Ultrafast spectroscopy of semiconductors has provided extensive new information about dynamics of coherent processes, relaxation processes and transport processes in semiconductors during the past 30 years. The field continues to thrive with emphasis on accessing new physics and on new experimental techniques. We provide a brief overview of some recent developments and discuss how one new technique, femtosecond spectral interferometry, has provided new insights into the physics of resonant Rayleigh scattering.

Coherent signatures in the semiconductor light emission are studied using a fully quantum mechanical theory for the system of photons and Coulomb interacting electron-hole pairs. The dominant light-matter correlations couple the semiconductor Bloch and luminescence equations yielding significant quantum corrections. A coherent excitation leads to squeezing of the emitted light as well as to entanglement between light and matter.

The normal modes in a nonperturbatively coupled quantum-well semiconductor microcavity are linear superpositions of the QW exciton and cavity mode when the QW exciton transition is resonant with the cavity mode. When the lower normal mode is excited by a phase-locked pair of optical pulses, the nonlinear response of a probe pulse tuned to the upper mode is controlled. Thus the normal modes are coupled in their nonlinear optical response due to the nonlinearity of the exciton underlying the two normal modes. The cavity enhancement of the excitonic nonlinearity gives rise to a large signal; modulating the relative phase of the excitation pulses produces a differential reflectivity of up to 10%. Besides the coherent control of normal modes which is explainable with in the frame of semiclassical models, we observe a purely quantum mechanical phenomenon in our system. The quantum correlations between the field and carrier density lead to intraband coherences which live much longer than the interband dephasing time.

In this paper, we report ultrafast manipulation of coherent excitons in quantum-wells using phase locked pulse sequence prepared by a frequency domain shaping technique. The pulse shaping system with double liquid crystal spatial light modulator (SLM) and the Ti³⁺-sapphire laser are used to generate phase-locked pulses. Ultrafast coherent control of exciton population and exciton polarization is demonstrated by the observation of the reflectivity-change in the pump-probe and the diffracted power in the degenerate-four-wave-mixing measurements. In the single quantum wells, good coherent control characteristics with 87% of coherent carrier destruction is demonstrated at low excitation power of 0.15 mW (approximately 1.1 X 10¹⁰ photons/cm²). Also the preliminary manipulation of the exciton population is demonstrated by the phase locked multi-pulses. In case of coupled quantum-wells, the modulation of SLM is carefully controlled and coherent control of both the exciton population and the polarization are demonstrated in the two different coupled quantum well samples.

We verify the validity of the optical characterization of the 2D electron gas confined in modulation doped (GaAl)/As/GaAs heterojunctions designed for both high electron mobility devices. The properties of the 2D electron gas are evaluated from the magneto photoluminescence and the microwave influenced photoluminescence detected on the GaAs excitons in the flat band region. We evaluate the electron density and mobility from the oscillation of the photoluminescence intensity in a magnetic field, and the effective mass from the resonant change of the PL intensity under additional microwave radiation. We attribute these non-linear optical properties to the vertical transport and the bimolecular exciton formation of photoexcited carriers. We proved these specific transport and exciton formation processes by time-resolved photoluminescence experiments, where an extremely long exciton formation time and a strong signal of anti-stokes emissions under resonant excitations are observed. Furthermore, we realized a significant amplification of the photoluminescence intensity by exciting the heterojunction with two temporarily delayed laser pulses. The amplification of the luminescence intensity reflects unambiguously the recycling through the bimolecular exciton formation of the photo-excited carriers accumulated in the flat band region after vertical transport.

Semiconductors below the half band gap exhibit higher bond- electronic, off-resonant nonlinearities compare to silica fibers. They also exhibit low scattering losses, negligible two-photon absorption, and no linear absorption. Furthermore, due to the mature fabrication technology many complicated device designs can be easily fabricated. Thus, this material system offers unique opportunity to observe and test novel nonlinear phenomena and devices. In this paper, we will review the nonlinear optical properties of AlGaAs waveguides near the half band gap.

