Optical manipulation and defect creation in a liquid crystal on a photoresponsive surface

Light-induced modulations of the refractive index and pattern formation are desirable to generate complex photonic structures via exposure to light. Here we show that local modulations of the effective refractive index and reconfigurable defects can be locally induced in a hybridized thin birefringent film of a nematic liquid crystal (LC) on a photoresponsive (generating photoinduced electric fields) iron doped lithium niobate surface via exposure to a focused laser beam. Samples were studied with a tailored imaging approach, which provided the ability to investigate these optically excited, field-induced responses on a microscopic level. Upon exposure with a focused laser beam, the fluent LC was expanded on the substrate's surface and localized field-induced defects were optically created. Both umbilic (central) and line defects were observed. The formation of field-induced umbilic defects was modeled in numerical simulations. In addition, line defects were experimentally studied. It was seen that line defects interconnected the centers of two central defects (field-induced defects, which were present at the upper and lower surfaces of the LC layer). In addition, line disclinations separating reverse tilt domains (caused by the inhomogeneous distribution of the photogenerated fields) were seen. These line disclinations were pinned to the central defects. By exposure with two adjacent focused laser beams two umbilic defects were created side by side and interconnected with a line defect (the line defects pinned to each umbilic defect were joined in a single defect line). An alternative technique is presented to field-induce promising photonic motives (microlenses, resonators, line defects) in a liquid crystalline, hybridized birefringent film on a microscopic scale by using a low-power laser (opposed to the high power necessary to induce optical Kerr responses in a neat LC).

Figure 1

(a) Schematic of the open film sample where a Fe:LN substrate was covered with an open LC film. This sample was exposed to a focused laser beam near the center of the film (focus position 1) or near the film edge (focus position 2). (b) Schematic of the sample where the LC film was confined with two film spacers and a cover slip. (c) Photograph of a confined film sample between crossed polarizers. The alignment direction (of both the Fe:LN substrate and cover slip) is indicated by a double arrow. (d) Schematic of the exposure and imaging experiment. Samples were investigated using an inverted polarized optical microscope fitted with a focused laser beam optic and rotatable polarizers. Exposure was realized with a focused green diode laser, which was coupled to the transmitted light microscopic beam path.

Figure 2

Polarized optical micrographs recorded in open film samples, and associated photogenerated, field-induced defects. (a) Sample in the initial state. (b) Defect structure (central defect and defect line) observed upon exposure with a focused laser beam. (c) Closeup image of the defect center. (d) Desaturated image of the defect center. (e) Sequence of still images captured from a recorded video file showing the edge of the LC film. The edge of the LC film and the substrate surface (bare Fe:LN surface) are indicated. The focus position of the laser beam was scanned across the film edge (from left to right) within a few seconds; the film edge was thereby expanded. See Supplemental Material [25].

Figure 3

Polarized optical micrographs of photogenerated field-induced defects in open film samples. (a)–(c) Series of images recorded when the crossed polarizers were rotated counterclockwise, and in turn the Maltese cross pattern was rotated clockwise: A black line is shown in one brush of the Maltese cross pattern as guide to the eye. (d) through (f) Additional images of various types of line defects observed are shown.

Figure 4

Defect lines in samples with a confined LC film. The alignment direction of the LC is indicated (white double arrows) and a vector $l$ is shown, which is parallel to the defect line. (a,b) The sample was rotated in different positions and exposed to a single focused laser beam. (c) The sample was exposed to two adjacent focused laser beams resulting in defects connected by a merged defect line.

Figure 5

Defect formation in a confined film sample observed at increasing exposure intensity. (a) Sample in the initial state. (b) Defect-free reorientation of the LC. (c) Formation of defect pair, pinned disclination line and interconnecting line defect. A magnification of the area indicated is shown as an inset. (d) Disclination line pinned to central defect.

Figure 6

Simulation of LC reorientation. (a) Schematic of the simulated three-dimensional volume. A Gaussian charge distribution was considered at the top and bottom surface of the Fe:LN layer. The obtained electric field is shown. (b) Electric-field distribution inside the LC layer. (c) Initial state: Director field with alignment parallel to $d_1$ at the Fe:LN surface and perpendicular at the LC-air boundary. (d) Director field obtained with anchoring similar as in (c) but with charged surfaces. Please note, in (b)–(d) half of the director field is selectively shown for clarity.

Figure 7

(a) Director pattern in a hyperbolic defect, used as anchoring condition at the Fe:LN surface. (b) Director field in the case of hyperbolic anchoring. One quadrant is shown selectively. (c) Simulated pattern: Transmission of monochromatic light (589 nm) between crossed polarizers.