The properties of hot electrons in systems where electrons and phonons experience quantum confinement are reviewed. The modifications to the behaviour of electrons and phonons brought about by confinement are described, particularly with reference to the principal scattering mechanism. The latter include the interaction with longitudinal optical phonons and plasmons, along with carrier-carrier effects. Some conflict in the literature concerning Fuchs-Kliewer polaritons is discussed. Low-temperature interactions with acoustic phonons are described. A central topic is that of energy relaxation, and the experimental and theoretical data relating to this form a large part of the review. Energy relaxation mechanisms in the femtosecond to nanosecond regimes, including intersubband and well-capture processes, are eventually summarized. An equally large section deals with hot-electron transport; in which negative differential resistance and other instabilities associated with parallel transport are discussed before turning to ballistic transport and impact ionization.

https://iopscience.iop.org/article/10.1088/0034-4885/54/2/001/pdf

Hot electrons in low-dimensional structures

Spectroscopy of hot carriers in semiconductors

The application of femtosecond spectroscopy to the study of ultrafast dynamics in semiconductor materials and nanostructures is reviewed with particular emphasis on the physics that can be learned from it. Excitation with ultrashort optical pulses in general results in the creation of coherent superpositions and correlated many-particle states. The review comprises a discussion of the dynamics of this correlated many-body system during and after pulsed excitation as well as its analysis by means of refined measurements and advanced theories. After an introduction of basic concepts—such as coherence, correlation and quantum kinetics—a brief overview of the most important experimental techniques and theoretical approaches is given. The remainder of this paper is devoted to specific results selected in order to highlight how femtosecond spectroscopy gives access to the physics of coherences, correlations and quantum kinetics involving charge, spin and lattice degrees of freedom.First examples deal with the dynamics of basic laser-induced coherences that can be observed, e.g. in quantum beat spectroscopy, in coherent control measurements or in experiments using few-cycle pulses. The phenomena discussed here are basic in the sense that they can be understood to a large extent on the mean-field level of the theory. Nevertheless, already on this level it is found that semiconductors behave substantially differently from atomic systems. Subsequent sections report on the occurrence of coherences and correlations beyond the mean-field level that are mediated either by carrier–phonon or carrier–carrier interactions. The corresponding analysis gives deep insight into fundamental issues such as the energy–time uncertainty, pure dephasing in quantum dot structures, the role of two-pair or even higher correlations and the build-up of screening. Finally results are presented concerning the ultrafast dynamics of resonantly coupled excitations, where a combination of different interaction mechanisms is involved in forming new types of correlations. Examples are coupled plasmon–phonon and Bloch–phonon oscillations.The results reviewed in this paper clearly reveal the central role of many-particle correlations and coherences for the ultrafast dynamics of dense semiconductor systems. Both the presence of strong correlation effects and the formation of coherences in a genuine many-particle system have important implications for the controllability of optical signals from this class of materials, which is of utmost importance for applications in present-day and future optoelectronic devices.

https://iopscience.iop.org/article/10.1088/0034-4885/67/4/R01/pdf

Femtosecond spectroscopy in semiconductors: a key to coherences, correlations and quantum kinetics

The objective of this review article is to give an overview of the current state-of-the-art of photoluminescence (PL) spectroscopy as a characterization tool in the study of semiconductors. A detailed description of all of the PL spectroscopic techniques will be given together with examples from the literature of their use. These examples will be the primary tool used to convey the essential aspects and usefulness of the variety of PL techniques discussed throughout this review. The primary examples of semiconductor materials examined with these techniques will be of the prototypical direct-gap and indirect-gap semiconductors, gallium arsenide and silicon, respectively. Additionally, PL characterization of structures with lower dimensionality (quantum wells, superlattices, quantum wires, and quantum dots) will be shown. This article will not cover the history of this type of spectroscopy, inelastic light scattering, infrared spectroscopy, or reflectance, etc. The focus is intended to be experimental in nature, and will be supplemented by theory where appropriate and necessary. There are several fine review articles and books dealing with the subject of photoluminescence, and radiative recombination in general, and this article is only intended to supplement these sources.

Photoluminescence spectroscopy of crystalline semiconductors

Femtosecond carrier dynamics in GaAs and AlGaAs thin films are studied by time-resolved pump-probe spectroscopy. Measurements are performed using identical pulses as well as a continuum probe. Observations of the ultrafast absorption saturation response are related to the excitation of the carriers into specific nonthermal distributions, to their rapid scattering out of these states, and to subsequent cooling of the quasi-thermal carrier distribution to the temperature of the lattice. A strong dependence of the initial scattering rates on the mole fraction of Al and the excess electron energy is observed. >

I.Ja. Karlik

Papers

Hot electron photoluminescence in GaAs crystals