Non-Degenerate Two-Photon Absorption in ZnSe: Experiment and Theory

We experimentally and theoretically investigate the non-degenerate two-photon absorption coefficient $\beta ({\omega }_1,{\omega }_2)$ in the prototypical semiconductor ZnSe. In particular, we provide a comprehensive data set on the dependence of $\beta ({\omega }_1,{\omega }_2)$ on the non-degeneracy parameter ${\omega }_1\mathrm{/}{\omega }_2$ with the total frequency sum ${\omega }_1+{\omega }_2$ kept constant. We find a substantial increase of the two-photon absorption strength with increasing ${\omega }_1\mathrm{/}{\omega }_2$. In addition, different crystallographic orientations and polarization configurations are investigated. The nonlinear optical response is analyzed theoretically by evaluating the multiband semiconductor Bloch equations including inter- and intraband excitations in the length gauge. The band structure and the matrix elements are taken an eight-band k.p model. The simulation results are in very good agreement with the experiment.