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Annual Meeting Optical Society of America

Direct measurement of dispersive nonlinearities in GaAs

Y. H. Lee, B. K. Rhee, Arturo Chavez-Pirson, Hyatt M. Gibbs, Arthur C. Gossard, W. Wiegmann

Published Jan 1, 1986• 4 citations
Publisher

Abstract

Nonlinear refractive-index changes in GaAs are obtained at room temperature by measuring shifts of Fabry-Perot (F.P.) transmission. The 299-Å multiple-quantum-well GaAs sample was sandwiched between two dielectric mirrors (90% reflectivity) to form a F.P. etalon (finesse =10). The pump beam was tuned to 821 nm and modulated to obtain 1-μs pulses at 10 kHz. The photoluminescence from a focused pump spot at the GaAs etalon was used as a broadband probe source. Since the photoluminescence is due to carriers generated by a pump beam, the pump and probe beams automatically align themselves. The fast electronic nonlinearity exhibited blue shifts (negative index changes) of the F.P. transmission peaks, while the slow thermal nonlinearity showed red shifts (positive index changes). Nonlinear index changes were calculated from the measured shifts of the F.P. peaks for different pump intensities. A self-consistent F.P. phase function for this calculation was obtained assuming lengths of 2.9 and 1.5 μm for the GaAs and air gap, respectively. Optical bistability was most easily observed for the region of large index change (873-878 nm). The slow nonlinear index changes below the band gap showed nonresonant characteristics rather than resonant, saturable behavior.

