Planar Aligned Nematic Liquid Crystal Research Articles

Scanned conical illumination as a probe of electro-optic retro-reflection.

We describe a prototype element for use in probing electro-optic retro-reflection in sensor applications, illuminating a planar-aligned nematic liquid crystal electro-optic cell with convergent light having a single, tunable angle of incidence (tunable conical illumination). This illumination is generated using a 100X, high numerical aperture, long working-distance microscope objective under conditions of extreme spherical aberration. The electro-optic effect observed is multiple-beam optical interference between polarized reflections from the two bounding plates of the cell, rendered tunable with voltage-controlled refractive index changes induced by molecular reorientation of the liquid crystal. Characterization of the reflectivity vs. angle of incidence and applied voltage enables identification of conditions of high-contrast, low power, electro-optic reflectivity control applicable to fiber optics.

Shear-enhanced diffraction from thin permanent gratings in dye-doped nematic liquid crystal cells

Permanent gratings are recorded in planar-aligned dye-doped nematic liquid crystal cells under visible light illumination. By increasing the irradiation intensity and exposure time, several diffraction orders of the recorded gratings are obtained in the Raman-Nath diffraction regime. By applying a dynamic transverse shear on one of the confining plates of the cell, an enhancement of the diffraction efficiency is achieved, which follows the period of the grating. By microscope inspection under static displacement of the upper plate, surface gratings formed by the dye adsorption are revealed in both the front and rear windows of the cell, indicating that the diffraction amplification originates from a coherent superposition of the diffracted orders when the gratings are displaced half a period. The effect provides self-matched amplification of diffraction with a simple cell, a single photo-inscription stage, and elementary displacement steps.

In-Plane Switching of Chiral Nematic Liquid Crystals with Different Dielectric Anisotropies

The electro-optic response in planar-aligned chiral nematic liquid crystals in an electric field oriented perpendicular to the helical structure axis is studied as a function of the magnitude and sign of the dielectric anisotropy of liquid crystal in order to understand the mechanism of this electro-optic effect. The main contribution to the electro-optic response is shown to be due to the interaction of the electric field with the dielectric anisotropy. The flexoelectric effect also contributes to the electro-optic response, but its magnitude is much smaller.

Tunable angular shearing interferometer based on wedged liquid crystal cells.

The concept of a liquid crystal wedge as a tunable angular shearing interferometer is introduced and demonstrated to combine both high stability and high tunability. Different wedges are fabricated from planar aligned nematic liquid crystal cells with thickness gradients. These wedges are shown to produce stable interferograms from the polarization interference between the ordinary and extraordinary waves propagating in different directions at the output of the cell. The fringe periods, ranging from 70μm to 1.25mm, can be precisely controlled by a low voltage. Despite the wedge-shaped structure, no inhomogeneity has been detected when the wedge is driven adiabatically and the interferograms are uniform over regions as large as 5×5 mm. Moreover, dynamical measurements show that the wedges behave as a succession of multiple cells with different thickness, giving rise to a moving front of stabilizing fringes when driven dynamically. All the observations show that the device is suitable for large beam size and tunable shearing interferometry, with attractive features for applications such as phase sensing, photoalignment or photolithography.

Conoscopic analysis of electric field driven planar aligned nematic liquid crystal.

This paper illustrates the conoscopic observation of a molecular reconstruction occurring across a nematic liquid crystal (NLC) medium in the presence of an external electric field. Conoscopy is an optical interferometric method, employed to determine the orientation of an optic axis in uniaxial crystals. Here a planar aligned NLC medium is used, and the topological changes with respect to various applied voltages are monitored simultaneously. Homogenous planar alignment is obtained by providing suitable surface treatments to the ITO coated cell walls. The variation in the conoscopic interferometric patterns clearly demonstrates the transition from planar to homeotropic state through various intermediate states.

