Intersubband excitations play an important role for ultrafast carrier dynamics in quasi-two-dimensional semiconductors and for device applications. We present a study of the ultrafast coherent and incoherent dynamics of intersubband excitations in a pure electron plasma by means of femtosecond spectroscopy in the mid-infrared. The different relaxation processes following intersubband excitation of electrons in GaInAs/AllnAs quantum wells are observed in real-time and the relevant microscopic scattering mechanisms are identified. We find a decay of coherent intersubband polarizations on a time scale of several hundreds of femtoseconds which is governed by electron-electron scattering. Electrons excited to the n equals 2 conduction subband undergo intersubband scattering to the n equals 1 subband by emission of longitudinal optical phonons with characteristic time constants of 1 ps. This is followed by thermalization of the backscattered electrons on a similar time scale, involving both electron-electron and electron-phonon scattering. Eventually, the hot electron distribution cools down to lattice temperature within about 50 ps.

A microscopic many-body theory is presented which allows one to compute the linear and nonlinear optical properties of semiconductor superlattices in the presence of static and time-dependent electric fields applied in the growth direction. For static fields the Bloch-oscillation dynamics, the role of Coulomb effects, carrier relaxation, phonon scattering, and inter- and intraband polarization dephasing is analyzed. The observability of dynamic localization using optical spectroscopy is discussed for alternating applied fields.

A microscopic description of coherent phonon oscillations generated by femtosecond optical excitation in both polar and nonpolar semiconductors is presented. For nonpolar semiconductors such as Ge, we show that the coherent lattice displacement is related to a quantum-mechanical average of a single phonon creation operator and we derive the equation of motion for the coherent phonon amplitude. In polar materials such as GaAs there is also another driving force which is more effective, namely, the depolarization electric field created by the separation of electrons and holes in the applied DC field and we formulate a microscopic theory of the plasmon- phonon oscillations. Results show that for an idealized situation with homogeneous plasma density that plasmon-like oscillations dominate the transient behavior. However, once the inhomogeneous density distribution is taken into account, only density-independent LO phonon oscillations are present in the transient optical response.

Schemes for coherent control of carrier dynamics in semiconductors are investigated theoretically. We consider coherent control of carrier generation and charge oscillations by quantum interference of one- and three-photon absorption, as well as one- and two-photon absorption. It is shown that, for coherent light pulses shorter than characteristic phase breaking times, the relative phase between the two excitation routes can be used to control the extent of charge oscillations in semiconductor double wells. Furthermore, we show that optical gain and absorption can be controlled coherently by two coherent light fields. In particular, optical gain can be shifted from one intersubband transition to another.

The dynamics of coherent plasmon-phonon oscillations in compound semiconductors are investigated using a pump-probe technique. It is shown that the optically excited coherent carriers drive the thermal carriers introduced by the doping in forming coherent plasma oscillations. By analyzing the coupled oscillations with different hole densities a strong influence of electron-hole scattering on the dephasing of coherent plasmons is found. For the near-band edge detection the reflectivity change by these oscillations is strongly enhanced by the Franz-Keldysh effect. Finally, the second harmonic of the coherent longitudinal optical phonon frequency is observed as a result of the dynamical Franz-Keldysh effect.

The theory of pump-probe femto-second time-resolved experiment will be presented. For the case in which the pump and probe pulses do not overlap, the either can be referred to as the generalized linear response theory. The fs time-resolved spectra consist of the contributions from the dynamics of both population and coherence (or phase) of the nonstationary system. Unless the dephasing is fast, the quantum beat is often observed in fs time-resolved spectra. Recently, it has been found that some polymers could exhibit semiconducting properties. In particular, the conjugated polymers, poly (phenylene-vinylene), i.e., PPV, show strong photoluminescence and can form electroluminescent layer in light-emitting diodes (LED). To study the mechanisms of the photoluminescence of PPV, fs time-resolved experiments have been performed. In this paper, the theoretical analysis of these fs time-resolved spectra will be presented. 15

