Population inversion in optically pumped asymmetric quantum well terahertz lasers
Intersubband carrier lifetimes and population ratios are calculated for three- and four-level optically pumped terahertz laser structures. Laser operation is based on intersubband transitions between the conduction band states of asymmetric GaAs-Ga1−xAlxAs quantum wells. It is shown that the carrier lifetimes in three-level systems fulfill the necessary conditions for stimulated emission only at temperatures below 200 K. The addition of a fourth level, however, enables fast depopulation of the lower laser level by resonant longitudinal optical phonon emission and thus offers potential for room temperature laser operation.
Effect of electron-electron scattering on nonequilibrium transport in quantum-well systems.
We model nonequilibrium transport in a GaAs-Al x Ga 1−x As quantum-well structure using an ensemble Monte Carlo simulation of the full multisubband system in which we include electron-electron (e-e) scattering explicitly into the calculation. The e-e scattering cross section is calculated using the Born approximation and introduced into the transient Monte Carlo simulation via a self-scattering technique. This interaction is found to be especially effective in transferring energy between different subbands, thus thermalizing the carriers within a picosecond. The model described herein is applied to the study of laser-excited carriers in a quantum-well system and to the response of such a system to high parallel electric fields. In the case of laser excitation, e-e interaction may dominate the initial evolution, reducing the cascade of carriers via optical-phonon emission.
Dispersion and band-gap scaling of the electronic Kerr effect in solids associated with two-photon absorption.
Measurements of the nonlinear refractive index of several semiconductors using beam-distortion methods and four-wave mixing show a strong systematic dispersion in the bound-electronic nonlinearity (electronic Kerr effect ${\mathit{n}}_{2}$) near the two-photon-absorption edge. This eventually turns from positive to negative at higher frequencies. We find that by using the two-photon-absorption spectrum as predicted by a two-parabolic-band model, we can predict the observed universal dispersion, scaling, and values of ${\mathit{n}}_{2}$ that range over 4 orders of magnitude and change sign, using a simple Kramers-Kronig analysis (i.e., relating the real and imaginary parts of the third-order susceptibility). The resulting scaling rule correctly predicts the value of ${\mathit{n}}_{2}$ for all the 26 different materials we have examined. This includes wide-gap dielectrics which have 3 to 4 orders of magnitude smaller values of ${\mathit{n}}_{2}$ than semiconductors.