Dynamic studies of holographic gratings in dye-doped liquid-crystal films

Direct measurement of the propagation of the phase-transition region of liquid crystals

Many types of active matter, such as biological cells, have liquid-crystalline membranes, which are soft and flexible in their interactions with their surroundings and sometimes allow molecular-structural or -orientational changes to extend for long distances, owing to long-range molecular interactions. Despite the technological and fundamental importance of these long-range changes, there is no good physical property with which to express them for the liquid crystal. Here, we show direct measurements of the propagation of structural or orientational changes due to long-range molecular interactions in liquid crystals. We induced a patterned phase transition in a liquid crystal via illumination with a fringe pattern and observed the propagation of the phase-transition region. We determined that the propagation occurred in a ballistic manner with a velocity of 80–110 m/s and that two types of propagation—side-by-side and head-to-tail molecular interactions—were found.

Enhanced diffraction properties of photoinduced gratings in nematic liquid crystals doped with Disperse Red 1

Diffraction properties of photoinduced gratings recorded by overlapping two coherent beams at 532 nm in nematic liquid crystals doped with Disperse Red 1 were investigated with a probe beam at 632.8 nm. The grating was formed due to the alignment of dye molecules that leaded to the reorientation of the liquid crystal phase. The diffraction efficiency of the photoinduced grating was found to increase rapidly when the sample temperature was close to the clearing point in the nematic phase and a nearly 30-fold enhancement of the first-order diffraction efficiency was obtained. The pretransitional enhancement of the diffraction efficiency was discussed in terms of the reorientation of liquid crystals, optical nonlinearity effects and the onset of critical opalescence near the nematic-isotropic phase transition. Moreover, a peak shift of diffraction efficiency towards the lower temperature was observed with the increase of recording light intensity, which was attributed to laser induced photochemical disordering.

Photo-rewritable flexible LCD using indium zinc oxide/polycarbonate substrates

This work presents a photo-rewritable transmissive flexible-LCD based on the alignment effect of the photo-induced adsorption of azo dye on flexible indium zinc oxide/polycarbonate (IZO/PC) substrates. Images can be displayed without applying an external field and rewritten by changing the direction of the writing laser beam while the cell temperature is controlled. By using IZO/PC substrates, the writing and erasing processes can be achieved within 1 min with a high contrast.

Polarization-independent holographic gratings based on azo-dye-doped polymer-dispersed liquid-crystal films

We demonstrate polarization-independent holographic gratings (PIHGs) based on azo-dye-doped polymer-dispersed liquid crystal films. The PIHGs are recorded by irradiation under an intensity-modulated interference field, generated by two linearly polarized green optical beams, and by simultaneously applying a suitable AC voltage to the sample. The photoexcited azo dyes are adsorbed onto the UV-cured polymer film with their long axes parallel to the normal of the substrate. When the applied voltage is switched off, the PIHGs are generated with periodic modulation of liquid-crystal structures with transparently homeotropic and randomly scattered alignments. Additionally, the generated PIHGs can be completely switched off by an applied voltage.

Measurement of helical twisting power based on axially symmetrical photo-aligned dye-doped liquid crystal film

This investigation demonstrates a simple but accurate method for measuring the helical twisting power of chiral doped liquid crystals using axially symmetrical photo-alignment in azo dye-doped liquid crystal films. As reported in our previous paper, a reversed twist effect produces a disclination line in photo-aligned axially symmetrical liquid crystal films. The pitch and helical twisting power can be obtained by measuring the rotation angle of the disclination line in chrial doped liquid crystal. This method is independent of cell gap and provide an error below 0.5%.

Investigation of multiple holographic recording in azo-dye-doped nematic liquid-crystal film

We report multiple holographic recording and optical address recognition schemes in Methyl Red-doped liquid-crystal film without an applied electric field. Ten gratings are recorded at a single location of a 5 mm2 area by using optical multiplexing methods, and the diffraction efficiencies are studied for multiplexed gratings. Diffraction behavior of angular and peristrophic multiplexed gratings is discussed, and two methods that involve recording angles and peristrophic rotation angles are presented for optical recognition.

