in this context, very little experimental data exist about molecular-scale behavior of heavy ions and their complexes at interfaces, especially under technologically significant conditions. It has recently been shown that PtCl6 2− complexes adsorb at positively charged interfaces in a two-step process that cannot fit into well-known empirical trends, such as Hofmeister series. Here, a combined vibrational sum frequency generation and molecular dynamics study reveals that a unique interfacial water structure is connected to this peculiar adsorption behavior. A novel subensemble analysis of molecular dynamics simulation results shows that after adsorption PtCl6 2− complexes partially retain their first and second hydration spheres and that it is possible to identify three different types of water molecules around them on the basis of their orientational structures and hydrogen-bonding strengths. These results have important implications for relating interfacial water structure and hydration enthalpy to the general understanding of specific ion effects.

This in turn influences interpretation of heavy metal ion distribution across, and reactivity within, liquid interfaces.

Published: 2018.   Source: J. Phys. Chem. C 2018, 122, 29228−29236

Vibrational fingerprint of localized excitons in a two-dimensional metal-organic crystal

Related products:  SFG Spectrometer PL2230 series PGx01 series

Authors:  M. Corva, A. Ferrari, M. Rinaldi, Z. Feng, M. Roiaz, C. Rameshan, G. Rupprechter, R. Costantini, M. Dell’Angela, G. Pastore, G. Comelli, N. Seriani, E. Vesselli

Long-lived excitons formed upon visible light absorption play an essential role in photovoltaics, photocatalysis, and even in high-density information storage. Here, we describe a self-assembled two-dimensional metal-organic crystal, composed of graphene-supported macrocycles, each hosting a single FeN4 center, where a single carbon monoxide molecule can adsorb. In this heme-like biomimetic model system, excitons are generated by visible laser light upon a spin transition associated with the layer 2D crystallinity, and are simultaneously detected via the carbon monoxide ligand stretching mode at room temperature and near-ambient pressure. The proposed mechanism is supported by the results of infrared and time-resolved pump-probe spectroscopies, and by ab initio theoretical methods, opening a path towards the handling of exciton dynamics on 2D biomimetic crystals.

Published: 2018.   Source: Nature Communications, vol. 9, art. 4703 (2018)

Aggregation States of Poly(4-methylpentene-1) at a Solid Interface

Related products:  SFG Spectrometer PL2250 series

Authors:  K. Yamamoto, D. Kawaguchi, K. Sasahara, M. Inutsuka, S. Yamamoto, K. Uchida, K. Mita, H. Ogawa, M. Takenaka, K. Tanaka

A thin film of poly(4-methylpentene-1) (P4MP1) was prepared on a quartz substrate, which was a model system of an interface in filler-reinforced semicrystalline polymer composites. Grazing-incidence wide-angle X-ray diffraction measurements revealed that P4MP1 in the thin film after isothermal crystallization formed a Form I crystal polymorph composed of a tetragonal unit cell with a 72 helix, in which the chain axis was oriented along the direction parallel to the quartz interface. Combining sum-frequency generation vibrational spectroscopy with molecular dynamics simulation enabled us to gain access to the local conformation of P4MP1 chains at the quartz interface and the changes that occurred with isothermal crystallization. Finally, the way in which the initial chain orientation at the substrate interface impacted the crystalline structure in the thin film was discussed.

Published: 2018.   Source: Polymer Journal vol. 51, pp. 247–255 (2019)

Segregation of an amine component in a model epoxy resin at a copper interface

Related products:  SFG Spectrometer

Authors:  M. Aoki, A. Shundo, K. Okamoto, T. Ganbe, K. Tanaka

We have studied the chemical composition of the epoxy and amine components, HDGEBA and CBMA, of an epoxy resin in close proximity to a copper interface by using ADXPS in conjunction with SFG vibrational spectroscopy. A bilayer sample of epoxy resin and copper was first prepared on a solid substrate before etching the copper layer just before the interface with Ar+ beams. Using ADXPS, in which an incident X-ray was guided from the copper surface, it was found that the CBMA component was preferentially segregated at the copper interface, with the segregation extending over ~10 nm. SFG spectroscopy was used to confirm the above observation. Postulating that copper ions diffused from the metal copper into the internal phase during the curing process and reacted with amine groups to form copper complexes, the interfacial segregation of CBMA can be understood. This knowledge should be useful for understanding and controlling the adhesive properties of epoxy resins.

Published: 2018.   Source: Polymer Journal vol. 51, pp. 359–363 (2019)

High-performance graphdiyne-based electrochemical actuators

Related products:  SFG Spectrometer

Authors:  C. Lu, Y. Yang, J. Wang, R. Fu, X. Zhao, L. Zhao, Y. Ming, Y. Hu, H. Lin, X. Tao, Y. Li, W. Chen

Electrochemical actuators directly converting electrical energy to mechanical energy are critically important for artificial intelligence. However, their energy transduction efficiency is always lower than 1.0% because electrode materials lack active units in microstructure, and their assembly systems can hardly express the intrinsic properties. Here, we report a molecular-scale active graphdiyne-based electrochemical actuator with a high electro-mechanical transduction efficiency of up to 6.03%, exceeding that of the best-known piezoelectric ceramic, shape memory alloy and electroactive polymer reported before, and its energy density (11.5 kJ m−3) is comparable to that of mammalian skeletal muscle (~8 kJ m−3). Meanwhile, the actuator remains responsive at frequencies from 0.1 to 30 Hz with excellent cycling stability over 100,000 cycles. Furthermore, we verify the alkene–alkyne complex transition effect responsible for the high performance through in situ sum frequency generation spectroscopy. This discovery sheds light on our understanding of actuation mechanisms and will accelerate development of smart actuators.

Published: 2018.   Source: Nature Communications vol. 9, Art. nr: 752 (2018)

Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces

Related products:  SFG Spectrometer

Authors:  C. Zhang

Sum frequency generation vibrational spectroscopy (SFG-VS) has become one of the most appealing technologies to characterize molecular structures at interfaces. In this focal point review, we focus on SFG-VS studies at buried polymer interfaces and review many of the recent publications in the field. We also cover the essential theoretical background of SFG-VS and discuss the experimental implementation of SFG-VS.

Published: 2017.   Source: Applied Spectroscopy 2017, Vol. 71(8) 1717–1749

Structure of the Fundamental Lipopeptide Surfactin at the Air/Water Interface Investigated by Sum Frequency Generation Spectroscopy

Related products:  SFG Spectrometer PL2230 series PGx01 series

Authors:  S. A. Goussous, M. T. L. Casford, A. C. Murphy, G. P. C. Salmond, F. J. Leeper and P. B. Davies

The lipopeptide surfactin produced by certain strains of Bacillus subtilis is a powerful bio-surfactant possessing potentially useful antimicrobial properties. In order to better understand its surface behaviour, we have used surface sensitive Sum Frequency Generation (SFG) vibrational spectroscopy in the C-H and C=O stretching regions to determine its structure at the air/water interface. Using surfactin with the leucine groups of the peptide ring perdeuterated we have shown that the majority of the SFG signals arise from the 4 leucine residues. We find that surfactin forms a robust film, and that its structure is not affected by the number density at the interface or by pH variation of the sub-phase. The spectra show that the ring of the molecule lies in the plane of the surface rather than perpendicular to it, with the tail lying above this, also in the plane of the interface.

Published: 2017.   Source: J. Phys. Chem. B 2017, 121, 19, 5072-5077

A structural and temporal study of the surfactants behenyltrimethylammonium methosulfate and behenyltrimethylammonium chloride adsorbed at air/water and air/glass interfaces using sum frequency generation spectroscopy

Related products:  SFG Spectrometer PL2230 series PGx01 series

Authors:  S. A. Goussous, M. T.L. Casford, S. A. Johnson, P. B. Davies

Molecular scale information about the structure of surfactants at interfaces underlies their application in consumer products. In this study the non-linear optical technique of Sum Frequency Generation (SFG) vibrational spectroscopy has been used to investigate the structure and temporal behaviour of two cationic surfactants used frequently in hair conditioners. SFG spectra of films of behenyltrimethylammonium methosulfate (BTMS) and behenyltrimethylammonium chloride (BTAC) were recorded at the air/water interface and on glass slides following Langmuir Blodgett (LB) deposition. The assignment of the BTMS and BTAC spectral features (resonances) to the C--H stretching modes of the surfactants was consolidated by comparison with the SFG spectrum of deuterated cetyltrimethylammonium bromide (d-CTAB) and by recording spectra on D2O as well as on water. The C--H resonances arise from the methylene and methyl groups of the tail and head-groups of the surfactants. A slow collapse mechanism was observed following film compression of both BTAC and BTMS. The change in molecular structure of the films undergoing this slow collapse was followed by recording sequential SFG spectra in the C--H region, and by monitoring the SFG intensity at specific wavenumbers over time. Additionally, LB deposition onto glass was used to capture the state of the film during the slow collapse, and these SFG spectra showed close similarity to the corresponding spectra on water. Complementary Atomic Force Microscopy (AFM) was used to elucidate the layering of the compressed and relaxed films deposited onto mica by LB deposition.

Published: 2017.   Source: Journal of Colloid and Interface Science 488 (2017) 365–372

Discovery of Cellulose Surface Layer Conformation by Nonlinear Vibrational Spectroscopy

Related products:  SFG Spectrometer

Authors:  Libing Zhang, Li Fu, Hong-fei Wang, Bin Yang

Significant questions remain in respect to cellulose’s structure and polymorphs, particularly the cellulose surface layers and the bulk crystalline core as well as the conformational differences. Total Internal Reflection Sum Frequency Generation Vibrational Spectroscopy (TIR-SFG-VS) combined with conventional SFG-VS (non-TIR) enables selectively characterizing the molecular structures of surface layers and the crystalline core of cellulose, revealing their differences for the first time. From the SFG spectra in the C-H and O-H regions, we found that the surface layers of Avicel are essentially amorphous while the surface layers of Iβ cellulose are crystalline but with different structural and spectroscopic signatures compared with its crystalline core. The differences between hydrogen bonding networks of cellulose surface and crystalline core were also shown by the SFG signal. The discovery here represents yet another instance of the importance of spectroscopic observations in transformative advances to understand the structure of the cellulosic biomass.

Published: 2017.   Source: Scientific Reports, vol. 7, art. 44319 (2017)

Sum frequency generation vibrational spectroscopy (SFG-VS) for complex molecular surfaces and interfaces: Spectral lineshape measurement and analysis plus some controversial issues

Related products:  SFG Spectrometer

Authors:  Hong-Fei Wang

Sum-frequency generation vibrational spectroscopy (SFG-VS) was first developed in the 1980s and it has been proven a uniquely sensitive and surface/interface selective spectroscopic probe for characterization of the structure, conformation and dynamics of molecular surfaces and interfaces. In recent years, there have been many progresses in the development of methodology and instrumentation in the SFG-VS toolbox that have significantly broadened the application to complex molecular surfaces and interfaces. In this review, after presenting a unified view on the theory and methodology focusing on the SFG-VS spectral lineshape, as well as the new opportunities in SFG-VS applications with such developments, some of the controversial issues that have been puzzling the community are discussed. The aim of this review is to present to the researchers and students interested in molecular surfaces and interfacial sciences up-to-date perspectives complementary to the existing textbooks and reviews on SFG-VS.