Laser light has been used as a probe of atoms, molecules, and solids since the invention of the laser. The use of laser light in a more active role, to modify and process surfaces, and initiate chemical reactions, followed shortly thereafter. But usually it is the intensity and the directionality of the laser light that is employed, not necessarily its coherence, and not particularly the fact that it has a well-defined phase. 'Coherence control' can be broadly understood as the set of processes whereby light modifies matter in a way that is critically dependent on the incident light beams possessing well-defined phases. While in a laser matter is manipulated to produce light of the desired properties, in coherent control light is manipulated -- in particular, its phase and intensity is adjusted -- to produce a material response of the desired type. Of the various coherent control processes that are currently being investigated, some involve a transition in the material medium from an initial state to a final state by two or more possible processes. With each of these is associated a quantum mechanical amplitude, and hence the probability for the transition can show interference effects between the two amplitudes, just as in the familiar two-slit interference experiment the probability for the electron to be observed at a given position involves a probability that is the square of the sum of two amplitudes. In quantum interference control (QUIC), the relative phase of the two amplitudes is adjusted by adjusting the relative phase of two polarizations of a single beam, or the relative phase of two beams at different frequencies. It is this particular type of coherent control that is of interest in this communication.

We study the coherence times of CuBr nanocrystals (quantum dots) in a borosilicate matrix under resonant excitation. Measurements are performed in two different experiments: Degenerate four-wave mixing with pulses of approximately 100 fs duration and spectral hole burning using a nanosecond excitation. Our results show a dependence of the dephasing times on the excitation intensity, and, when extrapolated to low intensity, a longer coherence time under fs than ns excitation. This result is tentatively explained as being due to spectral diffusion in the case of ns excitation.

Femtosecond four-wave-mixing (FWM) is used to study the coherent dynamics of excitons in thin epilayers of GaN grown by metalorganic chemical vapor deposition on sapphire substrates. Temperature dependent FWM is used to accurately measure the exciton homogeneous linewidth and it is shown that the exciton-LO phonon interaction is larger in GaN than in other III-V materials. Furthermore, the excitonic resonances in our samples are shown to be very nearly homogeneously broadened even at low temperature. We have observed strong beating behavior in the FWM signal corresponding to the energy separation of the A and B free exciton transitions. The beats were studied as a function of relative position across the B exciton linewidth in order to determine that the beating is due to a coherent exchange of population between the A and B excitons and not to, so called, polarization interference. The quantum beats were further studied as a function of polarization geometry and a phase shift of 180 degrees was observed when changing from collinear to cross-linear polarization geometries. The FWM signal was calculated in the ultrashort pulse limit in order to theoretically model the observed phase change.

We investigate the spatial displacement dynamics of optically excited wave packets in semiconductor superlattices. A short laser pulse exciting semiconductor superlattice induces quantum beats between different excitonic states that in turn leads to formation of a time-varying coherent wave packet. The real space oscillation of the excited wave packet identifies these quantum beats of the Wannier-Stark states as Bloch oscillations: We present an experimental technique which measures directly the displacement of the wave packet center- of-mass. The oscillating Bloch wave packets create a microscopic dipole moment which can be detected using the shift of the Wannier-Stark ladder transition energy as a sensitive field detector. We show that the Bloch wave packet undergoes harmonic spatial motion, proving for the first time the predictions of Bloch and Zener. The influence of an experimental conditions on displacement of the Bloch wave packet is discussed.

Ultrafast relaxation of photoexcited nonequilibrium holes is selectively investigated in bulk GaAs and InP using a high- sensitivity two-color absorption saturation technique. Measurements of the hole characteristic thermalization time as a function of the lattice temperature and of the carrier density and initial average energy show that nonequilibrium hole relaxation is dominated by hole-optical phonon interactions in the range 100 - 300 K. Comparison of the experimental results with a numerical model of carrier dynamics permits the determination of the optical deformation potential in both of these compounds.

