Experimental observation of specular optical activity

We report the first positive experimental observation of the optical activity effect on normal reflection. The experiment was performed along the optic axis in a gyrotropic semiconductor crystal of α-HgS, cinnabar, in a spectral region of strong absorption. Reflected light polarization azimuth rotation resulting from the reflection is in the order of ${10}^{\mathrm{-}4}$ rad and has pronounced dependence on ħω-${\mathit{E}}_{\mathit{g}}$.