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 (${E}_{g}=2.5$ eV) using a pulsed ruby laser ($\ensuremath{\hbar}\ensuremath{\omega}=1.78$ eV). The radiative recombination emission from exciton and impurity levels subsequent to the simultaneous absorption of two quanta of $\ensuremath{\hbar}\ensuremath{\omega}=1.78$ eV was observed as a function of laser intensity and compared to the emission excited by single-quanta absorption for photons of $\ensuremath{\hbar}\ensuremath{\omega}>{E}_{g}$. 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.

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