- Integrated OPO system
- High energy
- Ultrabroad tuning range from 192 to 4400 nm
- Up to 50 mJ in VIS
- Up to 20 Hz repetition rate
- Integrated OPO system
- High energy
- Ultrabroad tuning range from 192 to 4400 nm
- Up to 50 mJ in VIS
- Up to 20 Hz repetition rate
Features & Applications
- Hands-free no gap wavelength tuning from 192 to 4400 nm
- Up to 50 mJ pulse energy in visible spectral range
- Up to 10 mJ pulse energy in UV spectral range
- Up to 15 mJ pulse energy in MIR spectral range
- 3 – 5 ns pulse duration
- Up to 20 Hz pulse repetition rate
- Remote control via key pad or PC
- Optional separate shared output port for 532/1064 nm beam (separate output port for the 355 nm beam is standard)
- OPO pump energy monitoring
- Hermetically sealed oscillator cavity protects non-linear crystals from dust and humidity
- Laser-induced fluorescence
- Flash photolysis
- Remote sensing
- Time-resolved spectroscopy
- Non-linear spectroscopy
- The system is widely tunable 192 – 2600 nm and delivers high pulse energy (up to 50 mJ) that allows the investigation of an extensive range of materials
- Up to 18 μm customization possibility enables studies of IR vibrations of molecules
- Narrow linewidth (down to 3 cm⁻¹) and superior tuning resolution (1 – 2 cm⁻¹) allows recording of high quality spectra
- Flashlamps replacement without misalignment of the laser cavity saves on maintenance costs
- High integration level saves valuable space in the laboratory
- In-house design and manufacturing of complete systems, including pump lasers, guarantees on-time warranty and post warranty services and spares supply
- Variety of control interfaces: USB, RS232, LAN and WLAN ensures easy control and integration with other equipment
- Attenuator and fiber coupling options facilitate incorporation of NT340 systems into various experimental environments
The NT340 series tunable wavelength nanosecond laser seamlessly integrates the nanosecond optical parametric oscillator and the Nd:YAG Q-switched nanosecond laser – all in a compact housing.
The main system features are: hands-free wavelength tuning from UV to IR, high conversion efficiency, optional fiber-coupled output and separate output port for pump laser beam.
NT340 has a linewidth of less than 5 cm⁻¹, which is ideal for many spectroscopic applications.
The laser is designed for convenient use. It can be controlled from remote keypad or PC using LabView™ drivers that are supplied with the system. The remote keypad features a backlit display that is easy to read even through laser safety googles. The OPO pump energy monitoring system helps to control pump laser parameters. Replacement of laser flashlamps can be done without misalignment of the laser cavity and/or deterioration of laser performance.
Tuning range extensions
|-SH||Second harmonic generator for 210 – 410 nm range|
|-SF||Sum-frequency generator for 300 – 410 nm range with high pulse energy|
|-SH/SF||Combined option for highest pulse energy in 210 – 410 nm range|
|-DUV||Deep UV option for 192 – 210 nm range|
|-MIR||Mid infrared option for 2500 – 4400 nm range output|
Accessories and Options
|-FC||Fiber coupled output in 350 – 2000 nm range|
|-ATTN/FC||Fiber coupled attenuator|
|-H, -2H||Separate shared output port for Nd:YAG pump laser harmonics (532 or 1064 nm wavelengths)|
|-AW||Air cooled power supply|
|Wavelength range 2)|
|Signal||410 – 710 nm 3)|
|Idler||710 – 2600 nm|
|SH generator (optional)||210 – 410 nm|
|SH/SF generator (optional)||210 – 410 nm|
|DUV generator (optional)||192 – 210 nm|
|MIR generator (optional)||2500 – 4400 nm|
|Output pulse energy|
|OPO 4)||30 mJ||50 mJ|
|SH generator (optional) 5)||4 mJ||6.5 mJ|
|SH/SF generator (optional) 6)||6 mJ||10 mJ|
|DUV generator (optional) 7)||0.6 mJ||1 mJ|
|MIR generator (optional) 8)||15 mJ|
|Linewidth||<5 cm-1 9)|
|Tuning resolution 10)|
|Signal (410 – 710 nm)||1 cm⁻¹|
|Idler (710 – 2600 nm)||1 cm⁻¹|
|SH/SF/DUV beam (192 – 410 nm)||2 cm⁻¹|
|MIR (2500 – 4400 nm)||1 cm⁻¹|
