Optical Double-Photon Absorption in CdS

Observations have been made of the two-photon excitation of an electron from the valence to the conduction band in CdS ($E_g=2.5\mathrm{}$ eV) using a pulsed ruby laser ($\hslash \omega =1.78\mathrm{}$ eV). The radiative recombination emission from exciton and impurity levels subsequent to the simultaneous absorption of two quanta of $\hslash \omega =1.78\mathrm{}$ eV was observed as a function of laser intensity and compared to the emission excited by single-quanta absorption for photons of $\hslash \omega >E_g$. It was found that the intensity of the recombination radiation is proportional to ${I_0}^n$ for single-quanta excitation and ${I_0}^{2n}$ for double-quanta excitation, where $I_0$ is the excitation intensity and $n$ is a constant which differs for different groups of emission lines. The observed cross section for double-quanta excitation is compared with theory utilizing the band parameters of CdS.