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Topical Meeting on Optical Bistability (OB)

Optical Bistability in CdS

Claus F. Klingshirn, F. Fidorra, J.-Y. Bigot, M. Wegener, Claus Dörnfeld, M. Lambsdorff

Published Jan 1, 1985• 0 citations

Abstract

Optical bistability (ob) is investigated in CdS in a wide range of intensities. Under pulsed excitation (total pulse width 20 nsec) we find at 5K for ħωexc ≈ 2.550 eV on the flank of the AΓ5 exciton ob of the induced absorption type for incident intensities of Iexc ≈ 1kW/cm2. The nonlinearity is in this case a collision-broadening of the free exciton resonance [1]. The commutation times τ↓ and τ↑ are about 1 nsec and 5 nsec, respectively. For ħωexc ≈ 2.546 eV we observe absorptive ob due to the formation of an electron-hole plasma [2] for Iexc ≈ 1MW/cm2. Single shot experiments show τ↓ ≲ 500 psec being thus to our knowledge one of the shortest commutation times reported for single beam ob so far. The observed hysteresis loops are compared to model calculations with variable pulse lengths. The comparison indicates true ob in our experiments.

DOI: 10.1364/obi.1985.mb3

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Optical bistability (ob) is investigated in CdS in a wide range of intensities. Under pulsed excitation (total pulse width 20 nsec) we find at 5K for ħωexc ≈ 2.550 eV on the flank of the AΓ5 exciton ob of the induced absorption type for incident intensities of Iexc ≈ 1kW/cm2. The nonlinearity is in this case a collision-broadening of the free exciton resonance [1]. The commutation times τ↓ and τ↑ are about 1 nsec and 5 nsec, respectively. For ħωexc ≈ 2.546 eV we observe absorptive ob due to the formation of an electron-hole plasma [2] for Iexc ≈ 1MW/cm2. Single shot experiments show τ↓ ≲ 500 psec being thus to our knowledge one of the shortest commutation times reported for single beam ob so far. The observed hysteresis loops are compared to model calculations with variable pulse lengths. The comparison indicates true ob in our experiments.

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A general discussion of the linear-optical properties of free and bound excitons in direct-gap semiconductors at low temperature is presented. This discussion then specializes to CdS. The nonlinear-optical response of CdS is introduced by first discussing the saturation behavior of bound excitons. A new type of cavityless optical bistability based on induced absorption near the free and bound excitons is described. Finally, fast all-optical switching (<1 nsec) at very low switching energies (<8 pJ) is demonstrated, using the giant nonlinearity associated with bound excitons.

In this paper we present our investigations concerning thermally induced optical bistability (OB) in CdS and ZnSe. With CdS platelet type single crystals basic boolean functions are realized. The possibilities of cascadability and parallelity of the information processing and the switching dynamics of the system are examined. Further on the effect of optical noise upon the switching process and the instable branch of the hysteresis loop are measured. In ZnSe both absorptive and dispersive OB are observed. An efficient opto-electric modulator is built using the photothermal SEED-effect.