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Based on the coupled density and energy balance equations, a dynamical model is proposed for exploring many-body effects on optical carrier cooling not lattice cooling in steady state in comparison with the earlier findings of current-driven carrier cooling in doped semiconductors X. L. Lei and C. S. Ting, Phys. Rev. B 32, 1112 1985 and tunneling-driven carrier cooling through discrete levels of a quantum dot H. L. Edwards et al., Phys. Rev. B 52, 5714 1995 . This dynamical carrier-cooling process is mediated by a photoinduced nonthermal electron-hole composite plasma in an intrinsic semiconductor under a thermal contact with a low-temperature external heat bath, which is a generalization of the previous theory for a thermal electron-hole plasma H. Haug and S. Schmitt-Rink, J. Opt. Soc. Am. B 2, 1135 1985 . The important roles played by the many-body effects such as band-gap renormalization, screening, and excitonic interaction are fully included and analyzed by calculating the optical-absorption coefficient, spontaneous emission spectrum, and thermalenergy exchange through carrier-phonon scattering. Both the optical carrier cooling and heating are found with increasing pump-laser intensity when the laser photon energy is set below and above the band gap of an intrinsic semiconductor. In addition, the switching from carrier cooling to carrier heating is predicted when the frequency detuning of a pump laser changes from below the band gap to above the band gap.

This work generalizes our previous study of the effect of plasma-wave induced longitudinal fields [D. H. Huang et al. J. Appl. Phys. 100, 113711 (2006)]. We incorporate a conductive grating on top of a conducting sheet for generating and mixing Bragg modes arising from a reflected and/or transmitted electromagnetic field. It also generalizes our recent work on the coupling of electronically modulated two-dimensional plasmons with the surface plasmon [G. Gumbs and D. H. Huang, Phys. Rev. B 75, 115314 (2007)] by including the retardation and longitudinal-field effects in calculating the diffracted near and far electromagnetic fields. The main result of this paper is the prediction of large enhancements at the band edges of a coupled Bloch-surface-plasmon-polariton band in the spectrum of the reflected far electromagnetic field due to anticrossing gaps induced by the strong coupling between the continuous surface-plasmon-polariton mode and discrete Bloch-like modes. The existence of these Bloch-like modes is a direct consequence of the nonlocal mixing of specular and diffraction modes of the reflected electromagnetic field by free-electron induced optical polarization and the interference of a pair of surface optical-polarization waves with opposite Bragg order numbers in the presence of a grating. The interference of these two counterpropagating surface waves leads to the formation of a Wannier-like state with associated electromagnetic fields localized within the grating-gap regions. The effects of the sheet density, plasmon instability, grating period, angle of incidence, and absorption loss on the optical enhancements of both transmitted and reflected electromagnetic fields are investigated.

A successive four-step model is proposed for spatially selective laser cooling of carriers in undoped semiconductor quantum wells. The four physical steps include the following processes: (1) cold electrons with nearly zero kinetic energy are initially excited across a bandgap in a coherent and resonant way by using a weak laser field; (2) the induced cold carriers in two different bands are heated via inelastic phonon scattering to higher-energy states above their chemical potentials; (3) the resulting hot electrons and holes radiatively recombine to release photons, thus extracting more power from the quantum well than that acquired during the weak pump process; and (4) hot phonons in two surrounding hot barrier regions thermally diffuse into the central cool quantum well, thereby cooling the entire lattice with time. Based on this model, a thermal-diffusion equation for phonons including source terms from the carrier-phonon inelastic scattering and the thermal radiation received by the lattice from the surrounding environment is derived to study the evolution of the lattice temperature. At the same time, an energy-balance equation is applied to adiabatically find the spatial dependence of the carrier temperature for a given lattice temperature at each moment. There are two interesting findings in thismore » paper. First, a V-shape feature in the carrier temperature is predicted by numerical calculations, which becomes apparent only for initial lattice temperature above 150 K. Second, a thermal-drag of the carrier temperature is found as a result of the strong carrier-phonon scattering. The difference between the lattice and carrier temperatures resulting from the thermal-drag effect is larger in the barrier regions than in the well region.« less

