Polarization-independent optical fiber modulator by use of polymer-dispersed liquid crystals

Ultrasmall light modulators have been made by sandwiching a polymer-dispersed liquid crystal (PDLC) between two ferrules with optical fibers. The device can modulate light independent of the state of polarization, because the PDLC becomes transparent or opaque when either sufficient or no voltage is applied to the film. The PDLC was prepared by mixing and annealing a prepolymer and nematic liquid crystal with large anisotropy. An optical fiber modulator with a 30-mum thick PDLC film had an extinction ratio of 8:1-33:1, an insertion loss of 1.3 dB, and rise and decay times of 4 ms at a wavelength of 1.3 mum.     

All-optical switchable holographic Fresnel lens based on azo-dye-doped polymer-dispersed liquid crystals

Fabrication of an all-optical switchable holographic liquid crystal (LC) Fresnel lens based on azo-dye-doped polymer-dispersed LCs is reported using a Michelson interferometer. It is found that, upon circularly polarized photoirradiation, the diffraction efficiency of the fabricated Fresnel lens was increased significantly in a reversible manner. We believe this is due to the anisotropy induced by reorientation of the LC molecules coupled with azo-dye molecule orientation due to trans-cis-trans photoisomerization, which modulates the refractive index of the LC-rich regions. We also studied the effect of azo dye on the polarization dependency of the fabricated lens.     

Optically induced changes of absorption in polymer-dispersed liquid crystals doped with inorganic nanocrystals

A principal role of the nanocrystallites of La₂CaB₁₀O₁₉ on single pulse photoinduced changes of the liquid crystals was shown. It was established that the effect of the photoinduced changes is sensitive to the concentration of the nanocrystallites and to the type of liquid crystalline matrices. The principal role in the observed effects should play local thermoheating which determines effective exchange of energy between the nanocrystallites and the polymer matrices.     

Multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals: static and dynamic studies

The optimization of the experimental parameters of two multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals is investigated. Two methods are used to record the holograms: simultaneous and sequential multiplexing. These two processes are optimized to produce two multiplexed Bragg gratings that have the same and the highest possible diffraction efficiencies in the first order. The two methods show similar results when suitable recording parameters are used. The parameters of the recorded gratings (mainly the refractive-index modulation) are retrieved by use of an extension of the rigorous coupled-wave theory to multiplexed gratings. Finally, the response of the holograms to an electric field is studied. We demonstrate few coupling effects between the behavior of both gratings, and we expect a possibility of switching from one grating to the other.     

Study of the dynamic formation of transmission gratings recorded in photopolymers and holographic polymer-dispersed liquid crystals

Local and nonlocal models for the diffusion of photopolymers are applied to the dynamic formation of transmission gratings recorded in photopolymers and holographic polymer-dispersed liquid crystals (H-PDLCs). We retrieve the main parameters of H-PDLCs (refractive-index modulation and diffusion coefficient) by combining a solution of the one-dimensional diffusion equation and the rigorous coupled-wave theory applied to transmission gratings. The rigorous coupled-wave theory method provides us with information on higher harmonics of the refractive profile (not only on the first harmonic as when the classical Kogelnik theory is applied). Measurements concerning the second harmonic validate the modeling.     

Kerr modulators based on polymer-dispersed liquid crystal complexes

We have demonstrated a possibility to design light modulator based on Kerr effect using borate La₂CaB₁₀O₁₉ (LCBO) nanocrystallites incorporated into two different polymer-dispersed liquid crystal composites (PDLC). For this reason we have used different amount of the LCBO nanocrystallites to find the optimal conditions for the Kerr modulators. It was established that for the both PDLC systems the borate nanocrystals may be used for effective operation by elecrooptical susceptibilities varying the content of the nanocrystals. At the same time the frequency shift of the Kerr coefficient seems to be sensitive only for the LCBO nanocrystals content about 5% by weight. The maximal value of the Kerr coefficients were varied within the 4 × 10⁻⁹ to 6 × 10⁻¹⁰ m V^−2.     