The charge transfer (CT) process in organic semiconductor thin film structures is an important problem for applications such as photoreceptors and light-emitting devices. The operation of a photoreceptor structure is based on a CT process between a donor molecule and an acceptor transport molecule. We have investigated such a structure formed by vacuum-grown thin films of two organic molecules, N,N'-diphenethyl-3,4,9,10-perylenetetracarboxylic-diimide (DPEP) and N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'- biphenyl-4,4'-diamine (TPD), with femtosecond time-resolved photoemission spectroscopy. By measuring the lifetimes of the excited electron states in the mixtures of these molecules we are able to determine the time-scale for the electron transfer (ET) between the excited states of the TPD and DPEP molecules. We show that the biexponential ET dynamics consist of a short component of less than 100 fs and a long component of several hundreds fs in length.

We present a new heterodyne technique for detecting weak signals beyond the standard shot-noise limited by employing amplitude-squeezed light as the local oscillator (LO) field. The amplitude-squeezed LO field at 428.8 nm is generated from a highly efficient (up to 60% conversion efficiency) single-pass second-harmonic generation in a nonlinear crystal (KNbO₃) pumped by femtosecond (130 fs) pulses at 857.7 nm. The weak signal field is introduced from the entrance port of the crystal to combine with the generated squeezed LO field. An enhancement of 0.7 dB (1.4 dB inferred) in signal-to-noise ratio beyond the shot-noise limit is directly observed for this new heterodyne detection scheme. Practical applications of proposed technique in laser radar systems are discussed.

We have consistently observed quasi-phase-matched second- and third-order second-harmonic generation in the reflection geometry from GaAs/AlAs multilayers. We have measured the second-harmonic pulse energies and polarizations for different pump polarizations, incident angles, and pulse energies.

For the first time to the best of our knowledge, we show that it is feasible to efficiently generate THz waves based on transition radiation when a nonrelativistic electron beam moves through an interface formed by two dielectric layers. If one uses multilayers one can achieve narrow-linewidth output.

Fundamental properties of phonons in III-V nitrides are examined with a view toward understanding processes important in the operation of III-V nitride devices. Firstly, confined, interface and propagating modes in wurtzite quantum wells are described in terms of Loudon's model for uniaxial semiconductors and the dielectric continuum model. Basic properties of the phonon modes and carrier-phonon interactions are considered in the basis of this treatment of dimensionally-confined phonons in wurtzite structures. A key feature of these phonon modes is their enhanced dispersion and its origin from the non-isotropic nature of the wurtzites. As will be discussed, this dispersion has important consequences for phonon propagation and phonon energy spectra. Secondly, the second-order phonon decay process of combined point defect scattering and anharmonic decay is examined as a means of estimating line broadening associated with the decay of phonons in III-V nitrides of wurtzite structure containing point defects. Thirdly, an analysis of Raman linewidths measured for AlN and GaN wurtzites is made to estimate phonon lifetimes.

Calculations of the electronic mobility and drift velocity have been carried out for bulk GaN and AlGaN-GaN heterojunctions based on a Monte Carlo approach. The bulk calculations were intended to serve as a validity check of the simulation model and yielded a set of best-fit transport parameters. Wurtzite GaN has been shown to have superior steady state drift velocity characteristics over both GaAs and the zinc blende phase of GaN. Electron mobility in HFET structures has ben analyzed taking account of polarization effects, degeneracy and interface roughness scattering. Degeneracy is shown to play an important role, especially at large gate bias values. Very good agreement with available experiments has been obtained. Our results underscored the dominance of interface roughness scattering, and demonstrated that a parameterized model based on a weak- perturbation, Born approximation theory can yield sufficiently accurate results.

The intersubband transition (ISBT) in nitride quantum wells (QWs) is considered to be an excellent device mechanism for ultrafast optical switches capable of 1 Tb/s operation at room temperature. The 1.55-micrometers ISBT is feasible because of a large (approximately 2 eV) conduction band discontinuity in AlGaN/GaN QWs. The intersubband relaxation time in AlGaN/GaN QWs was calculated to be about 100 fs, which is 25 times shorter than that in AlAs/(In)GaAs QWs. The fast relaxation in nitride semiconductors is due to the strong interaction between electrons and LO-phonons. Intersubband absorption in the wavelength range of 3 - 7 micrometers was observed in MOCVD-grown AlGaN/GaN QWs, and the ultrafast response of the ISBT in nitrides was experimentally verified. The ISBT wavelength in the nitride QWs, however, was found to be affected by a strong built-in field (approximately MV/cm) caused by the spontaneous polarization and piezoelectric effect. A design to realize the ISBT at the communication wavelength in AlGaN/GaN QWs with a strong built-in field is discussed. Next, we report on an ultrashort pulse propagation model for nonlinear optical waveguides utilizing the intersubband absorption in AlGaN/GaN QWs. The finite-difference time-domain approach in conjunction with the rate equations describing the ISBT was adopted. Ultrafast optical gate operation in the waveguide was simulated.

