Realistic simulations of semiconductor plasmas require detailed, many-species descriptions of the phonon and electronic systems. Limited numerical power then usually requires simplifying approximations. One approximation is the use of a screened Coulomb interaction. When an accurate screening function is not available, or when a better electrostatics treatment is needed, one can perform ensemble Monte Carlo (EMC) simulations that use a phase-space- trajectories or 'molecular dynamics' (MD) evolution of the electronic ensemble. In these EMC/MD simulations, Coulomb scattering events are treated continuously in the MD evolution of electron trajectories rather than by instantaneous scattering in EMC. Dynamic scattering effects are then included accurately by the explicit correlated motion of the electron ensemble. The electron trajectories simulated by MD have until recently been completely classical. We describe extensions of EMC/MD into the semiclassical regime, thus incorporating quantum effects such as position-momentum uncertainty. The method takes account of the Fermi statistics of the many-electron ensemble, yielding spin-dependent exchange contributions to the forces and effective mass. We describe effects of these corrections on the velocity autocorrelation function and on thermalization of satellite-valley electrons.

We studied the lifetimes and the luminescence spectra of photoexcited carriers in H⁺ bombarded InP and GaAs for different damage doses by means of femtosecond luminescence spectroscopy. For InP the lifetime decreases down to 95 fs for the highest dose, whereas for GaAs no shorter lifetime than 650 fs could be observed. With decreasing lifetime we observe an increase of the high energy tail of the time-integrated luminescence spectrum which is even inverted for the 95 fs InP sample.

Hot electron luminescence spectroscopy of GaAs shows polarization dependent lineshape variations of 0.5 approximately 1.0 meV. It is shown how a lineshape model which includes a k.p calculation of the band structure, optical transition matrix elements in the dipole model, and lifetime broadening, is able to explain these polarization effects. For linearly polarized excitation, the observation arises from the optical alignment of the hot electron momenta, while for circularly polarized excitation the effect is caused by transitions between specific electronic spin states identifiable by a selection rule.

The pump-probe technique is used to perform a series of measurements on intrinsic GaAs samples at room temperature with a temporal resolution of 75 - 100 fs. Changes of both absorption coefficient and refractive index are measured over a wide spectral region (550 - 950 nm) for various carrier densities (<10¹⁶ to 10¹⁹ cm^-3) injected at 2 eV. These measurements provide insight on the fundamental properties of nonequilibrium carriers, including electron-electron scattering, electron-hole scattering, electron-phonon scattering, hole-phonon scattering, band-gap renormalization, plasma screening of Coulomb interactions, and free-carrier absorption.

We have previously reported on the interaction of a one-component (electron) plasma with optical phonons. We now report on more recent results on the interaction of a two-component (electron-hole) plasma with coherent optical phonons. The plasma scattering rate appears to be in roughly the same in the two cases.

The initial generation of hot LO phonons by the relaxation of hot carriers in GaAs and Al_xGa_1-xAs alloy semiconductors is studied. Within the initial 2 ps of photoexcitation, only those electrons originating from the split-off hole bands are found to generate a significant number of (Gamma) -valley hot phonons when photon energies of 2.33 eV are used. A picosecond Raman scattering technique is used to determine the hot phonon occupation number in a series of MBE grown Al_xGa_1-xAs samples with 0 17 cm^-3. A model based upon the instantaneous thermalization of hot electrons photoexcited from the split-off hole bands is used to fit our data. We have obtained very good agreement between experiment and theory. This work provides a clear understanding to the relaxation of (Gamma) valley hot electrons by the generation of hot phonons on subpicosecond and picosecond time scales, which has long standing implications to previous time resolved Raman experiments.

Femtosecond photoexcitation experiments have been used by many groups to investigate ultrafast scattering processes in semiconductors. Information on intervalley scattering rates can readily be deduced by monitoring valley populations in real time, and particularly, a number of groups have made measurements for (Gamma) -L and (Gamma) -X intervalley scattering in GaAs. However, due to the direct gap, the L-X scattering in GaAs can not be directly monitored. Recently, experiments to monitor the X valley population in indirect AlGaAs have been performed, and utilized to set up an upper bound for the L-X scattering lifetime. We have used an ensemble Monte Carlo (EMC) technique to calculate the evolution of valley populations in indirect AlGaAs illuminated by a femtosecond pulse laser. The time evolution of electron populations in the (Gamma) , L and X valleys is studied by varying the intervalley coupling constants. The L-X intervalley deformation potential is found to be D_XL equals 1.5 +/- 0.5 X 10⁸ eV/cm.

