Characterization of the vortex-pair interaction law and nonlinear mobility effects

Abstract

Employing nematic liquid crystals in a homeotropic cell with a photosensitive wall, dissipative vortex pairs are selectively induced by external illumination and the interaction law is characterized for pairs of opposite topological charges. Contrary to the phenomenological fit with a force inversely proportional to the distance, the data provide evidence that nonlinear mobility effects must be taken into account. The observations lead to a reconciliation of experiments with theory.

Light–matter interaction induces a single positive vortex with swirling arms

“…Numerical simulations of the amplitude equation show a fairly good agreement with the experimental observations. Vortexes with swirling arms are usually observed in vortex interactions [32]. They have also been observed in LCs with an active surface [33] and in singular birefringent patterns generated by non-singular light beams [34].…”

Optical vortex induction via light–matter interaction in liquid-crystal media

“…The different colors observed experimentally are due to the different optical paths produced by the different orientations of the molecules. Moreover, from the Ginzburg-Landau equation, one deduces that the interaction between vortices is symmetric [7,71], but it has been reported that the speeds of 1 umbilic defects in the process of collision are different [70]. Numerical simulations that consider the dynamic of the nematic liquid crystal show the same result, where the speed asymmetry arises from backflow effects and anisotropy in the elastic constants [70].…”

“…Nematic liquid crystals with negative anisotropic dielectric constant and homeotropic anchoring are a natural physical context where dissipative vortices are observed [54,55]. Two types of stable vortices with opposite charges are observed; these are characterized by being attracted (repulsed) to the opposite (identical) topological charge [7,70,71].…”

“…It is worth noting that vortices are always created by pairs to conserve the topological charge, even though one vortex has more energy than the other. Furthermore, the scenario of the collision of opposite vortices described by the isotropic Ginzburg-Landau (see [7,71] and references therein) does not account for the whole picture of the collision of opposite nematic umbilics, as is shown in [70]. The characterization of vortex interaction in the anisotropic Ginzburg-Landau equation is an open problem.…”

Harnessing Optical Vortex Lattices in Nematic Liquid Crystals

“…Owing to this photoconductive substrate, when the LCLV is illuminated by a Gaussian light beam, the effective voltage drop across the LC layer acquires a bell shaped profile, peaked in the center of the illuminated area and able to overcome the critical value of the Fréedericksz transition for which the molecules start to reorient owing to the torque exerted by the electric field [23]. As we employ a liquid crystal with Á" < 0, the torque exerted along the short molecular axis is larger than that on the long axis; therefore, the molecules tend to (re)align perpendicularly to the electric field, leading to a 2-degenerate reorientation and the formation of topological defects in the nematic texture [24]. Besides, the Gaussian profile of the incoming beam also produces a transverse component of the electric field, thus giving rise to an effective potential able to pin the topological defect close to the optical intensity peak [20,25].…”