Published: 2016.   Source: Progress in Surface Science, Vol. 91, Is. 4, Dec. 2016, pp. 155-182

The complex nature of calcium cation interactions with phospholipid bilayers

Related products:  SFG Spectrometer

Authors:  Adéla Melcrová, Sarka Pokorna, Saranya Pullanchery, Miriam Kohagen, Piotr Jurkiewicz, Martin Hof, Pavel Jungwirth, Paul S. Cremer, Lukasz Cwiklik

Understanding interactions of calcium with lipid membranes at the molecular level is of great importance in light of their involvement in calcium signaling, association of proteins with cellular membranes, and membrane fusion. We quantify these interactions in detail by employing a combination of spectroscopic methods with atomistic molecular dynamics simulations. Namely, time-resolved fluorescent spectroscopy of lipid vesicles and vibrational sum frequency spectroscopy of lipid monolayers are used to characterize local binding sites of calcium in zwitterionic and anionic model lipid assemblies, while dynamic light scattering and zeta potential measurements are employed for macroscopic characterization of lipid vesicles in calcium-containing environments. To gain additional atomic-level information, the experiments are complemented by molecular simulations that utilize an accurate force field for calcium ions with scaled charges effectively accounting for electronic polarization effects. We demonstrate that lipid membranes have substantial calcium-binding capacity, with several types of binding sites present. Significantly, the binding mode depends on calcium concentration with important implications for calcium buffering, synaptic plasticity, and protein-membrane association.

Published: 2016.   Source: Scientific Reports, vol. 6, art. 38035 (2016)

Platelet-adhesion behavior synchronized with surface rearrangement in a film of poly(methyl methacrylate) terminated with elemental blocks

Related products:  SFG Spectrometer

Authors:  Hisao Matsuno, Ryota Tsukamoto, Shinichiro Shimomura, Tomoyasu Hirai, Yukari Oda, Keiji Tanaka

Poly(methyl methacrylate) (PMMA) terminated with elemental blocks containing polyhedral oligomeric silsesquioxane (POSS), hereafter referred to as PPMP, was synthesized by living anionic polymerization. Combining modern interfacial-sensitive spectroscopy with traditional contact angle measurements, static and dynamic structures at the surface of PPMP films in water were examined. The surface of the well-annealed PPMP films, where the POSS end groups were preferentially segregated, was flat at the sub-nanometer level. Once the PPMP film was immersed in water, the surface was reorganized, and the rate was much slower for PPMP than for the conventional PMMA. This implies that the POSS units hindered the interfacial dynamics of the polymer segments. Then, platelet-adhesion tests were performed on the PPMP films. The number of platelets adhered to the PPMP film was dependent on the pre-immersion time in phosphate-buffered saline before the platelet seeding, whereas that of the reference PMMA film was unaffected by the pre-immersion time. These results could be explained in terms of the aggregation states of water at the interface.

Published: 2016.   Source: Polymer Journal, vol. 48, pp. 413–419 (2016)

Retrieval of complex χ(2) parts for quantitative analysis of sum-frequency generation intensity spectra

Related products:  SFG Spectrometer PL2230 series PGx01 series PGx11 series

Authors:  M. J. Hofmann, P. Koelsch

Vibrational sum-frequency generation (SFG) spectroscopy has become an established technique for in situ surface analysis. While spectral recording procedures and hardware have been optimized, unique data analysis routines have yet to be established. The SFG intensity is related to probing geometries and properties of the system under investigation such as the absolute square of the second-order susceptibility |χ(2)|2 . A conventional SFG intensity measurement does not grant access to the complex parts of χ(2) unless further assumptions have been made. It is therefore difficult, sometimes impossible, to establish a unique fitting solution for SFG intensity spectra. Recently, interferometric phase-sensitive SFG or heterodyne detection methods have been introduced to measure real and imaginary parts of χ(2) experimentally. Here, we demonstrate that iterative phase-matching between complex spectra retrieved from maximum entropy method analysis and fitting of intensity SFG spectra (iMEMfit) leads to a unique solution for the complex parts of χ(2) and enables quantitative analysis of SFG intensity spectra. A comparison between complex parts retrieved by iMEMfit applied to intensity spectra and phase sensitive experimental data shows excellent agreement between the two methods.

Published: 2015.   Source: J Chem Phys. 2015 Oct 7; 143(13): 134112.

Quantitative Sum-Frequency Generation Vibrational Spectroscopy of Molecular Surfaces and Interfaces: Lineshape, Polarization, and Orientation

Related products:  SFG Spectrometer PL2230 series PGx01 series

Authors:  H. Wang, L. Velarde, W. Gan, L. Fu

Sum-frequency generation vibrational spectroscopy (SFG-VS) can provide detailed information and understanding of the molecular composition, interactions, and orientational and conformational structure of surfaces and interfaces through quantitative measurement and analysis. In this review, we present the current status of and discuss important recent developments in the measurement of intrinsic SFG spectral lineshapes and formulations for polarization measurements and orientational analysis of SFG-VS spectra. The focus of this review is to present a coherent description of SFG-VS and discuss the main concepts and issues that can help advance this technique as a quantitative analytical research tool for revealing the chemistry and physics of complex molecular surfaces and interfaces.

Published: 2015.   Source: Annu. Rev. Phys. Chem. 2015. 66:189–216

Unified treatment and measurement of the spectral resolution and temporal effects in frequency-resolved sum-frequency generation vibrational spectroscopy (SFG-VS)

Related products:  SFG Spectrometer PL2230 series PGx01 series PGx11 series

Authors:  L. Velarde, H. F. Wang

The lack of understanding of the temporal effects and the restricted ability to control experimental conditions in order to obtain intrinsic spectral lineshapes in surface sum-frequency generation vibrational spectroscopy (SFG-VS) have limited its applications in surface and interfacial studies. The emergence of high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) with sub-wavenumber resolution [Velarde et al., J. Chem. Phys., 2011, 135, 241102] offers new opportunities for obtaining and understanding the spectral lineshapes and temporal effects in SFG-VS. Particularly, the high accuracy of the HR-BB-SFG-VS experimental lineshape provides detailed information on the complex coherent vibrational dynamics through direct spectral measurements. Here we present a unified formalism for the theoretical and experimental routes for obtaining an accurate lineshape of the SFG response. Then, we present a detailed analysis of a cholesterol monolayer at the air/water interface with higher and lower resolution SFG spectra along with their temporal response. With higher spectral resolution and accurate vibrational spectral lineshapes, it is shown that the parameters of the experimental SFG spectra can be used both to understand and to quantitatively reproduce the temporal effects in lower resolution SFG measurements. This perspective provides not only a unified picture but also a novel experimental approach to measuring and understanding the frequency-domain and time-domain SFG response of a complex molecular interface.

Published: 2013.   Source: Phys. Chem. Chem. Phys., 2013, 15, 19970-19984

Alkanethiols as Inhibitors for the Atmospheric Corrosion of Copper Induced by Formic Acid: Effect of Chain Length

Related products:  SFG Spectrometer

Authors:  S. Hosseinpour, C. M. Johnson, Ch. Leygraf

Self assembled monolayers (SAMs) of n-alkanethiols of different chain length (4, 6, 8, 12, and 18 carbons in the chain) have been explored as corrosion inhibitors for copper exposed to humidified air containing formic acid, an environment used to mimic accelerated indoor atmospheric corrosion. Near-surface sensitive in-situ infrared reflection/absorption spectroscopy combined with interface sensitive vibrational sum frequency spectroscopy revealed unique molecular information on the role of each SAM during ongoing corrosion. All SAMs protect copper against corrosion, and this ability increases continuously with chain length. Their structural order is high prior to exposure, but an increased disorder is observed as a result of the corrosion process. The protection ability of the SAMs is attributed to a selective hindrance of the corrosion stimulators water, oxygen gas, and formic acid to reach the copper-SAM interface through each SAM, which results in different corrosion mechanisms on SAM protected copper and unprotected copper. This significantly retards the formation of the corrosion products copper hydroxide and copper formate, and results in essentially no formation of cuprite.

Published: 2013.   Source: J. Electrochem. Soc. 160, C270 (2013)

Simultaneous measurement of magnitude and phase in interferometric sum-frequency vibrational spectroscopy

Related products:  SFG Spectrometer

Authors:  P. A. Covert, W. R. FitzGerald, D. K. Horea

We present a visible-infrared sum-frequency spectroscopic technique that is capable of simultaneously determining the magnitude and phase of the sample response from a single set of experimental conditions. This is especially valuable in cases where the phase stability is high, as in collinear beam geometries, as it enables multiple experiments to be performed without re-measuring the local oscillator phase or the reference phase. After illustrating the phase stability achievable with such a geometry, we provide a technique for quantitatively determining the magnitude and phase from a single set of two-dimensional spectral-temporal interference fringes. A complete demonstration is provided for the C–H stretching frequency region at the surface of an octadecyltricholosilane film.

Published: 2012.   Source: J. Chem. Phys. 137, 014201 (2012)

Phase measurement in nondegenerate three-wave mixing spectroscopy

Related products:  SFG Spectrometer

Authors:  K. C. Jena, P. A. Covert, D. K. Horea

A detailed model is presented that describes the temporal and spectral interference patterns resulting from phase-recovery infrared–visible sum-frequency spectroscopy. Included in this model are the effects of dispersive elements other than the phase shifting unit placed between the sample and local oscillator signals. This inclusion is critical when considering the interference patterns arising from studies of buried interfaces. Furthermore, in the midinfrared where it is difficult to have high visibility of the fringes, it is demonstrated that local field corrections have a significant effect on the shape of the interference pattern. By collecting and subsequently fitting a two-dimensional interference pattern displaying both temporal and spectral fringes, a complete characterization of all these effects is possible.

Published: 2011.   Source: J. Chem. Phys. 134, 044712 (2011)

Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy

Related products:  SFG Spectrometer PL2230 series PGx01 series PGx11 series

Authors:  Z. Chen

This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liquid and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liquids. It has been found that molecular interactions at such polymer/liquid interfaces dictate interfacial polymer structures. The molecular structures of silane molecules, which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing molecularlevel understanding of polymer adhesion promotion. The molecular structures of polymer/solid interfaces have been examined using SFG with several different experimental geometries. These results have provided molecularlevel information about polymer friction, adhesion, interfacial chemical reactions, interfacial electronic properties, and the structure of layerbylayer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive analytical techniques.

Published: 2010.   Source: Prog Polym Sci. 2010 Nov 1; 35(11): 1376–1402.

Probing the Orientation and Conformation of α-Helix and β-Strand Model Peptides on Self-Assembled Monolayers Using Sum Frequency Generation and NEXAFS Spectroscopy

Related products:  SFG Spectrometer PL2230 series PGx01 series

Authors:  Tobias Weidner, Julia S. Apte, Lara J. Gamble, David G. Castner

The structure and orientation of amphiphilic α-helix and β-strand model peptide films on self-assembled monolayers (SAMs) have been studied with sum frequency generation (SFG) vibrational spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The α-helix peptide is a 14-mer, and the β-strand is a 15-mer of hydrophilic lysine and hydrophobic leucine residues with hydrophobic periodicities of 3.5 and 2, respectively. These periodicities result in the leucine side chains located on one side of the peptides and the lysine side chains on the other side. The SAMs were prepared from the assembly of either carboxylic acid- or methyl-terminated alkyl thiols onto gold surfaces. For SFG studies, the deuterated analog of the methyl SAM was used. SFG vibrational spectra in the C-H region of air-dried peptides films on both SAMs exhibit strong peaks near 2965, 2940, and 2875 cm-1 related to ordered leucine side chains. The orientation of the leucine side chains was determined from the phase of these features relative to the nonresonant gold background. The relative phase for both the R-helix and β-strand peptides showed that the leucine side chains were oriented away from the carboxylic acid SAM surface and oriented toward the methyl SAM surface. Amide I peaks observed near 1656 cm-1 for the R-helix peptide confirm that the secondary structure is preserved on both SAMs. Strong linear dichroism related to the amide π* orbital at 400.8 eV was observed in the nitrogen K-edge NEXAFS spectra for the adsorbed β-strand peptides, suggesting that the peptide backbones are oriented parallel to the SAM surface with the side chains pointing toward or away from the interface. For the α-helix the dichroism of the amide π* is significantly weaker, probably because of the broad distribution of amide bond orientations in the α-helix secondary structure.

