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Applied Physics Letters

Intrinsic absorptive optical bistability in CdS

Klaus M. Bohnert, H. Kalt, Claus F. Klingshirn

Published Dec 15, 1983• 105 citations
Publisher

Abstract

An intrinsic absorptive optical bistability is observed in the transmission of high intensity, nanosecond laser pulses through thin CdS platelets at liquid helium temperature. The bistability is due to a renormalization of the band gap which is caused by the formation of an electron hole plasma and occurs in a broad spectral range below the excitonic resonances. The switching times are in the subnanosecond range. The dependence on photon energy is investigated and discussed.

DOI
10.1063/1.94252

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An intrinsic absorptive optical bistability is observed in the transmission of high intensity, nanosecond laser pulses through thin CdS platelets at liquid helium temperature. The bistability is due to a renormalization of the band gap which is caused by the formation of an electron hole plasma and occurs in a broad spectral range below the excitonic resonances. The switching times are in the subnanosecond range. The dependence on photon energy is investigated and discussed.
The switching dynamics of thermally induced optical bistability due to increasing absorption is investigated in thin CdS platelets, at room temperature. Fast and slow components can be observed during the switching process, they depend on the holding point intensity and on the switching pulse energy. It is shown that the transverse relaxation of the temperature in the sample influences this dynamics. The spectral shift of the band to band absorption edge, at the origin of the bistable behavior, is observed in a three beam experiment.
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.
We have observed high-speed optical switching based on the optical bistability of CdSe at low temperatures due both to induced absorption and to dispersion changes. Using a streak camera to record the response of a thin CdSe platelet to high-intensity dye-laser pulses at near-band-gap energies, we have studied these phenomena on a picosecond time scale and measured switching times of several hundred picoseconds. We attribute our observations to nonlinearities caused by exciton interactions in the semiconductor material. Finally, we discuss the part played in these phenomena by thermal and other dynamic effects.
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.
Abstract The transmission spectra of CdS platelets are studied at room temperature with a time‐resolved pump‐probe technique. These measurements are also performed during the switching between the two transmission levels of the bistable crystal induced by the pump pulse. In this case, the third beam from a cw argon laser holds the sample around the switching threshold. These spectral and temporally resolved experiments allow the analysis of the switching dynamics of such absorptive bistable devices.