|Pulse duration 11)||3 – 5 ns|
|Typical beam diameter 12)||5 mm||7 mm|
|Typical beam divergence 13)||< 2 mrad|
|PUMP LASER 14)|
|Pump wavelength||355 nm|
|Typical pump pulse energy||100 mJ||150 mJ|
|Pulse duration||4 – 7 ns|
|Beam quality||Hat-top in near field, without hot spots|
|Beam divergence||< 0.6 mrad|
|Pulse energy stability (StdDev)||< 3.5 %|
|Pulse repetition rate||10 or 20 Hz||10 Hz|
|Unit size (W × L × H) 15)||456 × 821× 270 mm|
|Power supply size (W × L × H)||330 × 490 × 585 mm|
|Umbilical length||2.5 m|
|Water consumption (max 20 °C) 16)||< 10 l/min|
|Room temperature||18 – 27 °C|
|Relative humidity||20 – 80 % (non-condensing)|
|Mains voltage||200 – 240 VAC, single phase, 50/60 Hz|
|Power consumption||< 1.5 kVA|
- Due to continuous improvement, all specifications are subject to change. Parameters marked typical are illustrative; they are indications of typical performance and will vary with each unit we manufacture. Unless stated otherwise, all specifications are measured at 450 nm and for basic system without options.
- Hands-free tuning range is from 192 nm to 4400 nm. MIR option is not compatible with SF and DUV option. Inquire for custom IR option with tuning up to 18 µm.
- Tuning range extension to 400 – 709 nm is optional.
- Measured at 430 nm. See tuning curves for typical outputs at other wavelengths.
- Measured at 260 nm. See tuning curves for typical outputs at other wavelengths.
- Measured at 340 nm. SF generator is optimized for maximum output in 300 – 410 nm range. See tuning curves for typical outputs at other wavelengths.
- Measured at 200 nm. See tuning curves for typical outputs at other wavelengths.
- Measured at 3000 nm. See tuning curves for typical outputs at other wavelengths.
- Linewidth is <8 cm⁻¹ for 210 – 409 nm, 2500 – 4400 nm ranges.
- When wavelength is controlled from PC. When wavelength is controlled from keypad, tuning resolution is 0.1 nm for signal, 1 nm for idler, MIR and 0.05 nm for SH, SF and DUV.
- FWHM measured with photodiode featuring 1 ns rise time and 300 MHz bandwidth oscilloscope.
- Beam diameter is measured at 450 nm at the FWHM level. It is approximate and can vary depending on the pump pulse energy and wavelength.
- Full angle measured at the FWHM level at 450 nm, < 5 mrad at 3000 nm with MIR option.
- Separate output port for the 355 nm beam is standard. Outputs for 1064 nm and 532 nm beams are optional. Laser output will be optimised for the best OPO operation and specifications may vary with each unit we manufacture.
- Length from 821 to 1220 mm depending on configuration.
- Air cooled power supply is available as an option.
Note: Laser must be connected to the mains electricity all the time. If there will be no mains electricity for longer that 1 hour then laser (system) needs warm up for a few hours before switching on.
Performance & Drawings
The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites
Related applications: Time Resolved Photoconductivity
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.
Probing the Structural Evolution of the Hydrated Electron in Water Cluster Anions (H2O)n–, n ≤ 200, by Electronic Absorption Spectroscopy
Electronic absorption spectra of water cluster anions (H2O)n–, n ≤ 200, at T = 80 K are obtained by photodissociation spectroscopy and compared with simulations from literature and experimental data for bulk hydrated electrons. Two almost isoenergetic electron binding motifs are seen for cluster sizes 20 ≤ n ≤ 40, which are assigned to surface and partially embedded isomers. With increasing cluster size, the surface isomer becomes less populated, and for n ≥ 50, the partially embedded isomer prevails. The absorption shifts to the blue, reaching a plateau at n ≈ 100. In this size range, the absorption spectrum is similar to that of the bulk hydrated electron but is slightly red-shifted; spectral moment analysis indicates that these clusters are reasonable model systems for hydrated electrons near the liquid–vacuum interface.