Published: 2009.   Source: Langmuir 2010, 26(5), 3433–3440

Study of self-assembled triethoxysilane thin films made by casting neat reagents in ambient atmosphere

Related products:  SFG Spectrometer PL2230 series

Authors:  Y. Yang, A. M. Bittner, S. Baldelli, K. Kern

We studied four trialkoxysilane thin films, fabricated via self-assembly by casting neat silane reagents onto hydrophilic SiOx/Si substrates in the ambient. This drop-casting method is simple, yet rarely studied for the production of silane self-assembled monolayers (SAMs). Various ex-situ techniques were utilized to systematically characterize the growth process: Ellipsometry measurements can monitor the evolution of film thickness with silanization time; water droplet contact angle measurements reveal the wettability; the change of surface morphology was followed by Atomic Force Microscopy; the chemical identity of the films was verified by Infrared–Visible Sum Frequency Generation spectroscopy. We show that the shorter carbon chain (propyl-) or branched (2-(diphenylphosphino)ethyl-) silane SAMs exhibit poor ordering. In contrast, longer carbon chain (octadecyl and decyl) silanes form relatively ordered monolayers. The growth of the latter two cases shows Langmuir-like kinetics and a transition process from lying-down to standing-up geometry with increasing coverage.

Published: 2007.   Source: Thin Solid Films, Vol. 516, Iss. 12, 2008, pp. 3948-3956
SHG Spectroscopy / Microscopy

Second harmonic generation (SHG) is a second order nonlinear optical effect where two photons of frequency ω are converted to one photon of frequency 2ω. SHG is allowed only in media without inversion symmetry. This optical method it is non‐invasive, can be applied in situ, and can provide real time resolution.

SHG measurements provide information about: surface coverage, molecular orientation, adsorbtion-desorbtion processes, and reactions at interfaces. SHG has the ability to detect low concentrations of analytes, such as proteins, peptides, and small molecules, due to its high sensitivity, and the second harmonic response can be enhanced through the use of target molecules that are resonant with the incident (ω) and/or second harmonic (2ω) frequencies.

SHG microscopy allows for selective probing of a non-centrosymmetric area of sample. This type of nonlinear optical microscope was first used to observe ferroelectric domains and has been applied to various specimens including the biological samples to date. Imaging of the endogenous SHG of biological tissue can be utilized for the selective observation of filament systems in tissues such as collagen, myosin, and microtubules, which exhibit a polar structure. It has been reported that, by imaging exogenous SHG of the membrane, sensitive detection of membrane damage could be realized using the SHG microscope.

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Industrial Lasers
FemtoLux series – microjoule class industrial grade femtosecond fiber laser

FemtoLux 3 laser for the rapid wide-field second harmonic generation microscopy

Related products:  FemtoLux series

Authors:  D. Rutkauskas, A. Dementjev

FemtoLux 3 laser was used as an illumination source in the wide-field second harmonic generation (SHG) microscope. Relatively high laser pulse energy at a medium pulse repetition frequency allowed for a faster single image acquisition compared to that using laser-scanning. It was also possible to acquire images of relatively large sample areas, which paved the way for the rapid imaging of macroscopic sample areas with microscopic resolution.

Published: 2020.   Source: Ekspla Application notes. Issue № AN2001IL01.
Pump-Probe Spectroscopy

Ultrafast spectroscopy is based on the use of light pulses that have shorter temporal duration compared to the underlying dynamics of the system. Shorter pulse- better time resolution, longer pulse- better spectral resolution. Two pulses separated by time delay are used. First pulse excites (electronic and/or vibrational levels, ionises, make Coulombic explosions, creates plasma, etc.) sample, second delayed pulse probes- checks what happened at the certain time moment. Dynamics of investigated system is recorded by changing delay between pulses. Various experimental techniques are employed to record time-resolved signals: for example transient absorption, four-wave mixing, sum frequency generation. Pump and probe spectral range can vary from UV or even X-ray to far infrared.

The advantage of pump-probe spectroscopy is direct investigation of dynamics. For example: excitation relaxation, energy transfer, photochemical reactions dynamics and movement of particles, structural changes. Tunability of pump and probe pulses opens two dimensional pump-probe spectroscopy where is possible to obtain temporally resolved energy map of system which shows separated and coupled states.

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Picosecond Lasers
PL2210 series – diode pumped picosecond kHz pulsed Nd:YAG lasers
PL2230 series – diode pumped high energy picosecond Nd:YAG lasers
Tunable Wavelength Devices – standalone and integrated OPO sytems
PGx11 series – transform limited broadly tunable picosecond OPA
NT230 series – high energy broadly tunable DPSS lasers
NT242 series – broadly tunable kHz pulsed DPSS lasers
NT342 series – high energy broadly tunable lasers
Laser Spectroscopy
SFG Spectrometer – Sum frequency generation vibrational spectrometer

Vibrational Relaxation Lifetime of a Physisorbed Molecule at a Metal Surface

Related products:  SFG Spectrometer PL2230 series PGx11 series

Authors:  S. Kumar, H. Jiang, M. Schwarzer, A. Kandratsenka, D. Schwarzer, A.M. Wodtke

Previous measurements of vibrational relaxation lifetimes for molecules adsorbed at metal surfaces yielded values of 1–3 ps; however, only chemisorbed molecules have been studied. We report the first measurements of the vibrational relaxation lifetime of a molecule physisorbed to a metal surface. For CO(v=1) adsorbed on Au(111) at 35 K the vibrational lifetime of the excited stretching mode is 49±3  ps. The long lifetime seen here is likely to be a general feature of physisorption, which involves weaker electronic coupling between the adsorbate and the solid due to bonding at larger distances.

Published: 2019.   Source: Phys. Rev. Lett. 123, 156101

Near infrared light induced plasmonic hot hole transfer at a nano-heterointerface

Related products:  PL2210 series

Authors:  Zichao Lian, Masanori Sakamoto, Hironori Matsunaga, Junie Jhon M. Vequizo, Akira Yamakata, Mitsutaka Haruta, Hiroki Kurata, Wataru Ota, Tohru Sato, Toshiharu Teranishi

Localized surface plasmon resonance (LSPR)-induced hot-carrier transfer is a key mechanism for achieving artificial photosynthesis using the whole solar spectrum, even including the infrared (IR) region. In contrast to the explosive development of photocatalysts based on the plasmon-induced hot electron transfer, the hole transfer system is still quite immature regardless of its importance, because the mechanism of plasmon-induced hole transfer has remained unclear. Herein, we elucidate LSPR-induced hot hole transfer in CdS/CuS heterostructured nanocrystals (HNCs) using time-resolved IR (TR-IR) spectroscopy. TR-IR spectroscopy enables the direct observation of carrier in a LSPR-excited CdS/CuS HNC. The spectroscopic results provide insight into the novel hole transfer mechanism, named plasmon-induced transit carrier transfer (PITCT), with high quantum yields (19%) and long-lived charge separations (9.2 μs). As an ultrafast charge recombination is a major drawback of all plasmonic energy conversion systems, we anticipate that PITCT will break the limit of conventional plasmon-induced energy conversion.

Published: 2018.   Source: Nature Communications, vol. 9, art. 2314 (2018)

Capturing an initial intermediate during the P450nor enzymatic reaction using time-resolved XFEL crystallography and caged-substrate

Related products:  NT230 series

Authors:  Takehiko Tosha, Takashi Nomura, Takuma Nishida, Naoya Saeki, Kouta Okubayashi, Raika Yamagiwa, Michihiro Sugahara, Takanori Nakane, Keitaro Yamashita, Kunio Hirata, Go Ueno, Tetsunari Kimura, Tamao Hisano, Kazumasa Muramoto, Hitomi Sawai, Hanae Takeda, Eiichi Mizohata, Ayumi Yamashita, etc.

Time-resolved serial femtosecond crystallography using an X-ray free electron laser (XFEL) in conjunction with a photosensitive caged-compound offers a crystallographic method to track enzymatic reactions. Here we demonstrate the application of this method using fungal NO reductase, a heme-containing enzyme, at room temperature. Twenty milliseconds after caged-NO photolysis, we identify a NO-bound form of the enzyme, which is an initial intermediate with a slightly bent Fe-N-O coordination geometry at a resolution of 2.1 Å. The NO geometry is compatible with those analyzed by XFEL-based cryo-crystallography and QM/MM calculations, indicating that we obtain an intact Fe3+-NO coordination structure that is free of X-ray radiation damage. The slightly bent NO geometry is appropriate to prevent immediate NO dissociation and thus accept H from NADH. The combination of using XFEL and a caged-compound is a powerful tool for determining functional enzyme structures during catalytic reactions at the atomic level.

Published: 2017.   Source: Nature Communications, vol. 8, art. 1585 (2017)

Luminescence upconversion in colloidal double quantum dots

Related products:  NT342 series

Authors:  Zvicka Deutsch, Lior Neeman, Dan Oron

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved.

Published: 2013.   Source: Nature Nanotechnology, vol. 8, pp. 649–653 (2013)

Photogeneration and reactions of benzhydryl cations and radicals: A complex sequence of mechanisms from femtoseconds to microseconds

Related products:  NT230 series NT242 series NT342 series

Authors:  Christian F. Sailer, Eberhard Riedle

Benzhydryl radicals and cations are reactive intermediates central to the understanding of organic reactivity. They can be generated from benzhydryl halides by UV irradiation. We performed transient absorption (TA) measurements over the range from femtoseconds to microseconds to unravel the complete reaction scheme. The 290–720-nm probe range allows the unambiguous monitoring of all fragments. The appearance of the radical is delayed to the optical excitation, the onset of the cation signal is found even later. Ab initio calculations show that this non-rate behavior in the 100 fs range is due to wavepacket motion from the Franck–Condon region to two distinct conical intersections. The rise of the optical signal with a quasi-exponential time of 300 fs is assigned to the planarization and solvation of the photoproducts. The bond cleavage predominantly generates radical pairs. A subsequent electron transfer (ET) transforms radical pairs into ion pairs. Due to the broad interradical distance distribution and the distance dependence, the ET is strongly non-exponential. Part of the ion pairs recombine geminately. The ET and the recombination are terminated by the depletion of close pairs and diffusional separation. The remaining free radicals and cations undergo further reactions in the nanosecond to microsecond regime.

Published: 2013.   Source: Pure Appl. Chem., Vol. 85, No. 7, pp. 1487–1498, 2013.
Solid-phase Photoluminescence Spectroscopy

Time-resolved Photoluminescence Spectroscopy (TRPL) is a contactless method to characterize recombination and transport in solid materials. Measuring TRPL requires exciting luminescence from a sample with a pulsed light source and then measuring the subsequent decay in photoluminescence as a function of time. Most experiments excite the sample with a pulsed laser source and detect the PL with a photodiode, streak camera, or photomultiplier tube set up for upconversion or single-photon counting. The system response time, wavelength range and sensitivity vary widely for each configuration.

It is possible to apply the general methodology of time-resolved photoluminescence for lifetime imaging of the charge carrier dynamics. Nonradiative surface recombination at the boundaries of a semiconductor device can be a major factor limiting the efficiency in light-emitting and laser diodes (LEDs and LDs), photovoltaic cells, and photodetectors. Therefore, the effective lifetime is a crucial parameter to obtain solar cells with a high conversion ratio.

Photoluminescence microscopy also is a powerful optical method for the study of crystal defects in semiconductors and organometallic complexes, with important applications in the manufacturing process of nanostructures, optoelectronic devices and solar cell systems.