The optical properties of Al_xGa_1-xN alloys with x varied from 0 to 0.35 have been investigated by picosecond time-resolved photoluminescence (PL) spectroscopy. Our results revealed that while the PL intensity decreases with an increase of Al-content, the low temperature PL decay lifetime increases with Al-content. These results can be understood in terms of the effects of tail states in the density of states due to alloy fluctuation in the Al_xGa_1-xN alloys. The Al content dependence of the energy tail state distribution parameter, E₀, which is an important parameter for determining optical and electrical properties of the AlGaN alloys, has been obtained experimentally. The PL decay lifetime increases with the localization energy and consequently increases with Al content. The implications of our findings to III-nitride optoelectronic device applications are also discussed.

A picosecond modulation of interband (IB) resonant light (approximately 800 nm) by intersubband (ISB) resonant light (approximately 7 micrometers ) in n-doped GaAs/AlGaAs quantum wells is demonstrated. Two-color pump-probe measurements are carried out by using ultrashort (approximately 120 fs) ISB (pump) and IB (probe) light pulses at room temperature. Ultrafast modulation (FWHM approximately 1.3 ps) of the IB light is clearly observed with a low pump pulse energy of about 4 fJ/micrometers ². The observed modulation depth is approximately 8.5% which corresponds to the absorption coefficient change of as large as approximately 1000 cm^-1. The modulation depth decreases when the pump pulse wavelength is detuned from the ISB absorption peak. The modulation dependence on the ISB light pulse energy is also measured. The carrier relaxation mechanism in high and low excitation conditions is discussed by employing a numerical simulation of the relaxation process of electron-- longitudinal optical phonon systems. The results indicate that the utilization of the intersubband transition is promising for the ultrashort all-optical modulation and switching.

We propose new methods of frequency conversion in compound III-V semiconductor nonlinear waveguides and high mid- infrared absorbing nonlinear waveguides. In the first scheme we use higher order waveguide mode as an intermediary to achieve phasematching in nonlinear waveguides. In the second scheme we generate mid-infrared radiation in high absorption nonlinear waveguides by coupling the absorbing nonlinear waveguide to a linear waveguide with low absorption. We show that high conversion efficiencies can be achieved in both schemes.

InGaAsP/InP slab-type coupled waveguides consisting of two dissimilar waveguides were experimentally studied as potential dispersion compensators. A narrowing of the autocorrelation signals from the coupled waveguides was experimentally observed and the smallest narrowing factor was 0.233. The narrowing agrees well with the calculated narrowing obtained by a theoretical model based on group- velocity dispersions (GVDs) associated with supermodes. This agreement strongly indicates that the observed narrowing is attributed to pulse compression by dispersion compensation by using the GVDs of the supermodes. Furthermore, controllability of the GVDs of the coupled waveguide was preliminarily demonstrated by changing the spacing between the dissimilar waveguides: pulse-broadening and pulse- compression experiments showed that the GVDs of two coupled waveguides with different spacings are significantly different. In other words, the GVDs of a coupled waveguide increase with increasing spacing. this result is consistent with the theoretically predicted increase in the GVDs.

This paper summarizes algorithms, which extend the finite- difference time-domain calculation method to solve Maxwell's equations for terahertz (THz) electromagnetic radiation from photoconductive materials pumped by femtosecond laser pulses. In the calculations, we include transient electric conductivity by considering carrier dynamics. By using the newly proposed algorithms, we calculated the THz radiation from photoconductive antennas. The preliminary calculation results about the THz pulse generation, propagation, and radiation pattern are presented.