We have previously calculated intervalley scattering (IVS) times for GaAs, resulting in good agreement with various experiments, although some details are still under discussion. In this paper, we consider a semiconductor alloy like Al_1-xGa_xAs, where scattering due to disorder competes with phonon-assisted intervalley scattering. We have calculated the scattering times from the (Gamma) point in indirect Al_1-xGa_xAs (x > 0.4) to the satellite valleys at L and X using a second-order perturbational approach. Other methods have also been investigated. We find that disorder-induced scattering is about twice as effective as phonon-assisted scattering, even at room temperature. We also calculate the lifetime broadenings caused by these processes and compare them to experimental broadenings of the E₀ gap obtained by resonant Raman scattering and spectroscopic ellipsometry. IVS times in InP from the (Gamma) valley to side valleys around L and X are given as a function of energy at 0 and 300 K as well as the return times from the L and X points to the (Gamma) valley from 0 to 400 K. Our results suggest that the strength of the electron-phonon interaction in InP is similar to that in GaAs. Therefore, the IVS times in InP can also be obtained from those of GaAs by a simple extrapolation allowing for the different position of the satellite valleys. Conclusions about IVS times in Al_1-xGa_xAs, however, cannot be drawn from data on GaAs, because most IVS in Al_1-xGa_xAs is due to alloy scattering, which does not occur in GaAs.

The hot carrier dynamics in the satellite X valley in Al_xGa_1-xAs was measured by femtosecond pump-probe infrared absorption spectroscopy. The dynamics of the X valley electrons for samples with x = 0.439. The critical value of x_c corresponding to the direct-to-indirect band gap transition for Al_xGa_1-xAs was determined to be 0.412 +/- 0.006.

Current biased films of YBa₂Cu₃O_7-(delta ) are illuminated by short (300 fsec) pulses of variable fluence laser light. Transient voltages are measured over the temperature range 10 K < T < 250 K. Near T_c a bolometric response is observed, but below T_c signals are observed, with characteristic decay times of the order of hundreds of psec, that cannot be explained by a bolometric mechanism. Furthermore, a fast precursor, with full width of tens of ps, is observed when the bias current approaches the critical current. These signals are characterized and discussed.

Picosecond voltage pulses were generated in YBCO microstriplines by irradiation with a picosecond laser. An autocorrelation method was proposed and applied to measure the temporal decay of these ultrafast voltage pulses. It was also ascertained that these pulses were generated in the superconducting state. Using a 40 ps laser, voltage pulses with fast components < 40 ps were obtained. This fast decay was dependent on the bias current. There were also significant thermal components in these pulses. Methods of generating even shorter pulses are discussed.

The temperature dependence of the electron-phonon relaxation and of the electron-phonon coupling constant for a metal (Nb) has been determined for temperatures ranging from room temperature (292 K) to below superconducting temperatures (4.5 K). The measured rise time of the transmittance change decreases from 370 fs at 292 K to 205 fs at 4.5 K. The electron- phonon coupling constant decreases from 20 X 10¹⁷ at 292 K to 0.5 X 10¹⁷ W m^-3s^-1 at 4.5 K. The cooling of the lattice after being heated by the pump pulse shows an increase in the cooling speed when the sample is below superconducting temperature. This faster decay may be attributed to the cooling due to formation of Cooper pairs.

The ultrafast dynamics of photoexcited carriers in a semiconductor is investigated by means of a Monte Carlo approach. In addition to a 'conventional' Monte Carlo simulation, the coherence of the external light field and the resulting coherence in the carrier system is fully taken into account. This allows us to treat the correct time-dependence of the generation process showing a time-dependent line-width associated with a recombination from states off- resonance due to stimulated emission. The subsequent dephasing of the carriers due to scattering processes is analyzed. In addition, the simulation contains the carrier-carrier interaction in Hartree-Fock approximation giving rise to a band-gap renormalization and excitonic effects which cannot be treated in a conventional Monte Carlo simulation.