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Picosecond Lasers
PL2230 series – diode pumped high energy picosecond Nd:YAG lasers
PL2250 series – flash-lamp pumped picosecond Nd:YAG lasers
Tunable Wavelength Devices – standalone and integrated OPO sytems
PGx01 series – high energy broadly tunable picosecond OPA
PGx11 series – transform limited broadly tunable picosecond OPA
NT230 series – high energy broadly tunable DPSS lasers
NT242 series – broadly tunable kHz pulsed DPSS lasers
NT342 series – high energy broadly tunable lasers

The detrimental effect of AlGaN barrier quality on carrier dynamics in AlGaN/GaN interface

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Authors:  Žydrūnas Podlipskas, Jonas Jurkevičius, Arūnas Kadys, Saulius Miasojedovas, Tadas Malinauskas, Ramūnas Aleksiejūnas

Carrier recombination and scattering at the semiconductor boundaries can substantially limit the device efficiency. However, surface and interface recombination is generally neglected in the nitride-based devices. Here, we study carrier recombination and diffusivity in AlGaN/GaN/sapphire heterointerfaces with AlGaN barriers of different quality. We employ the light induced transient grating and time-resolved photoluminescence spectroscopy techniques to extract carrier lifetime in different depths of the GaN buffer as well as in the AlGaN barrier, and to evaluate the carrier diffusion coefficient in the buffer. Moreover, we assess interface recombination velocity, Shockley-Read-Hall and radiative recombination rates. We reveal the adverse barrier influence on carrier dynamics in the underlying buffer: AlGaN barrier accelerates the nonradiative carrier recombination in the GaN buffer. The interface recombination velocity in the GaN buffer increases with decreasing AlGaN barrier quality, and the dominating recombination mechanism switches from Shockley-Read-Hall to interface recombination. These phenomena are governed by a cumulative effect of various interface-deteriorating barrier defects. Meanwhile, the carrier diffusivity in the GaN buffer is not affected by the AlGaN barrier. We conclude that barrier-accelerated interface recombination can become a major carrier loss mechanism in AlGaN/GaN interface, and may substantially limit the efficiency in nitride-based UV LEDs.

Published: 2019.   Source: Scientific Reports volume 9, Article number: 17346 (2019)

Superlinear Photoluminescence by Ultrafast Laser Pulses in Dielectric Matrices with Metal Nanoclusters

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Authors:  J. Bornacelli, C. Torres-Torres, H. G. Silva-Pereyra, G. J. Labrada-Delgado, A. Crespo-Sosa, J. C. Cheang-Wong, A. Oliver

An intense photoluminescence emission was observed from noble metal nanoclusters (Pt, Ag or Au) embedded in sapphire plates, nucleated by MeV ion-implantation and assisted by an annealing process. In particular, the spectral photoluminescence characteristics, such as range and peak emission, were compared to the behavior observed from Pt nanoclusters embedded in a silica matrix and excited by UV irradiation. Correlation between emission energy, nanoclusters size and metal composition were analyzed by using the scaling energy relation EFermi/N1/3 from the spherical Jellium model. The metal nanocluster luminescent spectra were numerically simulated and correctly fitted using the bulk Fermi energy for each metal and a Gaussian nanoclusters size distribution for the samples. Our results suggest protoplasmonics photoluminescence from metal nanoclusters free of surface state or strain effects at the nanoclusters-matrix interface that can influence over their optical properties. These metal nanoclusters present very promising optical features such as bright visible photoluminescence and photostability under strong picosecond laser excitations. Besides superlinear photoluminescence from metal nanoclusters were also observed under UV high power excitation showing a quadratic dependence on the pump power fluence.

Published: 2019.   Source: Scientific Reports, vol. 9, art. 5699 (2019)

Luminescence spectroscopy of oxazine dye cations isolated in vacuo

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Authors:  Ch. Kjær, S. B. Nielsen

Here we report gas-phase action and luminescence spectra of cationic dyes derived from oxazine: cresyl violet (CV+), oxazine 170 (Ox-170+), nile blue (NB+), darrow red (DR+), oxazine 1 (Ox-1+), oxazine 4 (Ox-4+), and brilliant cresyl blue (BCB+). The first four have a benzofused structure, which results in asymmetric charge distributions along the long axis. The positive charge is also asymmetrically distributed in BCB+ while Ox-1+ and Ox-4+ are symmetric. As the ions are isolated in vacuo, there are no interactions with solvent molecules or counter ions, and the effect of chemical modifications is therefore more easily revealed than from solution-phase experiments. The transition energy decreases in the order: DR+ > CV+ > Ox-4+ > Ox-170+ > BCB+ > Ox-1+ > NB+, and the fluorescence from BCB+ is less than from the others. We discuss the results based on electron delocalisation, degree of charge-transfer character, rigidity of the chromophore structure, and substituents.

Published: 2019.   Source: Phys. Chem. Chem. Phys., 2019,21, 4600-4605

Near infrared emission properties of Er doped cubic sesquioxides in the second/third biological windows

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Authors:  Daniel Avram, Ion Tiseanu, Bogdan S. Vasile, Mihaela Florea, Carmen Tiseanu

In the recent years, there is an extensive effort concentrated towards the development of nanoparticles with near-infrared emission within the so called second or third biological windows induced by excitation outside 800–1000 nm range corresponding to the traditional Nd (800 nm) and Yb (980 nm) sensitizers. Here, we present a first report on the near-infrared (900–1700 nm) emission of significant member of cubic sesquioxides, Er-Lu2O3 nanoparticles, measured under both near-infrared up-conversion and low energy X-ray excitations. The nanoparticle compositions are optimized by varying Er concentration and Li addition. It is found that, under ca. 1500 nm up-conversion excitation, the emission is almost monochromatic (>93%) and centered at 980 nm while over 80% of the X-ray induced emission is concentrated around 1500 nm. The mechanisms responsible for the up-conversion emission of Er - Lu2O3 are identified by help of the up-conversion emission and excitation spectra as well as emission decays considering multiple excitation/emission transitions across visible to near-infrared ranges. Comparison between the emission properties of Er-Lu2O3 and Er-Y2O3 induced by optical and X-ray excitation is also presented. Our results suggest that the further optimized Er-doped cubic sesquioxides represent promising candidates for bioimaging and photovoltaic applications.

Published: 2018.   Source: Scientific Reports, vol. 8, art. 18033 (2018)

Ultra-sensitive mid-infrared emission spectrometer with sub-ns temporal resolution

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Authors:  L. Chen, D. Schwarzer, J. A. Lau, V. B. Verma, M. J. Stevens, F. Marsili, R. P. Mirin, S. Woo Nam, A. M. Wodtke

We evaluate the performance of a mid-infrared emission spectrometer operating at wavelengths between 1.5 and 6 μm based on an amorphous tungsten silicide (a-WSi) superconducting nanowire single-photon detector (SNSPD). We performed laser induced fluorescence spectroscopy of surface adsorbates with sub-monolayer sensitivity and sub-nanosecond temporal resolution. We discuss possible future improvements of the SNSPD-based infrared emission spectrometer and its potential applications in molecular science.

Published: 2018.   Source: Optics Express Vol. 26, Issue 12, pp. 14859-14868 (2018)

Probing the pathways of free charge generation in organic bulk heterojunction solar cells

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Authors:  Jona Kurpiers, Thomas Ferron, Steffen Roland, Marius Jakoby, Tobias Thiede, Frank Jaiser, Steve Albrecht, Silvia Janietz, Brian A. Collins, Ian A. Howard, Dieter Neher

The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.

Published: 2018.   Source: Nature Communications, vol. 9, art. 2038 (2018)

Quenching of the red Mn4+ luminescence in Mn4+-doped fluoride LED phosphors

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Authors:  Tim Senden, Relinde J.A. van Dijk-Moes, Andries Meijerink

Red-emitting Mn4+-doped fluorides are a promising class of materials to improve the color rendering and luminous efficacy of white light-emitting diodes (w-LEDs). For w-LEDs, the luminescence quenching temperature is very important, but surprisingly no systematic research has been conducted to understand the mechanism for thermal quenching in Mn4+-doped fluorides. Furthermore, concentration quenching of the Mn4+ luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching in Mn4+-doped fluorides by measuring luminescence spectra and decay curves of K2TiF6:Mn4+ between 4 and 600 K and for Mn4+ concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K2TiF6:Mn4+ and other Mn4+-doped phosphors show that quenching occurs through thermally activated crossover between the 4T2 excited state and 4A2 ground state. The quenching temperature can be optimized by designing host lattices in which Mn4+ has a high 4T2 state energy. Concentration-dependent studies reveal that concentration quenching effects are limited in K2TiF6:Mn4+ up to 5% Mn4+. This is important, as high Mn4+ concentrations are required for sufficient absorption of blue LED light in the parity-forbidden Mn4+ dd transitions. At even higher Mn4+ concentrations (>10%), the quantum efficiency decreases, mostly due to direct energy transfer to quenching sites (defects and impurity ions). Optimization of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn4+ phosphors. The present systematic study provides detailed insights into temperature and concentration quenching of Mn4+ emission and can be used to realize superior narrow-band red Mn4+ phosphors for w-LEDs.

Published: 2018.   Source: Light: Science & Applications, vol. 7, art. 8 (2018)

How nature covers its bases

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Authors:  S. Boldissar, M. S. de Vries

The response of DNA and RNA bases to ultraviolet (UV) radiation has been receiving increasing attention for a number of important reasons: (i) the selection of the building blocks of life on an early earth may have been mediated by UV photochemistry, (ii) radiative damage of DNA depends critically on its photochemical properties, and (iii) the processes involved are quite general and play a role in more biomolecules as well as in other compounds. A growing number of groups worldwide have been studying the photochemistry of nucleobases and their derivatives. Here we focus on gas phase studies, which (i) reveal intrinsic properties distinct from effects from the molecular environment, (ii) allow for the most detailed comparison with the highest levels of computational theory, and (iii) provide isomeric selectivity. From the work so far a picture is emerging of rapid decay pathways following UV excitation. The main understanding, which is now well established, is that canonical nucleobases, when absorbing UV radiation, tend to eliminate the resulting electronic excitation by internal conversion (IC) to the electronic ground state in picoseconds or less. The availability of this rapid “safe” de-excitation pathway turns out to depend exquisitely on molecular structure. The canonical DNA and RNA bases are generally short-lived in the excited state, and thus UV protected. Many closely related compounds are longer lived, and thus more prone to other, potentially harmful, photochemical processes. It is this structure dependence that suggests a mechanism for the chemical selection of the building blocks of life on an early earth. However, the picture is far from complete and many new questions now arise.

Published: 2018.   Source: Phys. Chem. Chem. Phys., 2018,20, 9701-9716

Nanoscale insights into doping behavior, particle size and surface effects in trivalent metal doped SnO2

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Authors:  Bogdan Cojocaru, Daniel Avram, Vadim Kessler, Vasile Parvulescu, Gulaim Seisenbaeva, Carmen Tiseanu

Despite considerable research, the location of an aliovalent dopant into SnO2 nanoparticles is far to be clarified. The aim of the present study on trivalent lanthanide doped SnO2 is to differentiate between substitutional versus interstitial and surface versus bulk doping, delineate the bulk and surface defects induced by doping and establish an intrinsic dopant distribution. We evidence for the first time a complex distribution of intrinsic nature composed of substitutional isolated, substitutional associates with defects as well as surface centers. Such multi-modal distribution is revealed for Eu and Sm, while Pr, Tb and Dy appear to be distributed mostly on the SnO2 surface. Like the previously reported case of Eu, Sm displays a long-lived luminescence decaying in the hundreds of ms scale which is likely related to a selective interaction between the traps and the substitutional isolated center. Analyzing the time-gated luminescence, we conclude that the local lattice environment of the lattice Sn is not affected by the particle size, being remarkably similar in the ~2 and 20 nm particles. The photocatalytic measurements employed as a probe tool confirm the conclusions from the luminescence measurements concerning the nature of defects and the temperature induced migration of lanthanide dopants.

