Spin wave amplification by electrons due to interband absorption of laser radiation in magnetic semiconductors

We investigate here the possibility of generation of nonequilibrium magnons by electrons due to interband absorption of a laser field in magnetic semiconductors. It is found that under certain conditions the magnon excitation rate may become greater than the rate of relaxation and the magnon system may reach instability.     

Laser field induced interband absorption in a strained GaAs/GaAlAs double quantum well system

Effect of laser field intensity on exciton binding energies is investigated in a GaAs/ GaAlAs double quantum well system. Calculations have been carried out with the variational technique within the single band effective mass approximations using a two parametric trial wave function. The interband emission energy as a function of well width is calculated in the influence of laser field. The laser field induced photoionization cross-section for the exciton placed at the centre of the quantum well is computed as a function of normalized photon energy. The dependence of the photoionization cross-section on photon energy is carried out for the excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The intense laser field dependence of interband absorption coefficient is investigated. The results show that the exciton binding energy, interband emission energy, the photoionization cross-section and the interband absorption coefficient depend strongly on the well width and the laser field intensity. Our results are compared with the other existing literature available.     

Dynamics and damping of electromagnetic solitons in the carbon nanotube bundles

The possible existence of the electromagnetic solitons in carbon nanotubes is analyzed. Solitons appear as a result of a simultaneous change in the classical electron distribution function and the electric field produced by the nonequilibrium electrons in carbon nanotube. The effective equations are obtained for the dynamics of electromagnetic field with allowance for the interband transitions causing soliton damping. The numerical results are presented evidencing the existence of solitons in carbon nanotubes. The propagation dynamics is studied for the periodic electromagnetic waves in the bundles of carbon nanotubes. The shape of electromagnetic wave is found to change during its propagation.     

Influence of an electric field on the magnetoabsorption in a field of resonant laser radiation

The frequency dependence of the coefficient of interband magnetoabsorption of a weak electromagnetic wave propagating in a constant electric field and in a field of resonant laser radiation at a frequency equal to the cyclotron frequency (infrared magnetic resonance) is calculated. The specific features observed in interband absorption of the electromagnetic wave in a uniform electric field are considered for the case in which the frequency of laser radiation is equal to the confinement frequency in a parabolic quantum well (infrared quantum-well resonance).     

Resonant laser excitation and electron-hole kinetics of a semiconductor: II. Nonlinear particle transport and field propagation

Starting from the initial equations derived in a previous paper for an electron-hole plasma in a semiconductor resonantly excited in the interband absorption region by a monochromatic laser, the collision integrals of the Boltzmann equations for the quasiparticles as well as the nonlinearly defined optical functions are discussed in detail and related to each other. Explicit results are presented for the limiting case of linear absorption as well as for a stationary and homogeneous situation and compared with previous results in the literature. Especially for the latter case it is shown that it cannot provide a consistent understanding of the underlying physics and that consideration of inhomogeneous behaviour is essential.     

Flux-limited nonequilibrium electron energy transport in warm dense gold

An abrupt change in energy transport has been observed in femtosecond laser heated gold when the absorbed laser flux exceeds ~7×10(12) W/cm(2). Below this value, the absorbed flux is carried by ballistic motion of nonthermal electrons produced in interband excitation. Above this value energy transport appears to include ballistic transport by nonthermal electrons and heat diffusion by thermalized hot electrons. The ballistic component is limited to a flux of ~7×10(12) W/cm(2). This offers a unique benchmark for comparison with theory on nonequilibrium electron transport.     

Interband absorption in square and semiparabolic near-surface quantum wells under intense laser field

The exciton effects on the interband absorption spectra in near-surface square and semiparabolic quantum wells under intense laser field are studied taking into account the correct dressing effect for the confinement potential and electrostatic self-energy due to the repulsive interaction between carriers and their image charges. We found that for near-surface quantum wells with different shapes the laser field induces significant effects on their electronic and optical properties. The numerical results for the InGaAs/GaAs system show that the red-shift of the absorption peak induced by the increasing cap layer can be effectively compensated using the blue-shift caused by the enhanced laser parameter. In square quantum well without laser field our theoretical values for the absorption peak position are in good agreement with the available experimental data. As a key result, we conclude that the optical properties in near-surface quantum wells can be tuned by tailoring the heterostructure parameters: well shape, capped layer thickness and/or dielectric mismatch as well as the external field radiation strength.     

Prebreakdown generation of nonequilibrium electron-hole pairs: The multiphoton avalanche effect

A new mechanism of prebreakdown generation of electron-hole pairs is considered. It includes a cascade of interband multiphoton transitions as well as Auger-type processes involving two or three photons. A combination of these processes leads to the multiphoton avalanche effect. The threshold pumping radiation intensities required for triggering the avalanche mechanism lie in the range of 10¹¹–10¹²W/cm². The band population balance equations describing the kinetics of pair production are obtained and solved numerically. It is shown that the proposed mechanism of production of nonequilibrium electron-hole pairs is more effective than “conventional” multiphoton absorption for intensities exceeding the threshold values.     

