Optically induced twist Fréedericksz transitions in planar-aligned nematic liquid crystals

The optical-field-induced Fr\'eedericksz transition for a twist deformation by a normally incident laser beam in a planar-aligned nematic liquid crystal is studied. The Euler equation for the molecular director and the equations describing the evolution of the beam polarization in the birefringent medium are solved simultaneously in the small-perturbation limit. The stability of the undistorted state is investigated. An alternate series of stable and unstable bifurcations is found. This phenomenon has no analog in the Fr\'eedericksz transition induced by dc electric and magnetic external fields.