Dichroism of an LiF crystal under conditions of irreversible spatially selective bleaching of $F^-_2$ colour centres by $\lambda \sim$ 1.06 $\mu$m radiation

An investigation was made of the characteristics of an LiF:$F^-_2$ crystal irradiated with strong resonant radiation ($\lambda \sim$ 1.06 $\mu$m). This irradiation bleached the $F^-_2$ colour centres of all the orientations except one. The irradiated part of the crystal had the combined properties of a partial polariser and a phase plate. The propagation of weak radiation, which did not saturate the absorption by the colour centres, in the irradiated zone rotated the polarisation vector by 5$^o$. The irradiated zone also acted as a $\lambda$/15 phase plate when either weak or strong (saturating completely the absorption by the $F^-_2$ centres) radiation propagated in the crystal.