This paper presents original studies of the phase modulation of light by nematic liquid crystals (NLCs) with a sign change of the dielectric anisotropy (Δε) as a function of the frequency of the supplied sinusoidal voltage. The voltage dependences of the phase lag are obtained for a number of frequencies. The threshold voltage of the S effect is experimentally determined, and its frequency dependences are found to have a characteristic maximum that corresponds to the critical frequency at which the sign of Δepsi of the NLC mixture changes. It is shown that, in the frequency interval 100-5000 Hz where Δε>0, the phase lag and the threshold voltage are independent of the frequency of the supplied voltage, but they are affected by the conditions of the anisotropic-elastic interphase interaction associated with the orienting surface.
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