The effect of hopping energy relaxation on the diffusion-limited reaction A+B→0 is considered. The main effect of the relaxation is to make the diffusion constant a decreasing function of time. We find that fluctuation effects radically alter the decay law predicted from the rate-equation approach, as is already known to be the case for diffusion in thermal equilibrium. New long-time behavior is predicted for the concentration of reacting particles as compared to the thermal equilibrium case.

It is known that in disordered semiconductors with purely exponential energy distribution of localized band-tail states, as in amorphous semiconductors, all transport phenomena at low temperatures are determined by hopping of electrons in the vicinity of a particular energy level, called the transport energy. We analyse whether such a transport level exists also in materials with densities of localized states (DOSs) different from the purely exponential one. We consider two DOS functions with , typical for polymers, heavily doped semiconductors, and, probably, liquid semiconductors and , typical for mixed crystals. It is shown that in both cases the transport energy exists, implying that it also exists for all intermediate forms of the DOS. Special attention is paid to the dependences of the transport level and of its width on the DOS parameters and temperature.

We present measurements of the energy relaxation of quasi-two-dimensional excitons in the hopping regime. Under a variety of experimental conditions, the mean energy decays in proportion to In t. We interpret this result naturally in a model based on the dipole-dipole mechanism for energy loss hops. The model achieves semiquantitative agreement with the observed energy loss rates.

We study experimentally the slow relaxation of the mean energy and radiative rate of a system of interacting, two-dimensional electrons and holes in the presence of static disorder. The system evolves through two phases: an early, excitonic phase and a long-time, distant-pair phase. The results differ from the usual energy-loss hopping because of the dominant role of interactions in the present system.

Time-integrated (TI) and time-resolved (TR) degenerate-four-wave-mixing (DFWM) experiments performed on the exciton transition in GaAs quantum wells (QW) have shown strong variations of the signal features with the polarization configuration of the two excitation pulses that cannot be explained in the frame of the model describing the 2-D excitan by two noninteracting twofold-degenerate three-level systems with circular dipole transition rules.1

Rapid progress in the development of mode-locked laser systems during the past decade has boosted the time resolution attainable in nonlinear optical spectroscopy well below 100 fs. Ti: sapphire lasers which can directly generate pulses as short as 12 fs [1] mark the most recent milestone of this evolution. This new generation of fs solid state lasers surpasses the older colliding-pulse mode-locked (CPM) dye laser [2] by far with respect to output power and stability, and most importantly, by the broad tunability of the fs output between 680–1000 nm. Combining these lasers with frequency converters like harmonic generators [3] or optical parametric oscillators [4] extends the tunability range to the ultraviolet, visible, near and middle infrared regime.

Several recent two-pulse degenerate four- wave-mixmg experiments on 2D excitons in GaAs showed considerably faster dephasing rates for cross-polarized beams than for colinear polarization.1,2 Moreover, time-resolved analysis of the DFWM signal on some samples revealed a photon echo and a free-induction decaylike behavior for the parallel and perpendicular polarizations, respectively.1

The self-diffraction four wave mixing signal from excitonic transitions in GaAs quantum wells is investigated experimentally as a function of arbitrary linear polarization orientation of the incident light pulses. The polarization of the diffracted signal, and the phase of quantum beats between light and heavy hole excitons, are shown to strongly dependent on the magnitude of excitation intensity. The experimental results observed for excitation densities above 2 × 109 cm −2 are in accordance with theoretical conceptions of the polarization selection rules for light and heavy hole exciton transitions in a quantum well. For lower excitation, remarkable deviations from the predictions of the theoretical model occur.

A new echo phenomenon is suggested, which we call the current echo. A disordered one-dimensional tight-binding conductor subject to two very short voltage pulses is considered. While the current response following the first pulse decays due to scattering off the disorder, a delayed current pulse is predicted following the second voltage pulse, its delay being equal to the temporal separation of the two voltage pulses. This prediction is illustrated for an ensemble of finite, disordered chains.

We suggest an experimental method which should, at least in principle, be capable of measuring Bloch oscillations directly in the time domain. The method consists of measuring the spontaneous photon-echo signal in a time-resolved four-wave-mixing experiment on a semiconductor superlattice. We calculate the third-order nonlinear polarization of an idealized superlattice in the presence of a homogeneous electric field applied along the growth direction. In the limits of both vanishing and infinite fields, the echo signal produced by the nonlinear polarization should be independent of the delay time tau between the exciting pulses, if irreversible dephasing processes are disregarded. For finite fields, the echo signal should exhibit a modulation periodic in tau, with the periodicity of the Bloch oscillations. If observed experimentally, this would represent a direct manifestation of Bloch oscillations in the time domain.

Photon echo experiments can be used to study the interaction of optical excitations with other quasiparticles on a short time scale. Two short laser pulses impinge on the sample with a time separation τ. The sample then emits a photon echo at time 2τ. The decay of the photon echo amplitude with τ can be attributed to dephasing interactions.