Holographic image storage and multiple hologram storage in a planar Methyl Red-doped liquid crystal film

Holographic images are recorded and the diffraction characteristic of the hologram is investigated in a planar Methyl Red-doped liquid crystal film without an external electric field. Multiple holograms are stored in this film with no external electric field, and peristrophic multiplexing, angular multiplexing, and spatial multiplexing are demonstrated to be suited for the implementation of azo-dye doped liquid crystal films among the multiplexing techniques. The diffraction efficiency is theoretically studied for multiple storage. Optical reconstruction for peristrophic multiplexed holograms and angular multiplexed holograms is discussed, which involves the peristrophic rotation angles and the recording angles, respectively.

Three-dimensional vector holograms in anisotropic photoreactive liquid-crystal composites

In this paper, we describe the principle of the three-dimensional vector holograms formed in anisotropic recording medium. The polarization states of the interference light are three-dimensionally modulated due to both the polarization interference and optical anisotropy in the recording medium. The electric field of the polarized light reorients the director and forms the three-dimensional vector hologram in anisotropic photoreactive liquid-crystal composites. The theoretical consideration reveals the formation mechanism and optical characteristics of the resultant three-dimensional vector holograms.

Axially symmetric polarization converters based on photo-aligned liquid crystal films

This work demonstrates axially symmetric polarization converters based on photo-alignment in dye-doped liquid crystal (DDLC) films. A linear-shape and linearly polarized beam is applied onto a rotated homogeneous DDLC cell to achieve three axially symmetric polarizations - radial, azimuthal and vortical. Additionally, the spiral degree of the axially symmetric vortical polarization can be controlled by varying the polarization of the pumping light and the simulation results agree well with the experiment.

Polarization controllable spatial filter based on azo-dye-doped liquid-crystal film

This investigation establishes the feasibility of exploiting the surface-assisted photoalignment effect in dye-doped liquid-crystal (DDLC) films as spatial filters with controllable polarization in optical signal processing. The fabrication relies on the fact that the various intensities of the diffracted orders are responsible for various changes of the polarization state induced by the photoaligned DDLC film. Specific spatial orders in Fourier optical signal processing can be filtered by use of an analyzer placed behind the sample to control the polarization state of the diffracted orders. A simulation was performed, and the results agree closely with the experimental data.

Photorefractive effect induced by polarization gratings in dye-doped liquid crystals

We report on the photorefractive effect induced by a polarization grating in the presence of dc voltage in a dye-doped liquid-crystal (DDLC) film. The writing beams are two orthogonally (left- and right-circularly) polarized laser beams that create a spatially polarization-modulated interference field with constant intensity. The photorefractivity is ascribed to the absorption anisotropy of the azo dye. The unique dichroism of a DDLC cell causes a spatial variation in the absorption of light in response to a polarization-modulated interference field. Such a variation establishes a space-charge field in the presence of dc voltage, generating photorefractivity. Two-beam couplings were also verified and measured dynamically during the formation of the photorefractive grating in this study.

Laser-induced reorientation effect and ripple structure in dye-doped liquid-crystal films

The effects of light-induced reorientation on a homeotropical dye-doped liquid crystal (DDLC) cell are discussed. The photoexcited azo dye Methyl Red (MR) is diffused and adsorbed onto the substrate, thus forming a ripple structure. The adsorbed dye and the laser-induced ripple structure then reorient the liquid-crystal molecules and induce a holographic grating. Initially, the liquid-crystal directors are reoriented primarily by the adsorbed dye. However, given a sufficiently large ripple groove amplitude, the torque imposed by the ripple grooves overcomes that which is due to the adsorbed dyes, and the liquid crystals are realigned along the groove direction.