One-Step Holographic Photoalignment for Twisted Nematic Liquid Crystal Gratings

Abstract

Liquid crystal gratings, in which liquid crystal molecules are periodically aligned, are fabricated by highly efficient and practical one-step holographic photoalignment method using a photocrosslinkable polymer liquid crystal (PCLC). This method is an innovative fabrication technique for liquid crystal grating containing a twisted nematic alignment, which does not require a conventional complex fabrication process. In this chapter, three types of liquid crystal gratings with twisted nematic alignment are fabricated. Periodic director distributions of these liquid crystal gratings are analyzed based on the elastic continuum theory and observed experimentally using a polarized light optical microscope. Furthermore, the polarization diffraction properties were measured by illumination with a visible laser beam. The resultant liquid crystal gratings exhibit various polarization diffraction properties depending on the director distributions and the polarization states of the incident beams. These polarization diffraction properties are well explained by theoretical analysis based on Jones calculus. These resultant liquid crystal gratings exhibit great potential for application as a diffractive optical element that can simultaneously control the various parameters of the light wave, such as amplitude, polarization states, and propagation direction.

Photoinduced Reorientation and Photofunctional Control of Liquid Crystalline Copolymers with in Situ-Formed N-Benzylideneaniline Derivative Side Groups

“…Photoalignment of polymeric films is utilized to fabricate optical and display devices, where light exposure regulates the molecular reorientation direction of the material. − Conventionally, a command surface technique using thin photoalignment layers is applied to uniaxial liquid crystal (LC) alignment to fabricate LC displays and birefringent and optical devices combined with polymerizable LCs. − By contrast, the use of photoalignable LC polymers can enable direct fabrication of a photoinduced molecularly oriented structure of the polymeric film by means of linearly polarized (LP) light exposure and, in some cases, thermally stimulated self-organization. ,− …”

“…Many types of photosensitive materials have been explored to investigate photoinduced reorientation based on axis-selective trans–cis–trans photoisomerization of azobenzene-containing materials using LP light. − Photoreactive LC polymers containing cinnamate or phenyl benzoate mesogenic side groups exhibit a thermally stimulated photoinduced molecular reorientation, which is initiated by axis-selective photo-cross-linking or photo-Fries rearrangement. − Additionally, a new molecular orientation technique for photoinactive LC polymeric films from a free (air) surface using an azobenzene-containing polymer realizes a photoinduced molecular orientation of the inner side of the polymers and precise patterning. − Furthermore, Shishido et al have reported that dynamic photopolymerization attains molecularly oriented structures of the LC polymeric films, where the self-organized orientation of the mesogenic side groups occurs at the interface between the monomer and polymer using methacrylate monomers with mesogenic side groups. − These molecularly oriented films are applicable to birefringent optical devices and polarization-sensitive and polarization-controlled diffraction gratings when the film contains a periodical molecular orientation structure. ,,− …”

Polarized Fluorescence of N-Salicylideneaniline Derivatives Formed by In Situ Exchange from N-Benzylideneaniline Side Groups in Photoaligned Liquid Crystalline Copolymer Films

“…Many types of optical and display devices are fabricated by photoalignment of photosensitive polymeric films. − Two techniques have achieved photoalignment. One prepares a thin alignment layer for functional materials such as low-molecular-weight and polymerizable liquid crystals (LCs). − The other directly generates a molecularly oriented structure of a photosensitive film, and the photoinduced molecular reorientation produces a large optical anisotropy. ,,− In some cases, thermally stimulated photoinduced molecular reorientation occurs when a photoalignable material exhibits LC characteristics based on thermally induced self-organization. ,− Photoalignable LC polymers with cinnamate- or azobenzene-containing materials often display thermally stimulated photoinduced molecular reorientation. ,− These polymer films are applicable to birefringent optical devices and polarization-controlled diffraction gratings with a periodic molecular orientation structure. ,,− …”