Cholesterol-oleate-doped polymer-dispersed liquid-crystal voltage-controlled ring projector

We report a cholesterol-oleate-doped polymer-dispersed liquid-crystal (PDLC) device that, under laser illumination, exhibits a ring diffraction pattern, the radius of which is controlled by an externally applied voltage. This effect is accurately explained by modeling of the random distribution of spiral cells shown in the microstructure with a voltage-dependent shape variation.     

Small-angle light scattering from polymer-dispersed liquid-crystal films

A method is developed for modeling and computing the angular distribution of light scattered forward from a single-layer polymer-dispersed liquid-crystal (PDLC) film. The method is based on effective-medium approximation, anomalous diffraction approximation, and far-field single-scattering approximation. The angular distribution of forward-scattered light is analyzed for PDLC films with droplet size larger than the optical wavelength. The method can be used to study field-and temperature-induced phase transitions in LC droplets with cylindrical symmetry by measuring polarized scattered light intensity.     

Fast optical recording in dye-doped polymer-dispersed liquid-crystal films

We describe a fast optical recording material based on a dye-doped polymer-dispersed liquid-crystal film. A holographic grating is written in this film with a single Q -switched Nd:YAG laser pulse that has a duration of ~6 ns . Such a grating is due to the reorientation effect of the liquid-crystal molecules through interaction with the photoinduced adsorption of the doped azo dyes. Experimental results indicate that the grating thus formed is permanent but electrically switchable.     

Switchable holographic gratings formed in polymer-dispersed liquid-crystal cells by use of a He-Ne laser

We report on the formation of switchable holographic transmission gratings in polymer-dispersed liquid-crystal cells by use of the 633-nm wavelength of a He-Ne laser. We present results for the structure, diffraction efficiency, and switching characteristics of the gratings.     

Transfer functions of nonuniform transmission photonic structures in polymer-dispersed liquid-crystal materials

The transfer functions of nonuniform transmission photonic structures holographically formed in polymer-dispersed liquid-crystal materials are calculated. The diffraction properties of these structures are numerically calculated for different types of inhomogeneity, coupling parameters, and light beam divergences.     

Evidence for droplet reorientation and interfacial charges in a polymer-dispersed liquid-crystal cell

We observe a new feature ( plateau), sandwiched between two exponentially decaying components, in the switching response of a polymer-dispersed liquid-crystal (PDLC) cell. Upon application of square-wave electrical pulses, the cell response exhibits a complex decay whose characteristics are significantly different within and outside the applied field. We interpret these results in terms of reorientation dynamics of nonspherical droplets in the PDLC. Irradiation of the PDLC with gamma rays induces significant changes in the cell response in a manner that is consistent with redistribution of charges at the liquid-crystal-polymer interface in the PDLC.     

Focusing of light by polymer-dispersed liquid-crystal films with nanosized droplets

An analysis is presented of polarization-independent electrically tunable light focusing by polymerdispersed liquid-crystal films with nanosized liquid-crystal droplets. Polymer-dispersed liquid-crystal films with axially symmetric distributions of liquid-crystal droplet concentration and layers with axially symmetric thickness profiles are considered. The paraxial, Rayleigh, and Rayleigh-Gans approximations, as well as the Foldy-Twersky equation, are used to examine the dependence of focal length on lens geometry, droplet size, concentration of nematic liquid-crystal droplets, and applied field. The tunable focusing ranges are evaluated for both lens types considered in the study. Dependence of the transmittance of polymer-dispersed liquid-crystal film on its characteristics is analyzed. The results obtained are compared with those available from the literature.     

Photorefractive inorganic organic polymer-dispersed liquid-crystal nanocomposite photosensitized with cadmium sulfide quantum dots

We report on a new photorefractive medium consisting of a hole-transporting polymer composite matrix, electro-optically active nanodroplets of liquid crystals, and cadmium sulfide quantum dots as photosensitizers. This medium exhibits greater than 90% internal diffraction efficiency and a net two-beam coupling gain of 22.5 cm(-1) with a response time of <1 min . Data on optical transmission are also presented.     

Droplet deformation and alignment for high-efficiency polarization-dependent holographic polymer-dispersed liquid-crystal reflection gratings

Droplet deformation and alignment are achieved in holographic polymer-dispersed liquid-crystal reflection gratings by applying an in situ shear during recording. High diffraction efficiency (99%) is obtained for light polarized parallel to the shear, with nearly zero efficiency for perpendicular polarization, and no increase of incoherent scattering. Permanent polarization dependence is related to stress-induced morphology changes of liquid-crystal droplets that are frozen by polymerization. The system is studied by electron microscopy and modeled by anisotropic coupled-wave and scattering theory. The morphology is consistent with the theory of small deformations of liquid droplets in fluid flow. Diffraction efficiency measurements are in agreement with theory incorporating this morphology as well as concomitant orientation and alignment of liquid-crystal molecules.     

Recording of thin phase holograms in polymer-dispersed liquid-crystal composites based on fullerene-containing π-conjugated organic systems

The recording of thin phase holograms in promising fullerene-containing, polymer-dispersed liquid-crystal composites based on 2-cyclooctylamino-5-nitropyridine and N-(4-nitrophenyl)-(L)-prolinol using picosecond laser pulses is studied for the first time. The anisotropy of the refractive index substantially increases after laser irradiation and exceeds by a factor of five this anisotropy for liquid-crystal cells containing dyes. The results of our studies suggest that the fullerene-containing, polymer-dispersed liquid-crystal systems can be used as passive media for reversible data recording.     

