[[abstract]]The authors present theoretical and experimental results on quasistatic-electric- and optical-field-induced birefringence as well as quasistatic-electric-field-enhanced optical diffraction effects in a nematic-liquid-crystal film. For an inclined-incident pump beam, the enhancement of the diffraction effect and birefringence values due to the quasistatic electric field does not saturate at high quasistatic electric field strength. Instead, it decreases from the maximum, through zero (i.e. no effect), to a region of negative coupling between the fields. Locally this novel zero-crossing phenomenon occurs at a quasistatic electric field strength for which the nematic director has been reoriented by the electric field to align with the polarization direction of the optical field. The zero-crossing effect manifested by the sample as a whole is the net result of the field-coupling effect on the local director along the beam path, which locally could be positive, zero, or negative. This novel effect is also independent of the pump-laser intensity. The authors' experimental results are in quantitative agreement with theoretical predictions
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