Orientational optical nonlinearity of liquid crystals

The orientational optical nonlinearity of the mesophases of liquid crystals (LCs) is attracting ever more attention, owing to the high values of the constants characterizing it, owing to the many interesting specific features that LCs introduce even into traditional nonlinear optical effects, and owing to the potentialities of studying the physical properties of LCs. This article presents the state of the theory and experiment of orientational interaction of light waves with LCs. Especial attention is paid to the physical mechanisms of interaction—the light-induced Freedericksz effect, recording of director gratings, and the action of surface light waves on the orientation of the director. Varied manifestations of the effects of orientational self-action and interaction of light waves are discussed: self-focusing of light, nonlinear optical activity, mutual focusing, stimulated light scattering, and wave-front conjugation. Optical nonlinearities specific for the mesophase of a liquid crystal and associated with absorption of light quanta are also discussed. They include photoconformational nonlinearity, thermal-orientational nonlinearity, and liquid-crystal light valves. Especial attention is paid to the methodological problem of deriving the Euler–Lagrange–Rayleigh variational equations and to the correct choice of the free energy for the system LC + electromagnetic field. Section 8 traces the history of the studies of orientational optical nonlinearity of LCs and reviews the studies whose results are not directly reflected in the main text, and also reviews the studies on practical applications of light-induced orientational effects.