Planar Nematic Liquid Crystal Cells Research Articles

Behaviour of nematic liquid crystals doped with ferroelectric nanoparticles in the presence of an electric field

ABSTRACTA planar nematic liquid crystal cell (NLC) doped with spherical ferroelectric nanoparticles is considered. Polarisation of the nanoparticles are assumed to be along the NLC molecules parallel and antiparallel to the director with equal probability. The NLC molecules anchoring to the cell walls are considered to be strong, while soft anchoring at the nanoparticles surface is supposed. Behaviour of the NLC molecules and nanoparticles in the presence of a perpendicular electric field to the NLC cell is theoretically investigated. The electric field of the nanoparticles is taken into account in the calculations. Freedericksz transition (FT) threshold field in the presence of nanoparticles is found. Then, the director and particles reorientations for the electric fields larger than the threshold field are studied. Measuring the onset of the nanoparticles reorientation is proposed as a new method for the FT threshold measurement.

Interactions of single nanoparticles in nematic liquid crystal

We studied interactions of single silica nanoparticles with homeotropic surface anchoring in a confined planar nematic liquid crystal cell. Nanoparticles form stable pairs down to a size of 35 nm with the pair binding energy proportional to the size of the particle. 22 nm particles do not form stable pairs, only metastable states were observed due to strong thermal motion and electrostatic repulsion. These nanoparticles strongly interact with the confining surfaces and the larger clusters, which prevents the formation of stable homogenous dispersions. In addition we have shown that nanoparticles are strongly attracted by topological defects induced by the microparticles. The origin of the attractive forces in nanoparticle dispersions is the minimization of free energy. These forces strongly depend on the strength and type of the liquid crystal anchoring at the surface of the nanoparticles.

Optically-driven switching of a planar nematic liquid crystal cell with parallel rubbing

This letter reports on the switching of a planar nematic liquid crystal cell with parallel rubbing of the alignment layers, under the application of a voltage, when there is initially an optical field. The voltage application over the liquid crystal in such a cell leads normally to the formation of multiple domains because there is the two switching directions are equivalent. However, an incident optical field under an angle will locally reorient the director and break the symmetry between the equivalent switching directions. The subsequent application of a voltage pulse amplifies the tilt angle and leads to the formation of a dominant domain, with an order of magnitude larger size than the optical beam profile. Several switching conditions are demonstrated for different incident angles of the beam. It is shown that the final switching direction of the entire cell is determined by the tilt angle of the optical field. The lensing effects due to the modified director distribution in the domain walls is analyzed qualitatively. Full Text: PDF ReferencesI. C. Khoo, Liquid crystals (2nd ed. Hoboken (NJ), Wiley, 2007) CrossRef A. Zolotko, V. Kitaeva, N. Kroo et al. OCBP. JETP Lett. 32, 158?162 (1980). DirectLink J. Beeckman, K. Neyts, X. Hutsebaut X, et al. "Simulations and experiments on self-focusing conditions in nematic liquid-crystal planar cells", Opt Express, 12, 1011? 1018 (2004). CrossRef M. Peccianti, C. Conti, G. Assanto, et al. "Electrically assisted self-confinement and waveguiding in planar nematic liquid crystal cells", Appl. Phys. Lett. 77, 7 ? 9 (2000). CrossRef N. Kravets, A. Piccardi, A. Alberucci et al, "Bistability with Optical Beams Propagating in a Reorientational Medium", Phys. Rev. Lett. 113, 023901 (2014) CrossRef A. Piccardi, N. Kravets, A. Alberucci et al, "Voltage-driven beam bistability in a reorientational uniaxial dielectric", APL Photonics 1, 011302 (2016). CrossRef V. Nersesyan, T. Brans, F. Beunis, R. Drampyan , J. Beeckman, K. Neyts, "Light-controlled reorientation of nematic liquid crystal driven by an electric field", Liquid crystals, 43, 1422-1430 (2016). CrossRef J. Beeckman, K. Neyts, W. Cort, et al. "Non-linear light propagation and bistability in nematic liquid crystals", Proc SPIE 7414, 74140K (2009). CrossRef

Surface-induced structures in nematic liquid crystal colloids.

We predict theoretically the existence of a class of colloidal structures in nematic liquid crystal (NLC) cells, which are induced by surface patterns on the plates of the cell (like cells with UV-irradiated polyamide surfaces using micron sized masks in front of the cell). These bulk structures arise from nonuniform boundary conditions for the director distortions at the confining surfaces. In particular, we demonstrate that quadrupole spherical particles (like spheres with boojums or Saturn-ring director configurations) form a square lattice inside a planar NLC cell, which has checkerboard patterns on both its plates.

