Subpicosecond time-resolved photoluminescence studies of a symmetric multiple quantum well structure and an asymmetric quantum well with a similar well thickness have revealed several important experimental results on the behavior of photoexcited carriers in these microstructures: (1) The energy relaxation process is substantially suppressed due to the existence of a large population of nonequilibrium phonons after an initial rapid cooling. (2) The two different well structures do not show any difference in the energy relaxation process under the same experimental conditions. (3) The photoexcited carrier density deduced from exact and consistent fittings of the time-resolved photoluminescence profiles at various emitted photon energies decreases nonexponentially and very rapidly within the first 30 ps after the end of a 0.5 ps pulse. An effective carrier depletion time is determined to be as short as 10 ps in either well. A mechanism which leads to such short carrier depletion time is found to be associated with the nonequilibrium phonon enhanced phonon replica emission. Unlike conventional stimulated emission behavior, this mechanism is even more effective in exhausting carriers from the system at higher temperatures. (4) The difference in potential well profile corresponds to a very different behavior of carrier diffusion process in the lateral well plane. The carrier diffusion is enhanced in the well plane of the asymmetric well by restricting carrier diffusion in the growth direction. Diffusivity D of the photoexcited carriers in the asymmetric well has been directly determined to be 106cm2/s at 4.3K, which is about four orders of magnitude larger than the value in bulk GaAs.

The application of time-resolved luminescence techniques is illustrated for two superlattice structures, based on ZnSe/(Zn,Mn)Se and CdTe/ZnTe heteropairs, respectively, In the quantum well limit, both structures share in common strong excitonic character in their recombination spectra at low lattice temperatures. In the (Zn,Mn)Se the Mn-ion d-electron transition provides an alternate path for electronic energy relaxation. More generally, in II-VI strained layer superlattices with small valence band offset we find that excitonic localization phenomena are important and are directly accessible through time-resolved study.

Exciton lifetimes in Cdi-xMnxTe-CdTe superlattices (x = 0.06, 0.23 and 0.45) at 7 K have been studied by time-correlated single photon counting with high spectral resolution. The lifetimes decrease with well thickness, in agreement with earlier spectrally integrated luminescence decay measurements using a spectrometer/streak camera combination. Our spectral resolution is sufficient to show small structure for the Cd0.5Mn0.45Te-CdTe sample at energies near those predicted by a Kronig-Penny calculation for the subbands, and we report the time behavior of these states.

Recombination dynamics in 111-V quantum well structures have been investigated using a wide variety of optical techniques. Results of these studies have a number of implications for the design of light-emitting devices.

We examine the influence of dense electron-hole plasmas on the optical properties of AlxGa1-xAs for a variety of x-values using transient transmission spectroscopy and time-resolved photoluminescence. The measurements provide evidence for band-filling, nonlinear refraction, induced absorption, alloy disorder, and band-gap renormalization. The band gap renormalization is compared to existing theory, and good agreement is obtained by considering the influence of alloy disorder and the separate contributions of exchange and correlation.

The interaction of picosecond laser pulses with solid surfaces is characterized by ultrafast energy transfer from the electronic system to the lattice. Heating and melting occur in the picosecond time scale. In semiconductors, the transfer is mediated by the electron hole plasma. Its kinetics is dominated by nonlinear recombination mechanisms that reduce considerably the maximum carrier concentration at high irradiation levels. Experiments with time resolved optical and photoelectrical techniques have established firm evidence for ultrafast thermal surface heating and rapid carrier recombination.