Characterization of optically switchable holographic polymer-dispersed liquid crystal transmission gratings

This study characterizes the all-optical switching effect in holographic polymer-dispersed liquid crystal transmission gratings. The light-induced switching behaviors of these structures are due to the doped azobenzene-derived LC (azo-LC), which changes the refractive index of phase-separated LC within the polymer composite. This study also optimizes the polymer-dispersed liquid crystal formulation containing 15 wt.% azo-LC and 35 wt.% nematic LC to achieve a grating performance with a tunable diffraction efficiency of 78% and a fast switching-on time (0.5 s) with a relatively small light stimulus of 9 mW/cm².     

Optical reconfiguration by anisotropic diffraction in holographic polymer-dispersed liquid crystal memory

Holographic polymer dispersed liquid crystal (HPDLC) memory is fabricated by a photoinduced phase separation comprised of polymer and liquid crystal (LC) phases using laser light interference exposures. The anisotropic diffraction induced by the alignment of LC in the periodic structure of the HPDLC memory is applied to reconstruct the configuration contexts for the optically reconfigurable gate arrays. Optical reconfiguration for various circuits under parallel programmability is implemented by switching the polarization state of incident light on the HPDLC memory using a spatial light modulator.     

Multistability in polymer-dispersed cholesteric liquid crystal film doped with ionic surfactant

A memory effect has been discovered in composite films based on a polymer containing dispersed cholesteric liquid crystal, which is related to the modification of the surface anchoring by an ionic surfactant under the action of an applied electric field. Parameters of electric signals are selected that ensure the switching of the composite film between some stable states with different levels of optical transmittance.     

Effect of alignment layer on polymer-dispersed liquid crystal random laser

The surface orienting effect of rubbed polyimide alignment layer on the random lasing (RL) properties of dye-doped polymer-dispersed liquid crystal (DDPDLC) was studied experimentally. Two groups of experiment were done and nematic liquid crystal E7 was used. In the first group, the samples were 25 μm non-oriented DDPDLC sample (N-DDPDLC) and 25 μm one-side oriented DDPDLC sample (P-DDPDLC). In the second, the samples were 10 μm N-DDPDLC and 10 μm P-DDPDLC sample. The results showed that compared to the N-DDPDLC, P-DDPDLC has a lower threshold, and the 10 μm P-DDPDLC has a lower threshold compared to the 25 μm P-DDPDLC. Apparently, the P-DDPDLC cell has stronger RL spike. The oriented layer changed the phase separation structure of DDPDLC which formed hierarchical structure so as to enhance the scattering intensity of system. It is a good way to reach high efficient random laser emission by tailoring the size and morphology of PDLC through surface orientation and Polymerization-induced phase separation process.     

Analysis of dichroic dye-doped polymer-dispersed liquid crystal materials for display devices

Dichroic polymer-dispersed liquid crystals (DPDLCs) based on nematic liquid crystal materials with azo dye were investigated in detail for the application of display devices. Polarizing optical microscopy, differential scanning calorimeter and electro-optic experiments all have shown that the DPDLC containing low concentration of dyes modifies the basic properties of these materials like optical transmission, threshold voltage, contrast ratio and absorbance factor. A minimum amount of dye needs to be added to the liquid crystal with the polymer matrix for its effective phase separation and to minimize the transmittance in the OFF state and therefore gives rise to an overall improvement in contrast ratio of the devices. Molecular orientation and dynamics in droplet sizes are readily controlled in these DPDLC materials. These findings imply that the value of the threshold electric field E_th is approximately 8 V/μm for pure polymer-dispersed liquid crystal (without dye) where the threshold electric field E_th values are approximately 4.0 V/μm, 2.0 V/μm, 1.7 V/μm 1.0 V/μm, for 0.0625%, 0.125%, 0.25% and 0.5% with azo dye in DPDLCs, respectively. From the results we can also infer that the maximum contrast is approximately 2.55 times the minimum contrast observed in the experiment for DPDLCs. The results show that the DPDLC with proscribed dye concentration will be possibly suitable and promising functional electronic materials for green technology flexible liquid crystal display.     