Coupled energy-drift and force-balance equations that contain a frictional force for the center-of-mass motion of electrons are derived for hot-electron transport under a strong dc electric field. The frictional force is found to be related to the net rate of phonon emission, which takes away the momentum of a phonon from an electron during each phonon-emission event. The net rate of phonon emission is determined by the Boltzmann scattering equation, which depends on the distribution of electrons interacting with phonons. The work done by the frictional force is included into the energy-drift equation for the electron-relative scattering motion and is found to increase the thermal energy of the electrons. The importance of the hot-electron effect in the energy-drift term under a strong dc field is demonstrated in reducing the field-dependent drift velocity and mobility. The Doppler shift in the energy conservation of scattering electrons interacting with impurities and phonons is found to lead to an anisotropic distribution of electrons in the momentum space along the field direction. The importance of this anisotropic distribution is demonstrated through a comparison with the isotropic energy-balance equation, from which we find that defining a state-independent electron temperature becomes impossible. To the leading order, the energy-drift equation is linearized with a distribution function by expanding it into a Fokker-Planck-type equation, along with the expansions of both the force-balance equation and the Boltzmann scattering equation for hot phonons.

The use of a Boltzmann transport equation with a drift term is physically incorrect for optical-field frequencies. Also, the use of a simple energy-balance equation is found to lead to an inaccurate estimation of electron temperature. Therefore, we have established a Boltzmann-scattering equation for the accurate description of the relative scattering motion of electrons interacting with an incident optical field by including impurity- and phonon-assisted photon absorption as well as Coulomb scattering between two electrons. Multiple peaks on the high-energy tail of a Fermi-Dirac distribution are predicted and the effect of pair scattering is analyzed. Moreover, the effective electron temperature is calculated as a function of both the incident-field amplitude and the photon energy so that the thermodynamics of hot electrons may be investigated.

The dynamics of a many-electron system under both dc and infrared fields is separated into a center-of-mass and a relative motion. The first-order force-balance equation is employed for the slow center-of-mass motion of electrons, and the Fokker-Planck equation is used for the ultrafast relative scattering motion of degenerate electrons. This approach allows us to include the anisotropic energy-relaxation process which has been neglected in the energy-balance equation in the past. It also leads us to include the anisotropic coupling to the incident infrared field with different polarizations. Based on this model, the transport of electrons is explored under strong dc and infrared fields by going beyond the relaxation-time approximation. The anisotropic dependence of the electron distribution function on the parallel and perpendicular kinetic energies of electrons is displayed with respect to the dc field direction, and the effect of anisotropic coupling to an incident infrared field with polarizations parallel and perpendicular to the applied dc electric field is shown. The heating of electrons is more accurately described beyond the energy-balance equation with the inclusion of an anisotropic coupling to the infrared field. The drift velocity of electrons is found to increase with the amplitude of the infrared field due to a suppressed momentum-relaxation process (or frictional force) under parallel polarization but decreases with the amplitude due to an enhanced momentum-relaxation process under perpendicular polarization.

By including the effect of local fluctuations in the electron kinetic energies, the kinetic Fokker-Planck-type equation for excited conduction electrons in bulk semiconductors (such as GaAs, Si, etc.) is systematically derived in the presence of a pulsed laser beyond the classical limit. A new contribution from an antidiffusion process is found as a correction to the spontaneous-phonon emission from drifting electrons in addition to contributions from joule heating and a field-dependent diffusion of electrons. The stimulated interband optical transitions of electrons from single-photon absorption are included as one of the source terms of the equation. Some possible types of damage in semiconductors including optical, electrical, and structural damage are explored. The calculated results demonstrate the existence of a kinklike feature in the electron distribution function around the edge of the conduction band due to antidiffusion. The energy spectra of the electron distribution function are studied at different times and used to analyze the transient behavior of both the conduction electron density and the hot-electron temperature.