Polarization tunable circular Dammann grating generated from azo-dye doped nematic liquid crystals

Abstract

A polarization tunable circular Dammann grating (CDG) was generated from an azo-dye (Methyl Red from Aldrich) doped liquid crystal (LC, E7 from Merck) cell. A simple multi-exposure photo-aligned process, based on cell assembled with non-rubbing glass substrates, was used to fabricate the binary phase liquid crystal CDG zone plane consisted of even zone with homogenous LC structure and odd zone with TN LC structure. Different twist angle of fabricated TN structure for odd zone can be obtained by adjusting photo exposure intensity or time. CDG with equal-intensity rings was produced through a Fourier-transform and then captured by a charge-coupled-device in our experiment. The maximum 0^th and 1^st diffraction orders of obtained CDG can be separated achieved by rotating the analyzer's polarization direction. If the chosen analyzer's direction leads to a zero phase difference of output light from even and odd zones, the maximum 0^th diffraction order will be achieved, in contrast, if the chosen analyzer's direction leads to a π phase difference of output light from even and odd zones, the maximum 1^st diffraction order will be produced. The TN structure of azo-dye doped liquid crystal cell fabricated by photo alignment technique provides a new method to generate CDG with polarization-dependent property. A broad wavelength band of lasers used to generate CDG, if far away from MR azo-dye absorption peak, expands the device's application range and shows a great advantage comparing to previously reported CDG fabricated by fixed materials, where only one fixed working wavelength is allowed.