Phase-Type Gratings Formed by Photochemical Phase Transition of Polymer Azobenzene Liquid Crystals:  Enhancement of Diffraction Efficiency by Spatial Modulation of Molecular Alignment

Abstract

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.

Molecular Dynamics Simulations of Various Branched Polymeric Liquid Crystals

“…On the same time scale we observe contraction of the box along the field, due to repacking of the mesogens, see Figure 9, right. This effect is observed experimentally in nematic films under spatially modulated illumination when the film is examined by polarizing optical microscopy [23]. It also reproduces anisotropic shrinking of the P6a12 freely floating drop along the light polarization [21].…”

Light‐Responsive Block Copolymers

“…In the case of holographic gratings imprinted on azobenzene containing LC polymer films, the DE is mainly governed by two kinds of contributions ( Figure 21). [50,73,74] Firstly, a refractive-index grating (RIG) is imprinted by a spatially selective photoisomerization (alternation of LC and isotropic phases). Secondly, this RIG is usually accompanied by a deformation of the surface which contributes often in great part to the DE.…”

Anisotropic Photomechanical Response of Stretched Blend Film Made of Polycaprolactone‐Polyvinyl Ether with Azobenzene Group as Side Chain

“…[1][2][3][4][5][6][7][8][9][10] Especially, polymer films containing azobenzene moiety have been investigated in detail, because they have potential as photoactive materials for micropatterning, [6,11] reversible optical storage, [5,12,13] and actuators. [14] For example, a liquid crystalline polymer having azobenzene moiety was shown to induce the nematic-isotropic phase transition by UV irradiation.…”