Sulfuric Disazo Dye Stabilized Copper Nanoparticle Composite Mixture: Synthesis and Characterization

A copper nanoparticle–sulfuric disazo dye (Cu–SD1) composite was synthesized using the sol–gel method. Cu–SD1 nanocomposite formation was monitored by ultraviolet-visible spectroscopy (UV-vis). The acquired experimental results suggested that 8 h of reaction is needed for the synthesis Cu0 nanoparticles. Transmission electron microcopy (TEM) and atomic force microscopy (AFM) were employed to elucidate the morphology of the Cu–SD1 nanocomposite. It was found that the diameter of particle sizes were in the range of 2–4 nm. The interaction of SD1 with copper was confirmed by Fourier transform infrared spectroscopy (FTIR). The peak shift of O–H and C–OH functional groups indicated the interaction between SD1 and copper nanoparticles. Moreover, the azo group (N[double bond, length as m-dash]N) peaks were suppressed after the formation of the nanocomposite, suggesting that a strong linkage was formed between the functional groups and the copper nanoparticles. The surface composition and chemical states of the as-synthesized copper nanoparticles were elucidated by X-ray photoelectron spectroscopy (XPS). In addition, photo-switching of the composites was elucidated in the solution state. It was found that the Cu–SD1 nanocomposite has a faster switching response compared to the parent, SD1, in a solution

Photoinduced ordering and anchoring properties of azo-dye films

We study both theoretically and experimentally anchoring properties of photoaligning azo-dye films in contact with a nematic liquid crystal depending on photoinduced ordering of azo-dye molecules. In the mean field approximation, we found that the bare surface anchoring energy linearly depends on the azo-dye order parameter and the azimuthal anchoring strength decays to zero in the limit of vanishing photoinduced ordering. From the absorption dichroism spectra measured in the azo-dye films that are prepared from the azo-dye derivative with polymerizable terminal groups (SDA-2) we obtain dependence of the dichroic ratio on the irradiation dose. We also measure the polar and azimuthal anchoring strengths in nematic liquid crystal (NLC) cells aligned by the azo-dye films and derive the anchoring strengths as functions of the dichroic ratio. Though linear fitting of the experimental data for both anchoring strengths gives reasonably well results, it, in contradiction with the theory, predicts vanishing of the azimuthal anchoring strength at certain nonzero value of the azo-dye order parameter. By using a simple phenomenological model we show that this discrepancy can be attributed to the difference between the surface and bulk order parameters in the films.Comment: revtex4, 25 pages, 9 figure

Kinetics of photoinduced ordering in azo-dye films: two-state and diffusion models

We study the kinetics of photoinduced ordering in the azo-dye SD1 photoaligning layers and present the results of modeling performed using two different phenomenological approaches. A phenomenological two state model is deduced from the master equation for an ensemble of two-level molecular systems. Using an alternative approach, we formulate the two-dimensional (2D) diffusion model as the free energy Fokker-Planck equation simplified for the limiting regime of purely in-plane reorientation. The models are employed to interpret the irradiation time dependence of the absorption order parameters extracted from the available experimental data by using the exact solution to the light transmission problem for a biaxially anisotropic absorbing layer. The transient photoinduced structures are found to be biaxially anisotropic whereas the photosteady and the initial states are uniaxial.Comment: revtex4, 34 pages, 9 figure

Polarization-gratings approach to deformed-helix ferroelectric liquid crystals with subwavelength pitch

Electro-optical properties of deformed helix ferroelectric liquid crystal (DHFLC) cells are studied by using a general theoretical approach to polarization gratings in which the transmission and reflection matrices of diffraction orders are explicitly related to the evolution operator of equations for the Floquet harmonics. In the short-pitch approximation, a DHFLC cell is shown to be optically equivalent to a uniformly anisotropic biaxial layer where one of the optical axes is normal to the bounding surfaces. For in-plane anisotropy, orientation of the optical axes and birefringence are both determined by the voltage applied across the cell and represent the parameters that govern the transmittance of normally incident light passing through crossed polarizers. We calculate the transmittance as a function of the electric field and compare the computed curves with the experimental data. The theoretical and experimental results are found to be in good agreement

Switching dynamics of surface stabilized ferroelectric liquid crystal cells: effects of anchoring energy asymmetry

We study both theoretically and experimentally switching dynamics in surface stabilized ferroelectric liquid crystal cells with asymmetric boundary conditions. In these cells the bounding surfaces are treated differently to produce asymmetry in their anchoring properties. Our electro-optic measurements of the switching voltage thresholds that are determined by the peaks of the reversal polarization current reveal the frequency dependent shift of the hysteresis loop. We examine the predictions of the uniform dynamical model with the anchoring energy taken into account. It is found that the asymmetry effects are dominated by the polar contribution to the anchoring energy. Frequency dependence of the voltage thresholds is studied by analyzing the properties of time-periodic solutions to the dynamical equation (cycles). For this purpose, we apply the method that uses the parameterized half-period mappings for the approximate model and relate the cycles to the fixed points of the composition of two half-period mappings. The cycles are found to be unstable and can only be formed when the driving frequency is lower than its critical value. The polar anchoring parameter is estimated by making a comparison between the results of modelling and the experimental data for the shift vs frequency curve. For a double-well potential considered as a deformation of the Rapini-Papoular potential, the branch of stable cycles emerges in the low frequency region separated by the gap from the high frequency interval for unstable cycles.Comment: 35 pages, 15 figure

Photoalignment and photopatterning in liquid crystal photonics

We have already used photoalignment and photopatterning materials to align LC in superthin photonic holes, curved and 3D surfaces and as cladding layers in microring silicon based resonators. We have proposed optical switches, tunable-focus LC lenses, photo-patterned micropolarizer array for complementary metal-oxide-semiconductor (CMOS) image sensors and other photonics liquid crystal elements based on LC photoalignment and photopatterning