Photogeneration of High Pretilt Angles of Nematic Liquid Crystals by Non‐Polarized Light Irradiation of Azobenzene‐Containing Polymer Films

A vertical‐alignment (VA) cell of nematic liquid crystals (LCs) was prepared using photoirradiated thin films of a poly(methacrylate) with mesogenic moieties of 4‐trifluoromethoxyazobenzene as the side chains. Optical anisotropy was generated by oblique irradiation of the azobenzene‐containing polymer films with non‐polarized UV light, followed by annealing treatment to enhance the photodichroism, which displayed thermal stability. The combination of oblique exposure to non‐polarized UV light and subsequent annealing treatment brought about high pretilt angles of nematic LCs so that a photoaligned VA LC cell was fabricated. The photopatterned LC cell exhibited electro‐optical properties with excellent optical quality when a voltage was applied even after heating at 100 °C for several hours.     

Influence of Side‐Chain Structure and Irradiation Condition on Photoalignment of Ladder‐Like Polysiloxane Films

In this paper we consider the photo‐induced aligning capability of various ladder‐like polysiloxane‐based photoalignment films—which could be used in liquid‐crystal displays—bearing different photoreactive side chains, i.e., laterally grafted cinnamate/azobenzene‐based dual photoreactive side chains with a short or longer spacer, and terminally fixed coumarin‐containing side chains. Results from polarized optical microscopy (POM), Fourier‐transform infrared (FTIR) spectroscopy, surface‐enhanced Raman scattering (SERS), atomic force microscopy (AFM), etc., are integrated to elucidate the influence of side‐chain structure and the irradiation conditions on the photoalignment of ladder‐like polysiloxane films. It is demonstrated that the film containing the dual photoreactive group with a longer spacer exhibits better alignment properties. Reasonably, the concerted photoreactions of the dual photoreactive group and the longer spacer are beneficial to the cooperative motion of chromophores at the “domain level”, resulting in improved alignment facility and stability. The complicated effects of irradiation conditions and moderate annealing are also discussed. High‐quality alignment of the polysilsesquioxane (LPS)‐based photoalignment film LPS‐CA11 with a longer spacer between the LPS main chain and cinnamoyl/azobenzene side chains can be achieved only within an optimal range of exposure (5–8 J cm^–2), while the pretilt angles can be adjusted in the range 0.5°–7° by varying the incident light intensity. Additionally, moderate annealing before and after illumination can markedly improve the alignment uniformity by self‐healing of defects.     

Photoinduced cooperative reorientation in polymer blend films with hydrogen-bonded mesogenic side groups

Thermally enhanced photoinduced cooperative reorientation in hydrogen (H)-bonded polymer blend films was investigated. The films consisted of two kinds polymethacrylates with hexamethylene spacer groups terminated with 4-oxycinnamic acid (P6CA), 4-oxybenzoic acid (P6BA) or 4-(4′-oxyphenyl)benzoic acid (P6PBA) in the side chains. The films were subjected to linearly polarized ultraviolet (LPUV) light and subsequent annealing. Sufficient cooperative in-plane molecular reorientation in P6CA–P6BA blend films was achieved for the first time. In P6CA–P6PBA blend films, however, reorientation was not observed. The molecular weight, as well as the thermal properties of the homopolymers and the degree of photoreaction all played an important role in the cooperative reorientation behavior of the blend films. Finally, uniform alignment control of low-molecular-weight liquid crystals (LCs) on the molecularly reoriented polymer blend films perpendicular to the polarization E of LPUV light was obtained.     

Photoreplicated Anisotropic Liquid‐Crystalline Lenses for Aberration Control and Dual‐Layer Readout of Optical Discs

This work describes the functional characterization of novel photoreplicated anisotropic lenses obtained from molds with different curvatures. These lenses were prepared by the photopolymerization of mixtures of a reactive liquid‐crystalline bisacrylate and non‐reactive derivatives of liquid‐crystalline cyanobiphenyl. The phase diagrams, the polymerization kinetics as a function of temperature, and the birefringence as a function of temperature for various mixtures were determined. For the fabrication of lenses, the average liquid‐crystal (LC) director of the mixtures has to be aligned in a single direction. Photoalignment and rubbing techniques were investigated to create a monodomain of the reactive material. Finally, optimal mixtures were used for the fabrication of anisotropic lenses. Aberrations were below 20 mλ root mean square (rms), which makes these LC lenses interesting for application in light paths, for example, as spherical aberration correction devices for dual‐ (or multiple‐) layer optical storage.     

Photo‐aligned anisotropic optical thin films

Abstract— Photo‐alignment of anisotropic optical thin films enables realization of novel optical elements, which were not feasible up until now. Photo‐aligned anisotropic thin films can be applied to rigid or flexible substrates, which may be flat or curved. The optical performance of such films can be tailored to application‐specific requirements by introducing tilt profiles of the optical axis and/or generate patterned retarders with continuous or periodical in‐plane variation of the optical axis.     

Azo dye adsorption effect induced by elliptically polarized light in azo dye-doped liquid crystals

This study elucidates the azo dye adsorption effect induced by elliptically polarized light in azo dye-doped liquid crystals (ADDLCs). Experimental results reveal that the light-induced molecular reorientation that is caused by the adsorbed azo dyes declines as the absolute value of the ellipticity of the pumping beam, having a proper selectively fixed light intensity or a proper selectively fixed light component along the direction of major axis, increases. The long axes of the adsorbed dyes are found to be independent of the sign of the light ellipticity, but they do depend on the direction of the major or minor axis of the elliptically polarized light. Notably, the tilt angle in LC alignment is not observed in this experiment. Additionally, neither twist angles nor tilt angles can be induced using a circularly polarized pumping beam.