With the recent rapid development of GaN based optoelectronic devices, a full understanding of the dynamics of fundamental optical transitions in GaN epilayers and quantum wells becomes increasingly important. In this paper, the dynamics of fundamental optical transitions, probed by picosecond time- resolved photoluminescence (PL), in GaN and InGaN epilayers, In_xGa_1-xN/GaN and GaN/Al_xGa_1-xN multiple quantum wells (MQWs) are reviewed. For GaN epilayers, optical transitions in n- and p-type (Mg doped) and semi-insulating GaN epilayers are discussed. Time-resolved PL results on the fundamental optical transitions in these materials, including the impurity-bound excitons and free excitons transitions, are summarized. For MQWs, recombination dynamics of optical transitions in both In_xGa_1-xN/GaN and GaN/Al_xGa_1-xN MQWs grown by different methods (MOCVD vs. MBE) are compared with each other as well as with GaN and InGaN epilayers to extrapolate the mechanisms and quantum efficiencies of the optical emissions in these structures. The implications of these results on device applications, in particular on the blue LEDs and laser diodes as well on the lasing mechanisms in GaN blue lasers, are also discussed.

In_0.135Ga_0.865As/GaAs quantum wires with widely varying widths down to 29 nm have been studied by degenerate four-wave mixing. The exciton binding energy E_B in the wires has been determined by quantum beat spectroscopy. We find an increase of E_B with decreasing wire width, giving clear evidence for the lateral confinement of the excitons in the wire structures. Further, the exciton dephasing kinetics due to exciton-phonon and exciton-exciton scattering is studied. For both dephasing mechanisms we find an increase of the scattering rates with decreasing wire width. For the exciton-phonon scattering the increase is rather weak, while for the exciton-exciton scattering a drastic increase of the scattering rate is observed.

The effects of X6 and X7 intervalley scattering on the energy relaxation of electrons in GaAs were investigated in GaAs for excitation energy of 4.3 eV. An initial build up of electron population in the upper valleys (X6 and X7) following the excitation leads to non-equilibrium LO phonon. The initial LO phonon spectrums in different valleys show maximum build up at different q vectors and then relax to similar distributions at longer times. The Heating of the LO phonons leads to slower transfer of electrons back to the central valleys.

We report anti-Stokes photoluminescence (ASPL) phenomena for Al_xGa_1-xAs/GaInP₂ heterostructures with a staggered (type-II) band lineup. The ASPL properties were investigated by PL, PL excitation, and time-resolved PL spectroscopy. We found that the ASPL could appear in both the GaInP-2) and Al_xGa_1-xAs layers adjacent to the type- II heterojunction when the excitation photon energy is higher than the interface-related, below-bandgap (BBG) luminescence energy. From the excitation intensity dependent PL measurements, we found that the GaInP₂ ASPL intensity exhibits a nearly linear dependence on the excitation intensity. Time-resolved PL measurements were performed for the excitation energy between the bandgaps of GaInP₂ and Al_xGa_1-xAs, as well as above the bandgap of the GaInP₂ layer. The GaInP₂ ASPL decay time of more than 100 ns was found to follow closely the decay of the BBG luminescence, whereas for excitation above the GaInP₂ bandgap, the GaInP₂ luminescence decays very rapidly (less than 1 ns). From these results, we propose that the energy up- conversion for the ASPL is via a two-step two-photon absorption process involving localized, long-lived carries near the interface.

Time-resolved photoluminescence (PL) measurements were performed on a multiple GaAs/AlGaAs quantum well structure. The rise and decay profiles of PL centered at the free exciton line and the impurity related excitonic-complex line were investigated as a function of temperature at a low excitation density (approximately 10⁹ cm^-2). This study provides new information on how excitons are captured by impurities and on the kinetics of excitonic-complex formation and annihilation. The exciton capture time by impurities is about 250 ps and independent of temperature (less than 80 K) when the exciton density is comparable to the residual impurity density.

