S. Juršėnas

Optical Bandgap Formation in AlInGaN Alloys

We report on the spectral dynamics of the reflectivity, site-selectively excited photoluminescence, photoluminescence excitation, and time-resolved luminescence in quaternary AlInGaN epitaxial layers grown on GaN templates. The incorporation of a few percents of In into AlGaN causes significant smoothening of the band-bottom potential profile in AlInGaN layers owing to improved crystal quality. An abrupt optical bandgap indicates that a nearly lattice-matched AlInGaN/GaN heterostructure with large energy band offsets can be grown for high-efficiency light-emitting devices.

Optical monitoring of nonequilibrium carrier lifetime in freestanding GaN by time-resolved four-wave mixing and photoluminescence techniques

Optical monitoring of nonequilibrium carrier dynamics was performed in freestanding GaN. Four-wave mixing kinetics directly provided carrier lifetime of 5.4ns in the layer, while complementary measurements by photoluminescence technique revealed the fast transients with subnanosecond decay time. Numerical modeling of photoluminescence decay taking into account the carrier spatial-temporal dynamics allowed us to attribute an origin of the fast photoluminescence transients to carrier diffusion to the bulk and to reabsorption of the backward emission. The studies demonstrated carrier diffusion limited applicability of the time-resolved photoluminescence technique for carrier lifetime measurements in a high quality thick III-nitride layers.

Decay of stimulated and spontaneous emission in highly excited homoepitaxial GaN

The high-density effects in the recombination of electron–hole plasma in photoexcited homoepitaxial GaN epilayers were studied by means of transient photoluminescence at room temperature. Owing to the “backward” and “lateral” photoluminescence measurement geometries employed, the influence of stimulated transitions on the decay of degenerate nonthermalized plasma was revealed. The lateral stimulated emission was demonstrated to cause a remarkable increase in the recombination rate on the early stage of the luminescence transient. A delayed enhancement of the stimulated emission due to the cooling of plasma from the initial temperature of 1100 K was observed. After completion of the thermalization process and exhaustion of the stimulated emission, the spontaneous-luminescence decay exhibited an exponential slope that relates to the nonradiative recombination of the carriers. The homoepitaxially grown GaN layer featured a luminescence decay time of 445 ps that implies a room-temperature free-carrier lifetim...

Deep-ultraviolet light-emitting diodes for frequency domain measurements of fluorescence lifetime in basic biofluorophores

We report on the development of deep-ultraviolet light-emitting diodes with AlGaN multiple-quantum-well active region for real-time fluorescence lifetime sensing in natural biofluorophores. The peak wavelengths of the devices are 340 and 280 nm, linewidth at half maximum approximately 10 nm, wall-plug efficiency up to 0.9%, output power in the milliwatt range, peak-to-background ratio up to four orders of magnitude, and cutoff frequencies for electrical modulation in the range of 100 MHz. Devices with high-frequency modulated output were demonstrated for frequency domain fluorescence lifetime measurements in basic biological autofluorophores (nicotinamide adenine dinucliotide, riboflavin, tyrosine, and tryptophan) with subnanosecond resolution.

The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer Glasses

Light upconversion (UC) via triplet-triplet annihilation (TTA) by using noncoherent photoexcitation at subsolar irradiance power densities is extremely attractive, particularly for enhanced solar energy harvesting. Unfortunately, practical TTA-UC application is hampered by low UC efficiency of upconverting polymer glasses, which is commonly attributed to poor exciton diffusion of the triplet excitons across emitter molecules. The present study addresses this issue by systematically evaluating triplet exciton diffusion coefficients and diffusion lengths (LD) in a UC model system based on platinum-octaethylporphyrin-sensitized poly(methyl methacrylate)/diphenylanthracene (emitter) films as a function of emitter concentration (15-40 wt %). For this evaluation time-resolved photoluminescence bulk-quenching technique followed by Stern-Volmer-type quenching analysis of experimental data was employed. The key finding is that although increasing emitter concentration in the disordered PMMA/DPA/PtOEP films improves triplet exciton diffusion, and thus LD, this does not result in enhanced UC quantum yield. Conversely, improved LD accompanied by the accelerated decay of UC intensity on millisecond time scale degrades TTA-UC performance at high emitter loadings (>25 wt %) and suggests that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor.

