Optical birefringence and anisotropic scattering in acrylate based holographic polymer dispersed liquid crystals

In this work, we experimentally study the evolution of the diffraction efficiency and birefringence of a holographic polymer dispersed liquid crystal grating. It is shown that the onset of birefringence coincides closely with the moment when TE and TM polarizated probes start to diffract differently. Light scattering, caused by liquid crystal droplets, is also monitored to understand the dynamics of droplet formation and growth. It is shown that the onset of anisotropic scattering and birefringence also occur almost simultaneously.     

Coherent backscattering of light in nematic liquid crystals

Multiple light scattering by director fluctuations in nematic liquid crystals is considered. A uniform director orientation is assumed to be specified by an applied magnetic field. The coherent backscattering effect, which consists in the presence of a sharp light backscattering peak, is studied. The Bethe-Salpeter equation is used to calculate the multiple scattering intensity taking into account the contributions of ladder and cyclic diagrams. An analytical expression for the angular and polarization dependences of the coherent backscattering intensity is obtained in terms of the diffusion approximation. The calculation and experimental results are compared. The developed theory is shown to qualitatively describe the elliptical shape of the backscattering cone, to explain the absence of a coherent contribution for crossed polarizations, and to calculate the relative peak height.     

Control of extraordinary light transmission through perforated metal films using liquid crystals

We calculate the effective dielectric tensor of a metal film penetrated by cylindrical holes filled with a nematic liquid crystal (NLC). We assume that the director of the NLC is parallel to the film, and that its direction within the plane can be controlled by a static magnetic field, via the Freedericksz effect. To calculate the effective dielectric tensor, we consider both randomly distributed holes (using a Maxwell-Garnett approximation) and a square lattice of holes (using a Fourier technique). Both the holes and the lattice constant of the square lattice are assumed small compared to the wavelength. The films are found to exhibit extraordinary light transmission at special frequencies related to the surface plasmon resonances of the composite film. Furthermore, the frequencies of peak transmission are found to be substantially split when the dielectric in the holes is anisotropic. For typical NLC parameters, the splitting is of order 5–10% of the metal plasma frequency. Thus, the extraordinary transmission can be controlled by a static magnetic or electric field whose direction can be rotated to orient the director of the NLC. Finally, as a practical means of producing the NLC-filled holes, we consider the case where the entire perforated metal film is dipped into a pool of NLC, so that all the holes are filled with the NLC, and there are also homogeneous slabs of NLC on both sides of the film. The transmission in this geometry is shown to have similar characteristics to that in which the NLC-filled screen is placed in air.     

Small-angle light scattering symmetry breaking in polymer-dispersed liquid crystal films with inhomogeneous electrically controlled interface anchoring

We have described the method of analyzing and reporting on the results of calculation of the small-angle structure of radiation scattered by a polymer-dispersed liquid crystal film with electrically controlled interfacial anchoring. The method is based on the interference approximation of the wave scattering theory and the hard disk model. Scattering from an individual liquid crystal droplet has been described using the anomalous diffraction approximation extended to the case of droplets with uniform and nonuniform interface anchoring at the droplet–polymer boundary. The director field structure in an individual droplet is determined from the solution of the problem of minimizing the volume density of the free energy. The electrooptical effect of symmetry breaking in the angular distribution of scattered radiation has been analyzed. This effect means that the intensities of radiation scattered within angles +θ _s and–θ _s relative to the direction of illumination in the scattering plane can be different. The effect is of the interference origin and is associated with asymmetry of the phase shift of the wavefront of an incident wave from individual parts of the droplet, which appears due to asymmetry of the director field structure in the droplet, caused by nonuniform anchoring of liquid crystal molecules with the polymer on its surface. This effect is analyzed in the case of normal illumination of the film depending on the interfacial anchoring at the liquid crystal–polymer interface, the orientation of the optical axes of droplets, their concentration, sizes, anisometry, and polydispersity.     

