Absolute two-photon absorption line shape in ZnTe

By measuring, as a function of temperature, the nonlinear light transmittance from neodymium and ruby lasers, the ZnTe two-photon absorption (TPA) has been studied both near the direct band gap and close to the critical point $E_1$. The TPA absolute coefficients have been determined by means of the two-channel normalization technique. It has been found that for $2\hslash \omega \ge E_g$, the TPA spectrum line shape has a dependence that is peculiar to absorption involving exciton levels, while for $2\hslash \omega$ close to $E_1$, the line shape has a ${(2\mathrm{}\hslash \omega -E_g)}^{\frac{3}{2}}$ dependence, which is typical of the "allowed-forbidden" mechanisms corresponding to the two-band model. By comparing the TPA experimental and theoretical values, it has been found that the TPA is strongly influenced both by the exciton effect and by the valence-band degeneracy.