Iminothioindoxyl as a molecular photoswitch with 100 nm band separation in the visible range
Light is an exceptional external stimulus for establishing precise control over the properties and functions of chemical and biological systems, which is enabled through the use of molecular photoswitches. Ideal photoswitches are operated with visible light only, show large separation of absorption bands and are functional in various solvents including water, posing an unmet challenge. Here we show a class of fully-visible-light-operated molecular photoswitches, Iminothioindoxyls (ITIs) that meet these requirements. ITIs show a band separation of over 100 nm, isomerize on picosecond time scale and thermally relax on millisecond time scale. Using a combination of advanced spectroscopic and computational techniques, we provide the rationale for the switching behavior of ITIs and the influence of structural modifications and environment, including aqueous solution, on their photochemical properties. This research paves the way for the development of improved photo-controlled systems for a wide variety of applications that require fast responsive functions.
Black phosphorene as a hole extraction layer boosting solar water splitting of oxygen evolution catalysts
As the development of oxygen evolution co-catalysts (OECs) is being actively undertaken, the tailored integration of those OECs with photoanodes is expected to be a plausible avenue for achieving highly efficient solar-assisted water splitting. Here, we demonstrate that a black phosphorene (BP) layer, inserted between the OEC and BiVO4 can improve the photoelectrochemical performance of pre-optimized OEC/BiVO4 (OEC: NiOOH, MnOx, and CoOOH) systems by 1.2∼1.6-fold, while the OEC overlayer, in turn, can suppress BP self-oxidation to achieve a high durability. A photocurrent density of 4.48 mA·cm−2 at 1.23 V vs reversible hydrogen electrode (RHE) is achieved by the NiOOH/BP/BiVO4 photoanode. It is found that the intrinsic p-type BP can boost hole extraction from BiVO4 and prolong holes trapping lifetime on BiVO4 surface. This work sheds light on the design of BP-based devices for application in solar to fuel conversion, and also suggests a promising nexus between semiconductor and electrocatalyst.
Pulsed photo-ionization spectroscopy of traps in as-grown and neutron irradiated ammonothermally synthesized GaN
GaN-based structures are promising for production of radiation detectors and high-voltage high-frequency devices. Particle detectors made of GaN are beneficial as devices simultaneously generating of the optical and electrical signals. Photon-electron coupling cross-section is a parameter which relates radiation absorption and emission characteristics. On the other hand, photon-electron coupling cross-section together with photo-ionization energy are fingerprints of deep centres in material. In this work, the wafer fragments of the GaN grown by ammonothermal (AT) technology are studied to reveal the dominant defects introduced by growth procedures and reactor neutron irradiations in a wide range, 1012–1016 cm−2, of fluences. Several defects in the as-grown and irradiated material have been revealed by using the pulsed photo-ionization spectroscopy (PPIS) technique. The PPIS measurements were performed by combining femtosecond (40 fs) and nanosecond (4 ns) laser pulses emitted by optical parametric oscillators (OPO) to clarify the role of electron-phonon coupling. Variations of the operational characteristics of the tentative sensors, made of the AT GaN doped with Mg and Mn, under radiation damage by reactor neutrons have been considered.
Luminescence spectroscopy of oxazine dye cations isolated in vacuo
Related applications: Photoluminescence Spectroscopy
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.
Near infrared emission properties of Er doped cubic sesquioxides in the second/third biological windows
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.
Luminescence Spectroscopy of Rhodamine Homodimer Dications in Vacuo Reveals Strong Dye‐Dye Interactions
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.
Electronic spectroscopy and nanocalorimetry of hydrated magnesium ions [Mg(H2O)n]+, n = 20–70: spontaneous formation of a hydrated electron?
Related applications: Photolysis
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.
Charge carrier transport in polycrystalline CH3NH3PbI3 perovskite thin films in a lateral direction characterized by time-of-flight photoconductivity
Related applications: Time Resolved Photoconductivity
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.