23 ps/200 W clear single optical pulses were achieved from a Q-switched commercial single heterostructure laser with a standard peak power of 5 W by means of spectral filtering of the transient mode together with optimal pumping conditions. These conditions were found by analysis of the transient spectrum dynamics. A high-power picosecond range optical pulse appears near the trailing range of the pumping current pulse under certain conditions. Its intensity is found to be determined mainly by the transient spectrum width, which in turn depends on the lattice temperature. An increase in the temperature causes both an optical pulse delay with respect to the trailing edge of the current pulse and significant spectrum narrowing. This behavior is ascribed to the effect of saturable absorption and carrier recombination in the heavily doped and compensated active region of the laser diode. A recently suggested model used to interpret the experimental data explains Q-switching behavior by considering tail-state absorption together with carrier heating and cooling in the active region. The difference between the Q-switching mode observed here and the traditional one caused by diffraction losses in the cavity is discussed.

In this study, amplitude-phase relationship of laser diodes is analyzed under modulation with the help of the transfer functions. The dynamic behavior of laser diodes is observed as functions of both damping and relaxation resonance frequencies. Within the linear system approach, the amplitude-phase relationship is harmonious up to some extent and both of them have dependence on each other. Phase changes occurring within the system are not stable and cause oscillations which prevent the system from stable working. These oscillations have some threshold value and the system may be driven in to chaos after this point.

Carrier dynamics in single quasi-1D GaAs quantum wires are studied in a wide temperature range by near-field scanning optical microscopy using pico- and femtosecond pulses. Luminescence and pump-probe experiments with a spatial resolution of 250 nm and a time resolution of up to 200 fs allow for a separation of carrier transport along the quantum wire and in the embedding GaAs quantum well from local carrier relaxation phenomena. We demonstrate that local potential barriers close to the quantum wire strongly affect the lateral carrier transport into the quantum wire. This drift-diffusive motion occurs in the pico- to nanosecond regime with diffusion lengths of up to several microns. Diffusive transport along the quantum wire is characterized by carrier motion on a somewhat shorter picosecond time scale. In contrast, sub-picosecond relaxation times are found for the redistribution of carriers from high-lying to low-lying quantum wire states. This relaxation is governed by electron-electron and electron-phonon scattering.

High field transport in wide bandgap materials such as ZnS and SrS is of current importance for thin film electroluminescent devices currently used in flat panel display applications. Typically, carriers injected into the phosphor layer of such structures undergo acceleration in fields ranging from 1 - 2 MV/cm, with average carrier energies of 1 - 2 eV, and therefore high field transport is critical to the device operation. A major problem in the understanding of transport in such wide bandgap materials is the relative lack of experimental data for the electron- phonon coupling constants and impact ionization coefficients, particularly under high electric fields, where details of the full bandstructure are important. Hence, first-principles modeling of the electronic and transport properties is required for assessing the technological potential of these materials. In the present work, a review is given on the electronic and transport properties of three wide bandgap materials, ZnS, SrS, and GaN, simulated using full-band ensemble Monte Carlo (EMC) simulations. The impact ionization rates for both electrons and holes were derived directly from bandstructure calculated using the empirical pseudopotential method (EPM). To avoid arbitrary fitting parameters for the electron-phonon coupling, a microscopic rigid-ion model calculation is performed of the electron- phonon scattering rate directly from the EPM bandstructure, and a valence-shell model for the lattice dynamics. The momentum averaged scattering rate is input directly into the full-band EMC simulation. Results for the high field distribution functions for all three materials are calculated and compared. Further, the process of impact excitation of luminescent centers by hot carriers is included, and compared to experimental photo-induced- luminescence versus field data, where good agreement is obtained.

Experimental Raman spectroscopy studies, performed by two different groups, to measure the relative strength of the polar modes in Al_xGa_1-xAs differed in that one showed a linear behavior with the Al concentration while the other showed a non-monotonic behavior. We have used an ensemble Monte Carlo code to study this system. It is found that the excitation energy and the electron concentration all contribute to the experimental discrepancies, while the excitation pulse width had little effect on the measured relative strength of the Al-mode. Apparently the relative strength of the Al-mode scattering is density dependent for x >= 0.25.