Morphological and electro-optical responses of dichroic polymer dispersed liquid crystal films

Dichroic polymer dispersed liquid crystal (DPDLC) films containing different concentration of dichroic dye have been prepared by polymerization induced phase separation technique using nematic liquid crystal material in UV curable polymer NOA 65 and anthraquinone dichroic dye. The effects of applied voltage and temperature on the LC droplet morphology and its optical characteristics were studied in detail. Liquid crystal droplets containing low concentration of dye (0.25%) in polymer matrix exhibit bipolar configuration at lower applied voltage (10 V) whereas at relatively higher voltage (15 V) maltese type liquid crystal droplets were seen. In addition, low concentration DPDLC film shows better optical transmission and higher contrast ratio.     

Electro-optic study of acrylate based holographic polymer dispersed liquid crystals with broad band photosensitivity

We describe the electro-optic (EO) modulation of diffraction in holographic polymer dispersed liquid crystals, recently developed for broad band photosensitivity, ranging from visible to near infrared. More than 90% EO modulation achieved for relatively low electric fields confirms the phase separation and formation of liquid crystal sub-micrometer sized droplets in the polymer matrix. Molecular reorientation induced by the applied voltage and a residual electrical current induced heating are at the origin of the EO modulation.     

Interferometry of hyper-Rayleigh scattering by inhomogeneous thin films

The use of specific symmetry properties of optical second-harmonic generation (the s, s-exclusion rule) has allowed us to observe high-contrast hyper-Rayleigh interference patterns in completely diffuse light, an effect that has no analog in the case of linear (Rayleigh) scattering.     

Fast electro-optical switching and high contrast ratio in epoxy-based polymer dispersed liquid crystals

We report the observation of an electro-optical switching effect from an opaque to a transparent state occurring at a threshold value of the applied field in a polymer dispersed liquid crystal (PDLC). Optical responses of the composite film under the conditions of an externally applied ac electric field (2–5 V_p−p/μ) and a film thickness (50 μm), were determined using an Argon laser (wavelength 514 nm). The experimental results showed promising switching times with a rise time of 190 μs and a decay time of 2 ms and an exceptionally high contrast ratio up to 410. These results demonstrate the validity of employing this new PDLC in electro-optical devices.     

Optical properties and optimized conditions for polymer dispersed liquid crystal containing UV curable polymer and nematic liquid crystal

Polymer dispersed liquid crystal was synthesized by combining a UV curable polymer and a nematic liquid crystal. Optimized conditions for the optical properties of the PDLC were found to be the concentration ratio of LC and polymer at 7:3, UV curing time of 18 min, and the thickness less than 25 μm. In the case of the high LC concentration (≥70%) sample, the amount of liquid crystal segregated in the polymerization process was enough to form a spherical shape of droplet, and the threshold driving voltage was reduced. The response time for the turn-on process was nearly independent of the concentration, while the turn-off process was almost proportional to the concentration. From microscopic image and UV–visible spectrum analysis, the relation between LC droplet morphology and optical properties were explained.     

Coherent transmittance quenching effect in thin films of polymer-dispersed ferroelectric liquid crystals

The conditions for vanishing of the coherent component of radiation transmitted through a monolayer of polymer-dispersed ferroelectric liquid-crystal droplets are found. The influence of polydispersity and disorientation of axes of semispherical droplets on the conditions of interference quenching is studied. It is shown that these factors cause a change in the average size and concentration of droplets for which the quenching effect is realized. Data on the contrast ratio and the modulation depth of a monolayer, determined from the coherent component of transmitted radiation, are presented. It is shown that these characteristics can be considerably improved under quenching conditions for a film of polymer-dispersed ferroelectric liquid-crystal droplets.     

Polarization of light by a polymer film containing elongated drops of liquid crystal with inhomogeneous interfacial anchoring

An optico-mechanical model describing the coherent (directed) transmittance and the degree of polarization of forward-transmitted light by a polymer film with elongated liquid-crystal (LC) drops has been developed. This model, based on the Foldy–Twersky and anomalous-diffraction approximations, makes it possible to analyze the optical response of a film under extension as a function of the film thickness, refractive index of the polymer, the sizes and anisometry parameters of liquid-crystal drops, their concentration, internal structure, polydispersity, and orientation of optical axes. The model is verified based on the comparison of numerical and experimental data for the inverse modification of interfacial anchoring by an ion-forming surfactant. The internal drop structure is determined by solving the problem of minimizing the volume free energy density. A comparative analysis of the calculated transmittance and degree of polarization for films with uniform homeotropic and modified inhomogeneous interfacial anchoring is performed. The spectral polarization characteristics of a film with elongated LC drops and single-domain internal structure, formed under mechanical extension with the aid of surfactants, are investigated.     

