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We report the generation of conjugate wave fronts by degenerate four-wave mixing in the isotropic phase of the ne- matic substance p-methoxy-benzylidene p-n-butylaniline. The temporal and spatial properties of the conjugate wave fronts are verified. The dependence of the nonlinear reflectivity on the pump-wave power and the tempera- ture of the medium is discussed. Liquid-crystal materials are composed of molecules with strong anisotropy. An optical field can induce appreciable molecular alignment along the field, re- sulting 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 -methoxy- benzylidene 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 pre- transitional 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 com- ponents Ek (w), the third-order nonlinear polarization in an isotropic medium is9

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