Photochemistry and spectroscopy of small hydrated magnesium clusters Mg+(H2O)n, n = 1–5
Related applications: Photolysis
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.
Photodissociation of Sodium Iodide Clusters Doped with Small Hydrocarbons
Related applications: Photolysis
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.
Quenching of the red Mn4+ luminescence in Mn4+-doped fluoride LED phosphors
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+ d–d 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.
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
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).
Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers
Related applications: Seeding and pumping
Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.
Photochemistry of glyoxylate embedded in sodium chloride clusters, a laboratory model for tropospheric sea-salt aerosols
Related applications: Time Resolved Photochemistry
Although marine aerosols undergo extensive photochemical processing in the troposphere, a molecular level understanding of the elementary steps involved in these complex reaction sequences is still missing. As a defined laboratory model system, the photodissociation of sea salt clusters doped with glyoxylate, [NanCln−2(C2HO3)]+, n = 5–11, is studied by a combination of mass spectrometry, laser spectroscopy and ab initio calculations. Glyoxylate acts as a chromophore, absorbing light below 400 nm via two absorption bands centered at about 346 and 231 nm. Cluster fragmentation dominates, which corresponds to internal conversion of the excited state energy into vibrational modes of the electronic ground state and subsequent unimolecular dissociation. Photochemical dissociation pathways in electronically excited states include CO and HCO elimination, leading to [Nan−xCln−x−2HCOO]+ and [NanCln−2COO˙]+ with typical quantum yields in the range of 1–3% and 5–10%, respectively, for n = 5. The latter species contains CO2˙− stabilized by the salt environment. The comparison of different cluster sizes shows that the fragments containing a carbon dioxide radical anion appear in a broad spectral region of 310–380 nm. This suggests that the elusive CO2˙− species may be formed by natural processes in the troposphere. Based on the photochemical cross sections obtained here, the photolysis lifetime of glyoxylate in a dry marine aerosol is estimated as 10 h. Quantum chemical calculations show that dissociation along the C–C bond in glyoxylic acid as well as glyoxylate embedded in the salt cluster occurs after reaching the S1/S0 conical intersection, while this conical intersection is absent in free glyoxylate ions.
Nanoscale insights into doping behavior, particle size and surface effects in trivalent metal doped SnO2
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.
Sibling rivalry: intrinsic luminescence from two xanthene dye monoanions, resorufin and fluorescein, provides evidence for excited-state proton transfer in the latter
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.
11% efficiency solid-state dye-sensitized solar cells with copper(II/I) hole transport materials
Solid-state dye-sensitized solar cells currently suffer from issues such as inadequate nanopore filling, low conductivity and crystallization of hole-transport materials infiltrated in the mesoscopic TiO2 scaffolds, leading to low performances. Here we report a record 11% stable solid-state dye-sensitized solar cell under standard air mass 1.5 global using a hole-transport material composed of a blend of [Cu (4,4′,6,6′-tetramethyl-2,2′-bipyridine)2](bis(trifluoromethylsulfonyl)imide)2 and [Cu (4,4′,6,6′-tetramethyl-2,2′-bipyridine)2](bis(trifluoromethylsulfonyl)imide). The amorphous Cu(II/I) conductors that conduct holes by rapid hopping infiltrated in a 6.5 μm-thick mesoscopic TiO2 scaffold are crucial for achieving such high efficiency. Using time-resolved laser photolysis, we determine the time constants for electron injection from the photoexcited sensitizers Y123 into the TiO2 and regeneration of the Y123 by Cu(I) to be 25 ps and 3.2 μs, respectively. Our work will foster the development of low-cost solid-state photovoltaic based on transition metal complexes as hole conductors.
Non-Poissonian photon statistics from macroscopic photon cutting materials
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.
