A novel diffraction phenomenon with the first order Fredericksz transition in a nematic liquid crystal film

We present the experimental results of the Ar+ laser- and electric-field-induced diffraction effect in a planar-parallel-aligned nematic liquid crystal film. It is found that only one set of diffraction rings occurs in the second order Fredericksz transition; however, two sets of diffraction rings can occur in the first order Fredericksz transition. The appearance of two sets of rings distinguishes the first order Fredericksz transition from the second order. We propose a theoretical explanation for the phenomenon.