Ultrafast carrier dynamics in an unintentionally doped GaN sample was investigated using femtosecond transient transmission measurements. Special attention was focused on bandtail states. The transient responses suggest that the shallow bandtail states are extended states and deep bandtail states are localized states. The carriers in shallow bandtail states are found to externally thermalize within 500 fs, at the same rate as the above bandgap carriers. The carriers in deep bandtail states are, on the other hand, dominated by carrier transfer into the lower energy states through phonon assisted tunneling.

Time-resolved photoluminescence spectra have been measured for single crystal GaN layers fabricated by metalorganic chemical vapor deposition on GaN single-crystal substrates. The low temperature spectra consist of a donor-bound exciton (D⁰X) recombination at 3.472 eV, an acceptor bound exciton (A⁰X) at 3.467 eV and free excitons A and B (FX_A and FX_B) at 3.478 eV and 3.483 eV, respectively. Below 70 K, the D⁰K peak dominates. At higher temperatures, delocalization of excitons is observed, and the free exciton emission becomes very strong. At low excitation density (100 W/cm²), the PL decay times in GaN are in the range from 30 ps for the FX_B up to 1000 ps for the A⁰X. Time constants of FX_A and D⁰X are of the order of few hundred ps. They are longest at about T equals 50 K. Decay times of both free and bound excitons become longer at higher excitation densities (above 1 kW/cm²) what is explained by changes in exciton dynamics with the increase of the exciton temperature caused by the laser pulse. The exciton temperature T_X (determined from the slope of the blue wing of the FX peak) decays on a time scale of about 80 ps.

Indium segregation in InGaN is a crucial phenomenon controlling the optical characteristics of such a compound. Its formation is a natural process due to the large lattice mismatch between InN and GaN. In InGaN, indium compositional fluctuations and phase separated InN clusters can be observed, particularly when the nominal indium content is high. In this paper, we report the results of material characterization, including X-ray diffraction, high- resolution transmission electron microscopy, atomic force microscopy, etc., and optical studies, including photo- luminescence and stimulated emission.

In_xAl_yGa_1-xN quaternary alloys with different In and Al composites were grown on sapphire substrates with GaN buffer by metal-organic chemical vapor deposition. Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. Our studies have revealed that In_xAl_yGa_1-xN quaternary alloys with lattice matched with GaN (y approximately 4.7x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensities (or quantum efficiencies) in In_xAl_yGa_1-x-yN quaternary alloys than that of GaN.

We present an optically-detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in a p-i-n diode, and show how it may be used to determine the electron transit time, velocity overshoot, and velocity-field characteristic in GaN at room temperature. In a GaN homojunction p-i-n diode, the transit time drops with increasing electric field E in the intermediate field regime (50 - 100 kV/cm), and the electron velocity possesses a weak, quasi-linear dependence on E attributed to polar optical phonon scattering. In the high field regime the transit time and the electron velocity gradually become independent of E. The peak electron velocity of 1.9 X 10⁷ cm/s, corresponding to a transit time of approximately 2.5 ps across the 0.53 micrometers depletion region, is attained at approximately 225 kV/cm. The experimental results are in qualitative agreement with theoretical steady-state velocity-field characteristics found in the literature. A measurement of the high field (approximately 300 kV/cm) transient electron velocity overshoot was also performed using a semi-transparent p-contact AlGaN/GaN heterojunction p-i-n diode. The peak electron velocity of 6.25 X 10⁷ cm/s attained within the first 200 fs decays within 1 ps to a steady-state velocity of 3.2 X 10⁷ cm/s in this improved device.

Recombination dynamics of spontaneous and stimulated emissions have been assessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs), by employing time- resolved photoluminescence and pump and probe spectroscopy. As for an In_0.02Ga_0.98N-ultraviolet-LED, excitons are weakly localized by 15 meV at low temperature, but they become almost free at room temperature. It was found that addition of small amount of In results in the reduction of nonradiative recombination centers originating from point defects. The internal electric field does exist in InGaN active layers, and induces a large modification of excitonic transitions. However, it alone does not explain the feature of spontaneous emission observed in an In_0.3Ga_0.7N- blue-LED such as an anomalous temperature dependent of peak energy, almost temperature independence of radiative lifetimes and mobility-edge type behavior, indicating an important role of exciton localization.