Room-temperature single-photon sources with definite circular and linear polarizations based on single-emitter fluorescence in liquid crystal hosts

Definite circular and linear polarizations of room-temperature single-photon sources, which can serve as polarization bases for quantum key distribution, are produced by doping planar-aligned liquid crystal hosts with single fluorescence emitters. Chiral 1-D photonic bandgap microcavities for a single handedness of circularly polarized light were prepared from both monomeric and oligomeric cholesteric liquid crystals. Fluorescent emitters, such as nanocrystal quantum dots, nitrogen vacancy color centers in nanodiamonds, and rare-earth ions in nanocrystals, were doped into these microcavity structures and used to produce circularly polarized fluorescence of definite handedness. Additionally, we observed circularly polarized resonances in the spectrum of nanocrystal quantum dot fluorescence at the edge of the cholesteric microcavity's photonic stopband. For this polarization we obtained a ~4.9 enhancement of intensity compared to the polarization of the opposite handedness that propagates without photonic bandgap microcavity effects. Such a resonance is indicative of coupling of quantum dot fluorescence to the cholesteric microcavity mode. We have also used planar-aligned nematic liquid crystal hosts to align DiI dye molecules doped into the host, thereby providing a single-photon source of linear polarization of definite direction. Antibunching is demonstrated for fluorescence of nanocrystal quantum dots, nitrogen vacancy color centers, and dye molecules in these liquid crystal structures.

Photoinduced reorientation of nematic liquid crystals doped with an azo dye: A dynamic and steady-state study of reorientation and loss of liquid crystal order

We compare photoinduced reorientation of homeotropic and planar aligned nematic liquid crystal 4-pentyl-4-cyanobiphenyl (5CB) doped with trace amounts of azo-dye disperse orange 3 (DO3) by studying the optical nonlinearities of the sample. Theoretical and experimental analyses confirm the proposal that the trans and cis isomers can be treated as independent contributors to the enhancement factor. Dynamic measurements indicate three contributions to photoinduced optical nonlinearities, two of which are isotropic and a third corresponding director reorientation. We also measure a large negative enhancement factor for the trans isomer and a positive enhancement factor for the cis isomer, consistent with previous measurements. The latter indicates that the mean field for the cis isomer is very small. Planar aligned samples demonstrate zenithal gliding whereas homeotropic samples do not. In addition, steady-state and dynamic measurements indicate loss of liquid crystal order associated with absorption as well as possible out-of-plane reorientation.

Far-Field Patterns from Dye-Doped Planar-Aligned Nematic Liquid Crystals Under Nanosecond Laser Irradiation

High-definition patterns were observed under 10-Hz-pulse-repetition-rate, nanosecond laser irradiation of azodye-doped planar-nematic liquid crystal layers at incident intensities I ∼ 5–10 MW/cm2 in a single beam configuration and without any feedback involved. An incident polarization parallel to the nematic director was used. Under periodic pulsed laser irradiation, far-field beam patterns at the output of a dye-doped liquid crystal layer changed kaleidoscopically from rings and stripes to multiple hexagons. This pattern-formation regime had a buildup time of several seconds to minutes. We explain the observed effect by diffraction of the laser beam on light-induced micrometer-size inhomogeneities inside the liquid crystal layer with absorption and refraction properties different from the surrounding area. Possible mechanisms of the formation of the inhomogeneities are discussed.

Quasistatic electric-field-modulated optical pattern transition in a thin nematic liquid-crystal film with a single feedback mirror

We present here the pattern formation in the transverse profile of a continuous-wave laser beam passing through a parallel planar-aligned nematic liquid crystal (NLC) film biased by a quasistatic electric field and then reflected back to the sample cell by a single planar mirror. The effect of the biasing voltage is studied through the ability to change the nonlinearity by modulating the orientation of the NLC molecules electrically. By suitably modulating the quasistatic electric field the optical pattern transition from the hexagon to the roll is achieved. The pattern transition can be explained by the linear stability analysis for the nonlinear liquid crystal film with optical feedback.

Crucial effects of the anisotropy on optical field induced pattern formation in nematic liquid crystal films

We present here the crucial effects of material anisotropy on optical field induced pattern formation in the one-feedback-mirror arrangement which utilizes the nematic liquid crystal film as the nonlinear medium. By using the quasi-static electric-field-biased planar-aligned homogeneous nematic liquid crystal (NLC) films, we observe both the hexagon and the roll patterns which can be switched optically due to the intrinsic anisotropic distribution of the threshold intensity. The anisotropy comes from the anisotropic nonlinear response of the NLC film and is the crucial factor for such a one-feedback-mirror system to form both the roll and hexagon patterns. The observed phenomena can be explained from the linear stability analysis of the governing diffusion-like equation. The experimental results indicate that the stable roll patterns are formed at low input light power and the stable hexagon patterns formed at high input power.