Dye-sensitized solar cells for efficient power generation under ambient lighting
Solar cells that operate efficiently under indoor lighting are of great practical interest as they can serve as electric power sources for portable electronics and devices for wireless sensor networks or the Internet of Things. Here, we demonstrate a dye-sensitized solar cell (DSC) that achieves very high power-conversion efficiencies (PCEs) under ambient light conditions. Our photosystem combines two judiciously designed sensitizers, coded D35 and XY1, with the copper complex Cu(II/I)(tmby) as a redox shuttle (tmby, 4,4′,6,6′-tetramethyl-2,2′-bipyridine), and features a high open-circuit photovoltage of 1.1 V. The DSC achieves an external quantum efficiency for photocurrent generation that exceeds 90% across the whole visible domain from 400 to 650 nm, and achieves power outputs of 15.6 and 88.5 μW cm–2 at 200 and 1,000 lux, respectively, under illumination from a model Osram 930 warm-white fluorescent light tube. This translates into a PCE of 28.9%.
Luminescence spectroscopy of chalcogen substituted rhodamine cations in vacuo
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.
Photodissociation spectroscopy of protonated leucine enkephalin
Related applications: Photolysis
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.
Investigation into the Advantages of Pure Perovskite Film without PbI2 for High Performance Solar Cell
Related applications: Time Resolved Photoconductivity
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.
Nile blue shows its true colors in gas-phase absorption and luminescence ion spectroscopy
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.
Magneto-elasto-electroporation (MEEP): In-vitro visualization and numerical characteristics
A magnetically controlled elastically driven electroporation phenomenon, or magneto-elasto-electroporation (MEEP), is discovered while studying the interactions between core-shell magnetoelectric nanoparticles (CSMEN) and biological cells in the presence of an a.c. magnetic field. In this paper we report the effect of MEEP observed via a series of in-vitro experiments using core (CoFe2O4)-shell (BaTiO3) structured magnetoelectric nanoparticles and human epithelial cells (HEP2). The cell electroporation phenomenon and its correlation with the magnetic field modulated CSMEN are described in detail. The potential application of CSMEN in electroporation is confirmed by analyzing crystallographic phases, multiferroic properties of the fabricated CSMEN, influences of d.c. and a.c. magnetic fields on the CSMEN and cytotoxicity tests. The mathematical formalism to quantitatively describe the phenomena is also reported. The reported findings provide insights into the underlying MEEP mechanism and demonstrate the utility of CSMEN as an electric pulse-generating nano-probe in electroporation experiments with a potential application toward accurate and efficient targeted cell permeation.
Flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels based on an organic bicomponent blend
Related applications: Time Resolved Photoconductivity
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.
Optoacoustic effect is responsible for laser-induced cochlear responses
Optical stimulation of the cochlea with laser light has been suggested as an alternative to conventional treatment of sensorineural hearing loss with cochlear implants. The underlying mechanisms are controversially discussed: The stimulation can either be based on a direct excitation of neurons, or it is a result of an optoacoustic pressure wave acting on the basilar membrane. Animal studies comparing the intra-cochlear optical stimulation of hearing and deafened guinea pigs have indicated that the stimulation requires intact hair cells. Therefore, optoacoustic stimulation seems to be the underlying mechanism. The present study investigates optoacoustic characteristics using pulsed laser stimulation for in vivo experiments on hearing guinea pigs and pressure measurements in water. As a result, in vivo as well as pressure measurements showed corresponding signal shapes. The amplitude of the signal for both measurements depended on the absorption coefficient and on the maximum of the first time-derivative of laser pulse power (velocity of heat deposition). In conclusion, the pressure measurements directly demonstrated that laser light generates acoustic waves, with amplitudes suitable for stimulating the (partially) intact cochlea. These findings corroborate optoacoustic as the basic mechanism of optical intra-cochlear stimulation.
A cylindrical quadrupole ion trap in combination with an electrospray ion source for gas-phase luminescence and absorption spectroscopy
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.
Recent Advances in Laser Utilization in the Chemical Modification of Graphene Oxide and Its Applications
Related applications: Material Processing
A dramatic rise in research interest in laser‐induced graphene oxide (GO) reduction and modification requires an overview of the most recent works on this subject. Typical methods for the recognition and confirmation of modified graphene and its derivatives, such as Raman, Fourier‐transform infra‐red (FTIR), X‐ray photoelectron (XP), and ultraviolet‐visible (UV–vis) spectroscopies, are introduced briefly in this review. A major part of the survey is devoted to the main modification ways and the laser parameters used in the literature. A discussion of possible reduction and modification mechanisms is also presented. Recent applications, especially in the biomedical field such as cell therapy treatment, as well as significant results of GO modification, are discussed in detail. Finally, perspectives for the application of laser‐induced GO modifications in passive THz photonics and biomedicine are briefly addressed.
