Optical Properties of Nontwisted Nematic Layers

Let the plane TM wave be incident at an angle ie onto a nematic layer with the initially homeotropic director distribution (Fig. 4.34) 

FIGURE 4.34. An initially homeotropic nematic layer with oblique light incidence at an angle ie. The electric vector of the TM light wave is in the plane of incidence. 9 z) characterizes the spatial distribution of the optical ellipsoid. I = R + T, where R is reflectivity and T is transmittance, provided that absorption is absent. 

Let us note that 9 is not the director angle at the boundary. This is so only for special types of director distributions, e.g., in the flexoelectric effect (Fig. 4.30(a, b)) or in the quasi-homeotropic reverse pretilt configuration (Fig. 4.3(b)). In the B effect, on the contrary, the director angle is maximum in the middle of the layer (Fig. 4.1(b)), and, consequently, we cannot analyze the boundary region with the TIR method. 

The transmittance angular spectra T i) of the initially homeotropic cell in electric field is given in Fig. 4.35 (i-incident angle). According to the energy conservation law, in nonabsorbing media the reflectance (or reflectivity) spectra R are complementary to the transmittance spectra, as 

The transmittance calculated for the director distribution in the B effect (Fig. 4.35) strongly depends on the applied voltage. For a purely homeotropic monodomain sample and U Ub Ub is the B effect threshold, Fig. 4.1(b)) the curve T(i) markedly oscillates with the incident angle i when i i. The amplitude of the oscillations reaches the maximum possible value T = 1 (R = 0) for the angles i close to ig. The oscillations appear due to the interference effect between the waves reflected from the upper z = 0 and lower z = d boundaries of the cell (Fig. 4.34). This effect becomes more pronounced for i = ig where the reflectance and transmit- 

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