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 ®lms 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 con®rmed that the isotropic phase induced photochemically was arranged at well de®ned separations. The polymer azobenzene liquid crystal showed a storage capability of w100 Mbits cm 22 which corresponds to a resolution of v1 mm and a spatial frequency of w1000 lines mm 21 . We also attempted holographic image storage using a photomask as an object.

Unique characteristics were observed in phase-type gratings that were formed in a polymer azobenzene liquidcrystal (PALC) film. Generation of sinusoidal variation of the surface of the film was confirmed by atomic force microscopy. However, comparison of the diffraction efficiency with the surface modulation revealed that the gratings could not be characterized only as conventional surface-relief gratings. In the glassy state of the film, moderate efficiency (∼18%) was obtained with large surface modulation (68-76 nm), whereas the gratings recorded in the nematic (N) phase showed high diffraction efficiency (∼28%) with slight surface modulation (33-53 nm). The diffraction efficiency was enhanced in the liquid-crystalline phase. Dynamics of the first-order diffraction beam exhibited that the grating formation was associated with photochemical phase transition of PALC. It was assumed that the isotropic (I) phases were formed by photochemical reaction of azobenzene moieties in the interference pattern at an appropriate interval. The grating would be made up by alternate arrangement of the I and the N phases as well as slight modulation of surface structure. Our speculation was supported by atomic force and polarizing optical microscopy. Anisotropy in the diffraction efficiency with respect to the linearly polarized readout beam also supported our hypothesis. It was revealed that the large enhancement of the efficiency was attributable to spatial modulation of molecular alignment.

The rapid formation (and erasure) of a dynamic phase grating is one condition required of materials for dynamic holography. The photochemical nematic–isotropic transition of the side‐chain polymer liquid crystal (PLC) shown in the Figure, caused by the trans–cis photoisomerization of the azobenzene moiety, is reported to cause a dynamic grating with fringe spacing of 1.4 μm to form in ∼150 ms.

Dynamic holographic gratings were investigated with thin films of polymer azobenzene liquid crystals by periodic induction of photochemical nematic-to-isotropic phase transition. On irradiation of writing beams (488 nm), multiple diffraction beams of a reading beam (633 nm) were immediately observed. The firstorder diffraction efficiency reached to a maximum value within several tens of milliseconds: the rise time was approximately 50 ms for PM6AB2 films and 30 ms for PA6ABCN films. Quite large modulation in refractive index (∼0.08) was obtained in both polymer films. Studies on effects of temperature and light intensity on the grating formation suggested that photochemical reactions of azobenzene moieties followed by photochemical phase transition in bright fringes of the interference pattern would be responsible for the formation of holographic gratings. By turning on and off the writing beams, the diffraction beams could be switched dynamically without significant fatigue.

This article describes the syntheses and second-order nonlinear optical (NLO) properties of novel types of polyesters containing second-order NLO active chromophores with high density, which were obtained by condensation polymerization using two comonomers with the N,N-dialkyl-4-nitroaniline or N,N-dialkyl-4-((4-cyanophenyl)azo)aniline moiety. The polyesters were synthesized by condensation polymerization between isophthalic acid derivatives and N-substituted diethanolamines using triphenylphosphine and diethyl azodicarboxylate as the condensation reagents in dimethyl sulfoxide (DMSO) or 1-methyl-2-pyrrolidinone (NMP). The obtained amorphous polyesters exhibited good solubility in common organic solvents and provided optical-quality films by spin-coating. The SHG measurements of the spin-coated films of the resulting polyesters were carried out by the Maker fringe method using a Q-switched Nd:YAG laser (1064 nm) as an exciting beam after corona-poling. One of the polyesters, which was prepared from 5-[2-(N-methyl-4-nitroanilino)ethoxy]isophthalic acid and N-[4-((4-cyanophenyl)azo)phenyl]diethanolamine, exhibited the large second-order NLO coefficient, d 33, of 202 pm/V. On the other hand, a polyester containing chromophore moieties with the highest density, which was prepared from 5-{[1,3-bis[4-((4-cyanophenyl)azo)-N-methylanilino]-2-propyl]oxy}isophthalic acid and N-[4-((4-cyanophenyl)azo)phenyl]diethanolamine, exhibited good temporal stability of the second-order nonlinearity at ambient temperature.