  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
$\omega_1/\omega_2$ with the total frequency sum $\omega_1+\omega_2$ kept
constant. We find a substantial increase of the two-photon absorption strength
with increasing $\omega_1/\omega_2$. 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.