We have investigated the carrier capture mechanism in quantum well lasers and its relevance for device characteristics. It is demonstrated that the dependence of the threshold current on the structure parameters of the layers in the active region is highly correlated with the electron capture efficiency. From our calculations it appears that not only the LO-phonon induced capture process but also the carrier-carrier scattering induced capture process oscillate as a function of quantum well width. The predicted structure parameters for an optimum capture efficiency are equivalent for these scattering processes, because in both capture mechanisms these oscillations arise from oscillations in the wave function overlap. The carrier-carrier scattering starts to dominate the capture process for carrier densities larger than 1.10¹¹ cm^-2 in the quantum well. As a result an efficient capture process enhances the cooling of the carriers after injection, giving rise to the reduction of the carrier temperature and thus to a low threshold current. We find that a large capture efficiency improves the modulation response of a quantum well laser due to a smaller amount of carrier accumulation in the barrier. By maximizing the carrier capture efficiency in laser structures we for the first time are able to predict the structure parameters of the layers in the active region for an optimum laser performance.

Carrier energy relaxation times have been measured in In_0.53Ga_0.47As grown by MBE on InP. Layer thicknesses from 0.5 to 3 microns have been studied. An NaCl color center laser using additive pulse modelocking supplied 150 femtosecond pulses with photon energies between 780 and 806 meV. These were used for time resolved optical saturation measurements near the 750 meV material bandgap. Carrier densities between 0.4 X 10¹⁸ and 5.7 X 10¹⁸ were achieved. Lifetimes of about 150 femtoseconds are reported. These are observed to decrease with increasing carrier density and with decreasing photon energy.

We present a combined experimental and theoretical study of the ultrafast internal thermalization of high energy carriers created by laser excitation. Luminescence up-conversion is used to monitor the spectral and temporal evolution of the photoexcited carrier distributions with a time resolution of about 100 fs. A Monte Carlo simulation joined with a molecular dynamics approach is then used to interpret the experimental results. We show that the coulomb interaction among carriers is responsible for the initial ultrafast thermalization. The simulation allow us to distinguish between binary carrier-carrier collisions and plasmon losses and reconcile the results obtained with time resolved vs. c.w. hot (e, angstroms) luminescence.

A femtosecond continuum, with a bandwidth of 90 nm, has been used in a pump-probe geometry to investigate nondegenerate two-photon absorption (2 PA) from 660 nm to 570 nm, with pump wavelength (lambda) equals 620 nm, in hexagonal CdS at 300 K. A dependence upon probe wavelength and relative beam polarization is observed. The polarization anisotropy has allowed us to measure the relative magnitudes of particular (chi) ⁽³) tensor elements. The nondegenerate 2 PA coefficient increases by a factor of about two from 660 nm to 570 nm. The anisotropy and dispersion are discussed in terms of a two-parabolic-band model.

We report the observation in a-Si:H of a large thermal nonlinearity with a picosecond response time. The spectral dependence of the refractive and absorptive parts of this unusually fast thermal nonlinearity reveals that it arises from a picosecond nonradiative recombination of electrons and holes across the band gap. The optical and electronic processes that make this thermal nonlinearity possible and observable are discussed.

Light deflection and diffraction scattering techniques have been used in pump-probe geometries to measure the temporal and irradiance dependences of refractive index changes induced by intense 120 fsec (lambda) equals 620 nm pulses in 100 micrometers thick crystals of bulk CdS and CdS_0.75Se_0.25 at 295 K. Instantaneous and long lived negative refractive index changes are observed. The instantaneous changes are attributed to optical Stark and two- photon resonance effects while the long lived changes are attributed to free carrier bandfilling effects. The irradiance dependences of the different contributions are discussed in terms of a model in which attenuation is dominated by two-photon absorption. The rise and decay times associated with the bandfilling nonlinearity have allowed us to identify a carrier cooling time of 2 psec, and a carrier lifetime which can be as short as 6 psec for pump irradiances in excess of 300 GW/cm². Possible mechanisms for the carrier recombination are discussed.

Exciton-polariton photoluminescence kinetics under short-pulse excitation in pure epitaxial GaAs has been investigated. The observed delayed onset of the polariton luminescence is attributed to the energy relaxation of polaritons and photoexcited electrons. The electron energy relaxation is controlled by inelastic impurity scattering. In an ultrapure sample (N_D approximately 10¹² cm^-3) the maximum of luminescence is reached after a considerable delay of 4 ns. At high repetition rate the next excitation pulse causes a fast quenching of polariton luminescence in the vicinity of exciton resonance due to heating of excitons by photoexcited hot electrons. A model of exciton luminescence kinetics involving exciton-electron interaction has been proposed.

Excitonic effects on coherent oscillations of a photoexcited electron wave packet in a double quantum well structure was studied using the time dependent Schrodinger equation. The oscillation period of the electron wave packet is found to increase due to the electron-hole Coulomb interaction.