We present theoretical simulation of the femtosecond pump- probe spectroscopy in GaN systems for photo-excitation both far below and far above the band gap. Semiconductor Bloch equations for carrier distribution and exciton polarization are solved numerically. The simulation results are compared with experiment. The experiment for both cases was performed at 10 K to study the non-equilibrium carrier dynamics in bulk GaN. For pump below the band gap, prominent AC Stark effects are observed, and the theoretical simulation gives line-shapes of the differential absorption spectra in qualitative agreement with experiment. If the carrier screening and band renormalized effects are properly scaled, then good quantitative agreement between theory and experiment can be obtained for various pump intensities and detuning energies. For pump far above band gap, the theoretical simulation shows a fast carrier relaxation due to LO phonon emission and carrier-carrier scattering with scattering time on the order of 10 - 100 fs, while experimentally, we find that the hot carriers are strongly confined in a non-thermal distribution and they relaxed collectively to the band edge at a surprisingly slow rate (with relaxation time around 1 ps).

We present the results of picosecond time-resolved photoluminescence (PL) measurements for GaN/Al_xGa_1-xN MQWs with varying structural parameters, grown by metalorganic chemical vapor deposition under the optimal GaN-like growth conditions. We have shown that the optimal GaN/AlGaN (x approximately 0.2) MQW structures for UV light emitter applications are those with well widths ranging from 12 and 42 angstroms and barrier widths ranging from 40 to 80 angstroms. The decreased quantum efficiency in GaN/Al_xGa_1-xN MQWs with well width L_W < 12 angstroms is due to the enhanced carrier leakage to the underlying GaN epilayers, while the decreased quantum efficiency in MQWs with well width L_W > 42 angstroms is associated with an increased nonradiative recombination rate as L_W approaching the critical thickness of MQWs. For the barrier width dependence, when the barrier width is below the critical thickness, the nonradiative recombination rate increases with a decrease of the barrier width due to the enhanced possibilities of the electron and hole wavefunctions at the interfaces as well as in the AlGaN barriers. On the other hand, the misfit dislocation density increases as the barrier width approaches the critical thickness, which can result in an enhanced nonradiative interface recombination rate. Our optimized GaN/Al_xGa_1-xN MQW structures exhibited extremely high quantum efficiencies as well as a ratio of well emission intensity to barrier emission intensity exceeding 10⁴.

We report on the Raman analysis of the phonon lifetimes and decay channels of the A₁(LO) and E₂(high) phonons of single-crystalline bulk AlN grown using the sublimation- recondensation method. The temperature dependence of the phonon lifetimes was investigated from 10 K to 1275 K. Lifetimes of the A₁(LO) phonon and the E₂(high) phonon of 0.75 ps and 2.9 ps, respectively, were measured at 10 K. Our experimental results show that the A₁(LO) phonons of AlN decay primarily into two phonons of equal energy (Klemens' decay channel), most likely longitudinal- acoustic phonons. AlN is therefore in great contrast to GaN, where a symmetric decay of the A₁(LO) phonon is not possible due to a large energy gap between the acoustic and optical phonon branches. For the E₂(high) phonon, we find an asymmetric phonon decay. Contributions from two- and three-phonon decay channels were used for the modeling of the temperature dependence of the E₂(high) phonon lifetime. Phonon lifetimes and decay channels of the E₁(LO), A₁(TO) and E₁(TO) phonons of AlN were also investigated.

The non-equilibrium carrier dynamics in GaN epilayer for carrier densities ranging from 4 X 10¹⁷ to 10¹⁹ cm^-3 at 10 K was studied by femtosecond pump-probe transmission spectroscopy. Spectral hole burning was initially peaked roughly at the excitation energy for an estimated carrier density of 4 X 10¹⁸ cm^-3 and gradually redshifted during the excitation. Because of reduced carrier-carrier and carrier-phonon scattering, a very slow energy relaxation of the hot carriers at these densities were observed. We show that the hot carriers were strongly confined in a non-thermal distribution and they relaxed collectively to the band edge.

Non-equilibrium electron distributions and energy loss rate in a MOCVD-grown In_xGa_1-xAs_1-yN_y (x equals 0.03 and y equals 0.01) epilayer on GaAs substrate have been studied by picosecond Raman spectroscopy. It is demonstrated that for electron density n approximately equals 10¹⁸ cm^-3, electron distributions can be described very well by Fermi-Dirac distributions with electron temperatures substantially higher than the lattice temperature. From the measurement of electron temperature as a function of the pulse width of excitation laser, the energy loss rate in In_xGa_1-xAs_1-yN_y is estimated to be 64 meV/ps. These experimental results are compared with those of GaAs.

The effects of hot phonons on the ultrafast relaxation of photoexcited electrons in AlN has been investigated using ensemble Monte Carlo approach. The electrons are excited using infra-red laser pulses with different densities and energies. The build-up and decay of the hot phonon distribution at several phonon wavevectors is examined. The strong polar optical phonon scattering rates coupled with the short lifetimes of A(LO) leads to quick decay of the hot phonon distributions. Additionally, the rapid electron- electron scattering leads to fast thermalization of the carrier distributions.

