| A detailed analysis of the intensity dependent nonlinearity in doped liquid crystalline materials and its wave mixing effects is presented. The nonlinearity arises from the formation of space charge fields within the material. These space charge fields come from two main sources. First the formation, and subsequent dissociation, of charge transfer complexes between the liquid crystal and the dopant produce free charge carriers, which drift and diffuse to form a space charge field. Second, an applied field may reinforce these fields through the Carr-Helfrich effect, which states that when an electric field is applied to a medium with anisotropic conductivity and permittivity local transverse field components will be present. These fields, plus any flows within the material, will produce torques on the liquid crystal molecules that locally realign the uniaxial liquid crystal molecules producing a modulation of the index of refraction. The resulting nonlinearity is extremely large and may be used for wave mixing within the material.; The behavior of the wave mixing is studied for a variety of different liquid crystals and dopants. Theoretical models derived to describe the nonlinear process are substantiated with experimental results. Also, experiments to examine the various aspects of the wave-mixing phenomena including time response, beam amplification, and wavelength dependence are documented. It is also shown that persistent recordings may be formed in the material provided the participating field strengths are large enough. Optimum values for dimensions, wavelength, and other parameters that maximize the nonlinearity are obtained. Finally preliminary experiments on applications resulting from the large nonlinearity are presented. These applications include optical limiting, spatial light modulation, and incoherent to coherent optical conversion. |