DOI
10.1364/oam.1986.mt6

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jun 27, 1988•0 citations
Nonlinear refractive-index changes in GaAs are obtained at room temperature by measuring shifts of Fabry-Perot (F.P.) transmission. The 299-Å multiple-quantum-well GaAs sample was sandwiched between two dielectric mirrors (90% reflectivity) to form a F.P. etalon (finesse =10). The pump beam was tuned to 821 nm and modulated to obtain 1-μs pulses at 10 kHz. The photoluminescence from a focused pump spot at the GaAs etalon was used as a broadband probe source. Since the photoluminescence is due to carriers generated by a pump beam, the pump and probe beams automatically align themselves. The fast electronic nonlinearity exhibited blue shifts (negative index changes) of the F.P. transmission peaks, while the slow thermal nonlinearity showed red shifts (positive index changes). Nonlinear index changes were calculated from the measured shifts of the F.P. peaks for different pump intensities. A self-consistent F.P. phase function for this calculation was obtained assuming lengths of 2.9 and 1.5 μm for the GaAs and air gap, respectively. Optical bistability was most easily observed for the region of large index change (873-878 nm). The slow nonlinear index changes below the band gap showed nonresonant characteristics rather than resonant, saturable behavior.
A bistable etalon, consisting of an optically nonlinear semiconductor spacer layer sandwiched between two Bragg mirrors, has been optimized for use in reflection, using a numerical model for a self-consistent carrier density based on published experimental data for the nonlinear absorption alpha(I,lambda) and refraction DELTA-n(I,lambda). The saturation behavior of the refractive index and the absorption in bulk GaAs has been investigated at different wavelengths. High saturation values of DELTA-n(s) almost-equal-to .080 were found 20 meV below the gap. 30 meV above the gap we observed a comparable index change, together with an absorption saturation feature, leading to a highly nonlinear response close to the bistability threshold in our unoptimized etalon.
This dissertation studies the physics of room-temperature optical nonlinearities in GaAs and their application to the optical logic gates. The microscopic origins of the room-temperature optical nonlinearities in GaAs are investigated experimentally and theoretically. The data of nonlinear absorption measurement are analyzed in the framework of a semiconductor plasma theory in combination with excitation-dependent line broadening. The importance of the plasma screening of the continuum-state Coulomb enhancement and band filling are emphasized for GaAs at room temperature. Optical bistability and optical logic gating are direct consequences of the nonlinear refractive index changes in etalons. The nonlinear index changes are directly measured by a new technique of observing the Fabry-Perot transmission peak shift using the self-photoluminescence as a broad-band source. The validity of a Kramers-Kronig technique under quasi-steady state conditions is crosschecked by an independent measurement of Δn under identical pumping conditions. Thermal index changes are also directly measured to establish the criteria on the temperature stability condition that is needed for reliable operation of devices based on dispersive nonlinearities. Optical logic gates based on dispersive optical nonlinearities may be the critical components of an all-optical computer in the future. Five optical logic functions are demonstrated in a nonlinear GaAs/AlGaAs MQW etalon. Specially designed dielectric mirrors are used to observe low-energy (3-pJ) operation of optical logic gates. Parallel operation using as many as eight optical logic devices is achieved with Wollaston prisms. Toward practical devices, optical logic gating using diode lasers is demonstrated in a setup much smaller than the usual argon-laser pumped dye laser setup. The cycle time of optical logic devices is limited, not by the switch-on time, but by the switch-off time which depends on the carrier relaxation rather than the switch-on time. To reduce the carrier relaxation time windowless GaAs is employed to take advantage of the faster surface recombination of carriers at the GaAs/dielectric mirror interface compared to that at the GaAs/GaAlAs interface. The speed and effectiveness of the windowless GaAs are compared with those of the proton-bombarded GaAs as optical logic gates.
We report on an experimental study of the origin of the nonlinearities and the size of the nonlinear index at various wavelengths close to the exciton resonance and the material band gap. It is concluded that for intensities required for optical bistability, the nonlinearity is mainly excitonic for MQWs with well sizes W of 50 and 152 Å which are either smaller or slightly larger than the exciton radius a. For 299-Å MQW, (W ≃ 2a), where the MQW behaves more like bulk but still has distinct excitonic features, the nonlinearity is due to the combination of exciton screening, band filling, and screening of the continuum states. For bulk GaAs, the effect of exciton is comparatively small and the latter two effects are responsible almost equally for the change in the refractive index. The measured nonlinear indices are rather similar for all these samples with the bulk GaAs and 299-Å MWQ exhibiting slightly larger Δn (change in index) for similar carrier densities (or for similar light intensities). The 50-Å MQW has a slightly smaller value of nonlinear index because the nonlinearity which in this sample is due to the saturation of the exciton resonance occurs at higher excitation intensities due to larger exciton binding energy.
Optical bistability has been observed in a semiconductor for the first time. The bistable etalon consists of a GaAlAs-GaAs-GaAlAs molecular-beam-epitaxially-grown sandwich with 90% reflectivity coatings. The bistability is primarily dispersive with the nonlinear refractive index arising from light-induced changes in exciton absorption. Using light of frequency just below the exciton peak, we observed bistability from 5 to 120 °K with 40-ns turn-off and subnanosecond turn-on times with 1 mW/μm2 holding intensity.
Optical bistability can be obtained in nonlinear materials either with Fabry-Perot mirrors or without external feedback. When a material is placed between partially reflecting mirrors, a nonlinear Fabry-Perot etalon is formed with the etalon providing an external feedback. Optical bistability and gating occur as a result of a shift in the Fabry-Perot peak when the light intensity is increased (or when a second laser beam is introduced into the cavity). Optical bistability with the external feedback mechanism has been observed in many semiconductors such as bulk GaAs, GaAs-AIGaAs multiple-quantum-well (MQW) structures, InSb, InAs, CdS, CdHgTe, GaSe, and CuCI.
The nonlinear saturation of the excitonic absorbtion in a GaAs-InxGa 1-xAs multiple quantum well sample consisting of 80 wells was investigated in the vicinity of the excitonic resonance at 991 nm using a tuneable titanium sapphire laser probe. This characterization yields a saturation intensity s of less than 50 kW/cm and a maximum off-resonance nonlinear refractive index of 6.42 x 10-7 cm2/W. It is shown that this dispersion can lead to practical optically bistable etalons fabricated from these multiple quantum well structures.