Giant natural circular dichroism of vibronic transitions in HoAl$_3$(BO$_3)_4$

The giant natural optical activity of vibronic replicas of the $f{-}f$ ${}^5I_8\to{}^5F_2$ transition in HoAl$_3$(BO$_3)_4$ is observed. It exceeds the optical activity related to purely electronic transitions by two orders of magnitude and corresponds to the almost complete circular polarization of the transitions. This effect is explained by the local distortions of the crystal in the excited electronic state, which lead to the mixing of electron and vibrational wavefunctions and, as a consequence, to the delocalization of the vibronic wavefunction and the enhancement of spatial dispersion, which is responsible for the optical activity.