Published: 2017.   Source: Scientific Reports, vol. 7, art. 9598 (2017)

Excited State Dynamics of 6‑Thioguanine

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Authors:  F. M. Siouri, S. Boldissar, J. A. Berenbeim, M. S. de Vries

Here we present the excited state dynamics of jet-cooled 6-thioguanine (6-TG), using resonance-enhanced multiphoton ionization (REMPI), IR–UV double resonance spectroscopy, and pump–probe spectroscopy in the nanosecond and picosecond time domains. We report data on two thiol tautomers, which appear to have different excited state dynamics. These decay to a dark state, possibly a triplet state, with rates depending on tautomer form and on excitation wavelength, with the fastest rate on the order of 1010 s–1. We also compare 6-TG with 9-enolguanine, for which we observed decay to a dark state with a 2 orders of magnitude smaller rate. At increased excitation energy (∼+500 cm–1) an additional pathway appears for the predominant thiol tautomer. Moreover, the excited state dynamics for 6-TG thiols is different from that recently predicted for thiones.

Published: 2017.   Source: J. Phys. Chem. A 2017, 121, 28, 5257-5266

Non-Poissonian photon statistics from macroscopic photon cutting materials

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Authors:  Mathijs de Jong, Andries Meijerink, Freddy T. Rabouw

In optical materials energy is usually extracted only from the lowest excited state, resulting in fundamental energy-efficiency limits such as the Shockley–Queisser limit for single-junction solar cells. Photon-cutting materials provide a way around such limits by absorbing high-energy photons and ‘cutting’ them into multiple low-energy excitations that can subsequently be extracted. The occurrence of photon cutting or quantum cutting has been demonstrated in a variety of materials, including semiconductor quantum dots, lanthanides and organic dyes. Here we show that photon cutting results in bunched photon emission on the timescale of the excited-state lifetime, even when observing a macroscopic number of optical centres. Our theoretical derivation matches well with experimental data on NaLaF4:Pr3+, a material that can cut deep-ultraviolet photons into two visible photons. This signature of photon cutting can be used to identify and characterize new photon-cutting materials unambiguously.

Published: 2017.   Source: Nature Communications, vol. 8, art. 15537 (2017)

Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters

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Authors:  Marc K. Etherington, Flavio Franchello, Jamie Gibson, Thomas Northey, Jose Santos, Jonathan S. Ward, Heather F. Higginbotham, Przemyslaw Data, Aleksandra Kurowska, Paloma Lays Dos Santos, David R. Graves, Andrei S. Batsanov, Fernando B. Dias, Martin R. Bryce, Thomas J. Penfold, Andrew P. Monkman

Regio- and conformational isomerization are fundamental in chemistry, with profound effects upon physical properties, however their role in excited state properties is less developed. Here two regioisomers of bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide, a donor–acceptor–donor (D–A–D) thermally-activated delayed fluorescence (TADF) emitter, are studied. 2,8-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide exhibits only one quasi-equatorial conformer on both donor sites, with charge-transfer (CT) emission close to the local triplet state leading to efficient TADF via spin-vibronic coupling. However, 3,7-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide displays both a quasi-equatorial CT state and a higher-energy quasi-axial CT state. No TADF is observed in the quasi-axial CT emission. These two CT states link directly to the two folded conformers of phenothiazine. The presence of the low-lying local triplet state of the axial conformer also means that this quasi-axial CT is an effective loss pathway both photophysically and in devices. Importantly, donors or acceptors with more than one conformer have negative repercussions for TADF in organic light-emitting diodes.

Published: 2017.   Source: Nature Communications, vol. 8, art. 14987 (2017)

Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence

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Authors:  Marc K. Etherington, Jamie Gibson, Heather F. Higginbotham, Thomas J. Penfold, Andrew P. Monkman

Knowing the underlying photophysics of thermally activated delayed fluorescence (TADF) allows proper design of high efficiency organic light-emitting diodes. We have proposed a model to describe reverse intersystem crossing (rISC) in donor–acceptor charge transfer molecules, where spin–orbit coupling between singlet and triplet states is mediated by one of the local triplet states of the donor (or acceptor). This second order, vibronically coupled mechanism describes the basic photophysics of TADF. Through a series of measurements, whereby the energy ordering of the charge transfer (CT) excited states and the local triplet are tuned in and out of resonance, we show that TADF reaches a maximum at the resonance point, substantiating our model of rISC. Moreover, using photoinduced absorption, we show how the populations of both singlet and triplet CT states and the local triplet state change in and out of resonance. Our vibronic coupling rISC model is used to predict this behaviour and describes how rISC and TADF are affected by external perturbation.

Published: 2016.   Source: Nature Communications, vol. 7, art. 13680 (2016)

Nile blue shows its true colors in gas-phase absorption and luminescence ion spectroscopy

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Authors:  M. H. Stockett, J. Houmøller, S.B. Nielsen

Nile blue is used extensively in biology as a histological stain and fluorescent probe. Its absorption and emission spectra are strongly solvent dependent, with variations larger than 100 nm. The molecule is charged due to an iminium group, and it is therefore an obvious target for gas-phase ion spectroscopy. Here we report the absorption and emission spectra of the mass-selected bare ions isolated in vacuo, and based on our results we revisit the interpretation of solution-phase spectra. An accelerator mass spectrometer was used for absorption spectroscopy where the absorption is represented by the yield of photofragment ions versus excitation wavelength (action spectroscopy). The luminescence experiments were done with a newly built ion trap setup equipped with an electrospray ion source, and some details on the mass selection technique will be given which have not been described before. In vacuo, the absorption and emission maxima are at 580 ± 10 nm and 628 ± 1 nm. These values are somewhat blue-shifted relative to those obtained in most solvents; however, they are much further to the red than those in some of the most non-polar solvents. Furthermore, the Stokes shift in the gas phase (1300 cm−1) is much smaller than that in these non-polar solvents but similar to that in polar ones. An explanation based on charge localization by solvent dipoles, or by counterions in some non-polar solvents, can fully account for these findings. Hence in the case of ions, it is nontrivial to establish intrinsic electronic transition energies from solvatochromic shifts alone.

Published: 2016.   Source: J. Chem. Phys. 145, 104303 (2016)

Aerobic photoreactivity of synthetic eumelanins and pheomelanins: generation of singlet oxygen and superoxide anion

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Authors:  Grzegorz Szewczyk, Andrzej Zadlo, Michal Sarna, Shosuke Ito, Kazumasa Wakamatsu, Tadeusz Sarna

In this work, we examined photoreactivity of synthetic eumelanins, formed by autooxidation of DOPA, or enzymatic oxidation of 5,6‐dihydroxyindole‐2‐carboxylic acid and synthetic pheomelanins obtained by enzymatic oxidation of 5‐S‐cysteinyldopa or 1:1 mixture of DOPA and cysteine. Electron paramagnetic resonance oximetry and spin trapping were used to measure oxygen consumption and formation of superoxide anion induced by irradiation of melanin with blue light, and time‐resolved near‐infrared luminescence was employed to determine the photoformation of singlet oxygen between 300 and 600 nm. Both superoxide anion and singlet oxygen were photogenerated by the synthetic melanins albeit with different efficiency. At 450‐nm, quantum yield of singlet oxygen was very low (~10−4) but it strongly increased in the UV region. The melanins quenched singlet oxygen efficiently, indicating that photogeneration and quenching of singlet oxygen may play an important role in aerobic photochemistry of melanin pigments and could contribute to their photodegradation and photoaging.

Published: 2016.   Source: Pigment Cell & Melanoma Reseach, Vol. 29, Iss. 6, 2016, p. 669-678

A cylindrical quadrupole ion trap in combination with an electrospray ion source for gas-phase luminescence and absorption spectroscopy

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Authors:  M. H. Stockett, J. Houmøller, K. Støchkel, A. Svendsen, S. B. Nielsen

A relatively simple setup for collection and detection of light emitted from isolated photo-excited molecular ions has been constructed. It benefits from a high collection efficiency of photons, which is accomplished by using a cylindrical ion trap where one end-cap electrode is a mesh grid combined with an aspheric condenser lens. The geometry permits nearly 10% of the emitted light to be collected and, after transmission losses, approximately 5% to be delivered to the entrance of a grating spectrometer equipped with a detector array. The high collection efficiency enables the use of pulsed tunable lasers with low repetition rates (e.g., 20 Hz) instead of continuous wave (cw) lasers or very high repetition rate (e.g., MHz) lasers that are typically used as light sources for gas-phase fluorescence experiments on molecular ions. A hole has been drilled in the cylinder electrode so that a light pulse can interact with the ion cloud in the center of the trap. Simulations indicate that these modifications to the trap do not significantly affect the storage capability and the overall shape of the ion cloud. The overlap between the ion cloud and the laser light is basically 100%, and experimentally >50% of negatively charged chromophore ions are routinely photodepleted. The performance of the setup is illustrated based on fluorescence spectra of several laser dyes, and the quality of these spectra is comparable to those reported by other groups. Finally, by replacing the optical system with a channeltron detector, we demonstrate that the setup can also be used for gas-phase action spectroscopy where either depletion or fragmentation is monitored to provide an indirect measurement on the absorption spectrum of the ion.

Published: 2016.   Source: Review of Scientific Instruments 87, 053103 (2016)

Roles of reactive oxygen species in UVA‐induced oxidation of 5,6‐dihydroxyindole‐2‐carboxylic acid‐melanin as studied by differential spectrophotometric method

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Authors:  Shosuke Ito, Marina Kikuta, Shota Koike, Grzegorz Szewczyk, Michal Sarna, Andrzej Zadlo, Tadeusz Sarna, Kazumasa Wakamatsu

Eumelanin photoprotects pigmented tissues from ultraviolet (UV) damage. However, UVA‐induced tanning seems to result from the photooxidation of preexisting melanin and does not contribute to photoprotection. We investigated the mechanism of UVA‐induced degradation of 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA)‐melanin taking advantage of its solubility in a neutral buffer and using a differential spectrophotometric method to detect subtle changes in its structure. Our methodology is suitable for examining the effects of various agents that interact with reactive oxygen species (ROS) to determine how ROS is involved in the UVA‐induced oxidative modifications. The results show that UVA radiation induces the oxidation of DHICA to indole‐5,6‐quinone‐2‐carboxylic acid in eumelanin, which is then cleaved to form a photodegraded, pyrrolic moiety and finally to form free pyrrole‐2,3,5‐tricarboxylic acid. The possible involvement of superoxide radical and singlet oxygen in the oxidation was suggested. The generation and quenching of singlet oxygen by DHICA‐melanin was confirmed by direct measurements of singlet oxygen phosphorescence.

Published: 2016.   Source: Pigment Cell & Melanoma res., vol.29/3, p. 340-351

Room-temperature InP distributed feedback laser array directly grown on silicon

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Authors:  Zhechao Wang, Bin Tian, Marianna Pantouvaki, Weiming Guo, Philippe Absil, Joris Van Campenhout, Clement Merckling, Dries Van Thourhout

Fully exploiting the silicon photonics platform for large-volume, cost-sensitive applications requires a fundamentally new approach to directly integrate high-performance laser sources using wafer-scale fabrication methods. Direct-bandgap III–V semiconductors allow efficient light generation, but the large mismatch in lattice constant, thermal expansion and crystal polarity makes their epitaxial growth directly on silicon extremely complex. Using a selective-area growth technique in confined regions, we surpass this fundamental limit and demonstrate an optically pumped InP-based distributed feedback laser array monolithically grown on (001)-silicon operating at room temperature and suitable for wavelength-division-multiplexing applications. The novel epitaxial technology suppresses threading dislocations and anti-phase boundaries to a less than 20-nm-thick layer, which does not affect device performance. Using an in-plane laser cavity defined using standard top-down lithographic patterning together with a high yield and high uniformity provides scalability and a straightforward path towards cost-effective co-integration with silicon photonic and electronic circuits.

