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Physical review. B, Condensed matter

Adiabatic-following solutions for multisubband quantum wells excited by an ultrafast optical pump field

Leung Tsang, Chaiyuth Chansungsan, Shun Lien Chuang

Published May 15, 1992• 17 citations
Publisher

Abstract

The two-band effective semiconductor optical Bloch equations for a quantum well have been generalized to include multisubband effects. The ultrafast transient adiabatic-following solutions are obtained from the equations for interband polarizations, intersubband polarizations, and subband-carrier distribution functions of a ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As/GaAs quantum well, excited by an ultrafast optical pump field. Two conservation laws for the multisubband case are derived. The steady-state response and the \ensuremath{\pi}-pulse response are studied. It is found that these multisubband equations lead to quantitatively different results from the conventional two-band approximation, particularly for wide quantum wells and asymmetric quantum wells.

DOI
10.1103/physrevb.45.11918

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The two-band effective semiconductor optical Bloch equations for a quantum well have been generalized to include multisubband effects. The ultrafast transient adiabatic-following solutions are obtained from the equations for interband polarizations, intersubband polarizations, and subband-carrier distribution functions of a {\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}As/GaAs quantum well, excited by an ultrafast optical pump field. Two conservation laws for the multisubband case are derived. The steady-state response and the \ensuremath{\pi}-pulse response are studied. It is found that these multisubband equations lead to quantitatively different results from the conventional two-band approximation, particularly for wide quantum wells and asymmetric quantum wells.
We study theoretically the time evolution of nonlinear optical phenomena of a probe electromagnetic field pulse in intersubband transitions of a semiconductor quantum well structure under the influence of a pump electromagnetic field. More specifically, we take into consideration the effects of electron-electron interactions and consider the interaction of a two-subband system with rectangular electromagnetic fields. The spectral form of the linear absorption, dispersion, and nonlinear optical Kerr effect is described, from the time that the external field is applied up until the dynamics of the system has reached a steady state. In order to describe the dynamics of the system, the effective nonlinear Bloch equations are used, in which renormalized terms for the transition energy and the applied field have been added, due to the effects of electron-electron interactions. These equations are properly combined, and the differential equations of the density matrix elements for the several optical phenomena up to third order are derived. These are solved numerically in the whole time range up to the steady state for a GaAs/AlGaAs quantum well structure. We show that the form of the several optical spectra studied here has a significantly altered evolution over time, according to the value of the frequency of the pump field and of electron sheet density. Even in the case in which the spectral form in the steady state case is almost identical for two different sets of parameters, their evolution can be quite different, according to the precise value of these parameters.
Optical interband excitonic absorption of semiconductor quantum wells (QW’s) driven by a coherent pump field is investigated on the basis of semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW’s. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron–hole pairs in the QW structure is low, the pump field induces an Autler–Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron–hole pairs is increased, the split contrast (the ratio between the maximum and the minimum values) is decreased, because the exciton effect is suppressed at higher densities owing to the many-body screening.
We explore the transient characteristics of an interband resonant light modulation process by ultrashort intersubband resonant light pulses in semiconductor quantum wells (QW's). The modulation characteristics in a three-level semiconductor QW system, including the effects due to in-plane momentum, have been investigated by a numerical analysis of the coupled Bloch equations using a density-matrix approach. We have studied the effect of the carrier density and the nature of the intersubband coupling light on the transient absorption features of the interband light. The modulation process has been compared in doped and undoped QW's. The switching behavior of the strong interband light field in presence of a train of intersubband light pulses has also been discussed.