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 forme d in a polymer azobenzene liquid crystal (PALC) film. Generation of sinusoi dal variation of the surface of the film was confirmed by atomic force micr oscopy. However, comparison of the diffraction efficiency with the surface modulation revealed that the gratings could not be characterized only as co nventional surface-relief gratings. In the glassy state of the film, modera te efficiency (similar to 18%) was obtained with large surface modulation ( 68-76 nm), whereas the gratings recorded in the nematic (N) phase showed hi gh diffraction efficiency (similar to 28%) with slight surface modulation ( 33-53 nm). The diffraction efficiency was enhanced in the liquid-crystallin e phase. Dynamics of the first-order diffraction beam exhibited that the gr ating 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 appropria te 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 sp eculation was supported by atomic force and polarizing optical microscopy. Anisotropy in the diffraction efficiency with respect to the linearly polar ized readout beam also supported our hypothesis. It was revealed that the l arge enhancement of the efficiency was attributable to spatial modulation o f molecular alignment.