Monte Carlo simulation has been shown to be an effective approach to the study of ultrafast carrier relaxation in semiconductor bulk materials and in microstructures. We review the use of this methodology to study electron-electron and electron-hole interactions, non-equilibrium and confined phonons, and inter-subband relaxation in quantum wells. We also discuss the presence of the collision-duration on the short-time scale, and review the work of some other workers in the field. Finally, we discuss some of the limitations of the Monte Carlo technique.

We have experimentally studied the electron velocity overshoot and the mechanism of its degradation in the inversion layer of sub-100 nm metal-oxide-silicon field-effect-transistors (MOSFETs). Both silicon-on-insulator (SOI) and bulk structures were studied. At low transverse electric fields, that is, for low carrier densities in SOI devices under low gate drive conditions, it is possible to achieve electron velocity overshoot due to non-stationary transport in the sub-100 nm region. However, it is very difficult in MOS structures to improve electron velocity at high surface electron densities, because of the reduced electron mobility in high transverse fields. Moreover, the surface electron density of MOS structures is reduced when a low channel impurity concentration is chosen to improve low field mobility; this results from the expanded inversion layer width. These results indicate the physical limitations of scaled MOS structures as regards the realization of higher current capabilities. According to both the above discussion and the statistical performance fluctuation data, we introduce a new scaling scenario for sub-100 nm SOI devices and show its possibility for high current drivability and suppressed performance fluctuation in sub-100 nm ultra-large-scale-integrations (ULSIs).

Several interesting behaviors of resonant tunneling diodes (RTDs) are investigated through numerical simulation: high frequency self-oscillations, strong intrinsic hysteresis, and pronounced static bistability. Each of these behaviors has been observed experimentally in RTDs, but the measured effects have been slower (oscillations), weaker (hysteresis, bistability), or required external inductance to occur (oscillations, hysteresis). These simulations indicate that the effects occur strongly and intrinsically in an RTD when a narrow energy band in the emitter aligns just below a quantized energy state in the quantum well. Quantum system models and available computation power have only recently developed to a point where the necessary physical effects (inelastic scattering, self-consistency, and transient operation) can be properly included to simulate these behaviors in a quantum device. A 1-D Wigner function model is used for transient, self-consistent RTD simulations including inelastic scattering. One-dimensional transfer-matrix calculations are used to locate quantized energy levels. The physics behind the intrinsic oscillations, hysteresis and bistability are described for the simulated RTD. Simulation results are also presented for double-well RTD structures in an attempt to enhance these effects.

Arsenic-ion-implanted GaAs (or GaAs:As⁺), with excess- arsenic-related deep level defects, has recently emerged as a potential alternative to low-temperature molecular-beam- epitaxy (LTMBE) grown GaAs for ultrafast optoelectronic applications. In this paper, we review results of our structural, ultrafast optical and optoelectronic investigations of as-implanted and thermally annealed GaAs:As⁺. Picosecond photoconductive switching responses are reported for devices fabricated on thermally- annealed low-dose and high-dose implanted GaAs:As⁺. Novel sign reversals in near-bandgap ultrafast optical responses were observed and explained.

The high speed response of 50-nm gate AlInAs/GaInAs/InP pseudomorphic high electron mobility transistors (HEMTs) have been used in optical mixing experiments to generate difference frequencies to 211 GHz from two continuous wave laser beams. A 16 dB signal to noise ratio was achieved. To our knowledge, this is the highest frequency optical mixing ever obtained for three-terminal devices. A broadband three wave mixing technique was employed to detect the optically mixed signals at these high frequencies. This scheme involves the nonlinear interaction of the optically generated signal with a millimeter wave signal electrically injected at the gate. The resulting signal, downshifted to W band, was radiated into the waveguide input of an external millimeter wave receiver system. To demonstrate the wide tunability of our system a sweep of frequencies from 160 - 190 GHz was performed. The HEMTs exhibited a relatively flat response with signal to noise ratios of greater than 12 dB. Ultrafast response of the HEMTs as indicated by cw mixing results was also characterized in the time domain using a picosecond electro-optic sampling system. To illustrate the use of the HEMTs in optical millimeter wave systems, optically mixed signals at 97 GHz, both continuous wave and modulated, were radiated into free space using a horn antenna. Modulation was obtained by injecting a baseband signal into the gate of the HEMT. Electrical characterization of the devices yielded cut-off frequencies of 228 GHz and a maximum oscillation frequency of 124 GHz.