The dynamics of carriers in GaAs/(A1GaAs) heterostructures is of great interest because of the technological applications of these materials and because of their novel electronic properties. The higher potential of the A1GaAs layers confines the electrons (and holes) to the GaAs layers. As a result, the electron gas shares all the properties of a two dimensional system, and the interactions of the electrons and holes with the lattice are expected to differ from the bulk GaAs. Using time-resolved Raman scattering of optical phonons in bulk GaAs, J. Kash et al. determined an average emission time of an optical phonon by a photo-excited electron and found it equal to 165 fms. Acoustic phonon scattering is a much weaker mechanism, but becomes the important cooling process for electronic temperatures below 35 K. J. Shah et al. studied the cooling rates of carriers in 2D-heterostructures and found that the electrons cool 25 times more slowly than the holes. So far, little attention has been paid to the relative rates of the cooling processes which involve carriers distributed on several electronic subbands. This is the point we discuss in this paper. There are two cases of interest: the first one occurs when the energy separation of the two lowest electronic subbands is greater than the energy of an optical phonon (ELO=36.7 meV), the second one occurs when the energy separation is less. In this latter case, one expects the scattering of an electron by an acoustic phonon to be the dominant cooling mechanism. Another scattering mechanism arises from the electron-electron interactions. For the large electronic densities generated by photoexcitation (n ~ 1011 electrons/cm2/well), electron-electron collisions effectively randomize the electron momentum and redistribute the energy of the photoexcited electrons. Carrier thermalization typically takes place in less than one or two picoseconds.

Quantum Transport Equations for Bloch electrons interacting with randomly distributed impurities in the presence of a homogeneous electric field of arbitrary strength and time dependence are derived. The equations account for all possible quantum effects to lowest nonzero order in the scattering strength, including intra and interband scattering, interband Zener tunneling and non-linear transient transport, and contain effects previously not anticipated, such as coherent impurity scattering, and field and time dependent scattering matrix elements.

The non-commutation relationship between the Hamiltonian of the system and the electron momentum operator in the confinement z-direction was taken into account using random variables in the calculation of electron scattering by polar optical phonons in quantum wells. The rate at the onset of phonon emission is found to be 3 times smaller than previously reported. Furthermore, this rate becomes an increasing function of initial electron energy instead of a decreasing function.

Over the past decade a variety of physical phenomena have emerged which are exercising profound constraints on the speed of devices whose primary transient is electronic in origin. For example, the broad high frequency extrapolations of high speed arising from velocity overshoot were found to be limited by the effects of space charge and the imposition of fields that take a finite time to reach their specified level. Much of the latter problem may be minimized through optical processes, but in its place rests the constraints of electron-hole interaction and other processes that may require quantum coherence for implementation. In the case of electron-hole interaction it has been found that it provides an important channel through which electron energy is transferred to the lattice, and that the electron cooling rates at high hole concentrations are higher when the electron-hole interaction is included, than when it is ignored. At low hole concentrations the reverse is true. The effect of electron-hole interaction also influences velocity overshoot. In the case of quantum coherence, of interest is the effect, e.g., of temporal scatterers (phonons) on the time dependent wave functions and modifications of interference effects. The influence of space charge on transient velocity overshoot, the effects of electron-hole interaction on the relaxation of an electron-hole plasma, and the effects of time dependent and time independent scattering on quantum interference will be discussed.

We explore the physics of nonequilibrium electron transport in semiconductors with the aim of using this knowledge to develop new, ultrafast electronic and optoelectronic devices. In this paper a theory describing hot electron transport in n- and p-type semiconductors is discussed and operation of an n-type unipolar transistor with a 100Å wide base and current gain, β greater than 15 is demonstrated.

The role of the electron-electron (e-e), hole-hole (h-h), and electron-hole (e-h) interaction on ultrafast cooling of carriers in GaAs is examined for excess excitation energies of 40, 200, and 300 meV using an Ensemble Monte Carlo (EMC) approach. It is found that when the initial energy of the carrier is below the phonon emission threshold carrier-carrier (c-c) interactions stimulate either the optical phonon emission or absorption process depending on whether the initial energy of the carrier is above or below the thermal energy, respectively. The e-h interaction role is strong when excitation energy is below the LO phonon emission threshold.

