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Applied Physics Letters

Two-color midinfrared saturation spectroscopy of intersubband transitions in multiquantum wells

Konstantin L. Vodopyanov, V. Chazapis, C. C. Phillips

Published Nov 25, 1996• 13 citations
Publisher

Abstract

A two-color pump–probe study of bound-to-bound and bound-to-quasibound intersubband transitions in InGaAs/AlGaAs and GaAs/AlGaAs multiquantum wells (MQW), with transition energies near 250 meV has been performed. Electrons were optically excited from the lowest to the upper subband using short midinfrared pulses of intensities up to 108 W/cm2 from a dual optical parametric generator. Differential transmission spectra, taken with the weaker probe pulses, evidence homogeneous line broadening behavior. From the width of the coherence peak, centered near the pump frequency, and arising from a dynamic population grating, the upper state lifetime (bound-to-bound transition) was found to be 0.6 ps, in good quantitative agreement with absorption saturation data.

DOI
10.1063/1.117274

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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.

Journal of Applied Physics•Oct 1, 2012•8 citations
A two-color pump–probe study of bound-to-bound and bound-to-quasibound intersubband transitions in InGaAs/AlGaAs and GaAs/AlGaAs multiquantum wells (MQW), with transition energies near 250 meV has been performed. Electrons were optically excited from the lowest to the upper subband using short midinfrared pulses of intensities up to 108 W/cm2 from a dual optical parametric generator. Differential transmission spectra, taken with the weaker probe pulses, evidence homogeneous line broadening behavior. From the width of the coherence peak, centered near the pump frequency, and arising from a dynamic population grating, the upper state lifetime (bound-to-bound transition) was found to be 0.6 ps, in good quantitative agreement with absorption saturation data.
We have investigated intersubband absorptions between the conduction ground state and the first excited state of two AlxGa1−xAs/GaAs/AlxGa1−xAs multiple quantum well structures with x=0.3 and 85 Å well width, and with x=0.57 and 96 Å well width. Small-signal measurements show absorption peaks at 10.45 and 10.15 μm, respectively. Under an intense resonant excitation from a pulsed CO2 laser, saturation of the intersubband absorption occurred. The saturation intensity is estimated to be 340±120 kW/cm2 for the first sample and 375±120 kW/cm2 for the second. From these values, we have deduced subband decay times of the order of 10.6±3.5 ps for the first sample and 15.5±5 ps for the second.
The lowest energy two-dimensional exciton in GaAs/GaAlAs multiquantum wells (MQW) is inhomogeneously broadened at low temperature by fluctuations in the layer width. The broadening permits probing of the excitons as a function of position within the inhomogeneous line[1]. At high densities interaction between the excitons becomes important and leads to saturation and to an increase of the homogeneous linewidth[2]. Using the picosecond pump/probe technique we have measured the saturation behavior as a function of exciton energy within the inhomogeneously broadened line. Figure 1 shows the transmission spectrum T of a weak narrowband probe (dotted) and the change in transmission ΔT of the probe induced by the presence of a pump for two different pump wavelength for 102 Å thick layers of GaAs separated by 200 Å layers of AlGaAs. An increase in transmission (holeburning) at the pump wavelength is observed showing that the interaction between resonant excitons is much stronger than between nonresonant excitons. The strength of the hole decreases rapidly on passing through the center of the line towards the high energy side. This is consistent with an increase in the homogeneous linewidth of the higher energy excitons[1,3]. By delaying the probe we measured the rate of decay of the hole which was found to depend strongly on pump intensity. At low intensity the hole lifetime approaches the luminescent lifetime but becomes shorter at exciton densities greater than ~109 excitons/cm2/layer. We attribute this to exciton-exciton scattering leading to spectral diffusion. The behavior was similar in 51 Å and 102 Å layers. In addition to holeburning, Figure 1 shows an increased absorption in the wings of the line whose shape is almost independent of pump wavelength and which is similar to that observed in bulk GaAs[2].
We report the measurement of the transient intersubband absorption in a coupled quantum well semiconductor structure. Pulsed interband excitation is used to prepare a coherent population in the two lowest states of a three level system of electronic subbands. The time-dependent absorption to the upper state is probed by midinfrared time-domain spectroscopy. For certain pump–probe delay times we observe in the intersubband absorption spectra typical signatures of quantum interference: regions with reduced or enhanced absorption.
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.
Recent studies of the transient behavior of absorption saturation in low temperature Multiple Quantum Wells (MQW) samples have revealed a surprisingly fast recovery of absorption (within 10 ps at 15 K) after excitation by intense ultrashort light pulses1. This has been attributed to the effect of the large stimulated recombination rate caused by amplified spontaneous luminescence guided along the MQW structure, as confirmed by streak camera observations of the edge-emitted luminescence1, and by measurements of time-resolved luminescence spectra with subpicosecond resolution2. This effect could be used to increase the speed of optical gates based on absorption saturation or carrier-induced nonlinear refraction, but a more complete understanding of the mechanisms involved is clearly needed in order to evaluate the interest of this approach.
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.