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Nonlinearities in the absorption and photoluminescence spectra of GaAs/AlAs type II heterostructures

Greg R. Olbright, W. S. Fu, J. F. Klem, Adelbert Owyoung, George Ronald Hadley, Rolf H. Binder, S. W. Koch, Ian Galbraith

Published May 21, 1990• 0 citations
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Abstract

We present here the first observation of quasisteady state optical nonlinearities in the absorption and photoluminescence (PL) spectra of GaAs/AlAs type II heterostructures. Using pump-probe spectroscopy we observed deep modulation, ΔT ~ 0.5 (Δα =−50,000 cm−1), due to a >40-meV blue shift and bleaching of the heavy hole exciton absorption peak. In addition, a blue shift of up to ~10 meV of the indirect PL line is observed with increasing PL excitation intensity. These nonlinearities are understood in terms of spatially separated layers of electrons and holes that are produced by the optical excitation process.

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Applied Physics Letters•Aug 18, 1997•1 citation
We present here the first observation of quasisteady state optical nonlinearities in the absorption and photoluminescence (PL) spectra of GaAs/AlAs type II heterostructures. Using pump-probe spectroscopy we observed deep modulation, ΔT ~ 0.5 (Δα =−50,000 cm−1), due to a >40-meV blue shift and bleaching of the heavy hole exciton absorption peak. In addition, a blue shift of up to ~10 meV of the indirect PL line is observed with increasing PL excitation intensity. These nonlinearities are understood in terms of spatially separated layers of electrons and holes that are produced by the optical excitation process.
Abstract : In this paper we describe a detailed study of the complete time evolution of the optical nonlinearities of type-11 GaAs/AlAs quantum wells using nanosecond and femtosecond pump/probe spectroscopy. We explain these nonlinearities by solving the generalized Wannier equation for the case of a type-II quantum-well structure. In contrast to the usual type-I quantum-wells, we observe different shifts and saturations of light- and heavy-hole excitons as well as the absence of gain. The dominant effects are a blue shift and saturation of the heavy-hole (HH) exciton.
The bleaching of the n=1 and n=2 heavy-hole exciton absorption has been observed at room temperature in ternary alloy Al0.37Ga0.63As/AlAs multiple-quantum-well structures by nanoseconds pump-and-probe spectroscopy. The relative variation of the absorption coefficient [(−Δα)/α0] are about 15% and 8%, respectively. The nonlinear absorption coefficient (α2) and the nonlinear absorption cross section has been deduced. The value of the saturation density deduced from our spectra is in good agreement with the theoretical predictions. The influence of the staggered band alignment on the observed process is also discussed.
This dissertation presents investigations of fundamental optical nonlinearities in semi-conductor microstructures. Two distinct effects are studied. In the first part the excitonic optical Stark effect in InGaAs and GaAs multiple quantum-well structures is investigated by means of pump-probe spectroscopy. For nonresonant excitation below the excitonic transition the direction of the shift of the resonance depends on the polarization of the pump and probe pulses. In particular, for anti-circular polarization a surprising red-shift is observed. For resonant excitation, induced absorption energetically above and below the exciton transition and bleaching of the resonance is found. Experiments using both resonant and nonresonant excitation reveal the importance of bound and unbound two-exciton states in absorption changes of the 1s heavy-hole exciton resonance. It is found that higher-order Coulomb contributions determine the intensity as well as the time dependence of the differential excitonic absorption. In addition, the influence of light-hole excitons is analyzed. It is shown that the direction of the optical Stark shift for nonresonant excitation depends also sensitively on the heavy-hole to light-hole splitting and the detuning of the pump pulse. For very high pump intensities and nonresonant excitation the absorption is split when a circularly polarized pump and a linearly polarized probe beam are used. For co-circular excitation traces of hyper-Raman gain are observed. In the second part of this dissertation, the nonlinear optical response of semiconductor microcavities in the nonperturbative regime is studied in resonant single-beam transmission and pump-probe experiments. In both types of experiment, a pronounced third transmission peak lying spectrally between the two normal modes is observed. Its dependence on the probe intensity, pump intensity, pump-probe delay, exciton-cavity detuning and pump detuning is investigated. For single-beam transmission, the energy of the third peak parallels the position of the cavity resonance. It is more pronounced for circularly polarized excitation and lasts longer than the two normal modes. For pump-probe experiments, the third peak increases with decreasing probe intensity and increasing pump intensity. Its energy is close to the low-energy side of the pump spectrum and virtually unaffected by the cavity-exciton detuning. The appearance of the third peak requires temporal overlap of pump and probe pulses. The origin of this complex nonlinearity is the quantum nature of light, which induces intraband polarizations in the presence of a coherent driving field and a finite carrier density. It is found that the coupling of the intraband polarizations via guided modes to the polarization of the fundamental longitudinal mode is responsible for the third transmission peak. A fully quantized theory reproduces the experimental observations.
Optical nonlinearities of Al(x)Ga(1-x)As/GaAs asymmetric coupled quantum wells within a p-i-n structure have been observed by using time-resolved spectroscopy. Under photoexcitation, exciton absorption lines exhibit spectral shifts of as much as approximately 1 meV. The recovery times of these spectral shifts are of the order of hundreds of picoseconds when the excitation photon is above the lowest exciton state but become less than 10 psec when excitation is below the lowest exciton state, indicating a virtual process. The behavior of these spectral shifts is consistent with the presence of a polarization induced by optical pumping. The polarization is believed to be due to the excitons that have nonvanishing electric dipole moments along the axis of the asymmetric coupled quantum wells.
Abstract The dynamics of optical nonlinearities connected with the blue‐shift of the exciton line in resonantly excited GaAs–GaAlAs MQWS's is investigated theoretically simulating a two‐beam experiment. Jumps in the optical spectra of a weak test pulse (optical switching), which follows the intense pump pulse at different time delays, are observed, basing on a strong increase of the absorption. A similar mechanism of the bistability is observed in connection with the electro‐absorptive effect.