Time-resolved luminescence and Raman measurements have indicated in recent years the existence of nonequilibrium phonon distributions, as a result of the cooling of phototexcited electrons and holes in GaAs. While several analytic studies of hot phonons have appeared in the literature, we present here a novel Ensemble Monte Carlo calculation of nonequilibrium-phonon effects on the relaxation rate of photoexcited electrons. The build up of the phonon population on a picosecond scale is monitored, in parallel with the cooling of the electron distribution. No assumptions on the form of the phonon or the electron distributions are required. The strong phonon emission by the high-energy photoexcited electrons in the first stage of their relaxation (within a few tenths of a picosecond) is found to drive the phonon distribution strongly out of equilibrium. After the excitation, reabsorption of the emitted phonons by the carriers and nonelectronic phonon decay processes bring the distribution back to its equilibrium value. The time evolution of the calculated phonon distribution is in agreement with the available data obtained with time-resolved Raman spectroscopy. Moreover, the strongly perturbed phonon distribution can, for the moderate carrier excitation densities considered, fully account for the observed reduction of the cooling rate of the phototexcited carriers. Finally the arguments for and against a dominant role of free carrier screening are critically analysed in the light of the present and of foregoing theoretical hot-phonon theories.

A brief tutorial review will be given on the origin of optical non-locality in the resonant, excitonic region of a semiconductor. The consequences of resonant exciton interactions for propagation of optical transients and pulses in semiconductors will be high lighted.

Recently a great deal of attention has been given to a new class of switching, gating and modulating devices based on photoconductivity effect with picosecond optical pulses. These devices include switches, gates, samplers, electronic impulse function correlators, A/D converters, optical detectors and high speed photomixers, DC to RF converters and coherent microwave generators. In this talk, I will present three different types of applications. The generation of pulsed and CW coherent microwaves in complete time synchronization with the ultrashort optical pulses is reported. In the pulsed microwave generation, a single excitation pulse is capable of producing microwave radiation containing a small number of RF cycles up to a few hundred cycles. The generation of over 5 kilowatts of microwave bursts has been demonstrated. For the generation of CW microwaves by the optoelectronic techniques, repetitive excitations are obtained by the use of CW mode-locked dye laser. The phase noise of the microwave has been measured to be 3.5 ps. In another experiment, I will report on the ultrafast optoelectronic modulation of the millimeter-waves at 94 GHz in the silicon-on-sapphire (SOS) waveguides. Optical picosecond pulses from a Nd-Yag mode-locked laser are used to generate high density electron-hole plasma in the epitaxial layer of SOS. The use of layered structure is necessary in order to prevent carrier diffusion. Comparison of the experimental result with theory will be given.

An electro-optical sampler is used to measure the electric potential generated by a device under test. However, the sampler actually measures the change in polarization of a laser beam in the electro-optic crystal, caused by the electric field from striplines to which the device has been connected. In order to relate this measured effect to the actual potential produced by the device, it is necessary to have a detailed knowledge of the stripline characteristics. It will be shown that in a typical three strip geometry, the half wave voltage can vary by 10% across the striplines due to curved electric fields.

A review of ultrafast techniques of characterizing ultra-high-speed semiconductor lasers is presented. The origin of nonlinear gain effects and their influence on the direct modulation bandwidth is discussed.

The reflection of a finite duration optical pulse from a semi-infinite nonlocal medium for various ABC's is investigated theoretically. We have obtained explicit expressions for the amplitude and phase of the transient reflected field (local and nonlocal) and evaluated them numerically for different ABC's. The effects of spatial dispersion for the reflected transients associated with the light pulse are important for laser frequency at the vicinity of an excitionpolariton resonance. For various ABC's, we find quantitative differences in the magnitude of the amplitude and phase which can be used to analyse different ABC's experimentally. The time decay of the transients has also been obtained.