Optical gain in homoepitaxial GaN

Optical gain in GaN epilayers, grown by metalorganic chemical-vapor deposition technique on bulk GaN substrates is studied by means of time-resolved luminescence spectroscopy at room temperature. Both stimulated emission and carrier recombination rate are analyzed under high photoexcitation conditions that are close to laser operation regime. Homoepitaxial GaN shows a high value of optical gain coefficient g=7200cm−1 estimated under intense pulsed excitation by a variable stripe method. For comparison, a GaN epilayer grown under identical conditions on sapphire shows a significantly lower value, g=2300cm−1. Larger values of the optical gain coefficient achieved in homoepitaxial GaN are due to the lower density of nonradiative traps. This is proved by the carrier capture time that is estimated right after exhaustion of the inverted population, and has values of τe=970 and 195ps for homo- and heterolayers, respectively.

Dynamic behavior of hot-electron–hole plasma in highly excited GaN epilayers

The room-temperature spontaneous luminescence of electron–hole plasma was investigated in GaN epilayers under extremely high quasi-steady-state photoexcitation. The photoluminescence spectra were measured for excitation power densities up to 200 MW/cm2 both under quasiresonant and off-resonant excitation conditions. High carrier temperatures up to 1000 K were observed under off-resonant excitation. A nonmonotonous dependence of the luminescence band peak position Ep on the excitation power density was observed. We attribute this nonmonotonous behavior of Ep to two competing mechanisms: (i) band-gap shrinkage due to carrier screening effects (redshift); and (ii) nonequilibrium carrier heating (blueshift). The obtained results are in a good agreement with finite-temperature theory of the band-gap renormalization.

Nonradiative recombination of hot photoelectrons in CdS nanocrystals embedded in glass

Transient luminescence spectra are investigated in highly photoexcited CdS nanocrystals (average radii 5.4–100 nm) embedded in glass. Luminescence-intensity kinetics exhibits distortion attributed to carrier-temperature-activated nonradiative recombination due to multiphonon emission. The distortion enhances with reduced crystallite size indicating that the photomodified semiconductor-glass interface originates deep traps with localization barrier of 130 meV.

Carrier recombination and diffusion in GaN revealed by transient luminescence under one-photon and two-photon excitations

Carrier recombination and diffusion dynamics in a 100-μm-thick GaN grown by hydride vapor phase epitaxy on semi-insulating GaN:Mg substrate have been studied by means of transient photoluminescence under one-photon (1P) and two-photon (2P) excitations. For 2P bulk excitation the luminescence transients featured an exponential decay with the time constant of 1100ps, which was mainly due to carrier capture to nonradiative deep traps. Meanwhile for 1P surface excitation, the luminescence transients showed a nonexponential decay with the mean time constant of 440ps, which was shown to be due to both carrier in-depth diffusion and recombination.

Fluorene- and benzofluorene-cored oligomers as low threshold and high gain amplifying media

Deliberate control of intermolecular interactions in fluorene- and benzofluorene-cored oligomers was attempted via introduction of different-length alkyl moieties to attain high emission amplification and low amplified spontaneous emission (ASE) threshold at high oligomer concentrations. Containing fluorenyl peripheral groups decorated with different-length alkyl moieties, the oligomers were found to express weak concentration quenching of emission, yet excellent carrier drift mobilities (close to 10−2 cm2/V/s) in the amorphous films. Owing to the larger radiative decay rates (>1.0 × 109 s−1) and smaller concentration quenching, fluorene-cored oligomers exhibited down to one order of magnitude lower ASE thresholds at higher concentrations as compared to those of benzofluorene counterparts. The lowest threshold (300 W/cm2) obtained for the fluorene-cored oligomers at the concentration of 50 wt % in polymer matrix is among the lowest reported for solution-processed amorphous films in ambient conditions, wha...