A hot carrier relaxation dynamics are studied in both n-type and p-type GaN films grown on sapphire by molecular beam epitaxy. A novel femtosecond pump-probe technique is used in which the carriers are excited by an infrared pump and the carrier dynamics are monitored by a tunable near UV probe. Complex transients, showing bleaching and induced absorption, are observed in both samples. The electron dynamics are fitted by a model in which the LO-phonon emission is the dominant energy relaxation process. The hole dynamics also have been discussed and compared to the electron dynamics. Hot phonon effects seem play a major role to reduce the effective strength of the carrier-LO phonon scattering rate in both cases.

Femtosecond degenerate four-wave mixing experiment (FWM) has been performed at 11 K in transmission geometry, on bulk GaAs as a continuous function of the sample thickness. The FWM signals exhibit the transition from the real, excitonic regime to the virtual regime as the thickness increases from 3 micrometers to 17.5 micrometers . The results at the negative time delay show an extraordinary signal where the energy extends well above the bandedge in FWM transmittance although the thickness is an order of magnitude larger than the penetration depth. These above-the-band-gap signals are mostly confined to the negative time delay region and as the detection energy increases, so does the value of the negative time delay at which the signal peaks. These unusual phenomena can be understood by the third order frequency mixing (2(omega) ₂-(omega) ₁; (omega) ₂>(omega) ₁) between positively chirped spectral components.

We have examined the ultrafast relaxation of electrons photoexcited by infrared laser pulses in AlN using ensemble Monte Carlo approach. The effects of doping, excitation levels, energy, and pulse duration were investigated. It is found that even at excitation energies above the G-U separation, the role of intervalley scattering is weaker compared to polar optical scattering. This is more significant at longer pulse duration.

Electron ballistic transport and in a InP-based p-i-n nanostructure under the application of an electric field have been studied by time-resolved Raman spectroscopy at T equals 300 K. The time-evolution of electron distribution, electron drift velocity has been directly measured with subpicosecond time resolution. Our experimental results show that, for a photoexcited electron-hole pair density of n is congruent to 5 X 10¹⁶ cm^-3, electrons travel quasi- ballistically--electron drift velocity increases linearly with time, during the first 150 fs. After 150 fs it increases sublinearly until reaching the peak value at about 300 fs. The electron drift velocity then decreases to its steady-state value.

Decay of the longitudinal optical (LO) phonons in wurtzite GaN and Al_xGa_1-xN (x equals 0.1) has been studied by subpicosecond time-resolved Raman spectroscopy. In contrast to the usually-believed 2LA decay channel for LO phonons in other semiconductors, our experimental results show that, among the various possible decay channels, the LO phonons in wurtzite GaN and Al_xGa_1-xN (x equals 0.1) decay primarily into a large wavevector TO and a large wavevector LA or TA phonons. These experimental results are consistent with the recent theoretical calculations of the phonon dispersion curves.

Femtosecond pump-probe (P-P) and four-wave mixing (FWM) experiments were performed simultaneously at 11 K on gallium nitride epilayers to study the initial temporal line-shapes of the exciton. A-B exciton beats were found in both P-P and FWM experiments near the exciton resonance. However, the differential reflection spectra showed a much slower rise time that persisted at longer negative time delay than the FWM signal or differential transition spectra at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including all Hartree Fock nonlinearities shows that this slow rise results from excitation induced dephasing, that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

We report the near-infrared intersubband absorption characteristics in In (formula available in paper) heterostructures lattice matched in InP substrate. We have investigated for the first time the excitation power dependence of the intersubband transitions in Sb based quantum wells using a femtosecond optical parametric amplifier tuned over wavelength ranging from 1.8 micrometers - 2.4 micrometers . The bandgap of the InGaAs material system in the regime of 1.0 eV facilitates nonlinear interband optical absorption effects in the presence of strong near-infrared intersubband resonant optical excitation. We have observed a novel nonlinear optical phenomenon,- an intersubband transition induced interband absorption due to the two- photon interband excitation after the onset of the intersubband absorption saturation. The excitation wavelength-dependence of the absorption saturation characteristics has also been studied. The absorption saturation measurements have been performed in quantum wells with various well widths.