Exciton dynamics in various 121 angstroms single quantum wells (QWs): a AlGaAs/GaAs QW and two AlGaAs/GaAsP QWs, under different built-in biaxial tension, has been investigated using time resolved photoluminescence (PL) spectroscopy at 5 K. Heavy-hole (hh) and light-hole (lh) exciton formation times from free electron-hole pair, hh (lh) exciton to lh (hh) exciton inter-subband relaxation times, exciton localization times to interface islands, and localized exciton annihilation decay times in the strained and non-strained QWs have been determined by fitting the PL time profiles at the lowest emission energy with an analytical solution for the localized exciton population profile obtained by solving six level rate equations.

We present the results of a comprehensive investigation of spin-relaxation processes of electrons, holes and excitons in quantum wells using subpicosecond spectroscopy of luminescence polarization. Spin relaxation rates of electrons and holes are measured directly in modulation-doped quantum wells and give a good understanding of spin-relaxation processes of electrons and holes. We show that spin-relaxation dynamics of excitons, on the other hand, is quite complicated and is strongly influenced by their formation dynamics, many-body effects and localization dynamics. Although we have made good progress towards understanding exciton spin relaxation processes, some other outstanding issues will require further attention. We compare our results to those in bulk GaAs, and those in quantum wells obtained by other techniques.

Tunneling transfer in various GaAs/Al_0.35Ga_0.65As asymmetric double quantum well structure is studied by time-resolved photoluminescence measurements in the pico- and femtosecond regime. A large variety of electron and hole resonances is detected when electric fields of both signs are externally applied. The ground state resonance shifts, when the electrons tunnel in the reverse direction, revealing the importance of excitonic effects. Longitudinal optical phonon assisted tunneling plays a minor role for narrow quantum wells in comparison to impurity or interface roughness assisted transfer. Resonant electron tunneling times depend exponentially on the square root of integrated tunneling barrier height and are an order of magnitude faster than resonant hole tunneling times. The n equals 2 to n equals 1 electronic intersubband scattering time in a 10 nm quantum well is determined to be 550 fs measuring the transfer time through a thin barrier.

We have used femtosecond optical spectroscopy to probe high-field transient parallel transport in quantum wells from the ballistic to the quasi-equilibrium regime, by studying both the carrier and electric field dynamics.

Cool hole effect on hot electron energy relaxation dynamics in GaAs quantum wells grown on Si was investigated using photoreflectance and time-resolved photoluminescence spectroscopies. It was demonstrated that for the quantum wells in which there is a two- dimensional light (heavy) mass hole gas the electron energy relaxation is dominated by electron-hole (electron-longitudinal optical phonon) energy exchange.

An investigation of hot carrier relaxation in GaAs/Al_xGa_1-xAs multiple quantum wells and bulk GaAs in the high carrier density limit is presented. Two techniques have been employed: luminescence upconversion with < 80 fs temporal resolution has been used to cover the range from 100 fs to 100 ps, and time-correlated single-photon counting to cover the range from 100 ps to 2 ns. Electron temperatures as a function of time were determined from the slope of the high energy tail of the time-resolved photoluminescence spectra. Our results show that hot electron cooling rates in the quantum wells begin to become significantly slower than that in the bulk when the photogenerated carrier density is above a critical value of approximately 2 X 10¹⁸ cm^-3; the difference in cooling rates increases rapidly with increasing carrier density. The time constant characterizing the power loss of hot carriers is also determined and discussed. A comparison is made with previous publications to resolve the confusion concerning the difference in cooling rates between quasi-2D and 3D systems.

The role of free carrier screening, in the ultrafast relaxation of optically excited carriers, is reassessed using the ensemble Monte Carlo technique. The conventional static screening approximation is compared to a new dynamic screening model. Evolution of the nonequilibrium dynamic dielectric function and its consequences for the carrier scattering are examined. It is shown that dynamic screening results in significant enhancement of both the carrier-carrier and polar optic phonon scattering rates. Relaxation times for the dynamic screening model are found to be dramatically shorter than those for the static screening model. Methods of experimentally differentiating between the two models are proposed.

We present a detailed study of phonons in rectangular quantum wires within the dielectric continuum model and within a fully microscopic approach. From the latter, we calculate the phonon dispersions and the potentials associated to the individual modes. The classification of such modes is much more complex than in the corresponding two dimensional case, owing to the intrinsic coupling of confined and interface modes associated to both directions perpendicular to the wire. This indicates a failure of the current implementations of the macroscopic dielectric continuum model.