Full electrodynamic Monte Carlo simulations have been performed to evaluate the electrical response of nanoscale GaAs photomixers driven by dual laser beams. Low temperature grown GaAs structures are shown to be capable of providing terahertz signals. Photocurrent results indicate an advantage in increasing input laser power. However, bulk heating due to carrier trapping and phonon emission is shown to be fairly significant, and should set a natural limit to laser input scaling. Finally, the simulations reveal that special antenna structure would be necessary for the efficient coupling and transmission of radiative energy.

We present the recent progress of terahertz (THz) sensors in free-space electromagnetic-field applications. We introduce the basic concepts and recent results for the detection of pulsed electromagnetic radiation with electro-optic and magneto-optic sensors. Ultrafast detection system to characterize the temporal and spatial distribution of free- space, broadband, pulsed electromagnetic radiation (GHz/THz beams) will be discussed.

The transient behavior of semiconductor devices within the framework of density operators, the density matrix and the quantum hydrodynamic moment equations are reviewed. A capacitive-inductive transition, near 4 THz for two terminal double barrier structures within the framework of the quantum hydrodynamic equations is discussed.

Ultrafast optoelectronic devices are crucial for fulfilling the future requirement of network throughput exceeding 1 Tb/s. A variety of ultrafast phenomena in semiconductors are attractive for developing such new optoelectronic devices. This paper discusses requirements of ultrafast optical communication and signal processing systems and devices necessary for them. Recent advances in the development of ultrafast semiconductor-based optoelectronic devices including lasers and optical switches are described with an emphasis on new types of ultrafast all-optical switches.

We review our recent results on ultrafast surface-emitting second-harmonic generation from mode-locked laser pulses with arbitrary pulse shapes. The spatial dependence of the SH energy density per pulse is almost the same as the temporal profile of the SH power emitted from the entire surface per waveguide width for a long waveguide. For a sinc fundamental pulse, we have obtained, rich and unique, temporal, spatial, and spectral behavior of the second harmonic. We have found that both the temporal and spectral profiles of the emitted second harmonic depend on the location at the surface. These effects can be observed with lasers currently available, and used for auto-correlation, in optical signal processing, optical communications, and practical frequency doubling of ultrafast optical pulses with an advantage of no broadening of the SH pulse. We have compared these results with those for Gaussian, Lorentzian, and hyperbolic secant fundamental pulses. After introducing saturation intensity to quantify the saturation regime, we have obtained quantitative expression for the second-harmonic intensity in terms of the pump and saturation intensities.

We describe the application of an optical second-harmonic sampling technique that allows for the detection of subpicosecond electrical transients.

The ability to engineer the free carrier lifetime of epitaxially grown semiconductors without significantly affecting the desirable nonlinear optical properties would allow the development of an entire new class of high-speed photonic devices. The primary method of achieving this is the controlled introduction of mid-gap defects via a variety of techniques including low temperature growth. We report on a systematic investigation of low-temperature-grown materials including bulk GaAs and Be-doped In_0.53Ga_0.47As/In_0.52Al_0.48As multiple quantum wells. Using both wavelength-dependent time-resolved nonlinear bandedge absorption spectroscopy and far infrared Terahertz spectroscopy, we unambiguously discriminate between recombination and trapping events and determine the carrier lifetime and mobility in a contactless fashion. We correlate the far infrared response and the bandedge response and thereby explain the apparent discrepancies with previous measurements and clarify the physical origin of the optical nonlinearity as well as the defect densities, carrier lifetimes and mobility.

