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We studied the behavior of photoinduced alignment change of a polymer liquid crystal (PLC) containing an azobenzene moiety in the side chain and formation of a grating by means of the photoinduced alignment of PLCs. The alignment change was evaluated by irradiation with s-polarized light at 488 nm. We investigated the effects of light intensity and incident angles of writing beams (two s-polarized (s + s) configuration) on grating formation in PLC films, and explored the relation between the alignment change and diffraction efficiency. The diffraction efficiency showed a maximum when the change in photoinduced alignment was almost complete in the bright area of interference patterns. The maximum value obtained for the modulation of the refractive index was 0.04. These results indicate that a PLC with an azobenzene moiety is effective under (s + s) polarization conditions, compared with other azopolymer systems.

Phase-type gratings formed by photochemical phase transitions of a polymer azobenzene liquid crystal have been characterized. The sensitivity of the material was improved by a factor of 23 in the liquid-crystalline phase owing to effective induction of the photochemical phase transition in the bright fringes of the interference pattern. A mechanism for grating formation in films based on dynamics both in grating formation and for the photochemical phase transition has been proposed. Observation of the grating structure with a polarizing optical microscope confirmed that the isotropic phase induced photochemically was arranged at well defined separations. The polymer azobenzene liquid crystal showed a storage capability of >100 Mbits cm−2 which corresponds to a resolution of 1000 lines mm−1. We also attempted holographic image storage using a photomask as an object.

The composition of a crosslinked azobenzene liquid-crystalline polymer and a flexible polymer film can provide a variety of simple devices that can walk in one direction like an ‘inchworm’ and move like a ‘robotic arm’ induced by light.

The formation of real-time holographic gratings was investigated in an azobenzene polymer liquid crystal (PLC) possessing a siloxane spacer in the side chain. Irradiation of thin films of the PLCs created a phase grating which is ascribed to periodic induction of the nematic-to-isotropic (N–I) phase transition due to photochemical trans–cis isomerization of the azobenzene moiety (photochemical phase transition). It was found that the N–I phase transition temperature of the PLC with the siloxane spacer was lower than that of the PLC without the siloxane spacer owing to flexibility of the siloxane units. This property resulted in a decrease in the irradiation temperature, leading to effective induction of the photochemical N–I phase transition by suppression of thermal cis–trans back-isomerization of the azobenzene moiety. Thus formation of the grating in the PLC having the siloxane spacer could be achieved by low intensity of the interference light compared to that in conventional PLCs including only alkyl spacers. Moreover, a grating with narrower fringe spacing could be created by depression of propagation of perturbation due to trans–cis photoisomerization of the azobenzene moiety in the LC phase.

The synthesis, optical and electrical properties of azobenzene-substituted polythiophene derivatives at the 3-position are reported: one without flexible spacer (PT0) and the other with a hexamethylene spacer (PT6). Photoirradiation of PT6 at 366 nm caused trans-cis isomerization of the azobenzene moiety and this photochromic reaction induced a change in conductivity of the thin film of PT6: the conductivity was 1.1×10 –7 S cm –1 before irradiation and 8.3×10 –7 S cm –1 after photoirradiation. After annealing, the conductivity of the film returned to the initial value. The conductivity of a compound without the azobenzene moiety, α-terthienyl (αT3), showed no change on photoirradiation at 366 nm.

The alignment direction of liquid crystals on polyimide alignment films prepared by in situ exposure to linearly polarized light (LPL) during thermal imidization of poly(amic acid)s has been found to be parallel to the direction of polarization of the LPL, whereas the conventional method, in which the alignment layers are exposed to LPL after imidization, leads to alignment perpendicular to the direction of polarization of the LPL.

The optical switching behavior of azobenzene liquid crystals (LCs) was explored in terms of stability by means of reflection-mode analysis. The optical switching of the LC in the reflection-mode has been found to be repeated over 10 000 cycles, which is 10 times as stable as that obtained in the transmission-mode analysis although the same chromophore was used. It was found that the optimization of the optical system for material used is an effective approach to realize more stable optical switching.

Photoinduced bending and unbending behavior of crosslinked liquid-crystalline polymers containing azotolane moieties in side chains occurred upon irradiation with sunlight, according to the trans–cisphotoisomerization of the azotolane moieties.

We have synthesized highly birefringent liquid-crystalline polymers (LCPs) containing an azobenzene group directly connected to a tolane moiety (P3ATm), in which the alkyl spacer length (CH2)m was varied as m = 6, 9 and 12. All LCPs showed a nematic phase in a broad temperature range (>170 °C). The homogeneously aligned LCP films showed a high value of birefringence (about 0.4) and a large change in the birefringence could be induced by photoinduced alignment change. Grating formation of LCPs has been explored in a glassy state. The diffraction efficiency (η) showed a maximum when an order–disorder change was completed in the bright areas of the interference patterns. Studies on the effects of light intensity on the grating formation suggested that the diffraction efficiency was high (η = 20%) even though the alignment change of the azo-tolane moieties was small. Furthermore, the behavior of the grating formation was found to be dependent on the alkyl spacer length of P3ATm.

Two types of photomodulation of orientations of liquid crystals (LCs) are reviewed: 1) order–disorder phase transitions of LCs induced by photochemical reactions of photochromic molecules and 2) order–order alignment change of LCs (change in LC directors) induced by photochemical reactions or without photochemical events. Both processes produce a large refractive-index modulation, which forms the basis of a range of photonic applications. Various modes of photomodulation of orientations of LCs with plausible mechanisms and their possible applications in photonics are described.