Electro-optical characteristics of polymer-dispersed liquid crystal film controlled by ionic-surfactant method

The electro-optical characteristics of a composite film based on a polymer containing dispersed nematic liquid crystal (NLC) doped with an ionic surfactant have been studied. The threshold character of the reorientation of NLC droplets as well as the dependence of the modulation amplitude of light transmission on the electric field strength has been revealed for the effect of ionic modification of surface anchoring. The dynamical parameters of the optical response have been determined.     

Photorefractive liquid crystals

Photorefractive liquid crystals represent the newest class of photorefractive materials. The low electric fields required for reorienting liquid crystals, combined with their high birefringence, results in the observation of photorefractivity with very low optical intensities and low applied fields. Photoinducing efficient charge transport over macroscopic distances is the primary hurdle for producing the space-charge field required for photorefractivity.     

Photoconducting liquid crystals

Major recent advances: ‘High’ mobility photoconduction in the columnar mesophases of disc-shaped (discotic) liquid crystals in which the charge carriers are holes or electrons was discovered in 1995. Prior to this photoconduction in liquid crystals was attributed to photo-generated ions and associated with ‘low’ mobilities. Over the last 7 years our understanding of the mechanism of carrier generation and transport in these novel, self-assembling systems has progressed to the point where we are able to design and manufacture organic semi-conductors with well-defined electronic and physical properties. Serious commercial devices incorporating conducting liquid crystals are finally on the horizon.     

Cherenkov radiation in liquid crystals

The up-to-date state of investigations of charged particle coherent radiation in liquid crystals is presented with most attention being paid to coherent radiation in chiral liquid crystals. A detailed analysis of Cherenkov radiation and “structure Cherenkov” radiation in cholesteric liquid crystals is given. It is shown that there are essential differences in the characteristics of Cherenkov radiation in cholesterics compared to Cherenkov radiation in homogeneous media. These reveal themselves in the spectral and angular distributions of the radiation and in its polarization properties. Many of these, in particular the two cones of emission, the divergence of the differential with respect to frequency (or angle) of the emission intensity and the shift of the threshold velocity for Cherenkov radiation, are common to Cherenkov radiation in any periodic medium. The prospects for experimental observation of the examined Cherenkov radiation peculiarities in chiral liquid crystals are briefly discussed, and the significance of the relevant measurements for studying coherent radiation from charged particles in other periodic media and other frequency ranges are noted.     

Electrooptical properties and microstructure of polymer-dispersed liquid crystal doped with various reinforcing materials

Different types of reinforcing material, such as hydrophilic silica (Aerosil 200), (1-propylmethacrylate)-heptaisobutyl-substituted PSS (POSS-1), octavinyl-substituted PSS (POSS-8), octamethyl-substituted PSS (POSS-octa), and poly(dimethylsiloxane) (PDMS) were incorporated into a conventional polymer-dispersed liquid crystal (PDLC) system to enhance electrooptical properties by increasing phase separation, resulting from increasing the gel content and decreasing the viscosity of the mixture. The mixtures with POSS-1, POSS-octa, and Aerosil 200 show lower viscosity than the neat mixture, caused by the weak interaction of monomer molecules because of inserting these particles into the monomer chains, whereas the mixtures with POSS-8 and PDMS show an increase in viscosity. The PDLC film with POSS-1 represents the lowest off-transmittance value because the gel content is above 94% and the droplet size of the LC is optimal. However, when the gel content is decreased, the droplet size of the LC in the film becomes large because of unreactive monomer flowing into the LC, giving rise to the increase in off-transmittance value.     

Layer-by-layer transitions in liquid crystals

Employing our free-standing film calorimetric system. we have identified three remarkable layer-by-layer transitions in several liquid crystal compounds. Each of these transitions can be well described by the simple power-law form:L = L ₀ t. The layering transitions liMind near the smectic-A- hexatic-B and smectic-A crystal-B transitions are well characterized by the exponent v 1 3. This value is consistent with models based on a van der Wads-like dominant intermolecular interaction. Another novel layer-by-layer thinning transition has been discovered above the hulk smectic-A isotropic transition of a perfluorinated liquid crystal compound. The value of the exponent obtained, v 3 4. cannot he easily explained using familiar models.Paper presented at the Twelfth Symposium on Thermophysical Properties. June 19–24, 1994 Boulder Colorado U.S.A.