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Applied Physics Letters

Cavityless optical bistability due to light-induced absorption in cadmium sulfide

M. Dagenais, Wayne F. Sharfin

Published Aug 1, 1984• 70 citations
Publisher

Abstract

Cavityless, whole-beam optical bistability in thin, uncoated cadmium sulfide platelets is observed at milliwatt power levels when a cw laser is tuned near a sharp extrinsic excitonic resonance in cadmium sulfide (the I2 bound exciton). Very large contrast ratios between the on and off states are measured. The detuning and temperature dependence of the bistability results are reported. Preliminary transient measurements are also discussed.

DOI
10.1063/1.95185

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Optical bistability in the presence of an optically induced absorption is observed when a thin 13-μm CdS sample is inserted between two mirrors of 90 % reflectivity, in our experiment, a modulated cw laser of ~15-mW peak power is tuned on each side of the free exciton resonance in CdS, and the transmission characteristics are studied. A theoretical calculation including both absorptive and dispersive effects is presented and good agreement is obtained. The absorption coefficient and the index of refraction of the material are connected by Kramers-Kronig relations. Propagation effects are taken into account. Sufficiently long pulses (40 ms) are considered to allow the thermally induced absorption to occur. For detunings above the free exciton resonance, we predict that counterclockwise hysteresis loops will first be observed as the intensity rises; for detunings below the free exciton resonance, only clockwise hysteresis loops are predicted. These predictions were experimentally verified. Power levels of the order of 5–10 mW were sufficient to observe bistability. Cavity effects were also observed.

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Low-power saturation of bound excitons and observation of optical bistability

M. Dagenais, Herbert G. Winful

Very large nonlinearities are observed in the vicinity of a bound exciton resonance in cadmium sulfide. Power levels of the order of 8 μW are sufficient for saturating the bound exciton resonance in a tight focusing geometry. In a more loosely focused geometry, power levels of the order of 250 μW are sufficient for saturating the bound exciton resonance and for observing optical bistability. Transverse effects are found to play a very important role. For all our measurements, no thermal contributions to the nonlinearity were observed. Since thermal effects are not playing a role and since the studied bound excitons decay radiatively in 500 psec, we infer subnanosecond switchoff and cycling times.

Conference on Lasers and Electro-Optics•Jan 1, 1983•1 citation
Cavityless, whole-beam optical bistability in thin, uncoated cadmium sulfide platelets is observed at milliwatt power levels when a cw laser is tuned near a sharp extrinsic excitonic resonance in cadmium sulfide (the I2 bound exciton). Very large contrast ratios between the on and off states are measured. The detuning and temperature dependence of the bistability results are reported. Preliminary transient measurements are also discussed.
The I2 bound exciton in cadmium sulfide has a giant oscillator strength of 6. It decays mostly radiatively in 500 psec. Such a large oscillator strength leads to a very large transition dipole moment and to very large nonlinear effects off-resonance. At 2K, this sytem saturates like an inhomogeneously broadened system for intensities up to 30 times the saturation intensity. The transition linewidth is 7.5 GHz. In a tight focusing geometry, only 3.6 μW is required to see the onset of saturation. This corresponds to a measured saturation intensity of about 60 W/cm2. The linear absorption coefficient right at the resonance peak was measured to be 3.2 x 103 cm-1. At higher temperatures, the transition linewidth broadens and the line becomes more and more homogeneously broadened because of phonon interaction. At intensities above 50 times the saturation intensity, the absorption coefficient stops decreasing and then starts increasing. At these high intensities, thermal effects appear to play a significant role in shifting the free and bound exciton resonances toward lower energies. At intensities less than 20 times the saturation intensity, thermal effects are believed to be insignificant in our saturation measurements.
Optical bistability in the presence of an optically induced absorption is observed when a thin 13-μm CdS sample is inserted between two mirrors of 90 % reflectivity, in our experiment, a modulated cw laser of ~15-mW peak power is tuned on each side of the free exciton resonance in CdS, and the transmission characteristics are studied. A theoretical calculation including both absorptive and dispersive effects is presented and good agreement is obtained. The absorption coefficient and the index of refraction of the material are connected by Kramers-Kronig relations. Propagation effects are taken into account. Sufficiently long pulses (40 ms) are considered to allow the thermally induced absorption to occur. For detunings above the free exciton resonance, we predict that counterclockwise hysteresis loops will first be observed as the intensity rises; for detunings below the free exciton resonance, only clockwise hysteresis loops are predicted. These predictions were experimentally verified. Power levels of the order of 5–10 mW were sufficient to observe bistability. Cavity effects were also observed.
Very large nonlinearities are observed in the vicinity of a bound exciton resonance in cadmium sulfide. Power levels of the order of 8 μW are sufficient for saturating the bound exciton resonance in a tight focusing geometry. In a more loosely focused geometry, power levels of the order of 250 μW are sufficient for saturating the bound exciton resonance and for observing optical bistability. Transverse effects are found to play a very important role. For all our measurements, no thermal contributions to the nonlinearity were observed. Since thermal effects are not playing a role and since the studied bound excitons decay radiatively in 500 psec, we infer subnanosecond switchoff and cycling times.