Nondegenerate two-photon absorption in ZnSe: Experiment and theory

Nonlinear optical processes often suffer from rather weak efficiencies. Here, the authors investigate two-photon absorption in ZnSe and demonstrate a tremendous enhancement of the two-photon absorption coefficient $\ensuremath{\beta}({\ensuremath{\omega}}_{1},{\ensuremath{\omega}}_{2})$ in the nondegenerate regime. Considering a constant frequency sum ${\ensuremath{\omega}}_{1}+{\ensuremath{\omega}}_{2}$, they measure an about tenfold increase of $\ensuremath{\beta}$ at a ratio ${\ensuremath{\omega}}_{1}/{\ensuremath{\omega}}_{2}=4$. These findings are in very good agreement with solutions of the semiconductor Bloch equations. The theoretical analysis predicts that even larger enhancements of $\ensuremath{\beta}$ should be possible when larger ratios ${\ensuremath{\omega}}_{1}/{\ensuremath{\omega}}_{2}$ become attainable.