Optical EngineeringOptical loss anisotropy of alumina films that contain liquid crystals
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Nematic liquid crystals were put into the micropores of anodic alumina films. Molecules of a liquid crystal tend to orient along the axis of columnar pores, since the pore diameter is smaller than 0.2 μm. Accordingly, the refractive index of the liquid crystal varies with the polarization of the light that propagates in the alumina film; i.e., the ordinary and extraordinary indices (no and ne) correspond to polarizations that are vertical and horizontal to the pores, respectively. The anisotropy in refractive index causes a prominent polarization function due to the index mismatch between a liquid crystal and alumina (n = 1.64). When a liquid crystal of no = 1.50 and ne = 1.64 is used, the optical loss for vertical polarization is larger than that for horizontal polarization by more than 10 times at 0.85- to 1.30-μm wavelengths. If one coats lecithin on the pore walls, molecules of a liquid crystal orient nonuniformly in the pores. Then the refractive index becomes nonuniform, and consequently, significant light scattering occurs inside the pores. The optical loss tends to decrease at shorter wavelengths and in alumina films with smaller pores.