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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Observation of intrinsic optical bistability induced by self-focusing in a semiconductor-doped glass filter

F. Pereira, José R. Salcedo, Michael Scott Belsley

Published Jan 1, 1994• 0 citations

Abstract

Wherret et al.1 have derived a general condition for the existence of intrinsic optical bistability, stipulating that the absorption length of the sample, αL, must be less than 0.18. Nonetheless several research groups have observed optical bistability in samples with absorption lengths significantly above this limit.2 It has been suggested2 that the observed bistability is the result of an initial self-focusing of the incident beam brought about by thermal refractive index change which then induces a bistable effect in the final portion of the sample. If the focal length of the laser induced thermal lens is comparable with the sample thickness, then the reduction of the laser beam size could then induce bistability at a lower threshold in the final portion of the sample. We have carried a systematic test of this hypothesis, using a modified version of the Z-scan technique, on a commercially available semiconductor-doped glass filter.

DOI: 10.1364/cleo_europe.1994.cthi79

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Wherret et al.1 have derived a general condition for the existence of intrinsic optical bistability, stipulating that the absorption length of the sample, αL, must be less than 0.18. Nonetheless several research groups have observed optical bistability in samples with absorption lengths significantly above this limit.2 It has been suggested2 that the observed bistability is the result of an initial self-focusing of the incident beam brought about by thermal refractive index change which then induces a bistable effect in the final portion of the sample. If the focal length of the laser induced thermal lens is comparable with the sample thickness, then the reduction of the laser beam size could then induce bistability at a lower threshold in the final portion of the sample. We have carried a systematic test of this hypothesis, using a modified version of the Z-scan technique, on a commercially available semiconductor-doped glass filter.

We have characterized the transverse spatial dependence of the real and imaginary parts of the complex nonlinear refractive index of a semiconductor doped glass filter, which exhibits absorptive bistability. Using the Z-scan technique, combined with an interferometric measurement of the integrated optical thickness, we are able to fit the observed experimental data assuming a quadratically varying transverse temperature profile in the sample. The transverse variations in the nonlinear refractive index do not scale directly with the size of the incident beam, but exhibit marked asymmetries depending on whether the incident beam is converging or diverging.

We have characterized the transverse spatial dependence of the real and the imaginary parts of the complex nonlinear refractive index of a semiconductor-doped glass filter that exhibits absorptive bistability. Using the Z-scan technique combined with an interferometric measurement of the integrated optical thickness, we are able to fit the observed experimental data, assuming a quadratically varying transverse temperature profile in the sample. The transverse variations in the nonlinear refractive index do not scale directly with the size of the incident beam but exhibit marked asymmetries that depend on whether the incident beam is converging or diverging.

We present a simple experiment which demonstrates the main features of intrinsic optical bistability obtained through thermally induced shifts in the band edge of semiconductor particles embedded in a glass matrix. This readily demonstrates how nonlinear optical effects can be used to transmit information, providing an instructive introduction to the main concepts of optical computing. Several possible extensions of the experiment suitable for advanced independent research projects are discussed.

Koch et al.1 have solved for the laser light intensity and the excitation density as a function of time and space (in the beam direction) in a semiconductor doped glass that exhibits increasing absorption2 optical bistability under pulsed operation. A kink in the excitation density that propagates as a discontinuity in the beam direction and manifests itself as a sawtooth variation in the output intensity is predicted. Gibbs et al.3 reported the first experimental observation of kinks in a sharp cut color filter. Kinks that are an emphatic illustration of the local nature of increasing absorption optical bistability are presented. Furthermore, kinks demonstrate an intrinsic dissimilarity between increasing absorption optical bistability and bistability dependent upon external feedback, where a longitudinal discontinuity cannot occur. If the input is an increasing ramp, then the front part of the sample reaches the condition, carrier density, and temperature, for switching down before the remainder, whose light intensity is reduced by absorption in the front. The discontinuity in the excitation density develops as a consequence of competition between transverse thermal conduction and heating of the sample by increasing absorption in the shielded back part of the sample. The number of discontinuities is limited by the pulse rise time, sample length, and conduction time, which scale as the square of the beam radius.