Electrically controlled microstructured liquid-crystal twist elements for phase conversion of light fields

Abstract

Subject of study. Various topologies are proposed for microstructured photoalignment of the directors in nematic liquid crystals; these topologies can be used in electrically controlled liquid-crystal elements to produce light beams with a specific phase singularity. Main results. Electrically controllable, azimuthally oriented liquid-crystal twist elements were developed to support the transformation of Gaussian beams into beams with a specific number of phase singularities in the wavefront. Polarization microscopy, analysis of the intensity distribution across the beam, and coherent superposition of the transformed light with a plane wave determines the operating voltage range supporting excitation of the beams with optical singularities. It is shown that the developed components perform the wavelength-independent topological phase transformation converting the wavefront associated with a circularly polarized Gaussian beam into optical vortices over a large portion of the optical spectrum without the need for precise adjustment of the control voltage. Diffractive optical elements based on microstructured twist-planar orientation of the liquid-crystal director enable the transformation of linearly polarized wavefronts into optical vortices with different topological charges. A diffractive topological component that produces optical vortices with the topological charge 4 was used as a sample for an experimental study to determine the diffraction efficiency of the componentas a function of the control voltage amplitude. The initial diffraction efficiency (at zero voltage) of the developed diffractive component is about 10% and the peak diffraction efficiency at the control voltage amplitude of a few volts is about 30%. There is no diffractive structure when the external voltage is increased up to 20 V. As revealed by analysis of the interference pattern resulting from coherent superposition of a phase singular beam and a plane wave, the excited optical vortices are stable with respect to the control voltage amplitude. It has been found experimentally that singular beams with high topological charges remain stable during the propagation to a distance approaching 3 m. Practical significance. The developed electrically controlled topological liquid-crystal elements can operate over a wide range of optical wavelengths and may be implemented to produce ultrashort-duration optical vortices and a supercontinuum. Besides, they may be used in the form of multi-trap optical tweezers or in technologies for protection of securities and documents.