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Physical review. B, Condensed matter

Influence of the depolarization effect on the nonlinear intersubband absorption spectra of quantum wells

M. Zal użny

Published Feb 15, 1993• 74 citations
Publisher

Abstract

The effect of the depolarization field on the nonlinear intersubband absorption spectrum of a rectangular quantum well (described by the two-subband model) is discussed. Calculations are performed in the framework of the density-matrix formulation with use of the relaxation-time approximation. Obtained results show that the depolarization effect not only shifts the peak by an amount depending on the intensity of the incident light but also leads to a significant distortion (asymmetry) of the line shape.

DOI
10.1103/physrevb.47.3995

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The effect of the depolarization field on the nonlinear intersubband absorption spectrum of a rectangular quantum well (described by the two-subband model) is discussed. Calculations are performed in the framework of the density-matrix formulation with use of the relaxation-time approximation. Obtained results show that the depolarization effect not only shifts the peak by an amount depending on the intensity of the incident light but also leads to a significant distortion (asymmetry) of the line shape.
The problem of saturation of intersubband absorption and optical rectification in asymmetric quantum wells is discussed theoretically using the density matrix formalism. The effects of the electron-electron interaction are taken into account. It is shown that near the resonance the spectral shapes of the nonlinear optical absorption coefficient and the nonlinear optical rectification coefficient are very similar. In the absence of the electron-electron interaction the spectral responses are nearly Lorentzian. The electron-electron interaction plays an important role in heavy doped systems. In structures with large spatial separation of the ground and the excited states wave functions, the depolarization effect is small and the absorption spectrum is affected mainly by the electrostatic Coulomb interaction. This interaction shifts the absorption peak to higher energy by an amount depending on the intensity of the incident light and leads to the significant distortion (asymmetry) of the line.
The effect of the depolarization field on the doubly resonant frequency mixing due to resonant intersubband transitions in asymmetric quantum wells, with only the ground subband occupied, is discussed. Calculations are performed in the framework of the perturbative density matrix formulation with relaxation time approximation. The results show that in the considered system the depolarization effect can be included by simple replacement (in the one-electron expression for the nonlinear susceptibility) of the intersubband energy by the depolarization shifted intersubband energy.
Infrared intersubband optical absorption between the quantized subbands and the Stark effects is of considerable interest because of their potential applications in high-speed IR modulators and photodetectors. Recently, we calculated the linear1 and third-order nonlinear2 intersubband optical absorptions with an applied electric field for the infinite quantum-well model with an effective well width (which gives the same ground state energy of a given finite quantum well). The observed intersubband absorption has a large dipole strength suggesting large optical nonlinearities which are in good agreement with our calculation based on the simple model. Distinct features of the intersubband absorption are as follows: (a) The absorption peak for the intersubband absorption increases in energy with increasing electric field over a wide range of the electric field because the energy difference between the two lowest subbands increases with the electric field. (b) The absorption peak for the intersubband absorption may (1) increase in magnitude with an increasing electric field if the intrasubband relaxation time τin is assumed to be constant or (2) decrease with an increasing electric field if the field broadening of the linewidth is taken into account. The nonlinear optical absorption coefficient with the electric field dependence is discussed.
In the past few years advance in nanofabrication techniques have made it possible to make well-like compound semiconductor structures of low nanometer size.In these structures,the state of the electron in the direction perpendicular to the length of the well is quantized with sufficiently large separation in energy levels and the motion of the electron parallel to the length is that of a quasi-free electron.One of the most remarkable properties of these one-dimensional electron system is that the optical transitions between the size-quantized subbands are feasible.The result shows there is a large optical nonlinearities in asymmetric quantum wells.The problem of optical saturation of intersubband absorption has recently attracted considerable attention.The problem is very important to the development of superlattice devices such as fast infrared detectors and(modulators) because one needs to know the range of input optical intensities for which linear operation is(possible.) The analytical expression of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained in Pschl-Teller wells using the density matrix formalism taking into account the(intrasubband) relaxation.Since there are two adjustable parameters κ and λ in Pschl-Teller wells,the shapes of the wells will change with the change of the parameters κ and λ,accordingly the linear and the third-order nonlinear optical absorption coefficients will change with the parameters κ and λ.It is shown that the optical absorption coefficient depends on the parameter,and the optical intensity strongly. The absorption saturation will occur with I increase.Numerical results are presented for a typical AlGaAs/GaAs Pschl-Teller well.
Abstract We study the nonlinear intersubband optical absorption of a single Si δ‐doped GaAs sheet placed in the middle of a GaAs quantum well and subjected to an electric field. The Schrödinger and Poisson equations are solved self‐consistently for various electric field strengths. The self‐consistent solutions provide us with the correct confining potential, the wave functions, the corresponding subband energies and the subband occupations. The nonlinear optical intersubband absorption spectra are discussed within the framework of the density matrix formulation for various electric field strengths, well widths, and sheet thicknesses in which Si atoms are distributed uniformly. We include both the depolarization and exciton shifts in calculations. The depolarization effect not only shifts the peak value but also makes the nonlinear absorption spectrum strongly asymmetric with the increasing intensity of the optical field. The blue‐shift associated with the depolarization shift is decreased when the intensity is increased. The absorption line shape becomes more asymmetric at smaller well widths and thicker doping layers. The electric field restores the symmetry of the absorption line shape at larger well widths and thinner doping layers, but it decreases the peak value of the line shape. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The analytic forms of the linear and the third-order nonlinear optical intersubband absorption coefficients were obtained for general asymmetric quantum well systems using the density matrix formalism.Based on the model,the linear and the third-order nonlinear intersubband optical absorptions in electric-field biased hyperbolic quantum wells were studied.The numerical results were presented for a typical AlxGa1-xAs/GaAs electric-field biased hyperbolic quantum wells.The results show that the contributors to the nonlinear absorption coefficient are due to the electric field,the shape of quantum well and the optical intensity.