Laser and Two-Photon Processes

The possibility of observing two-photon processes using the intense beam of the ruby optical maser is considered theoretically for two general types of experiment. The essential approximation in the theory is the hypothesis that excited states exist which connect both the initial and final states by electric dipole transitions having a total oscillator strength $f\sim 1$. Simple formulas are obtained for the two-photon absorption which do not depend on the details of the electronic structure. Recent experiments on the Ca${\mathrm{F}}_2$: ${\mathrm{Eu}}^{++}$ system are in quantitative agreement with the predictions of the theory.