Self-activated liquid-crystal cells with photovoltaic substrates

We show that photovoltaic fields are capable of efficiently reorienting liquid crystals, leading to new concepts of optically addressable light modulators. Using an arrangement consisting of a liquid-crystal layer between LiNbO3:Fe photovoltaic substrates, we observed spatial filtering due to self-phase modulation in a planar-oriented cell and nonlinear transmission between crossed polarizers in a twist-oriented cell. These processes do not require an external electric field. The substrates are arranged such that light propagates along the +c axis in each substrate, allowing a secondary process of power transfer to occur through contradirectional photorefractive two-beam coupling.     

Soliton blueshift in tapered photonic crystal fibers

We show that solitons undergo a strong blueshift in fibers with a dispersion landscape that varies along the direction of propagation. The experiments are based on a small-core photonic crystal fiber, tapered to have a core diameter that varies continuously along its length, resulting in a zero-dispersion wavelength that moves from 731 nm to 640 nm over the transition. The central wavelength of a soliton translates over 400 nm towards a shorter wavelength. This is accompanied by strong emission of radiation into the UV and IR spectral regions. The experimental results are confirmed by numerical simulation.     

Electrochromic cells with iridium oxide display electrodes

We report the fabrication and electrochromic behavior of iridium oxide film electrodes, including a specific display electrode design compatible with the high peak ion-insertion current densities accepted by this material and necessary for the shortest possible response time. The configuration of a complete two-terminal display cell, in which the high capacity counter electrode behaves as a pseudo-reference electrode, is presented. Cell performance, including the use of IR compensated constant-voltage address when response time is limited by impedances external to the iridium oxide itself, is reported.     

Visualization of the langmuir-blodgett film structure in liquid-crystal cells

The use of liquid-crystal (LC) cells for the visualization of the Langmuir-Blodgett (LB) film structure is described. Images appearing in LC cells upon LB film application are presented. The images are analyzed in relation to the LB film structure (application conditions) and to the relief formed by indium tin oxide (ITO) electrodes.     

Voltage sensitivity of the optical nonlinearity in nematic liquid-crystal cells

The influence of an external quasi-static electric field on the third-order nonlinear optical properties of a nematic liquid crystal has been studied. We demonstrate both in theory and experiment that the cell behaves like a Kerr-Medium. The sensitivity of the cell can be controlled by the applied voltage. If optical feedback is provided by the use of a dielectric mirror, optical bistability can be achieved. The switching behaviour of the cell may also be controlled by the applied voltage.     

Rotational reorientation of the director in twisted nematic liquid-crystal cells

The orientational relaxation of the director to its equilibrium orientation under electric, elastic, and viscous torques arising in twisted nematic liquid-crystal cells is investigated. It is shown that the relaxation time of the director depends strongly on the external electric field strength and weakly on the energy of anchoring of liquid-crystal molecules to the surfaces of the cell. The relaxation time of the director anomalously increases in electric fields close to the Fréedericksz threshold. It is established that, at specific strengths of the external electric field, the relaxation can occur in the form of traveling waves propagating from one edge to the other edge of the twisted nematic liquid-crystal cell. The calculations of the relaxation processes in the vicinity of the nematic-smectic A phase transition temperature demonstrate that the distortion of the director field is uniform over the entire cross section of the liquid-crystal cell and does not depend on the strength of anchoring of the liquid-crystal molecules to the surfaces of the cell.     

Optically implemented broadband blueshift switch in the terahertz regime

We experimentally demonstrate, for the first time, an optically implemented blueshift tunable metamaterial in the terahertz (THz) regime. The design implies two potential resonance states, and the photoconductive semiconductor (silicon) settled in the critical region plays the role of intermediary for switching the resonator from mode 1 to mode 2. The observed tuning range of the fabricated device is as high as 26% (from 0.76 THz to 0.96 THz) through optical control to silicon. The realization of broadband blueshift tunable metamaterial offers opportunities for achieving switchable metamaterials with simultaneous redshift and blueshift tunability and cascade tunable devices. Our experimental approach is compatible with semiconductor technologies and can be used for other applications in the THz regime.     

Abnormal blueshift of UV emission in single-crystalline ZnO nanowires

Single-crystalline ZnO nanowires (NWs) were synthesized by a facile vapor transport method. The good orientation and high crystal quality were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) measurements. Excitation-power-dependence photoluminescence spectra of ZnO NWs show that the UV emission displayed an evident blueshift with increasing excitation power and the corresponding energy shift might be as large as 10 meV. This anomalous phenomenon correlates to the band bending level caused by the surface built-in electric field due to the existence of substantial oxygen vacancies. By increasing the excitation power, the enhanced neutralization effect near the surface will reduce the built-in electric field and lead to a reduction of band bending which triggers the blueshift of the UV emission.     

Photorefractive polymer-dispersed liquid crystals

We report observation of the photorefractive effect in functionalized polymer-dispersed liquid crystals. The photoconductive properties are provided by the polymer matrix, and the field-dependent refractive-index changes are generated by the dispersed nematic liquid-crystal droplets. A high diffraction efficiency (8%) and a high refractive-index modulation amplitude (Dn = 2 x 10(-3)) are obtained in 53- microm-thick samples with an applied field of 22 V/microm.