Theoretical Prediction of Umbilics Creation in Nematic Liquid Crystals with Positive Dielectric Anisotropy

Abstract

Optically assisted electrical generation of umbilic defects, arising in homeotropically aligned nematic liquid crystal cells and known as topological templates for the generation of optical vortices, are reported in nematic liquid crystals with positive dielectric anisotropy in detail. It is shown that nematic liquid crystals with positive dielectric anisotropy can serve as a stable and efficient medium for the optical vortex generation from both linearly and circularly polarized input Gaussian beams. Hybrid cells made from a thin layer of nematic liquid crystal confined between a photoresponsive slab of iron-doped lithium niobate and a glass plate coated with an active material, i.e., indium tin oxide, were studied. Exposure to a laser beam locally induces a photovoltaic field in the iron-doped lithium niobate substrate, which can penetrate into the liquid crystal film and induce realignment of molecules. The photovoltaic field drives charge carrier accumulation at the interface of indium tin oxide with the liquid crystal, which effectively modifies the shape and symmetry of the electric field. The photovoltaic field has a continuous radial distribution in the transverse xy-plane, weakening with increasing distance from the light irradiation center, where the electric field is normal to the cell plane. Umbilics are created as a result of the liquid crystal tendency to realign parallel to the electric field. Numerical studies of the transmitted intensity profiles in between linear polarizers reveal optical vortex pattern (of four and eight brushes) characteristics for the umbilical defects. The application of crossed circular polarizers results in annular-shaped intensity patterns as a result of spin-to-orbital angular momentum conversions, which give rise to the optical vortices.

Photorefractive Effect in NLC Cells Caused by Anomalous Electrical Properties of ITO Electrodes

“…Photoinduced modulation of the conductivity of the ITO electrodes, as discussed in Section 2.2, can support the occurrence of the SPR effect. While the Burstein-Moss (B-M) effect (explained in Section 2.3) can be an important factor in the low effective voltage regime (in the ITO layer) by controlling the conductivity of the ITO as for example explained for the NLC cells with photoresponsive substrate [19].…”

“…The SPR effect can generate and support umbilical defects (characterized as non-singular topological defects) in the NLC cells with photo-responsive substrate of a z-cut ferroelectric material, i.e., iron-doped lithium niobate (Fe:LiNbO 3 or in short Fe: LN) covered with an ITO-equipped glass slab, as schematically shown in Figure 5. They act as context for the generation of OVs [19]. Here, photo-responsive substrate acts as a source of nonuniform photovoltaic electric field present in the LC film.…”

“…Here, photo-responsive substrate acts as a source of nonuniform photovoltaic electric field present in the LC film. In such test cells, the director field reorientation from an initial homeotropic alignment (provided by an anchoring agent) can take place under the action of an optically assisted fringe electric field distribution in the LC film (Figure 6) [19]. The fringe structure of the induced electric field is provided from both the photo-induced charge carriers' accumulation at the surfaces of the ferroelectric slab, as well as in the ITO layer in the beam spot [19].…”

“…In such test cells, the director field reorientation from an initial homeotropic alignment (provided by an anchoring agent) can take place under the action of an optically assisted fringe electric field distribution in the LC film (Figure 6) [19]. The fringe structure of the induced electric field is provided from both the photo-induced charge carriers' accumulation at the surfaces of the ferroelectric slab, as well as in the ITO layer in the beam spot [19]. As seen, the fringe electric field stemming only from the photo-induced charge carriers' distributions in the ferroelectric subtract is not capable of inducing the umbilical defects with spatial structure in the LC media as introduced by Rapini [66] in the cells without ITO equipment (the topological defects were formed only at the boundary surfaces of the LC layer) [17].…”

“…It was also experimentally shown that equipping a photoresponsive NLC cell with an ITO electrode can change the entity of the formed topological defects at the laser exposure spot from hyperbolic [17] to radial (with and without polymer coating as an anchoring agent) [18], while the impact of the ITO layer on modifying the electric field distribution in such LC test cells as a result of the induced space-charge field, E SC , was verified and simulated in an alternative work [19].…”