The hot carrier relaxation dynamics are studied in both intrinsic and n-type ZnO films grown on R-plane sapphire by metalorganic chemical vapor deposition. An ultrafast UV pump/UV probe experiment was used to study the relaxation process. Absorption saturation and band-gap renormalization are observed. A novel femtosecond pump-probe technique is also used in which the electrons present in n-type ZnO are excited by an infrared pump and the electron dynamics are monitored by a tunable near UV probe. Complex transients, showing bleaching and induced absorption, are observed. The results from those two samples are discussed.

We report simulations of the response of InSb, GaAs, and Si to 70-femtosecond laser pulses of various intensities. In agreement with the experiments of Mazur and coworkers, and other groups, there is a nonthermal phase transition for each of these semiconductors above a threshold intensity. Our simulations employ tight-binding electron-ion dynamics (TED), a technique which is briefly described in the text. In the experimental pump-probe observations, the dielectric function (epsilon) ((omega) ) and the second-order susceptibility (chi) ⁽²) can be measured. These same quantities can be calculated during a TED simulation, and there is good agreement in the behavior with respect to both time and frequency. The simulations provide much additional microscopic information which is experimentally inaccessible: for example, the time-dependence of the atomic pair-correlation function, electronic energy bands, occupancies of excited states, kinetic energy of the atoms, and excursions of atoms from their initial positions.

The properties of optically dense materials are influenced by interactions between elementary optical excitations (oscillators). Since such interactions are absent in the dilute limit, the resulting properties are unique to optically dense materials. While linear optical experiments can probe these effects, for example the Lorentz-Lorenz resonance shift, they are often more apparent in nonlinear experiments that are sensitive to coherence. Direct gap semiconductors are typically optically dense close to the fundamental gap and have been extensively studied using ultrafast coherent spectroscopy over the last ten years. However, their coherent optical properties are very complex because of many-body interactions among the extended excitations (electron-hole pairs or excitons). Dense atomic vapors have also been studied, but typically using frequency domain techniques. We present the results of using ultrafast techniques to study both semiconductors and dense atomic vapors. This reveals the similarities and differences of the two systems, yielding insight into the characteristics of each individually.

For the first time, we show that two-photon absorption strongly affects backward and forward optical parametric oscillation in certain wavelength ranges. We have introduced a normalized coefficient of two-photon absorption. We have obtained a simple expression for the threshold intensities in the presence of two-photon absorption, that is the same for both configurations of optical parametric oscillation. The threshold intensities increase as the two-photon absorption coefficient increases. As the same time, the conversion efficiencies decrease. We have shown that even if the threshold intensity is just increased by a very small factor due to two-photon absorption the conversion efficiency can be reduced considerably. We have considered several nonlinear optical materials as examples to illustrate how to optimize the performance of optical parametric oscillators in the presence of strong two-photon absorption in the extended wavelength ranges.

Several different schemes for detecting ultrafast optical events are available. Streak cameras and schemes utilizing photomultipliers tubes (PMTs), such as time-correlated photon counting, pump-probe, up-conversion, phase- modulation, are found in most ultrafast labs. The selection of which method depends on several parameters of the event; such as the amount of collectable photons, wavelength, repeatability, the duration of the phenomena of interest, total duration of the event, available lasers used to initiate the event, and available budget. Streak cameras offer the most direct means of detecting ultrafast phenomena, with single photon sensitivity and time resolution down to 200-femtosecond. PMT schemes can offer a more cost-effective solution, depending on the above mentioned parameters and desired performance level. A newly developed NIR photomultiplier makes it possible to detect weak optical signals out to 1500 nm. This paper will discuss the tradeoffs between the various detection methods as well as cover a few illustrative applications.

In semiconductor research and industry the lifetime of carriers in the material is an important characteristic. Time-resolved photoluminescence (TRPL) is focused on the measurement and identification of electron-hole recombination processes. The length of time a photoexcited carrier remains in the valence band is directly related to material quality, purity and device performance. Band alignment, doping, stress and disorder in the semiconductor strongly influence the recombination dynamics. These features can be inferred by TRPL measurements on many materials and devices such as photovoltaic cells, heterojunction transistors, sensitive photodetectors, efficient laser diodes or bright LEDs. TRPL is contactless, nondestructive and highly sensitive. Nevertheless TRPL has mainly been used as a research tool with little incorporation into production and quality control since instrumentation was found to be expensive and difficult to use.

