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Integrated Photonics Research

Spectral shape of absorption tail in high-purity GaAs

Kishore K. Kamath, P. R. Vaya

Published Jan 1, 1991• 0 citations
Publisher

Abstract

Recently, there has been a renewed interest in the study of absorption spectrum, particularly in the absorption tail region due to the possibility of using the semiconductor etalons for optical bistability. It has been shown that both the dispersive and absorptive optical bistability are possible in semiconductors. However, it is very difficult to obtain absorption saturation in the tail region.1 Therefore, it is very important to study the mechanism of formation, and the density of states distribution in the absorption tail region.

DOI
10.1364/ipr.1991.tud17

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Recently, there has been a renewed interest in the study of absorption spectrum, particularly in the absorption tail region due to the possibility of using the semiconductor etalons for optical bistability. It has been shown that both the dispersive and absorptive optical bistability are possible in semiconductors. However, it is very difficult to obtain absorption saturation in the tail region.1 Therefore, it is very important to study the mechanism of formation, and the density of states distribution in the absorption tail region.
Optical signal processing using all-optical circuitry is an increasingly attractive goal with the recent progress in long- lived light sources, high quantum efficiency detectors, and low- loss high-bandwidth optical fibers. Devices which permit one light beam to control another or which provide an optical memory afford a capability essential to the success of all-optical logic systems. Much progress has been made toward practical bistable1 devices since the first observation2 of optical bistability in sodium vapor. (By optical bistability we refer to devices containing no inverted medium, so that bistable lasers3 are explicitly excluded.) The most practical device so far, a semiconductor etalon consisting of 4 μm of GaAs between 0.2 μm of Al0.42Ga0.53As with R = 0.9 coatings, is described herein.
An analysis of static room-temperature-operation characteristics of a reflection-mode semiconductor multiple-quantum well etalon by a simplified two-level system formula of optical nonlinear properties and a computer simulation is presented. In the calculation for a GaAs/AlGaAs multiple-quantum-well etalon using the parameters reported in experimental measurement, it is shown that the very large excitonic optical nonlinearity is not effectively utilized for the optical bistable operation. This occurs because the effective positive-feedback mechanism which increases the etalon internal light intensity is suppressed when a hardly saturable background interband absorption tail is larger than a few thousand cm/sup -1/ at a wavelength where the large excitonic dispersive nonlinearity is observed.>
The saturation behavior of the nonlinear index δnNL in the band tail of GaAs/AlGaAs multiple quantum wells is experimentally studied in quasi-steady state and in a low absorption region (below 300 cm−1), where dispersive optical bistability is observed. Measurements at different detunings from the exciton resonance allow us to study the absorption dependence of the saturation parameters. The saturation intensity IS is found inversely proportional to the linear absorption coefficient, while the saturating nonlinear index δnS is approximately constant. This evidences that the nonlinear index saturation is due to a saturation of the carrier density versus light intensity, due to the saturation of the band tail absorption itself.
We have derived Maxwell-Bloch equations of a two-subband model suitable for studying intersubband-transition induced optical-nonlinearities of semiconductor superlattices (SLs). Analytical expressions and numerical illustrations for absorptive and dispersive optical bistabilities (OBs) of SL etalons are given. Both OBs can be realized in infrared region with high frequency selectivity, and their bistable domains and hysteresis loops are much larger than those of a two-level system. The dispersive OB is preferable due to its lower value of the input power.