Through simultaneous analysis of picosecond, time-resolved reflectivity and Raman scattering data we have been able to self-consistently relate hot carrier and hot phonon kinetics in bulk, crystalline Germanium. We show that hot carrier diffusion and electron-hole interactions are essential to achieving a quantitative correlation.

We discuss the role of LO-phonons confinement in quantum well systems, by comparing two different phonon models that have been proposed in the literature. A critical discussion concerning the use of macroscopic approaches for the description of phonons in two dimensional systems is presented. We use a Monte Carlo simulation which includes nonequilibrium phonon effects as well as carrier-carrier scattering to determine the effect of phonon confinent on the relaxation of photoexcited carriers in A1GaAs-GaAs quantum wells. Good agreement with available experimental data is found. Even at low excitation densities, intercarrier scattering and phonon reabsorption are important, and need to be taken into account in the interpretation of experimental data.

The ultrafast relaxation of electron-hole plasma photoexcited by a subpicosecond laser pulse in GaAs is investigated using ont Carlo method. The photoexcited carrier concentration is assumed to be 5x10 cm , and thephoton energy is assumed to 1.82 eV. The interaction between the heavy-holes and hot LO phonons has a minor effect on the cooling rates and the shape of hot phonon distribution but leads to an increased energy loss rates through the deformation potential interaction to compensate for the energy gained via LO phonon absorption.

The transient expansion process of a photogenerated electron-hole plasma is examined using Ensemble Monte Carlo techniques. The effect of nonequilibrium phonons on the expansion behaviour is included for the first time. Both the spatial and temporal variations of the phonon population are taken into account. Numerical results over the picosecond time scale demonstrate that the nonequilibrium phonons significantly enhance the plasma expansion and alter the spatial carrier distribution profiles.

We have characterized through degenerate four wave mixing (DFWM) the nonlinear optical response of bulk GaAs at =1.O64 tim. At the lower intensities, for the compensated samples, the reflectivity is accounted by a 3rd order nonlinearity, mainly due to free carrier generation, with an effective x3 at 30 P5 of 2-3x1 0b0 esu and a decay time ≤ l ns. From high intensity data we derive for all samples a two photon absorption coefficient f310 cm/GW , in agreement with the most recent reported values.

Relaxation of hot carriers excited by subpicosecond laser pulses has been studied by Raman scattering in GaAs/AlAs multiple quantum wells with well widths varying between 100 and 1000 A. The hot phonon population observed by Raman scattering is found to decrease with the well width despite the fact that the hot electron temperature remains constant. The results are explained in terms of confinement of both electrons and optical phonons in quantum wells.

We describe a new technique for performing femtosecond transient measurements of nonlinear index and absorption in waveguide devices. Using a time division interferometry technique in conjunction with a tunable femtosecond laser source we have performed the first measurement of the wavelength dependent nonresonant nonlinear index in A1GaAs. Contributions to nonlinear index arise from both virtual as well as real population mediated processes depending on the wavelength detuning from resonance. Complementary pump-probe measurements of transient absorption provide information on excited state population as well as two-photon induced absorption processes. These measurements provide imformation on the mechanism and dynamics of fundamental nonlinear optical processes below the band edge in semiconductors and are relevant to possible all optical switching applications in waveguide devices.

We deduce the F-L intervalley and poiar optic phonon scattering times of hot electrons in bulk GaAs from cw hot (e, A°) luminescence spectra as a function of electron kinetic energy at low excitation densities. We obtain the lifetime broadening due to these two processes from comparison with lineshape calculations using a 16x16 k.p Hamiltonian, a full integration over k-space and a dipole model for the optical matrix elements. We find for the LO-phonon emission time tLO=(l32±lO)fS. The threshold for IT-*L scattering is determined as (330±1O)meV, above which a distinct decrease in total lifetime is observed. Minimum F÷L scattering times are l5Ofs to 200fs. We discuss an estimation for the deformation potential DTL.

We have used the rigid-pseudoion method (with q-dependent matrix elements and a realistic non-parabolic band structure) to calculate the lifetimes of electrons at the L- and X-points in GaAs as a function of temperature (L: 2.2±0.5 p5, X: 130±20 fs at room temperature). The contribution of the TA phonons to LF-scattering explains the discrepancy between the experiments of Shah and Kash, performed at two different temperatures. About 80% of the carriers at X scatter into the L-valleys. The intervalley scattering times in the F-valley for electrons with an energy of 165 meV above the L-point are found to be 750±100 fs at helium temperatures (100 fs for electrons with an energy of 270 meV). These results compare favorably with recent femtosecond and CW laser experiments.

The dynamics of electrons in the X 6 valley for a GaAs crystal was measured by time-resolved absorption spectroscopy. An infrared picosecond probe pulse was used to monitor the growth and decay of the population in the X o valley subsequent to excitation by a 527 nm pump pulse. The intervalley X , F 6' L 6 scattering time was determined from the time evolution of electrons in the x 6 valley to be 700 500 fs by a rate equation analysis. The X 6-X absorption spectrum of GaAs was obtained by the time-resolved pump-JR-probe technique by varying the probe wavelength from 2.16 m to 3.9 m. It gives the energy gap between the minima of the X and X bands to be 0.345 eV, and the density of states effective mass for the X band to be 0.48 m

Using ensemble Monte Carlo methods, coupled with a molecular dynamics (MD) approach for the carrier-carrier interaction, we investigate the ultrafast relaxation of photoexcited carriers in GaAs. The interaction of various scattering mechanisms and the dynamic screening of hot carriers in semiconductors is studied. At a density for which the GaAs is degenerate (in equilibrium), scattering out of the excitation volume is dominated in the initial tens of femtoseconds by electron-electron scattering, and the scattering rate increases with increasing density. This rate increase agrees both in magnitude and in density dependence with some recent experiments. The presence of electron-electron scattering modifies both the population transition rates and carrier densities in the satellite valleys, primarily by reshaping the energy distribution of carriers in the central valley. Intervalley processes also play a role in the initial decay and the same processes play a modified role in the picosecond-scale luminescence decay. The intervalley transition rates must be estimated carefully because the f-L population shift contains a significant fraction of electrons that reach the L valleys by way of the X valleys. The exchange effect further modifies the satellite valley populations and intervalley transition rates. It also reduces the rate for electrons to scatter out of the excitation volume.