Picosecond degenerate and nondegenerate transient gratings in wide band gap crystals are reviewed. The concepts focus on how these transient gratings can be used as spectroscopic tools to investigate the dynamics of photogenerated electron-holes to impurities in ZnSe:Cu, and coherent optical phonons. The nondegenerate moving transient grating method is used to generate Raman spectra of LiNbO3 and CaCO3 spanning 2000 cm-1 in a single 30 ps. pulse. A new real time phonon dynamic technique using these gratings in conjunction with a streak camera is discussed. This technique is used to measure the dephasing of the coherent optical phonons in CaCO3 of 7.5 ps. and LiNbO3, with a temporal resolution of 2 ps.

Glasses doped with CdSexS1-x microcrystallites have attracted much attention recently. Semiconductor doped glasses are interesting both because of device applications as well as fundamental physics. Room-temperature capability, relatively large optical nonlinearity (n2 ≈ 10-8 - 10-9 cm2/kW) , wavelength tunability (which is obtained by changing the composition x which changes the semiconductor bandedge and thus provides wavelength tunability) , rapid response time (≈10 ps) , ease and inexpensiveness of the fabrication make these materials suitable for guided-wave device application . The physics of quantum confinement effects in all three dimensions makes these materials particulary attractive from the fundamental understanding point of view . In order to observe quantum confinement effects, the crystallite sizes have to be small, with (f uniform size distribution. Commercially available glases have an average diameter of ≈120 Å with a FWHM size distribution of ≈50 Å. Therefore, quantum confinement effects are usually absent in commercial glasses . However, a more uniform size distribution and small crystal diameters can be obtained with careful heat treatment of the glass . Researchers at Corning have been able to fabricate glasses with average crystallite diameter ranging from 30 Å to 80 Å with 24 Å to 44 Å FWHM size distribution, respectively . Optical absorption, photoluminescence, x-ray diffraction and transmission electron microscopy have been conducted in order to examine microcrystallites as a function of composition and development .

The recombination lifetimes for the radial and angular quantum number conserving 1S→1S and 1P→1P transitions from three-dimensionally confined electrons in CdSxSel-x were measured by time-resolved photoluminescence. The assignment of the observed transitions is supported by calculations of eigen energy levels, squared matrix element ratio for those transitions, steady-state and picosecond spectroscopic studies.

Picosecond trapping of photogenerated carriers in gap states of doped, compensated and undoped amorphous hydrogenated silicon (a-Si:H) and of a-Si:H based superlattices was studied by the pump and probe photomodulation technique. In undoped a-Si:H the photogenerated carriers are trapped in bandtail states and in compensated a-Si:H in impurity states introduced by doping. In singly doped a-Si:H the photoexcited majority carriers are trapped in impurity states whereas the photoexcited minority carriers are trapped in charged dangling bond defects. In a-Si:H/a-SiNx:H superlattices photocarriers are trapped in interface related defects. In all cases we found that electron trapping is about 50 times faster than hole trapping. This intrinsic property of a-Si:H originates from a larger electron hopping rate among localized states in the conduction band-tail.

Photoluminescence kinetics measured by streak camera at high photogenerated carrier density in CdSe is attributed to screening of electron phonon interaction and enhanced diffusion.

Present situation in picosecond image converter photonics is analized on the basis of the results obtained by the authors. Picosecond radiation sources are described as well as various approaches in image-tube photocathodes fabrication are discussed. Some new image converter cameras and CCD read-out devices will be introduced together with recent examples of their application in physics research.

The dynamics of dense electron-hole plasmas in submicron thick films of InGaAsP are studied by time-resolved absorbance and photoluminescence techniques. The plasmas are created by excitation with a 0.5 ps optical pulse, and have initial densities between 5x 1020 and 1 x 1018 cm-3. At these high initial densities, significant band filling is observed. The plasma cools rapidly within ~1 ps. The plasma density then decays by rapid spatial expansion driven by the Fermi pressure of the plasma, until a density of ~6x1018 cm-3 is reached. In this initial phase, plasma expansion is much more rapid than Auger recombination. Subsequently, the plasma decays by a combination of Auger and bimolecular radiative recombination. Band gap renormalization and generation of acoustic phonons resonant with the thickness of the thin films are also observed.