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Conference on Lasers and Electro-Optics

Transverse effects in optical bistability: computations anti observations

Jerome V. Moloney, Hyatt M. Gibbs, Jack L. Jewell, Kuochou Tai, E. A. Watson, Arthur C. Gossard, W. Wiegmann

Published Jan 1, 1982• 1 citation
Publisher

Abstract

The spatial profile of the output intensity of a bistable optical GaAs etalon1 has been studied by imaging the output plane of the etalon onto a 15-μm diam aperture with a magnification of 7X. The aperture was translated across the image plane, and the transmission was monitored with an 8852 photomultiplier tube having ≃ 5-nsec resolution. In the low-intensity linear regime the HWHM radius a at the sample was determined to be ≃5 μm, corresponding to a Fresnel number F = na2/λL of 24 for the etalon of length L = 4.5 μm and refractive index n = 3.5.

DOI
10.1364/cleo.1982.fl4

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The spatial profile of the output intensity of a bistable optical GaAs etalon1 has been studied by imaging the output plane of the etalon onto a 15-μm diam aperture with a magnification of 7X. The aperture was translated across the image plane, and the transmission was monitored with an 8852 photomultiplier tube having ≃ 5-nsec resolution. In the low-intensity linear regime the HWHM radius a at the sample was determined to be ≃5 μm, corresponding to a Fresnel number F = na2/λL of 24 for the etalon of length L = 4.5 μm and refractive index n = 3.5.
Self-focusing effects in optical bistable hysteresis loops have been observed in a GaAs-GaAlAs superlattice.1 Experimentally, one sees drastic overshooting in the hysteresis loops and changes in the output profile as the imaging lens is translated along the propagation direction, thus imaging different object planes onto a fixed image plane (i.e. aperture). Nearly identical results are found by a one-transverse-dimension numerical simulation of good-cavity bistability in which the output is free-space propagated backwards in space, corresponding to the movement of the imaging lens. These effects arise from a radially-dependent phase shift caused by the excitonic nonlinear refraction and a gaussian beam profile.
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.
We demonstrate the simultaneous, independent, and nearly identical operation on nanosecond time scales of GaAs optical bistable devices. A Wollaston prism inserted into the input beam produced two equal-intensity beams which were focused to <10-μm diam spots centered ~40 μm apart. Achievement of equal switch-on intensities for the two spots was straightforward and easy. Elimination of either beam by a polarizer revealed no visible change in the response to the other; thus the switches operated independently. Operation of four spots did not yield uniform response in this initial attempt. The device consists of sixty-one periods of 299 Å GaAs and 98 Å AIGaAs multiple quantum well structure capped by ~2000-Å AI­GaAs windows and sandwiched between 94% reflectance mirrors. Using two independently controlled beams we could vary continuously the distance between two spots. Diffusive crosstalk was then identified by a decrease in switch-on intensity of one of the beams when the other beam was brought sufficiently close. The extent of the carrier diffusion varied with position on the sample, orientation of the spots, and the switch-on intensities. The critical spacing at which crosstalk became noticeable ranged from 35 μm to more than 100 μm.
We have observed bistability of the n-type Cd x Hg1− x Te in Auger-dominated in the following conditions: the input probe beam, obtained by splitting the CO2 laser with a ZnSe splitter, was incident upon the CdHgTe detector 1. The output probe beam, passing through the Fabry-Perot etalon, was detected by the CdHgTe detector 2. All beams were chopped using a mechanical chopper with a tunable mark/space ratio between 20:1 at a high speed synchronous motor. The signals of the incident and transmission intensity synchronously detected by using the CdHgTe detectors at 77 K were displayed on an oscilloscope. Our specimen was n-type Cd x Hg1− x Te (x = 0.225) at 77 K, the intrinsic carrier concentration n = 9.8 × 1014 cm-3, the mobility μ = 3.15 × 104 (cm2/V sec). The absorption coefficient α = 72 cm was polished plane parallel to form an etalon 215 μm thick, the reflectivities of which are R F = R B = R = 32%. In our experiment, the incident intensity I is 750 W/cm2 and both the laser mode and power stability met the standards.
Abstract : The interaction of a light beam with matter has varying degrees of dependence on the transverse profile of the beam. Two topics are considered here. The first topic, superfluorescence, involves the coherent emission generated by an initially inverted two-level medium. Calculations in the past have included either quantum fluctuations in the initial conditions or transverse effects, but never both. The calculations presented here include both transverse effects and quantum fluctuations. The results of the theoretical calculations are in good agreement with previous experimental results, both quantitatively in terms of time delay averages and distributions and also qualitatively in terms of pulse shapes. The second topic considered, optical bistability, involves devices which exhibit two stable output light intensities (a high and a low branch) over a limited range of input light intensities. Simulations performed here show the effect of diffraction, measured in terms of the Fresnel number, on how much of the device becomes bistable. It was found that for small Fresnel number cases, the entire device become bistable. For high Fresnel number cases, only part of the device becomes bistable while the rest of the device remains low transmitting. The results of an experiment performed are consistent with the simulations. (Author)
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.