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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

CW Degenerate Four Wave Mixing In Bulk GaAs And Gaas/AlGaAs

Frank G. Patterson, J Brock, Maria Zales Caponi

Published Apr 8, 1988• 0 citations
Publisher

Abstract

Room temperature bulk GaAs and GaAs/A1GaAs multiple quantum well (MQW) samples are studied using backward degenerate four wave mixing (DFWM) at moderately high pump intensities (4 kW/cm2). The DFWM reflectivity is measured in the band gap region as a function of laser wavelength and pump intensity. When the MQW is pumped at intensities sufficient to saturate the excitonic resonance, a dispersive DFWM spectrum consistent with conduction band contributions is observed. Preliminary experiments in bulk GaAs indicate that the phase conjugate signal strength is the same as that for the MQW at approximately the same pump intensity.

DOI
10.1117/12.943815

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Room temperature bulk GaAs and GaAs/A1GaAs multiple quantum well (MQW) samples are studied using backward degenerate four wave mixing (DFWM) at moderately high pump intensities (4 kW/cm2). The DFWM reflectivity is measured in the band gap region as a function of laser wavelength and pump intensity. When the MQW is pumped at intensities sufficient to saturate the excitonic resonance, a dispersive DFWM spectrum consistent with conduction band contributions is observed. Preliminary experiments in bulk GaAs indicate that the phase conjugate signal strength is the same as that for the MQW at approximately the same pump intensity.
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Detailed measurements have been made of degenerate four-wave mixing (DFWM) in a resonant homogeneously broadened gas. The measurements were performed in SF6{\mathrm{F}}_{6}F6​ using a pulsed CO2{\mathrm{O}}_{2}O2​ laser operated on the 10.4-\ensuremath{\mu} m branch. A pulse width of 1.2 nsec was used to reduce effects of collisional relaxation. Data are also presented for pulse widths of 140 nsec. At high-energy fluences (\ensuremath{\sim}0.1 J/cm2{\mathrm{cm}}^{2}cm2) pulse breakup is observed and is believed to arise from multiple-photon effects. The data obtained with the short pulse width are compared to a four-level saturable-absorber model where excellent agreement is obtained when pump absorption is included. The qualitative behavior of DFWM due to dispersion is also demonstrated in the wings of the SF6{\mathrm{F}}_{6}F6​ absorption spectrum. Peak reflectivities of 38% were readily obtained. In addition contributions due to a coherent three-level nonlinearity are presented showing a contribution equal to the measured saturable contribution.
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Semiconductor multiple quantum well structures exhibit very large, room temperature, nonlinear refractive effects associated with resolvable excitonic features [1]. These nonlinearities may find use in all-optical devices if sufficient refractive index change can be achieved before the exciton completely saturates [2]. A unique aspect of band gap resonant optical, nonlinearities in quantum wells is the spatial confinement of the generated carriers within the layers. We have exploited the excitonic refractive nonlinearity in GaAs/AlGaAs multiple quantum wells in two separate degenerate four wave mixing configurations. These compare the intra-well and cross-well contributions to highly anisotropic carrier diffusion and also allow measurement of carrier relaxation rates and the saturation of the excitonic nonlinear refraction with increasing intensity.
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.