Phase-conjugate reflection by degenerate four-wave mixing in a nematic liquid crystal in the isotropic phase.

Liquid-crystal materials are composed of molecules with strong anisotropy. An optical field can induce appreciable molecular alignment along the field, resulting in a change of the index of refraction that is proportional to the optical intensity. Third-order nonlinear optical phenomena, such as self-focusing of light,1'2 stimulated scattering,3 and third-harmonic generation,4-6 have been observed in these materials. In this Letter, the generation of phase-conjugate wave fronts by degenerate four-wave mixing in p -methoxybenzylidene p-n-butylaniline (MBBA) is reported. MBBA is a nematic liquid crystal at room temperature. The experiment, however, utilized the isotropic phase of this material, which manifests some field-induced short-range order above the transition temperature, T. The nonlinear optical properties of MBBA in this pretransitional state have been investigated elsewhere,7-9 and this material has been found to possess a large nonlinear optical susceptibility, about an order of magnitude larger than that of carbon disulfide. In the presence of an applied optical field with components Ek (w), the third-order nonlinear polarization in an isotropic medium is9