Structural transitions in holographic polymer-dispersed liquid crystals

Multiple Bragg Diffractions with Different Wavelengths and Polarizations Composed of Liquid Crystal/Polymer Periodic Phases

We first fabricated holographic polymer-dispersed liquid crystals (HPDLCs) that produce multiple Bragg diffractions with different polarization states for every angle of incidence, through a photopolymerization-induced phase separation by one-time interferential exposure. The polarizations of the Bragg diffractions were well-controlled at individual wavelengths in the fabrication process by the compositional ratio of LCs to monomers. The raw mixtures of extremely low-functionality monomers having very different viscosities were used to reduce the domain size in phase separation and subsequently to form elaborate periodic structures of the LC and polymer phases. A cross-linker (1-vinyl-2-pyrrolidione) and a prepolymer with urethane groups were employed to strengthen the polymer network. Note that the diffractions of our HPDLCs are regarded as not purely but mostly Bragg type, according to the evaluation with the established criteria. The devices, which are monolithic but versatile in diffractive behaviors, have advantages of simple manufacturing and handling.

Holographic Gratings for Slow-Neutron Optics

Recent progress in the development of holographic gratings for neutron-optics applications is reviewed. We summarize the properties of gratings recorded in deuterated (poly)methylmethacrylate, holographic polymer-dispersed liquid crystals and nanoparticle-polymer composites revealed by diffraction experiments with slow neutrons. Existing and anticipated neutron-optical instrumentations based on holographic gratings are discussed.

Optical diffractometry of highly anisotropic holographic gratings formed by liquid crystal and polymer phase separation

Optical diffractometry is proposed as a practical method of quantitatively analyzing the microscopic structural origins of a wide range of highly efficient and linearly polarized optical diffraction grating produced from holographic polymer-dispersed liquid crystal. The structure is organized by a spatially periodical distribution of submicrometer-scale liquid crystal (LC) droplets in a polymer matrix. Six independent Bragg diffraction spectra were obtained at two orthogonal polarization states at temperatures below, at, and above the nematic-to-isotropic phase transition point. These spectra were simultaneously analyzed by employing anisotropic diffraction theory under the restraint of a simple and widely useful structural model constructed on the basis of the previously reported microscopic observations. The refractive indices of spatially periodic LC- and polymer-rich phases were analyzed using Cauchy's equation as a function of optical wavelength. The present diffractometry was demonstrated for a variety of holographic structures, and the structural parameters were discussed such as the filling ratio of LC droplets to polymer matrix, the orientational order in the droplets, and the thermo-optic properties in the LC droplets. Furthermore, the higher order Bragg diffractions were measured and discussed. The proposed method was examined in consistency by comparisons with polarizing optical microscopy and scanning electron microscopy.

Effects of anisotropic diffractions on holographic polymer-dispersed liquid-crystal gratings

Volume gratings fabricated by interferometric exposure using composite materials composed of nematic liquid crystals (LC) and LC diacrylate monomers are discussed in the effects of diffraction properties on different grating formations, such as varying LC content ratios, film thicknesses, and the surface conditions composed of alignment layers and rubbing directions. Diffraction properties are experimentally investigated in the viewpoints of anisotropic diffraction and LC orientation. The polarization-azimuth dependence of diffraction efficiencies as functions of the incident polarization states shows the controllability of anisotropic diffractions based on the effects of different surface conditions.

Colossal light-induced refractive-index modulation for neutrons in holographic polymer-dispersed liquid crystals

We report strong diffraction of cold neutrons from an only 30 micro m thick holographic polymer-dispersed liquid crystal (H-PDLC) transmission grating. The light-induced refractive-index modulation for neutrons is about 10(-6), i.e., nearly 2 orders of magnitude larger than in the best photo-neutron-refractive materials probed up to now. This makes H-PDLCs a promising candidate for fabricating neutron-optical devices.

Temperature dependence of optical anisotropy of holographic polymer-dispersed liquid crystal transmission gratings

We measured the angular dependence of the 0th, +/-1 st, and +/-2 nd optical diffraction orders from a 50 microm thick transmission grating recorded in a UV-curable holographic polymer-dispersed liquid crystal (HPDLC) made from commercially available constituents. The analysis was performed for two orthogonal polarizations of the probe beams. The emphasis was laid on the temperature dependence of the grating anisotropy. Above the nematic-isotropic phase transition, the grating is optically isotropic. At lower temperatures the grating strength for the optical polarization perpendicular to the grating vector decreases with decreasing temperature, while for orthogonal polarization it increases with decreasing temperature. As a consequence, a regime of diffraction with strongly overmodulated gratings is observed. Our investigations indicate that the anisotropy of the refractive-index modulation scales with the optical anisotropy of the liquid crystal medium forming the phase-separated domains. We further demonstrate that light scattering effects, which are profound only in the nematic phase, must not be neglected and can be taken into account via a Lorentzian line-shape broadening of the probing wave vector directions in the framework of the diffraction theory for anisotropic optical phase gratings.

Visible and near-infrared characterization and modeling of nanosized holographic-polymer-dispersed liquid crystal gratings

We have studied the electro-optical and angular behavior of holographic-polymer-dispersed liquid crystal gratings at different wavelengths, in the visible and in the near-infrared range. As usual in these kinds of materials, a strong polarization dependent behavior was observed. Our samples showed very high diffraction efficiency for p-polarized radiation at 1.55 microm, which is very interesting for many possible applications in the telecom field. However, we also observed a very unusual behavior for visible p-polarized light and we try to suggest some explanation for that. By analyzing the angular dependence of the diffraction efficiency, we could measure the components of the permittivity modulation tensor and infer important information about the main parameters involved in the grating structure: the degree of phase separation and the anisotropy in the liquid crystal droplet distribution. In our opinion, this simple and nondestructive methodology can be very useful for studying these kinds of materials and getting information on their morphology, in view of optimizing their properties. Finally, we discuss the role of the refractive index optical dispersion in order to describe the behavior of these materials at different wavelengths. These remarks are especially important when properties in the infrared range are extrapolated from measurements in the visible.