We discuss the role of many-body spin correlations in nonlinear optical response of a Fermi sea system with a deep impurity level. Due to the Hubbard repulsion between electrons at the impurity, the optical transitions between the impurity level and the Fermi sea states lead to an optically-induced Kondo effect. In particular, the third- order nonlinear optical susceptibility logarithmically diverges at the absorption threshold. The shape of the pump- probe spectrum is governed by the light-induced Kondo temperature, which can be tuned by varying the intensity and frequency of the pump optical field. In the Kondo limit, corresponding to off-resonant pump excitation, the nonlinear absorption spectrum exhibits a narrow peak below the linear absorption onset.

Adaptive holographic quantum-well films are high-sensitivity dynamic holographic media that can be used to self- compensate time-varying disturbances such as mechanical vibration in interferometers, laser speckle, fsec pulse broadening and arrival-time drift. The holograms are quasi- steady-state with a compensation bandwidth up to 1 MHz. Dispersion to all orders is compensated by forming a dynamic spectral-domain hologram of a signal pulse (that has a time- varying dispersion) referenced to a stable clock pulse. The hologram is read out using forward scattering phase conjugation to remove phase distortion to all orders, including linear order that induces drift in the time of flight. We also have used adaptive interferometry to combine two ultrashort pulses into a pulse train. The relative phase between the two pulses in the train are self-adaptively phase locked, and is immune to drifting optical path differences or delay times between the two input pulses.

Second-order nonlinearity of CuCl nanocrystal-doped silica and indium tin oxide (ITO) films were examined by utilizing Maker fringe method. The films were prepared by means of the rf sputtering method with silica or ITO target on which CuCl pellets were placed. X-ray diffraction patterns of the films indicate that the mean crystallite size of CuCl nanocrystals was 20 - 30 nm based on the Sherrer's equation. The films exhibit second-harmonic generation. As for the films prepared at a substrate temperature of room temperature, the dependence of second-harmonic intensity on incident angle shows that the films have an optical uniaxial anisotropy with an axis perpendicular to the film surface. An orientation of CuCl nanocrystals was confirmed by X-ray diffraction patterns, but the direction of orientation depends on the sputtering condition. In contrast, the films prepared at a substrate temperature higher than 200 degree(s)C shows different incident angle dependence of second-harmonic intensity; a maximum of second-harmonic intensity appears at the incident angle of 0 degree(s). The spacings of (111), (220), and (311) planes estimated from X-ray diffraction patterns are smaller than the values of the bulk cubic structure.

The dynamic properties of low-temperature-grown GaAs (LT- GaAs) depend critically on growth and annealing conditions, such as substrate temperature during the MBE process (T_s), annealing temperature (T_a) and duration ((tau) _a). Previous empirical models based on carrier rate- equations introduce parameters, such as carrier lifetime, that cannot be directly correlated with growth and annealing conditions. The Schockley-Read-Hall model we use here introduces deep donor (N_DD) and acceptor (N_A) concentrations, instead of lifetimes. In LT-GaAs, deep donors are in majority constituted by an As-antisite related defect. The acceptors, in absence of doping, are most likely constituted either by Ga vacancy defects or by the residual doping during MBE growth. The samples studied are grown on GaAs semi-insulating substrates at different T_s and annealed under different conditions. For each sample, we first measure N_DD using X-ray diffraction analysis. Then we fit both continuous photoconductivity and pump-probe reflectometry measurements using N_A as the only adjustable parameter. From the set of data obtained, we can relate T_s, T_a and (tau) _a with N_DD and N_A. This gives us a way to predict LT-GaAs dynamics from growth and annealing continues. This approach has been used to fabricate ultrafast photoconductive switches showing high sensitivity and good insulation.

We have observed first-, second- and third-order quasi- phase-matched second-harmonic generation in the reflection geometry from GaAs/AlAs multilayers. We have measured phase- matching curves and identified all the peaks. The linewidth for the first order is limited only by wave-vector mismatch. We have demonstrated two-order-of-magnitude enhancement solely using quasi-phase-matched multilayers. We have also achieved cavity-enhanced quasi-phase-matched second-order and non-phase-matched second-harmonic generation from GaAs/Al_0.8Ga_0.2As multilayers. We have determined the element of the second-order susceptibility tensor used for quasi-phase matching. We have measured the conversion efficiencies and discussed possibilities for further enhancements.

The phenomenon of electromagnetically induced transparency, which was discovered recently in atomic systems, is demonstrated for the first in a subband quantum well system. Applying strong coupling field (Rabi frequency is on the same order as the intersubband linewidth), which is two- photon-resonant with the 1 - 3 intersubband transition, produces dramatic change in the 1 - 2 intersubband absorption profile. This effect can be accounted for in terms of 1 - 2 and 2 - 3 dipoles being driven into coherence by a strong coupling field; it is not related to self- induced transparency or spectral hole-burning (the frequency of the coupling field is quite different from that of 1 - 2 resonance) and, similar to Fano-type interference, is a pure result of destructive interference of probability amplitudes.