Time-integrated and spectrally resolved degenerate-four-wave mixing experiments at liquid helium temperature on the heavy- hole exciton resonance of a wedged (In,Ga)As/GaAs quantum well Bragg structure reveal a strong dependence of the dynamic and amplitude of the coherent nonlinear optical response in dependence on sample position. In the vicinity of the Bragg resonance the signal amplitude depends on both the rapid dephasing due to enhanced superradiant decay of the excitons and on the constructive interference of the signal amplitude in the backward Bragg reflection direction. We find that even in the presence of moderate inhomogeneous broadening of the excitonic resonance the superradiant decay dominates the excitonic nonlinearity at Bragg resonance. Our experimental results can be fairly well described by solutions based on the semiconductor Maxwell-Bloch equations taking disorder into account.

SOI MOSFET device structures with SiO₂/AlN composite back insulator with wide range thermal conductivity values were investigated. The proposed structures resulted in different amount of self-heating depending on the fraction of SiO₂ in the back insulator. For a 0.8 micron FD-SOI MOSFET devices, increases in temperature from 23 C to 173 C for 1 milliwatt of power dissipation were obtained through numerical two dimensional device simulation. Additionally, simulation results show strong variation in mobility and generation current in devices with different back gate oxide composition. Similar structures could be used to determine the temperature dependence of transport parameters such as velocity overshoot, impact ionization rate, and other device parameters.

We have studied transient electron transport in an InP p-i-n nanostructure semiconductor by subpicosecond Raman spectroscopy at T equals 300 K. Both the non-equilibrium electron distribution and electron drift velocity in the regime of electron velocity overshoot have been directly measured. It is demonstrated that electron drift velocity in an InP p-i-n nanostructure is significantly larger than that in a GaAs p-i-n nanostructure sample, as a result of the larger central to satellite valley energy separation in InP.

We have used electric-field-induced Raman scattering to quantitatively assess the effects of carrier screening on the average electric fields in a GaAs-based p-i-n nanostructure semiconductor under the subpicosecond laser photoexcitation. Our experimental results demonstrated that the effects of carrier screening on the average electric field were negligible for photoexcited electron-hole pair density of n less than or equal to 10¹⁵ cm^-3. As the density of photoexcited carriers increased we observed a significant decrease of the average electric field. In particular, for n equals 10¹⁸ cm^-3, a decrease of electric field of about 50% was found. All of these experimental results were explained by ensemble Monte Carlo simulations and very good agreement has been obtained.

Electron transport and phonon dynamics in a GaAs-based p-i-n nanostructure under the application of an electric field have been studied by time-resoled Raman spectroscopy at T equals 80 K. The time-evolution of electron density, electron distribution, electron drift velocity, and LO phonon population has been directly measured with subpicosecond time resolution. Our experimental results show that, for a photoexcited electron-hole pair density of n approximately equals 10¹⁷ cm^-3, the effects of the drifting of electrons and electron intervalley scattering processes govern electron transport properties as well as the LO phonon dynamics. All of the experimental results are compared with ensemble Monte Carlo simulations and satisfactory agreement is obtained.

We have studied non-equilibrium electron distributions and electron-longitudinal optical phonon scattering rates in wurtzite GaN by subpicosecond time-resolved Raman spectroscopy. Our experimental result show that for electron densities n greater than or equal to 5 X 10¹⁷ cm^-3, the non-equilibrium electron distributions in wurtzite GaN can be very well described by Fermi-Dirac distribution functions with the effective electron temperature much higher than the lattice temperature. In addition, we find that the total electron-longitudinal optical phonon scattering rate in GaN is about one order of magnitude larger than that in GaAs. We attribute this enormous increase in the electron- longitudinal optical phonon scattering rate to the much larger ionicity in GaN.