Optical model of transient light scattering in ferroelectric liquid crystals

A static optical model is developed for the effect of field-induced transient scattering on coherent light transmission through ferroelectric liquid crystals. Scattering processes are described by introducing an optically anisotropic medium containing scatterers (transient domains). The results presented in the paper are obtained for a plane parallel layer of ferroelectric liquid crystals with a planar helicoidal structure under normal illumination with a linearly polarized plane wave. An analysis is presented of the coherent transmittance of the layer in static applied electric fields.     

Study on thulium oxide nanoparticles modulating electro-optics characteristics of polymer dispersed liquid crystals

By adding the thulium oxide nanoparticles (NP) of Gd2 O3 and Eu2O3 and Nd2O3 to polymer dispersed liquid crystals (PDLC), and the transmittances of PDLC and NP-PDLC changing with voltage in visible light was tested, and the thulium oxide modulation for PDLC was studied. The results show that transmittance of Gd2O3-NP-PDLC decreases with increasing voltage when the voltage is less than 10 V. The relaxation phenomenon of the sample appears at 15 V, and its transmittance curve fluctuates slightly around that at 10 V. The transmittance of Gd2O3-NP-PDLC increases rapidly at 20 V, and its filter phenomenon emerges. And the transmittance of Eu2O3-NP-PDLC decreases with increasing voltage, but the decrease is only slight. And the transmittance of Nd2O3-NP-PDLC changes with voltage insignificantly.     

Computer simulation of structure and mechanical properties of polymer liquid crystals

In this contribution, we report on a study of the self-organization and mechanical properties of polymer liquid crystals (PLCs). Both processes are computer simulated by the method of molecular dynamics. We investigated two real longitudinal PLCs (thermotropic polyesters) with macromolecules that consist of rigid and flexible parts arranged in a regular way. One rigid and one flexible part form a monomer containing 45 or 47 atoms. The total number of atoms in the macromolecules studied was 4700 (100 monomers) and 5400 (75 monomers). The self-organization was similar to that obtained earlier for a beads-on-a-string model, so compression calculations were done using this simpler model containing 1200 beads (100 monomers). Macroscopic characteristics such as the stress-strain relation, temperature change during deformation, as well as microscopic changes in structure, were investigated.     

Effect of polymer viscosity on morphological and electro-optic properties of aligned polymer dispersed ferroelectric liquid crystal composite films

A room temperature ferroelectric liquid crystal mixture was dispersed in UV curable polymers of different viscosity in 30:70 wt/wt ratio. These polymer dispersed ferroelectric liquid crystal (PDFLC) composite films were prepared by polymerization induced phase separation technique. It was found that the polymer viscosity influences the droplet size and the electro-optic properties. The spontaneous polarization of PDFLC decreases with an increase in polymer viscosity. The droplet morphology and electro-optic properties of these materials have been investigated in an aligned configuration.     

Holographic storage of three-dimensional image and data using photopolymer and polymer dispersed liquid crystal films

We present holographic storage of three-dimensional(3D) images and data in a photopolymer film without any applied electric field.Its absorption and diffraction efficiency are measured,and reflective analog hologram of real object and image of digital information are recorded in the films.The photopolymer is compared with polymer dispersed liquid crystals as holographic materials.Besides holographic diffraction efficiency of the former is little lower than that of the latter,this work demonstrates that the photopolymer is more suitable for analog hologram and big data permanent storage because of its high definition and no need of high voltage electric field.Therefore,our study proposes a potential holographic storage material to apply in large size static 3D holographic displays,including analog hologram displays,digital hologram prints,and holographic disks.