  We demonstrate that one-dimensional photonic crystals with pure nematic
liquid-crystal defects can operate as all-optical switching devices based on
optical orientational nonlinearities of liquid crystals. We show that such a
periodic structure is responsible for a modulated threshold of the optical
Fr\'eedericksz transition in the spectral domain, and this leads to all-optical
switching and light-induced multistability. This effect has no quasi-statics
electric field analogue, and it results from nonlinear coupling between light
and a defect mode.
