Phase-type gratings formed by photochemical phase transition of polymer azobenzene liquid crystals: Enhancement of diffraction efficiency by spatial modulation of molecular alignment

Unique characteristics were observed in phase-type gratings that were formed in a polymer azobenzene liquid crystal (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 (similar to 18%) was obtained with large surface modulation (68-76 nm), whereas the gratings recorded in the nematic (N) phase showed high diffraction efficiency (similar to 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.