Polarisation characteristics of two-photon absorption in an LiF:$F^-_2$ crystal at the 1.06 $\mu$m wavelength

Strong 1.06 $\mu$m radiation was shown to be capable of orientationally selective bleaching of the $F^-_2$ colour centres in a crystal of LiF. Conditions were found under which only identically oriented $F^-_2$ colour centres remained unbleached. Crystals in which the $F^-_2$ centres remained identically oriented were used to find the polarisation characteristics of two-photon absorption. The rate of bleaching of the colour centres as a result of two-photon absorption by an excited state was proportional to $\cos^2\theta$ where $\theta$ is the angle between the orientation of the electric vector of the strong wave and the axis joining the fluorine vacancies in the $F^-_2$ centres. The two-photon absorption cross section of the excited $F^-_2$ centres was estimated to be $\sigma_2 = 3.5\times10^{-51}$ s cm$^4$.