“…Our observation indicated that in the in-plane orientation the main axes of the liquid crystal molecules are in the direction of the capillary flow, which is orthogonal to the dipping direction. This is different from previously reported results in which the LCs are aligned in the dipping direction. 27a,, A possible explanation for this difference is that the capillary flow of the LC molecules in our case plays a similar role as a piece of soft cloth in a rubbing process. Meanwhile, the in-dipping-direction planar alignment mostly takes place if the main chain of the polymer is very stiff .…”
To generate command surfaces for all-optical switching, highly ordered polymeric Langmuir-Blodgett-Kuhn (LBK) multilayers were fabricated onto silicon substrates and gold-coated optical glass slides from novel azobenzene-bearing polyamic acid systems. Pronounced Bragg peaks and well-defined Kiessig fringes observed in the X-ray reflectivity measurement for samples on silicon substrates indicate that these films possess a regularly repeated Y-type LBK multilayer structure and ultrasmooth surfaces. Fourier transform infrared (FT-IR) spectra taken by grazing incidence reflection suggest specific orientations of the functional groups in the layers. The excellent film-forming properties of the polyamic acid allow for a smooth buildup of several hundreds of layers of the LBK films onto gold-coated glass slides, which in turn allows for determining the geometrical thickness and the anisotropic refractive indices of the films by using optical waveguide spectroscopy. Interestingly, the probe laser beam induced a distinct fluorescence signal from the films, which remained even after the film underwent a thermal imidization process at 160 degrees C for 8 h in vacuo. LBK films fabricated from these compounds can be successfully applied for all-optically switching the alignment of liquid crystals by irradiation with light of different wavelengths.
“…Our observation indicated that in the in-plane orientation the main axes of the liquid crystal molecules are in the direction of the capillary flow, which is orthogonal to the dipping direction. This is different from previously reported results in which the LCs are aligned in the dipping direction. 27a,, A possible explanation for this difference is that the capillary flow of the LC molecules in our case plays a similar role as a piece of soft cloth in a rubbing process. Meanwhile, the in-dipping-direction planar alignment mostly takes place if the main chain of the polymer is very stiff .…”
To generate command surfaces for all-optical switching, highly ordered polymeric Langmuir-Blodgett-Kuhn (LBK) multilayers were fabricated onto silicon substrates and gold-coated optical glass slides from novel azobenzene-bearing polyamic acid systems. Pronounced Bragg peaks and well-defined Kiessig fringes observed in the X-ray reflectivity measurement for samples on silicon substrates indicate that these films possess a regularly repeated Y-type LBK multilayer structure and ultrasmooth surfaces. Fourier transform infrared (FT-IR) spectra taken by grazing incidence reflection suggest specific orientations of the functional groups in the layers. The excellent film-forming properties of the polyamic acid allow for a smooth buildup of several hundreds of layers of the LBK films onto gold-coated glass slides, which in turn allows for determining the geometrical thickness and the anisotropic refractive indices of the films by using optical waveguide spectroscopy. Interestingly, the probe laser beam induced a distinct fluorescence signal from the films, which remained even after the film underwent a thermal imidization process at 160 degrees C for 8 h in vacuo. LBK films fabricated from these compounds can be successfully applied for all-optically switching the alignment of liquid crystals by irradiation with light of different wavelengths.
“…This would bring to the reorientation of LC molecular cluster in the illuminated area giving rise modulation of ''reading'' light polarization. Similar dyedriven phenomena have been observed using the photoisomerization of different azo derivatives as a command layer on the LC cell substrates, 4,5 or in liquid crystalline polymers, 6 etc.…”
Opto-dielectric effect on a nematic liquid crystal doped with a photoactive azo mesogen Dynamic holography and polarimetric techniques are used to study the photoisomerization kinetics of D2 bisazo dye and the excitation transfer from dye to nematic liquid crystal host. As a result of this transfer, transient modulation of the orientational order parameter, three-dimensional long-scale molecular transient rotation and equilibrated two-dimensional molecular rotations are identified in the host. Amplified rotation of nematic molecules is observed which is much more efficient for the dye photoexcitation with a wavelength in the red rather than in the blue side of its absorption band. The possible role of dye photoisomerization mechanism in these processes is discussed. The dynamic character of the dye induced torque and the presence of long-living intermediate states during photoisomerization and their diffusion are suggested.
“…Even though the in-plane orientation was changed by the linearly polarized UV light irradiation, the acquired orientation again reverted to the original direction imposed by the dipping process after a couple repetitions of alternating nonpolarized visible and UV light illumination for both 6Az10-PVA and 6Az10-PVA- d 3 (unpublished results). Sekkat et al demonstrated this memory effect by surface plasmon resonance waveguide spectroscopy using the 6Az10-PVA monolayered LB films. Appearance of such an orientational memory effect would be more naturally understood by assuming a preferred orientation of the polymer backbone.…”
Section: Discussionmentioning
confidence: 98%
“…Waveguide spectroscopy has been applied to the 6Az10-PVA/nematic LC systems by Knobloch et al They presented experiments on the dynamics of the photoinduced LC orientation process and reported a continuous transition character of this surface-mediated process. Furthermore, Sekkat et al carried out the same measurements, particularly focusing on the sensitivity to the linearly polarized light illumination. 1 …”
Langmuir−Blodgett (LB) films consisting of an amphiphilic
azobenzene (Az) polymer having
a deuterated poly(vinyl alcohol) were prepared at varied lateral
Az packing density, and their initial and
light-modulated structures were evaluated by X-ray reflectometry and
Fourier-transform infrared (FT-IR) spectroscopy. X-ray reflectometry indicated that the layer
structure of the LB film is strongly
dependent on the lateral packing density. The more densely packed
film was characterized by the better-defined structure in terms of the order of the local lamellar patterns
and surface smoothness of the total
film. The overall film thickness decreased and increased upon
exposure to UV and visible light,
respectively; however, the lamellar structure was irreversibly
disordered by the first exposure to UV
light. The deuterated PVA chain was found to have no appreciable
orientational preference in the in-plane direction, as proven by polarized transmission FT-IR, although a
nematic liquid crystal (LC) placed
on these LB films exhibited a definitive homogeneous in-plane alignment
depending on the Az packing
density [Seki, T.; et al. Thin Solid Films
1994,
243, 675]. UV light (365 nm) irradiation on the LB
films
induced a tilt of the alkyl part in the Az side chain from the surface
normal. The time course observation
of the UV light induced changes in the IR signals revealed that the
motion of the alkyl part in the most
densely packed LB film was retarded from that of the Az aromatic rings.
The IR signals from the alkyl
part reverted to the original state on subsequent visible light (436
nm) illumination, but reproducibility
of the signals from the aromatic ring was poor. Finally, an
increase in the Az packing density substantially
retarded the response of the UV light induced homeotropic → planar
alignment change of a nematic LC
placed on these Az LB films. The retarding effect was more
manifest in this intermolecularly driven
system.
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