Nonlinear optical absorption and energy structure of LiNbO$_3$ and $\alpha$-LiIO$_3$ crystals

An experimental investigation was made of the nonlinear absorption in LiNbO$_3$ and $\alpha$-LiIO$_3$ crystals in the photon energy range $\hbar(\omega_1+\omega_2)\le5.2$ eV. In the case of LiNbO$_3$ the nonlinear absorption was due to two-photon allowed-forbidden transitions. The magnitude of the two-photon absorption and its spectral dependences were used to determine some of the parameters of the energy band structure. In the case of $\alpha$-LiIO$_3$ the two-photon absorption coefficient was $\beta<4\times10^{-4}$ cm/MW in the range $\hbar(\omega_1+\omega_2)<5.2$ eV. This indicated that frequency conversion in the ultraviolet range could be carried out in $\alpha$-LiIO$_3$ over a wider frequency range than in LiNbO$_3$.