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Conference on Lasers and Electro-Optics

Operation of a GaAs-AIAs optical logic gate fabricated completely by molecular beam epitaxy

S. L. McCall, Arthur C. Gossard, John H. English, Jack L. Jewell, J. F. Duffy

Published Jan 1, 1986• 4 citations
Publisher

Abstract

A nonlinear Fabry-Perot etalon fabricated entirely by molecular beam epitaxy (MBE) when operated as a pump-probe optical logic gate1 has shown 5:1 contrast with <20 pJ incident. The device should be especially well suited for optical bistability.2 However, this mode of operation has not yet been tried. The design consists of ten pairs of quarterwave thickness AlAs and GaAs layers grown on a GaAs substrate followed by a ~2-μm GaAs spacer layer and seven more quarterwave pairs. The mirrors formed by the quarter-wave stacks both have ~90% reflectivity. Fewer layers are needed on the second grown mirror due to the higher intrinsic reflectivity at the GaAs-air interface compared with that on the substrate side. The GaAs spacer was intentionally wedged to provide tunability of the transmission peak. The substrate in its present form is unsulted for transmitting either the pump or probe beams, so we used the reflected probe signal as the output.

DOI
10.1364/cleo.1986.fk3

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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.

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Direct measurement of dispersive nonlinearities in GaAs

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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.

Annual Meeting Optical Society of America•Jan 1, 1986•4 citations

ROOM-TEMPERATURE OPTICAL NONLINEARITIES IN GALLIUM-ARSENIDE AND FAST OPTICAL LOGIC GATES.

Yong Hee. Lee

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.

•Jan 1, 1986•0 citations

Recent advances in optical testability

Nasser N. Peyghambarian

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.

Conference on Lasers and Electro-Optics•Jan 1, 1986•0 citations
A nonlinear Fabry-Perot etalon fabricated entirely by molecular beam epitaxy (MBE) when operated as a pump-probe optical logic gate1 has shown 5:1 contrast with <20 pJ incident. The device should be especially well suited for optical bistability.2 However, this mode of operation has not yet been tried. The design consists of ten pairs of quarterwave thickness AlAs and GaAs layers grown on a GaAs substrate followed by a ~2-μm GaAs spacer layer and seven more quarterwave pairs. The mirrors formed by the quarter-wave stacks both have ~90% reflectivity. Fewer layers are needed on the second grown mirror due to the higher intrinsic reflectivity at the GaAs-air interface compared with that on the substrate side. The GaAs spacer was intentionally wedged to provide tunability of the transmission peak. The substrate in its present form is unsulted for transmitting either the pump or probe beams, so we used the reflected probe signal as the output.
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.
Epitaxial methods allow the fabrication of nonlinear interferomethc structures which include the mirrors and the GaAs Multiple Quantum Wells (MQW) nonlinear medium in a single crystal. This results in very compact nonlinear microcavities with typically 5im overall thickness. Optical bistability is observed in these structures at mW optical power. We review our latest experimental results on these structures including the observation of high contrast switching in the reflection mode the direct measurement of dispersive optical nonlinearities in MQWs evidencing their saturation behavior at high intensities. Finally we report external-beam switching experiments which allow to assess the potential of these structures as all-optical gates.
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.
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.
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.