Transmittance, luminescence, and photocurrent in CdS under two-photon excitation

Two-photon transitions in CdS have been investigated at a single wavelength by measuring the nonlinear transmittance, photocurrent, and luminescence. The exciting source was the beam of a $Q$-switched ruby laser ($\hslash \omega =1.78\mathrm{}$ eV). The nonlinear cross section ${\gamma }_2$ for two-photon transitions has been measured and compared with the predictions of existing theories. The luminescence shows, at room temperature, stimulation effects at a pumping level $I_s\ge 4\times {10}^{25}$ photons/${\mathrm{cm}}^2$ sec. On the other hand, the photocurrent increases quadratically at low excitation levels and shows saturation effects near the threshold of stimulated emission. This saturation can be correlated to the light-emission stimulation.