On the collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field

The collisional transfer of nonequilibrium in the velocity distribution of resonant particles in a laser radiation field is investigated theoretically. It is shown numerically that the transfer effect is weak. This makes it possible to use simpler approximate one-dimensional quantum kinetic equations instead of three-dimensional equations to solve spectroscopy and light-induced gas kinetics problems, where it is important to take account of the velocity dependence of the collision frequency. It is shown for anomalous light-induced drift, calculations of which are most sensitive to neglecting the transfer effect, that in a wide range of spectroscopy and light-induced gas kinetics problems the transfer of nonequilibrium can be neglected without risking the loss of important fine details of the phenomena being described.     

Intense laser field effect on the interband absorption in differently shaped near-surface quantum wells

The 1S-exciton properties and interband absorption spectra in differently shaped near-surface quantum wells (NSQWs) with symmetrical/asymmetrical barriers, under intense laser field, are investigated taking into account the correct dressing effect for the confinement potential and electrostatic interaction between carriers and their image-charges. We found that: i) the 1S-exciton binding energy is significantly reduced by the laser intensity in InGaAs NSQWs of different asymmetrical shape; ii) the red-shift of the absorption peak induced by the asymmetry diminution or by increasing cap layer thickness can be effectively compensated using the blue-shift caused by enhancing laser parameter. Therefore, the optical properties of the differently shaped NSQWs could be tuned by proper tailoring of the heterostructure parameters (well shape, barrier asymmetry) and/or dielectric mismatch as well as by varying the laser field intensity.     

THEORY OF INTERBAND COHERENCE AND INVERSIONLESS BISTABILITY IN SEMICONDUCTOR LASERS

Coherent transitions of charge carriers between the conduction and valence bands of a semiconductor medium are essential for the operation of a semiconductor laser. In this paper, we study how such interband coherence can be set up by an injection current and a coherent pump-field. In the absence of the pump-field, the injection current is the only source to establish the interband coherence in a semiconductor laser system. A laser threshold is obtained, which shows that a strongly coupled high-Q microcavity has a low threshold value. However, when an external pump-field serving as another mechanism to create the interband coherence is applied, the threshold value of the injection current can be lowered and it vanishes for sufficiently strong field. Besides, if the pump-field exceeds a threshold value, it is even possible to achieve a bistability in the inversionless region. Some fundamental macroscopic properties, including polarization, absorption and dispersion for the semicoductor system, are also obtained analytically.     

The influence of resonance IR laser radiation on magnetoabsorption in quantum wires

The coefficient of interband absorption of a weak electromagnetic wave by quantum wires in a transverse magnetic field and an intense laser radiation field is calculated. It is shown that, if the laser radiation frequency is equal either to the size quantization frequency (dimensional infrared resonance) or to a hybrid frequency (magnetoinfrared resonance), laser illumination can determine the shape of absorption oscillations. In particular, it is shown that the second magnetoabsorption peak is split into two peaks, the half-widths of which and the distance between which depend on the intensity of resonance laser radiation. The influence of the polarization of IR radiation on the interband absorption in quantum wires is discussed. The dynamics of the frequency dependence of the optical absorption coefficient with increasing intensity of resonance laser radiation is studied.     

Interband phase modes and nonequilibrium soliton structures in two-gap superconductors

We predict a new dynamic state in current-carrying superconductors with a multicomponent order parameter. If the current density J exceeds a critical value J(t), an interband breakdown caused by charge imbalance of nonequilibrium quasiparticles occurs. For J>J(t), the electric field penetrating from current leads gives rise to various static and dynamic soliton phase textures, and voltage oscillations similar to the nonstationary Josephson effect. We propose experiments to observe these effects which would probe the multicomponent nature of the superconducting order parameter.     

Coherent photogalvanic valley Hall effect

The theory of the coherent photogalvanic valley Hall effect in two-dimensional systems with the Dirac spectrum of charge carriers is formulated. The study deals with a two-dimensional sample irradiated by two electromagnetic waves, at the fundamental and doubled frequencies. Both frequencies exceed the band gap of the material, whereas the wave with the fundamental frequency having circular polarization and a high intensity is taken into account in a nonperturbative manner. The wave at the doubled frequency is linearly polarized and the electrical conductivity of the two-dimensional system is calculated with respect to it. The effect under study manifests itself as the dc Hall current in the direction orthogonal to the electric field of the weak electromagnetic wave. It is assumed that, in equilibrium, the sample is in the insulating state with the completely occupied valence band and empty conduction band. The strong electromagnetic wave induces a nonequilibrium filling of the bands and the system passes to a strongly nonequilibrium steady state. The behavior of the Hall current in the case of nonequilibrium distribution functions is analyzed both including and disregarding the intraband relaxation and interband recombination.     

Dielectric enhancement of the exciton energies in laser-dressed near-surface quantum wells

A theoretical study of the intense high-frequency laser field effect on the interband transitions and on the ground (1S-like) and excited (2S-like) exciton states in InGaAs/GaAs near-surface quantum wells is performed within the effective mass approximation. The carrier confinement potentials and image charge contributions to the Coulomb interaction can significantly be modified and controlled by the capped layer thickness and laser field intensity. We found that: (i) the interband and exciton transition energies monotonically enhance with the laser amplitude; (ii) for small capped layers the splitting between the 2S and 1S exciton lines are more sensitive to the dressing laser parameter, and (iii) for high enough laser intensities the dressing effects on both confining potential and Coulomb interactions can yield entirely different exciton emission spectra depending on the cap layer thickness. Our results are compared with the theoretical and experimental data obtained in the absence of the laser field and a good agreement is reached.