Published: 2015.   Source: Nature Photonics, vol. 9, pp, 837–842 (2015)

Multi-photon quantum cutting in Gd2O2S:Tm3+ to enhance the photo-response of solar cells

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Authors:  De-Chao Yu, Rosa Martín-Rodríguez, Qin-Yuan Zhang, Andries Meijerink, Freddy T Rabouw

Conventional photoluminescence (PL) yields at most one emitted photon for each absorption event. Downconversion (or quantum cutting) materials can yield more than one photon by virtue of energy transfer processes between luminescent centers. In this work, we introduce Gd2O2S:Tm3+ as a multi-photon quantum cutter. It can convert near-infrared, visible, or ultraviolet photons into two, three, or four infrared photons of 1800 nm, respectively. The cross-relaxation steps between Tm3+ ions that lead to quantum cutting are identified from (time-resolved) PL as a function of the Tm3+ concentration in the crystal. A model is presented that reproduces the way in which the Tm3+ concentration affects both the relative intensities of the various emission lines and the excited state dynamics and providing insight in the quantum cutting efficiency. Finally, we discuss the potential application of Gd2O2S:Tm3+ for spectral conversion to improve the efficiency of next-generation photovoltaics.

Published: 2015.   Source: Light: Science & Applications, vol. 4, p. 344 (2015)

Luminescence upconversion in colloidal double quantum dots

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Authors:  Zvicka Deutsch, Lior Neeman, Dan Oron

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved.

Published: 2013.   Source: Nature Nanotechnology, vol. 8, pp. 649–653 (2013)

Study of GaN : Eu3+  Thin Films Deposited by Metallorganic Vapor-Phase Epitaxy

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Authors:  J. Laski, K. Klinedinst, M. Raukas, K. C. Mishra, J. Tao, J. McKittrick, J. B. Talbot

Using metallorganic vapor-phase epitaxy, thin films of gallium nitride activated by Eu3+ (GaN : Eu3+) have been deposited on sapphire substrates at atmospheric pressure. Luminescence from Eu3+ ions in GaN has been investigated using photoluminescence (PL) and PL excitation spectroscopy. Experimental results show that Eu3+ ions are excited via energy transfer from the host. Analyses of the observed emission and excitation spectra indicate occupancy of multiple sites in the nitride lattice. Using a pulsed laser source, variation of emission intensity with increasing excitation intensity has also been examined. The possibility of emission saturation at high excitation intensity is discussed from the perspective of application in light-emitting diode sources.

Published: 2008.   Source: J. Electrochem. Soc. 155, J315
Gas-phase Ion Luminescence Spectroscopy
Principle of Gas-phase Ion Luminescence Spectroscopy

Gas-phase ion luminescence spectroscopy is used to determine intrinsic electronic transition energies in the absence of a disturbing environment. Large ions produced by electrospray ionization are stored and mass-selected in a cylindrical Paul trap. Here they are irradiated by light from a 20-Hz pulsed tunable wavelelength laser from EKSPLA followed by detection of the emitted photons. The 20-Hz repetition rate of the tunable laser allows for mass selection in between every irradiation event, which implies that there is no fluorescence contribution from ion impurities. The tunability of the laser makes it possible to photo-excite a variety of dyes that absorb at different wavelengths. The figure shows spectra of oxazine dye cations that display emissions in a range from 500 nm to 750 nm. The technique can also be used to study three-dimensional structures of peptides and nucleic acids in the gas phase based on Förster Resonance Energy Transfer (FRET). The biomolecules are labeled with donor-acceptor dye pairs such as rhodamines 575 and 640. Structure information is provided as the efficiency of energy transfer depends on the distance between the donor and acceptor to the inverse power of six.

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Luminescence Spectroscopy of Rhodamine Homodimer Dications in Vacuo Reveals Strong Dye‐Dye Interactions

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Authors:  Ch. Kjær, H. Lissau, N.K.G. Salinas, A.Ø. Madsen, M.H. Stockett, F.E. Storm, T.H. Hansen, J.U. Andersen, B.W. Laursen, K.V. Mikkelsen, M.B. Nielsen, S.B. Nielsen

Being alone or together makes a difference for the photophysics of dyes but for ionic dyes it is difficult to quantify the interactions due to solvent screening and nearby counter ions. Gas‐phase luminescence experiments are desirable and now possible based on recent developments in mass spectrometry. Here we present results on tailor‐made rhodamine homodimers where two dye cations are separated by methylene linkers, (CH2)n. In solution the fluorescence is almost identical to that from the monomer whereas the emission from bare cation dimers redshifts with decreasing n. In the absence of screening, the electric field from the charge on one dye is strong enough to polarize the other dye, both in the ground state and in the excited state. An electrostatic model based on symmetric dye responses (equal induced‐dipole moments in ground state) captures the underlying physics and demonstrates interaction even at large distances. Our results have possible implications for gas‐phase Förster Resonance Energy Transfer.

Published: 2018.   Source: ChemPhysChem, Vol. 20 (4), p. 533-537, 2019

Gas-phase Ion Spectroscopy of Flexible and Nonflexible Nitrophenolates: Effect of Locking the Two Phenyl Units in 4’-nitro-[1,1’-biphenyl]-4-olate by a Bridging Atom

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Authors:  B.M. Pedersen, S.B. Nielsen

Nitrophenolates (NPs) are molecular anions that can undergo charge-transfer (CT) transitions determined by the degree of electron delocalization between the phenolate oxygen (donor group) and the nitro group (acceptor). Here we have studied four different NPs: 4’-nitro-[1,1’-biphenyl]-4-olate (1), 7-nitro-9H-carbazol-2-olate (NH linker, 2), 7-nitrodibenzo[b,d]furan-3-olate (oxygen linker, 3), and 7-nitrodibenzo[b,d]thiophen-3-olate (sulphur linker, 4), and recorded their electronic absorption spectra when isolated in vacuo to determine the effect of locking the biphenyl spacer group between the donor and acceptor on transition energies. Absorption was identified from ion dissociation (action spectroscopy) using a homebuilt setup (sector mass spectrometer combined with pulsed laser). We find that the absorption is broad in the visible region for all four NPs with significant vibronic features. The lowest energy peak is at 601 ± 4 nm, 606 ± 4 nm, 615 ± 4 nm, and 620 ± 4 nm, for 3, 4, 2, and 1, respectively. NP 1 is flexible, and its lowest energy structure is nonplanar while the other three NPs are planar according to density functional theory calculations. Hence in the case of 1 the electronic transition has a higher degree of CT than for the other three, accounting for its absorption furthest to the red. Our work demonstrates that oxygen and sulphur are best at conveying the electronic coupling between the donor and acceptor sites as 3 and 4 absorb furthest to the blue (i.e., the degree of CT is lowest for these two NPs). Based on the average spacing between the peaks in the vibrational progressions, coupling occurs to skeleton vibrational modes with frequencies of 649 ± 69 cm-1 (3), 655 ± 49 cm-1 (4), and 697 ± 52 cm-1 (2).

Published: 2018.   Source: Journal of Self-Assembly and Molecular Electronics, Vol. 6_1, 1–12.

Sibling rivalry: intrinsic luminescence from two xanthene dye monoanions, resorufin and fluorescein, provides evidence for excited-state proton transfer in the latter

Related products:  NT342 series

Authors:  Ch. Kjær, S.B. Nielsen, M.H. Stockett

While the emission spectrum of fluorescein monoanions isolated in vacuo displays a broad and featureless band, that of resorufin, also belonging to the xanthene family, has a sharp band maximum, clear vibronic structure, and experiences a small Stokes shift. Excited-state proton transfer in fluorescein can account for the differences.

Published: 2017.   Source: Phys. Chem. Chem. Phys., 2017,19, 24440-24444

Luminescence spectroscopy of chalcogen substituted rhodamine cations in vacuo

Related products:  NT342 series

Authors:  M.H. Stockett, Ch. Kjær, M.K. Linder, M.R. Detty, S.B. Nielsen

Intrinsic optical properties of several rhodamine cations were probed by measuring their dispersed fluorescence spectra in vacuo. Three different rhodamine structures were investigated, each with four different chalcogen heteroatoms. Fluorescence band maxima were blue-shifted by between 0.15 and 0.20 eV (1200–1600 cm−1) relative to previous solution-phase measurements. Trends in emission wavelengths and fluorescence quantum yields previously measured in solution are generally reproduced in the gas phase, confirming the intrinsic nature of these effects. One important exception is gas-phase brightness of the Texas Red analogues, which is significantly higher than the other rhodamine structures studied, despite having similar fluorescence quantum yields in solution. These results expand the library of fluorophores for which gas-phase photophysical data is available, and will aid in the design of experiments utilizing gas-phase structural biology methods such as Förster resonance energy transfer.

Published: 2017.   Source: Photochem. Photobiol. Sci., 2017,16, 779-784
Supercontinuum Generation

Supercontinuum “White Light Lasers” have become a well-established turn-key fiber-laser technology addressing a wide range of applications from biomedical imaging to optical device characterization. They add value due to their unique combination of optical parameters, including an extremely wide spectral coverage from 400 nm to 2400 nm, several W of optical output power, and focus down to the diffraction limit of a perfect Gaussian beam.

The majority of commercial supercontinuum lasers are fully fiber-based systems consisting of a modelocked fiber oscillator as the master seed laser, providing ps pulses at ∼1064 nm and repetition rates in the tens of MHz regime. Injected into a fiber amplifier giving rise to high peak power, and finally a few meters of a specially designed index-guiding PCF with suitable dispersion landscape. The supercontinuum source, when combined with a tunable spectral filter, transforms into a widely tunable laser, making it a versatile laser tool for a wide range of applications.

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Femtosecond Lasers
LightWire FFS – compact fiber seeders for femtosecond lasers
Picosecond Lasers
PL2210 series – diode pumped picosecond kHz pulsed Nd:YAG lasers
Tunable Wavelength Devices – standalone and integrated OPO sytems
PGx11 series – transform limited broadly tunable picosecond OPA

Mid-infrared, super-flat, supercontinuum generation covering the 2–5 μm spectral band using a fluoroindate fibre pumped with picosecond pulses

Related products:  PL2210 series PGx11 series

Authors:  Maria Michalska, Janusz Mikolajczyk, Jacek Wojtas & Jacek Swiderski

Broadband, mid-infrared supercontinuum generation in a step-index fluoroindate fibre is reported. By using ~70-picosecond laser pulses at 2.02 μm, provided by an optical parametric generator, a wide spectrum with a cut-off wavelength at 5.25 μm and a 5-dB bandwidth covering the entire 2–5 μm spectral interval has been demonstrated for the first time. The behaviour of the supercontinuum was investigated by changing the peak power and the wavelength of the pump pulses. This allowed the optimal pumping conditions to be determined for the nonlinear medium that was used. The optical damage threshold for the fluoroindate fibre was experimentally found to be ~200 GW/cm2.

Published: 2016.   Source: Scientific Reports | 6:39138

On-chip visible-to-infrared supercontinuum generation with more than 495 THz spectral bandwidth

Related products:  LightWire FFS

Authors:  Jörn P. Epping, Tim Hellwig, Marcel Hoekman, Richard Mateman, Arne Leinse, René G. Heideman, Albert van Rees, Peter J.M. van der Slot, Chris J. Lee, Carsten Fallnich, Klaus-J. Boller

We report ultra-broadband supercontinuum generation in high-confinement Si3N4 integrated optical waveguides. The spectrum extends through the visible (from 470 nm) to the infrared spectral range (2130 nm) comprising a spectral bandwidth wider than 495 THz, which is the widest supercontinuum spectrum generated on a chip.