On the non-symmetric planar aligned NLC cell

The planar aligned nematic liquid crystal cell with different anchoring for the two substrates (i.e. a non-symmetric NLC cell) is investigated by an analytical method. We deduce the basic equations and the boundary conditions of the tilt angle θ of the LC director. Expressions for threshold and saturation magnetic field are obtained, and numerical results of these two quantities with variation in anchoring parameters of the two substrates are given. A symmetry breaking parameter Δ is introduced and the relations between Δ and applied field, as well as the two sets of anchoring parameters are discussed in detail. A feasible experimental plan for measurement of anchoring strengths of a series of different substrates is proposed.

Field-induced effects in an eutectic nematic liquid crystal mixture

Electric-field effects were investigated in a planar-aligned nematic liquid crystal mixture. Under a DC electric field, we observed the presence of domains. The domain structure changed near the threshold field. The effects of charge injection from the substrate surfaces were investigated. The work function is calculated using Richardson's equation.PACS No.: 61.30.Hn

Preliminary communication: Magnetic-field-induced Freedericksz transition of a planar aligned liquid crystal doped with porphyrinatozinc(II): influence of the substituent of the porphyrin ring

The magnetic-field-induced Freedericksz transition of a planar aligned nematic liquid crystal doped with 1.0 wt % of a diamagnetic porphyrinatozinc(II) has been studied. It was observed that 5CB doped with 1.0 wt % of derivatives of Zn(II)TPP-5,10,15,20-tetraphenyl-porphyrinatozinc(II) - which have different substituents in one phenyl ring display sharp Freedericksz transition curves and dramatically decrease the critical magnetic field compared with pure 5CB.

Magnetic field induced Freedericksz transition of a planar aligned liquid crystal doped with metalloporphyrins FeTPPCl, MnTPPCl and ZnTPP

We observed that the planar aligned nematic liquid crystal (5CB) doped with a volume fraction of 1% of FeTPPCl [5,10,15,20-tetraphenylporphyriniron(III)chloride] or MnTPPCl [5,10,15,20-tetraphenylporphyrinmanganese(III)chloride] dramatically decreased the critical magnetic field for the magnetic field induced Freedericksz transition, while 5CB doped with ZnTPP [5,10,15,20-tetraphenylporphyrinzinc(II)] revealed no such effect, when compared with pure 5CB. In the guest-host (5CB) system, FeTPPCl and MnTPPCl as guests are both strong paramagnetic materials with an interaction through coordination of the -CN group in 5CB onto the metal ion of the porphyrin. As a result, the 5CB molecules are dragged to reorientate under a static magnetic field, while ZnTPP is a diamagnetic material without such a property. This phenomenon concerning magneto-optical components could be useful in liquid crystal displays.

A Liquid Crystal Broad Band Stokes-Meter

A device for the determination of the complete polarization state of light in a broad spectral interval is presented. Two tunable achromatic phase retarders with planar aligned nematic liquid crystals are used to modulate the polarization without moving components.

Damping of the twist director fluctuations in the nematic liquid crystal

The spectrum of Rayleigh scattered light in the planar aligned nematic liquid crystal octyloxycyanobiphenyl is studied by means of an optical homodyne technique. Experimental values are reported for the damping times and visco-elastic ratios associated with the twist elastic distortions in the nematic region as a function of temperature. The results obtained are discussed in relation to currently available theories concerning the elastic and viscous properties of nematics. It turns out that the measured visco-elastic properties are in reasonable agreement with the theoretical predictions.

All-optical bistability in nematic liquid crystals at 20 μW power levels

Optical switches at power levels as low as 14 μW, allowing bistable operation with compact disk laser systems, are reported in optimized cavities containing planar-aligned nematic liquid crystals.

Optically induced twist Fréedericksz transitions in planar-aligned nematic liquid crystals.

The optical-field-induced Freedericksz transition for a twist deformation by a normally incident laser beam in a planar-aligned nematic liquid crystal is studied. The Euler equation for the molecular director and the equations describing the evolution of the beam polarization in the birefringent medium are solved simultaneously in the small-perturbation limit. The stability of the undistorted state is investigated. An alternate series of stable and unstable bifurcations is found. This phenomenon has no analog in the Freedericksz transition induced by dc electric and magnetic external fields.