We have investigated the formation of intrinsic excitons following excitation of electron-hole pairs close to the bandgap by a subpicosecond laser pulse. We show that excitons form very rapidly ('c≤2Ops) and that they are initially in large wavevector states because of energy and momentum conservation requirements. These non-thermal excitons then interact with other excitons and acoustic phonons and relax very slowly (400ps) to the KO states which couple directly to light. This leads to an extremely slow rise of exciton luminescence and unusual dependence of this risetime on temperature, excitation density and excitation energy. These studies raise a number of fundamental issues related to excitons in semiconductors.

The wavelength of ultrafast laser probes near the fundamental (E0) absorption edge falls in the infrared and visible wavelengths in most diamond and zincblende structure semiconductors. Consequently the time-resolved reflectivity and transmission in a photo-excited sample are influenced by numerous factors: Drude reflectivity and free carrier absorption, in addition to interband saturation and band gap renormalization. In this work we demonstrate the utility of probing the higher (E1 and E2) absorption edges using two-photon absorption spectroscopy or ultraviolet wavelength probes. 1) In silicon we probe the E2 absorption edge by direct two-photon absorption using visible femtosecond pulses above the indirect edge. Longer pulses melt the sample before reaching peak intensities at which two-photon absorption becomes dominant. We extract the direct two-photon absorption coefficient over a wide spectral range, and distinguish other nonlinear absorption channels, and relate these results to the band structure of silicon. 2) Using an ultraviolet probe with visible pump, Drude effects and interband saturation become negligible, leaving renormalization of the E1 and E2 edges as the dominant influence on the probe. Time-resolved experiments compare the renormalization induced by hot carriers, cold carriers and lattice heating in 3D and 2D semiconductors.

We present experimental results for the thermalization of hot electron-hole plasma in bulk GaAs, AlGa1_As, GaAs/A1GaAs quantum wells and GaAs/AlAs superlattices. The results are compared with calculations of the thermalization of an electron-hole plasma with the lattice, taking into account different electron and hole temperatures and a nonequilibnum of the optical phonons. Several different situations are simulated: Cooling as a function of the excitation density and the excess photon energy, cooling in doped samples, and heating of a laser-excited plasma to an elevated lattice temperature. The qualitative agreement with our experimental results is generally good. However, experimentally observed reductions of the energy transfer rate seem to be larger than theory predicts.

The relaxation time of hot carriers in bulk Ga47In053As has been measured as a function of excitation energy near and above the conduction band minimum, and as a function of carrier density. The carrier relaxation rate increases dramatically with excess energy, due to the additional energy decay provided by the LO phonons. As a function of carrier density, the scattering rate is maximum at densities below 1018 cm3, but decreases for higher carrier concentrations, falling roughly by half at 1019 cm3.

I-lot hole energy relaxation dynamics are reported using time- and energy-resolved photoluminescence spectroscopy. The hole cooling rate is determined to be smaller than expected bed on hole scatterings with longitudinal optical phonons.

An ensemble Monte Carlo technique has been developed for the simulation of optical pulse-and-probe and dual pulse correlation experiments with tunable laser sources in InGaAs thin films lattice matched to InP. Electrons and holes, and all scattering mechanisms, including carrier-carrier scattering, have been included. The time evolution of electron and hole distributions, and optical absorption have been simulated. Results are correlated with measured data from the research group of C. Pollock at this conference. It was found that inhomogeneity effects and heavy and light holes are necessary for good agreement with experimental data. Finally the extraction of time constants from measurements is a1dresset

Ultrafast lasers have been used along with an electro-optic sampling technique to generate and characterize electrical waveforms with subpicosecond rise-times. We study these transients by coupling a three dimensional solution of Maxwell's equations in the time domain with a bipolar ensemble Monte- Carlo model. The physical parameters of interest can be accurately calculated and related to the measured data.

Using the results of previous experiments, CO2 laser induced impact ionization in InSb has been evaluated using measured dc ionization rate data. The laser field is scaled by 1/w'r, where o is the laser frequency and t is the momentum relaxation time of the hot electrons. Scaling is done to provide an ionization rate consistent with the experimental conditions. In this way, a momentum relaxation time t, 0.34 p5 can be deduced, in agreement with the value previously determined by four wave mixing experiments.

Lu this paper we discuss two issues important to high frequency quantum transport in ultrasmall systems. First, we discuss the question of the intrinsic high frequency cutoff in quantum systems and show that neither the traversal time proposed by Büttiker and Landauer nor the scattering time proposed by Wigner is adequate. This is confirmed in a model for a resonant tunneling device. We also consider the effect of charging, the role of 1W time constant, and point out the connection between this and the X-ray edge problem. Second, we study the connection between AC quantum transport and quantum chaos, and present some numerical results.

Abstract not available.