Multi-photon quantum cutting in Gd2O2S:Tm3+ to enhance the photo-response of solar cells
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.
Transparent Organic Photodetector using a Near-Infrared Absorbing Cyanine Dye
Related applications: Time Resolved Photoconductivity
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.
Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells
Related applications: Material Processing
Perovskite solar cells with submicrometre-thick CH3NH3PbI3 or CH3NH3PbI3–xClx active layers show a power conversion efficiency as high as 15%. However, compared to the best-performing device, the average efficiency was as low as 12%, with a large standard deviation (s.d.). Here, we report perovskite solar cells with an average efficiency exceeding 16% and best efficiency of 17%. This was enabled by the growth of CH3NH3PbI3 cuboids with a controlled size via a two-step spin-coating procedure. Spin-coating of a solution of CH3NH3I with different concentrations follows the spin-coating of PbI2, and the cuboid size of CH3NH3PbI3 is found to strongly depend on the concentration of CH3NH3I. Light-harvesting efficiency and charge-carrier extraction are significantly affected by the cuboid size. Under simulated one-sun illumination, average efficiencies of 16.4% (s.d. ± 0.35), 16.3% (s.d. ± 0.44) and 13.5% (s.d. ± 0.34) are obtained from solutions of CH3NH3I with concentrations of 0.038 M, 0.050 M and 0.063 M, respectively. By controlling the size of the cuboids of CH3NH3PbI3 during their growth, we achieved the best efficiency of 17.01% with a photocurrent density of 21.64 mA cm–2, open-circuit photovoltage of 1.056 V and fill factor of 0.741.
Luminescence upconversion in colloidal double quantum dots
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.
Photogeneration and reactions of benzhydryl cations and radicals: A complex sequence of mechanisms from femtoseconds to microseconds
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.
A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials
Dye-sensitized solar cells are a promising alternative to traditional inorganic semiconductor-based solar cells. Here we report an open-circuit voltage of over 1,000 mV in mesoscopic dye-sensitized solar cells incorporating a molecularly engineered cobalt complex as redox mediator. Cobalt complexes have negligible absorption in the visible region of the solar spectrum, and their redox properties can be tuned in a controlled fashion by selecting suitable donor/acceptor substituents on the ligand. This approach offers an attractive alternate to the traditional I3−/I− redox shuttle used in dye-sensitized solar cells. A cobalt complex using tridendate ligands [Co(bpy-pz)2]3+/2+(PF6)3/2 as redox mediator in combination with a cyclopentadithiophene-bridged donor-acceptor dye (Y123), adsorbed on TiO2, yielded a power conversion efficiency of over 10% at 100 mW cm−2. This result indicates that the molecularly engineered cobalt redox shuttle is a legitimate alternative to the commonly used I3−/I− redox shuttle.
Spectral characterization of surfaces using laser multi-photon ionization
Multi-photon ionization (MPI) spectroscopy of solid surfaces under ambient conditions and in nitrogen has been established and exemplified for a variety of materials. This was accomplished using a dedicated experimental setup that monitors the photoelectron yield as a function of the laser wavelength. The MPI spectra resemble the absorption characteristics, however, possess more peaks and are more detailed. This demonstrated the possibility to apply MPI spectroscopy for fast analysis of solids. The dependence of the signals upon the laser flux implies that the ionization mechanism depends on the examined molecule and in many cases it is a two-step process, via a long-living intermediate energy state. The method provides quantification in the pmole range and allows for surface imaging.
A novel method for direct nondestructive diagnosis of caries affected tooth surfaces by laser multiphoton ionization
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.
Study of GaN : Eu3+ Thin Films Deposited by Metallorganic Vapor-Phase Epitaxy
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.