Published: 2015.   Source: Opt. Express 23, 19596-19604 (2015)
Time-Resolved Photoconductivity

Time-resolved photoconductivity (TRMC) are key techniques used to perform the contactless determination of carrier density, transport, trapping, and recombination parameters in charge transport materials such as organic semiconductors and dyes, inorganic semiconductors, and metal-insulator composites, which find use in conductive inks, thin-film transistors, light-emitting diodes, photocatalysts, and photovoltaics.

The behavior of photoconductivity with photon energy, light intensity and temperature, and its time evolution and frequency dependence, can reveal a great deal about carrier generation, transport and recombination processes. Many of these processes now have a sound theoretical basis and so it is possible, with due caution, to use photoconductivity as a diagnostic tool in the study of new electronic materials and devices.

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Tunable Wavelength Devices – standalone and integrated OPO sytems
NT242 series – broadly tunable kHz pulsed DPSS lasers
NT342 series – high energy broadly tunable lasers

The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites

Related products:  NT342 series

Authors:  Zimu Wei, Dengyang Guo, Jos Thieme, Claudine Katan, Valentina M. Caselli, Jacky Even, Tom J. Savenije

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. Here we recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. A two-step upward trend is observed in the two-photon absorption spectrum for methylammonium lead iodide, and some analogues, which implies that the commonly used scaling law is not applicable to MHPs. This aspect is further confirmed by temperature-dependent conductivity measurements. Using an empirical multiband tight binding model, spectra for methylammonium lead iodide were calculated by integration over the entire Brillouin zone, showing compelling similarity with experimental results. We conclude that the second upward trend in the two-photon absorption spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Hence, valuable insight can be obtained in the opto-electronic properties of MHPs by sub-bandgap spectroscopy, complemented by modelling.

Published: 2019.   Source: Nature Communications, volume 10, article number: 5342 (2019)

Charge carrier transport in polycrystalline CH3NH3PbI3 perovskite thin films in a lateral direction characterized by time-of-flight photoconductivity

Related products:  NT342 series

Authors:  S.Emin, E.Pavlica, H.Okuyucu, M.Valant, G.Bratina

We used time-of-flight photocurrent measurements to determine the role of grain boundaries in charge carrier transport in thin layers of methyl ammonium lead iodide (CH3NH3PbI3). The measurement results were compared to Kinetic Monte Carlo simulations, based on a transport model, which disentangles the transport within crystallites and hopping across grain boundaries. The observed mobilities of electrons are in the order ∼2.5 × 10−1 cm2V−1s−1. The hopping across grains is modeled with an Arrhenius-type probability rate, characterized by activation energy (Ea). It was found that the Ea estimated from the slope of a mobility-temperature dependence is in the range of ∼56–70 meV. The factors contributing to Ea are shunting pathways and the grain-size variations including energy level misalignments at the grain boundaries. These results represent a step toward a design of novel windowless organic-inorganic perovskite solar cells.

Published: 2018.   Source: Materials Chemistry and Physics, vol. 220, pp. 182-189 (2018)

Evidence of enhanced photocurrent response in corannulene films

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Authors:  N. Pastukhova, L. Martin Samos, L. Zoppi, E. Pavlica, J. Mathew, G. Bratina, J. S. Siegel, K. K. Baldridge

Photoconductivity spectra measured in non-crystalline corannulene thin layers are compared to optical absorption in solution phase and thin films. The unexpected enhanced photoconductivity is correlated with GW–BSE theoretical predictions of corannulene gas-phase excitonic spectra. Theoretical analysis reveals a consistent contribution involving transitions to Super Atomic Molecular Orbitals (SAMOs), a unique set of diffuse orbitals typical of curved conjugated constructs. Results suggest SAMO population via direct photoexcitation as a potential mechanism towards exploiting these diffuse orbitals as conducting channels in suitably assembled quantum nanostructures or solids.

Published: 2017.   Source: RSC Advances, vol. 7, issue 72, pp. 45601-45606 (2017)

Investigation into the Advantages of Pure Perovskite Film without PbI2 for High Performance Solar Cell

Related products:  NT342 series

Authors:  Ujwal Thakur, Uisik Kwon, Md Mehedi Hasan, Wenping Yin, Dasom Kim, Na Young Ha, Soonil Lee, Tae Kyu Ahn, Hui Joon Park

In CH3NH3PbI3-based high efficiency perovskite solar cells (PSCs), tiny amount of PbI2 impurity was often found with the perovskite crystal. However, for two-step solution process-based perovskite films, most of findings have been based on the films having different morphologies between with and without PbI2. This was mainly due to the inferior morphology of pure perovskite film without PbI2, inevitably produced when the remaining PbI2 forced to be converted to perovskite, so advantages of pure perovskite photoactive layer without PbI2 impurity have been overlooked. In this work, we designed a printing-based two-step process, which could not only generate pure perovskite crystal without PbI2, but also provide uniform and full surface coverage perovskite film, of which nanoscale morphology was comparable to that prepared by conventional two-step solution process having residual PbI2. Our results showed that, in two-step solution process-based PSC, pure perovskite had better photon absorption and longer carrier lifetime, leading to superior photocurrent generation with higher power conversion efficiency. Furthermore, this process was further applicable to prepare mixed phase pure perovskite crystal without PbI2 impurity, and we showed that the additional merits such as extended absorption to longer wavelength, increased carrier lifetime and reduced carrier recombination could be secured.

Published: 2016.   Source: Scientific Reports, vol. 6, art. 35994 (2016)

Flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels based on an organic bicomponent blend

Related products:  NT342 series

Authors:  Tim Leydecker, Martin Herder, Egon Pavlica, Gvido Bratina, Stefan Hecht, Emanuele Orgiu, Paolo Samorì

Organic nanomaterials are attracting a great deal of interest for use in flexible electronic applications such as logic circuits, displays and solar cells. These technologies have already demonstrated good performances, but flexible organic memories are yet to deliver on all their promise in terms of volatility, operational voltage, write/erase speed, as well as the number of distinct attainable levels. Here, we report a multilevel non-volatile flexible optical memory thin-film transistor based on a blend of a reference polymer semiconductor, namely poly(3-hexylthiophene), and a photochromic diarylethene, switched with ultraviolet and green light irradiation. A three-terminal device featuring over 256 (8 bit storage) distinct current levels was fabricated, the memory states of which could be switched with 3 ns laser pulses. We also report robustness over 70 write–erase cycles and non-volatility exceeding 500 days. The device was implemented on a flexible polyethylene terephthalate substrate, validating the concept for integration into wearable electronics and smart nanodevices.

Published: 2016.   Source: Nature Nanotechnology, vol. 11, pp. 769–775 (2016)

Competition between recombination and extraction of free charges determines the fill factor of organic solar cells

Related products:  NT242 series

Authors:  Davide Bartesaghi, Irene del Carmen Pérez, Juliane Kniepert, Steffen Roland, Mathieu Turbiez, Dieter Neher, L. Jan Anton Koster

Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Here we quantify the competition between charge extraction and recombination by using a single parameter θ, and we demonstrate that this parameter is directly related to the fill factor of many different bulk-heterojunction solar cells. Our finding is supported by experimental measurements on 15 different donor:acceptor combinations, as well as by drift-diffusion simulations of organic solar cells in which charge-carrier mobilities, recombination rate, light intensity, energy levels and active-layer thickness are all varied over wide ranges to reproduce typical experimental conditions. The results unify the fill factors of several very different donor:acceptor combinations and give insight into why fill factors change so much with thickness, light intensity and materials properties. To achieve fill factors larger than 0.8 requires further improvements in charge transport while reducing recombination.

Published: 2015.   Source: Nature Communications, vol. 6, art. 7083 (2015)

Transparent Organic Photodetector using a Near-Infrared Absorbing Cyanine Dye

Related products:  NT342 series

Authors:  Hui Zhang, Sandra Jenatsch, Jelissa De Jonghe, Frank Nüesch, Roland Steim, Anna C. Véron, Roland Hany

Organic photodetectors are interesting for low cost, large area optical sensing applications. Combining organic semiconductors with discrete absorption bands outside the visible wavelength range with transparent and conductive electrodes allows for the fabrication of visibly transparent photodetectors. Visibly transparent photodetectors can have far reaching impact in a number of areas including smart displays, window-integrated electronic circuits and sensors. Here, we demonstrate a near-infrared sensitive, visibly transparent organic photodetector with a very high average visible transmittance of 68.9%. The transmitted light of the photodetector under solar irradiation exhibits excellent transparency colour perception and rendering capabilities. At a wavelength of 850 nm and at −1 V bias, the photoconversion efficiency is 17% and the specific detectivity is 1012 Jones. Large area photodetectors with an area of 1.6 cm2 are demonstrated.

Published: 2015.   Source: Scientific Reports, vol. 5, art. 9439 (2015)

Efficient charge generation by relaxed charge-transfer states at organic interfaces

Related products:  NT242 series

Authors:  Koen Vandewal, Steve Albrecht, Eric T. Hoke, Kenneth R. Graham, JohannesWidmer, Jessica D. Douglas, Marcel Schubert, William R. Mateker, Jason T. Bloking, George F. Burkhard, Alan Sellinger, Jean M. J. Fréchet, Aram Amassian, Moritz K. Riede, Michael D. McGehee, Dieter Neher, Alberto Salleo

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Published: 2013.   Source: Nature Materials, vol. 13, pp. 63–68 (2014)

Time-of-flight mobility of charge carriers in position-dependent electric field between coplanar electrodes

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Authors:  Egon Pavlica, Gvido Bratina
Time-of-flight measurements of the photocurrent in thin organic semiconductor layers represent an effective way to extract charge carrier mobility. A common method to interpret the time-dependence of the photocurrent in these material systems assumes a position-independent electric field between two coplanar electrodes. In this letter, we compare time-dependence of the photocurrent, measured in the samples comprising thin layers of poly-3-hexylthiophene, with the Monte Carlo simulations. In the simulations, we have used both, a position-independent and a position-dependent electric field. We obtained a favorable agreement between the simulations and the measurements only in the case of position-dependent electric field. We demonstrate that the charge carrier mobility may be underestimated by more than one order of magnitude, if a position-independent electric field is used in the calculations of the mobility.
Published: 2012.   Source: Appl. Phys. Lett. 101, 093304 (2012)

A novel method for direct nondestructive diagnosis of caries affected tooth surfaces by laser multiphoton ionization

Related products:  NT342 series

Authors:  Natalia Vinerot, Yuheng Chen, Vladimir V. Gridin, Valery Bulatov, Liviu Feller, Israel Schechter

A new nondestructive analytical method for diagnosis of tooth caries is presented. The method is based on the Multiphoton Ionization (MPI) fast conductivity signals measured from tooth surfaces. The signals are acquired for a series of laser wavelengths, thus obtaining full MPI spectra. The results indicate a good correlation between the MPI results and the degree of severity of the caries, as diagnosed using traditional inspection. Moreover, the spectral information can be reduced (using least squares fitting) to a single parameter that provides an objective quantitative estimation of the caries severity. The MPI data can be obtained for tiny points on the dental surface and it is suggested that mapping is possible by scanning method.

Published: 2010.   Source: Instrumentation Science & Technology, 38:2, 143-150
Z-Scan
Z-scan operating principle

In nonlinear optics, z-scan technique is used to measure the non-linear index n2 (Kerr nonlinearity) and the non-linear absorption coefficient Δα via the “closed” and “open” methods to measure both real and imaginary components of the nonlinear refractive index.

