Photoelectrically induced free carrier absorption and amplification of light in cadmium sulfide

It has been demonstrated that it is possible to control the intensity of visible and infrared light being transmitted through cadmium sulfide by controlled variations in photoelectrically induced free charge carrier densities which absorb the transmitted light. The free carrier optical absorption coefficient is extended to fit the case of photoelectrically induced free carrier absorption in semiconductors. This results in a theoretical modulation parameter which predicts the extent to which light transmission can be controlled by induced free carrier absorption. An experimental technique is presented which demonstrates that photoelectrically induced variations in the density of free carriers in CdS can indeed bring about a consequential control of the light transmission. Complete control over the transmission, in addition to light amplification by stimulated absorption, is demonstrated in CdS for both infrared and visible light between 0.6 micro and 4.0 micro. Theoretically computed and experimentally observed values of modulation are in fair agreement, considering the limitation of the theory and the nature of the solid. The observed transmitted wavelength dependence in CdS is discussed on the basis of both the free carrier absorption theory and various photoconductivity mechanisms, and it is shown to be reasonably consistent with modern theory.