Picosecond laser systems

The ultrafast photochemical reaction mechanism, transient spectra, and transition kinetics of the human blue cone visual pigment have been recorded at room temperature. Ultrafast time-resolved absorption spectroscopy revealed the progressive formation and decay of several metastable photo-intermediates, corresponding to the Batho to Meta-II photo-intermediates previously observed with bovine rhodopsin and human green cone opsin, on the picosecond to millisecond timescales following pulsed excitation. The experimental data reveal several interesting similarities and differences between the photobleaching sequences of bovine rhodopsin, human green cone opsin, and human blue cone opsin. While Meta-II formation kinetics are comparable between bovine rhodopsin and blue cone opsin, the transition kinetics of earlier photo-intermediates and qualitative characteristics of the Meta-I to Meta-II transition are more similar for blue cone opsin and green cone opsin. Additionally, the blue cone photo-intermediate spectra exhibit a high degree of overlap with uniquely small spectral shifts. The observed variation in Meta-II formation kinetics between rod and cone visual pigments is explained based on key structural differences.

Photolabile chelating cages or protecting groups need complex chemical syntheses and require UV, visible, or two-photon NIR light to trigger release. Different cages have different solubilities, reaction rates, and energies required for triggering. Here we show that liposomes containing calcium, adenosine triphosphate, or carboxyfluorescein are tethered to plasmon-resonant hollow gold nanoshells (HGN) tuned to absorb light from 650 – 950 nm. Picosecond pulses of near infrared (NIR) light provided by a two-photon microscope, or by a stand-alone laser during flow through microfluidic channels, trigger contents release with spatial and temporal control. NIR light adsorption heats the HGN, inducing vapor nanobubbles that rupture the liposome, releasing cargo within milliseconds. Any water-soluble molecule can be released at essentially the same rate from the liposome-HGN. By using liposomes of different composition, or HGN of different sizes or shapes with different nanobubble threshold fluences, or irradiating on or off resonance, two different cargoes can be released simultaneously, one before the other, or in a desired ratio. Calcium release from liposome-HGN can be spatially patterned to crosslink alginate gels and trap living cells. Liposome-HGN provide stable, biocompatible isolation of the bioactive compound from its surroundings with minimal interactions with the local environment.

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.

The black body remains the most prominent source of light for absolute radiometry. Its main alternative, synchrotron radiation, requires costly and large facilities. Quantum optics offers a new radiometric source: parametric down-conversion (PDC), a nonlinear optical process, in which pairwise photon correlations enable absolute calibration of photodetectors. Since the emission rate crucially depends on the brightness of the electromagnetic field, quantum-mechanical fluctuations of the vacuum can be seen as a seed of spontaneous PDC, and their amplitude is a natural radiometric standard. Thus, they allow for the calibration of the spectral radiance of light sources by measuring the ratio between seeded and unseeded PDC. Here, we directly use the frequency spectrum of the electromagnetic vacuum to trigger spontaneous PDC and employ the generated light to infer the spectral response of a spectrometer over a broad spectral range. Then, we deduce the absolute quantum efficiency from the spectral shape of PDC in the high-gain regime, without relying on a seed or reference detector. Our results compare well with the ones obtained with a reference lamp, demonstrating a promising primary radiation standard.

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.

Organic solid-state lasers (OSSLs) with distributed feedback (DFB) structures or distributed Bragg reflectors (DBRs) are promising for potential application in bio-sensing and hazardous materials detection. Here, the laser performances of the all-organic DFB waveguide lasers with various grating heights ranging from 0.4 to 4.7 μm were investigated. The grating structures used as the lasing cavity were fabricated using a two-photon absorption (TPA) direct laser writing (DLW) method with an SU-8 negative photoresist. The laser active layer consisted of a rhodamine 6G (R6G) laser dye and a cellulose acetate (CA) matrix. The R6G/CA solution was spin-coated onto the quartz substrate with the cavity (grating) structures to fabricate the DFB waveguide laser devices. The diffraction order of lasing ranged from m = 4 to 7. As the grating height was increased to 1.9 μm, the slope efficiency increased for all diffraction orders and the threshold decreases for each diffraction order. The dependence of the cavity (grating) length on the laser performances was investigated. The slope efficiency increased as the cavity length increased to 300 μm. The effect of the cavity (grating) position on the slope efficiency and the threshold position of the cavity (grating) was also studied. A maximum slope efficiency of 10.2% was achieved for the DFB waveguide laser device with a cavity (grating) length of 300 μm, a cavity position at 6 mm from the emission edge of the waveguide, and an aspect ratio ≈3 between the grating height of 1.74 μm and the grating width of 0.6 μm for the diffraction order m = 6 for lasing.

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.

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(υ=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.

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.

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.

We present a high peak and average power optical parametric chirped pulse amplification system driven by diode-pumped Yb:KGW and Nd:YAG lasers running at 1 kHz repetition rate. The advanced architecture of the system allows us to achieve >53 W average power combined with 5.5 TW peak power, along with sub-220 mrad CEP stability and sub-9 fs pulse duration at a center wavelength around 880 nm. Broadband, background-free, passively CEP stabilized seed pulses are produced in a series of cascaded optical parametric amplifiers pumped by the Yb:KGW laser, while a diode-pumped Nd:YAG laser system provides multi-mJ pump pulses for power amplification stages. Excellent stability of output parameters over 16 hours of continuous operation is demonstrated.

Quantitative concentrations measurements using time-resolved laser-induced fluorescence have been demonstrated for nitric oxide (NO) in flame. Fluorescence lifetimes measured using a picosecond Nd:YAG laser and optical parametric amplifier system have been used to directly compensate the measured signal for collisional quenching and evaluate NO concentration. The full evaluation also includes the spectral overlap between the ∼15 cm⁻¹ broad laser pulse and multiple NO absorption lines as well as the populations of the probed energy levels. Effective fluorescence lifetimes of 1.2 and 1.5 ns were measured in prepared NO/N₂/O₂ mixtures at ambient pressure and temperature and in a premixed NH₃-seeded CH₄/N₂/O₂ flame, respectively. Concentrations evaluated from measurements in NO/N₂/O₂ mixtures with NO concentrations of 100 – 600 ppm were in agreement with set values within 3% at higher concentrations. An accuracy of 13% was estimated by analysis of experimental uncertainties. An NO profile measured in the flame showed concentrations of ∼1000 ppm in the post-flame region and is in good agreement with NO concentrations predicted by a chemical mechanism for NH₃ combustion. An accuracy of 16% was estimated for the flame measurements. The direct concentration evaluation from time-resolved fluorescence allows for quantitative measurements in flames where the composition of major species and their collisional quenching on the probed species is unknown. In particular, this is valid for non-stationary turbulent combustion and implementation of the presented approach for measurements under such conditions is discussed.

We present a compact TW-class OPCPA system operating at 800 nm. Broadband seed pulses are generated and pre-amplified to 25 μJ in a white light continuum seeded femtosecond NOPA. Amplification of the seed pulses to 35 mJ at a repetition rate of 10 Hz and compression to 9 fs is demonstrated.

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.