For measuring the real part of the nonlinear refractive index, the z-scan setup is used in its closed-aperture form. The sample is typically placed in the focal plane of the lens, and then moved along the z axis, defined by the Rayleigh length. In this form, since the nonlinear material reacts like a weak z-dependent lens, the far-field aperture makes it possible to detect small beam distortions in the original beam. Since the focusing power of this weak nonlinear lens depends on the nonlinear refractive index, it is possible to extract its value by analyzing the z-dependent data acquired by the detector and by interpreting them using an appropriate theory.

For measurements of the imaginary part of the nonlinear refractive index, or the nonlinear absorption coefficient, the z-scan setup is used in its open-aperture form. In open-aperture measurements, the far-field aperture is removed and the whole signal is measured by the detector. By measuring the whole signal, the beam small distortions become insignificant and the z-dependent signal variation is due to the nonlinear absorption entirely.

The main cause of non-linear absorption is two-photon absorption. Due to high pulse intensity and cost effectiveness, picosecond high energy lasers are the most appropriate choice for z-scan measurements.

LIDAR
LIDAR operating principle

LIDAR is an acronym for “LIght Detection And Ranging”. LIDAR sends out short laser pulses into the atmosphere, where all along its path, the light is scattered by small particles, aerosols, and molecules of the air and is collected by telescope for analysis. Due to the constant velocity of light, time is related to the scatter’s distance, therefore, the spatial information is retrieved along the beam path. LIDAR uses ultraviolet, visible, or near infrared light to image objects. It can target a wide range of materials, including non-metallic objects, rocks, rain, chemical compounds, aerosols, pollutants, clouds, and even single molecules. LIDAR especially helps in those cases where access with conventional methods is troublesome.

LIBS
LIBS operating principle

Laser-induced breakdown spectroscopy (LIBS) utilizes a high intensity short laser pulse to convert a very small amount of target material to plasma for optical analysis of the spectra. LIBS can be used on solid, liquid, or gas samples, and, depending on the spectrograph and detector, can detect all elements. LIBS is non-contact, so it can be used in a wide variety of environments, including remote analysis and micro-sampling.

When coupled with appropriate optics and stages, elemental maps of a surface can be created. Multiple LIBS scans can effectively resolve material composition throughout the volume, building a full three dimensional elemental map.

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High Energy Laser Systems – Unique laser systems for extreme applications
NL120 series – SLM Q-switched Nd:YAG lasers
NL310 series – high energy Q-switched Nd:YAG lasers
Nanosecond Lasers
NL230 series – high energy Q-switched DPSS Nd:YAG lasers
NL300 series – compact flash-lamp pumped Q-switched Nd:YAG lasers
Picosecond Lasers
SL230 series – SBS compressed picosecond DPSS Nd:YAG lasers
Industrial Lasers
Atlantic series – high power industrial picosecond lasers

Adjusting the catalytic properties of cobalt ferrite nanoparticles by pulsed laser fragmentation in water with defined energy dose

Related products:  Atlantic series

Authors:  Friedrich Waag, Bilal Gökce, Chakrapani Kalapu, Georg Bendt, Soma Salamon, Joachim Landers, Ulrich Hagemann, Markus Heidelmann, Stephan Schulz, Heiko Wende, Nils Hartmann, Malte Behrens, Stephan Barcikowski

Highly active, structurally disordered CoFe2O4/CoO electrocatalysts are synthesized by pulsed laser fragmentation in liquid (PLFL) of a commercial CoFe2O4 powder dispersed in water. A partial transformation of the CoFe2O4 educt to CoO is observed and proposed to be a thermal decomposition process induced by the picosecond pulsed laser irradiation. The overpotential in the OER in aqueous alkaline media at 10 mA cm−2 is reduced by 23% compared to the educt down to 0.32 V with a Tafel slope of 71 mV dec−1. Importantly, the catalytic activity is systematically adjustable by the number of PLFL treatment cycles. The occurrence of thermal melting and decomposition during one PLFL cycle is verified by modelling the laser beam energy distribution within the irradiated colloid volume and comparing the by single particles absorbed part to threshold energies. Thermal decomposition leads to a massive reduction in particle size and crystal transformations towards crystalline CoO and amorphous CoFe2O4. Subsequently, thermal melting forms multi-phase spherical and network-like particles. Additionally, Fe-based layered double hydroxides at higher process cycle repetitions emerge as a byproduct. The results show that PLFL is a promising method that allows modification of the structural order in oxides and thus access to catalytically interesting materials.

Published: 2017.   Source: Scientific Reports, vol. 7, art. 13161 (2017)

Enhancement of Laser-Induced Breakdown Spectroscopy (LIBS) Detection Limit Using a Low-Pressure and Short-Pulse Laser-Induced Plasma Process

Related products:  NL120 series NL230 series NL310 series NL300 series SL230 series

Authors:  Z. Zhen Wang, Y. Deguchi, M. Kuwahara, J. Jie Yan, J. Ping Liu

Laser-induced breakdown spectroscopy (LIBS) technology is an appealing technique compared with many other types of elemental analysis because of the fast response, high sensitivity, real-time, and noncontact features. One of the challenging targets of LIBS is the enhancement of the detection limit. In this study, the detection limit of gas-phase LIBS analysis has been improved by controlling the pressure and laser pulse width. In order to verify this method, low-pressure gas plasma was induced using nanosecond and picosecond lasers. The method was applied to the detection of Hg. The emission intensity ratio of the Hg atom to NO (IHg/ INO) was analyzed to evaluate the LIBS detection limit because the NO emission (interference signal) was formed during the plasma generation and cooling process of N2 and O2 in the air. It was demonstrated that the enhancement of IHg/INO arose by decreasing the pressure to a few kilopascals, and the IHg/INO of the picosecond breakdown was always much higher than that of the nanosecond breakdown at low buffer gas pressure. Enhancement of IHg/INO increased more than 10 times at 700 Pa using picosecond laser with 35 ps pulse width. The detection limit was enhanced to 0.03 ppm (parts per million). We also saw that the spectra from the center and edge parts of plasma showed different features. Comparing the central spectra with the edge spectra, IHg/INO of the edge spectra was higher than that of the central spectra using the picosecond laser breakdown process.

Published: 2013.   Source: Applied Spectroscopy 67(11):1242-51

Overview of the application of laser-based techniques in plasma-wall interaction research program at IFPiLM

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Authors:  Monika Kubkowska, Paweł Gąsior, Agata Czarnecka, Marcin Rosiński, Jerzy Wołowski

The purpose of this paper is to give an overview of the progress in the application of the laser-based techniques which has been achieved in the research in the field of plasma-wall interaction (PWI) at the Division of Laser-Produced Plasmas (DLPP), Institute of Plasma Physics and Laser Microfusion (IFPiLM, Warsaw, Poland) since 2005. The evolution of the experimental set-up which started in a simple configuration for the laser ablative co-deposit removal is presented with stress on the milestones which led to subsequent modifications, namely installation of laser induced breakdown spectroscopy (LIBS), fast HR (high resolution) CCDs, pulsed fiber-laser and the common triggering system.

Published: 2012.   Source: Nukleonika, 2012; 57(2):163−166

Laser Induced Breakdown Spectroscopy and Applications Toward Thin Film Analysis

Related products:  NL230 series NL300 series

Authors:  T. N. Owens

Laser induced breakdown spectroscopy (LIBS) provides the opportunity to analyze almost any element, from any material, in any environment. Among the many applications of LIBS is the analysis of thin lms and multilayered structures. An automated system was designed and built to conduct LIBS using Nd:YAG and Ti:Sapphire lasers, broadband and high-resolution spectrometers and detectors. This system incorporates the sample manipulation as well as laser and spectrometer control and timing. A series of experiments were conducted to analyze the ability of nanosecond and femtosecond lasers to detect Mg impurities in thin TiO2 lms using LIBS. It was determined that optimal detection occurs early in the plasma ionic/atomic emission with detection capabilities in the parts-per-million range. Another series of experiments were conducted using LIBS to analyze thin transparent organic lms, with speci c emphasis on the e ect of lm thickness and interplay between lm and substrate. The challenges of ablating and measuring multiple layers have also been explored using various laser wavelengths. The e ectiveness of LIBS has been demonstrated for depth pro ling of CIGS solar cells. Ablation crater and ablation threshold analysis aided in understanding and overcoming some of the obstacles in depth pro ling. One of the challenges with LIBS is the identi cation and mitigation of matrix e ects. This problem was explored using a Mg tracer element and various compositions of the suspected elements Si, Ca, and Sr which can cause errors in LIBS analysis. The goal of this dissertation is to investigate the ability of LIBS to conduct detailed thin lm analysis for a variety of materials and potential applications. This includes analyzing trace elements from a traditionally noisy background, measuring dicult to ablate thin lms, and the unique challenges associated with multilayered structures.

Published: 2011.   Source: UC Berkeley Electronic Theses and Dissertations
Terahertz Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) probes inter-molecular interactions within solid materials. THz-TDS covers the spectral region of 0.1-10 THz which is a low energy and non-ionizing region of the electromagnetic spectrum. THz-TDS is a powerful technique for material characterization and process control and has several distinct advantages for use in spectroscopy. It can give the amplitude and phase information of the sample simultaneously. Many materials are transparent at terahertz wavelengths, and this radiation is safe for biological tissue being non-ionizing (as opposed to X-rays). It has been used for contact-free conductivity measurements of metals, semiconductors, 2D materials, and superconductors. Furthermore, THz-TDS has been used to identify chemical components such as amino acids, peptides, pharmaceuticals, and explosives, which makes it particularly valuable for fundamental science, security, and medical applications.

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Femtosecond Lasers
LightWire FFS – compact fiber seeders for femtosecond lasers

Terahertz Spectroscopy for Gastrointestinal Cancer Diagnosis

Related products:  LightWire FFS

Authors:  F. Wahaia, I. Kašalynas, G. Valušis, C.D. Carvalho Silva, P.L. Granja

In this chapter, we present a number of sensitive measurement modalities for the study and analysis of human cancer-affected colon and gastric tissue using terahertz (THz) spectroscopy. Considerable advancements have been reached in characterization of bio-tissue with some accuracy, although too dawn, and still long and exhaustive work have to be done towards well-established and reliable applications. The advent of the THz-time-domain spectroscopy (THz-TDS) test modality at a sub-picosecond time resolution has arguably fostered an intensive work in this field’s research line. The chapter addresses some basic theoretical aspects of this measurement modality with the presentation of general experimental laboratory setup diagrams for THz generation and detection, sample preparation aspects, samples optical parameters calculation procedures and data analysis.

Published: 2017.   Source: A Cutting Edge Technology, Jamal Uddin, IntechOpen

Green light stimulates terahertz emission from mesocrystal microspheres

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Authors:  X. L. Wu, S. J. Xiong, Z. Liu, J. Chen, J. C. Shen, T. H. Li, P. H. Wu, Paul K. Chu

The discovery of efficient sources of terahertz radiation has been exploited in imaging applications1, and developing a nanoscale terahertz source could lead to additional applications. High-frequency mechanical vibrations of charged nanostructures can lead to radiative emission, and vibrations at frequencies of hundreds of kilohertz have been observed from a ZnO nanobelt under the influence of an alternating electric field2. Here, we observe mechanical resonance and radiative emission at 0.36 THz from core–shell ZnO mesocrystal microspheres excited by a continuous green-wavelength laser. We find that 0.016% of the incident power is converted into terahertz radiation, which corresponds to a quantum efficiency of 33%, making the ZnO microspheres competitive with existing terahertz-emitting materials. The mechanical resonance and radiation stem from the coherent photo-induced vibration of the hexagonal ZnO nanoplates that make up the microsphere shells. The ZnO microspheres are formed by means of a nonclassical, self-organized crystallization process, and represent a straightforward route to terahertz radiation at the nanoscale.

Published: 2011.   Source: Nature Nanotechnology, vol. 6, pp. 103–106 (2011)