Nonlinear optical amplification in a nematic liquid crystal above the Frederiks transition

We have observed a new nonlinear light amplification process in a nematic liquid crystal, p-methoxybenzylidene-p-n-butylaniline, maintained above the Frederiks transition by a magnetic field. Substantial probe-beam gain (10% or more) was observed for a moderate same-frequency pump beam of intensity on the order of W/cm², and is attributed to the molecular anisotropy associated with the director axis reorientation. The observed gain and nonlinear intensity dependence are in excellent quantitative agreement with theoretical predictions. An analogous effect is also observed for the case when the Fredericks transition is caused by a dc electric field.