Self-organised formation of an edge-dislocation array and optical vortex generation by reentrant-type nematic liquid crystal.

Fork-shaped diffraction gratings generate optical vortices, i.e., light fields with helical wavefronts. To fabricate such a grating, we focus here on the periodic deformation induced near the phase transition between the nematic and smectic A liquid-crystalline phases. Although the periodic structure typically appears over a temperature range of only ∼0.1 K, which is too narrow for practical use as a grating, we show that this range can be extended to several kelvins by using a reentrant-type nematic liquid crystal. Moreover, by introducing a sandwich cell with a step in the substrate, we successfully create edge dislocations in the periodic pattern owing to the mismatch in its period across the step. Upon illuminating a dislocation with laser light, optical vortices are observed in the first-order diffracted beams, whose intensity distributions strongly depend on the incident polarisation. The resulting dislocations thus function as polarisation-sensitive fork-shaped gratings.