Two mechanisms of internal feedback in cadmium sulfide in electric field have been studied, namely, thermal shift of absorption lines and electron state transformation due to multiparticle interaction in the high carrier concentration system.
Optical bistability caused by thermal heating is observed when the CdS crystal is excited near the I2 bound exciton absorption line [1]. Impact ionization of free and bound excitons occurs in the crystal upon its illumination in electric field. The probability of impact ionization depends in a threshold way on exciton and free carrier concentration. At a certain field dependent carrier concentratiοn avalanche occurs, the electric current therewith increases the crystal temperature by 5-20 K [2] which is followed by a red I2 shift. The shift leads to an increase in absorption, carrier density and current. Therefore, feedback results from impact ionization current heating the crystal (Fig. 1).
Self-defocusing of ordinary and extraordinary laser beams have been investigated in cadmium sulfide semiconductors in the near-band-edge absorption region. First it has been shown experimentally, that effects of the anisotropy nonlinear refraction induced by charge carriers created by single photon absorption is described whole set components of the third order nonlinear susceptibility tensor. New type of transverse optical bistability with increased amout of feedback due to nonlinear anisotropy was demonstrated.
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.
Using computer simulation, the feasibility of absorption optical bistability based on the dependence of the absorption coefficient on an electric field induced by a high-intensity laser pulse acting on a semiconductor and shifting the energy levels of atoms is demonstrated. Unlike the known mechanisms behind absorption optical bistability, here enhanced diffusion of charged particles does not change the rate of switching the system from the lower to upper state. Radiation-semiconductor interaction conditions are found when, after being switched to the upper state, the system remains in it until the laser pulse disappears.