Properties of nematicons in low-birefringent nematic liquid crystals

We investigate nonlinear self-focusing and nematicon generation in low birefringencenematic liquid crystals (NLCs), where n=0.04. are obtained for larger optical powers but they are more than in NLCs with standard birefringence, of the order of n=0.2 Moreover, in low-birefringent NLCs, polarization components exchange energy in a nonlinear regime. Such behaviour is not observed in typical NLCs, where ordinary and extraordinary waves tend to propagate independently due to large walk off.The results of our experimental rest are also compared with numerical simulations, using a fully vectorial BPM. Calculations and experimental results are in good qualitative and quantitative agreement. Full Text: PDF References G. Assanto and M. A. Karpierz, Nematicons: self-localised beams in nematic liquid crystals, Liq. Cryst. 36, 1161 (2009). CrossRef M. Peccianti and G. Assanto, Nematicons, Phys. Rep. 516, 147(2012). CrossRef M. Peccianti, G. Assanto, A. De Luca, C. Umeton and I. C. Khoo, Electrically assisted self-confinement and waveguiding in planar nematic liquid crystal cells, Appl. Phys. Lett. 77, 7 (2000). CrossRef M. A. Karpierz, M. Sierakowski, M. Swillo and T. Wolinski, Self Focusing in Crystalline Waveguides, Mol. Cryst. Liq. Cryst. 320, 157 (1998). CrossRef U. A. Laudyn, M. Kwasny, M. A. Karpierz, Nematicons in chiral nematic liquid crystals, Appl. Phys. Lett. 94, 091110 (2009). CrossRef U. A. Laudyn, M. Kwasny, K. Jaworowicz, K. A. Rutkowska, M. A. Karpierz and G. Assanto, Properties of spatial solitons in chiral nematic liquid crystal cells, Photon. Lett. Poland 1, 7 (2009). CrossRef I. C. Khoo, Liquid Crystals, Second Edition, Wiley & Sons, New Jersey (2007). CrossRef A. Piccardi, A. Alberucci, G. Assanto, Soliton self-deflection via power-dependent walk-off, Appl. Phys. Lett.96, 061105 (2010). CrossRef F. A. Sala and M. A. Karpierz, Modeling of molecular reorientation and beam propagation in chiral and non-chiral nematic liquid crystals, Opt. Express 20, 13923 (2012). CrossRef A. Piccardi, M. Trotta, M. Kwasny, A. Alberucci, R. Asquini, M. Karpierz, A. D'Alessandro and G. Assanto, Trends and trade-offs in nematicon propagation, Appl. Phys. B 104, 805 (2011). CrossRef R. Dąbrowski, J. Dziaduszek and T. Szczucinski, Mesomorphic Characteristics of Some New Homologous Series with the Isothiocyanato Terminal Group, Mol. Cyst. Liq. Cryst. 124, 241 (1985). CrossRef

Effects of carbon nanotubes on electro-optical properties of dye-doped nematic liquid crystal

Electro-optical effects of planar nematic liquid crystal cells doped with dye and carbon nanotubes (CNTs) were investigated by utilizing the absorption property of the dye. Voltage-dependent transmittance and hysteresis measurements under applied ac and dc fields were presented. Cells doped with both Methyl Red and CNTs showed increased conductivity and capacitance, slower ac-rise switching time and dc-backflow on turn-off. The cells exhibited little hysteresis but those containing CNTs reduced residual dc by about 50%. First-order probe diffraction of permanent gratings revealed a correlation of ac-field-enhanced diffraction efficiency and Freedericksz threshold voltage.

Polarization-resolved conoscopic patterns of planar oriented liquid crystal cells

We theoretically and experimentally investigate the angular structure of polarization of light transmitted through planar nematic and cholesteric liquid crystal (LC) cells by analyzing the polarization state as a function of the incidence angles and the polarization characteristics of the incident wave (the ellipticity and the azimuth of polarization). The geometry of polarization-resolved conoscopic (angular) patterns emerging after the planar nematic and cholesteric LC cells is described in terms of the polarization singularities such as C-points (points of circular polarization) and L-lines (lines of linear polarization). The characteristic feature of polarization-resolved conoscopic patterns under consideration is that the conditions of C-point formation are very sensitive to polarization parameters of the incident wave. Full text: PDF

Breaking the degeneracy of nematic liquid crystals by means of actuated anisometric paramagnetic colloids

We demonstrate experimentally that the director of a planar nematic liquid crystal cell with degenerated azimuth can be resolved by means of sparse paramagnetic inclusions actuated upon by weak magnetic fields. The use of ellipsoidal particles enables the characterization of the reorientation of the mesogen following particle rotation by means of a simple elastic model. Complex structures like targets and double-armed helices can be generated and controlled by tuning an out-of-plane electric field.

Polarization-controlled contrasted images using dye-doped nematic liquid crystals

We explore the polarization dependence of the nonlinear response of a planar nematic liquid crystal cell doped with 1% wt of methyl red dye. The results obtained show that the refractive index change can be switched from a positive value to a negative one as the polarization of the beam changes from parallel to perpendicular with respect to the rubbing direction. This property is exploited in a phase contrast system, where a dynamic phase filter is photoinduced in a liquid crystal cell placed in the system's Fourier plane. Real-time contrast inversion in the resulting images is demonstrated.

Electric-field enhancement and modulation of permanent grating diffraction in dye and carbon nanotube-doped liquid crystal

Permanent gratings were written, without the aid of external fields, in planar nematic liquid crystal cells doped with only Methyl Red (MR) as well as cells doped with both Methyl Red and single-wall carbon nanotubes (CNTs). Grating formation based on trans-cis photoisomerism of MR followed by surface adsorption of the cis-isomer was attempted. The diffraction efficiency was easily enhanced and modulated by application of ac fields covering wide ranges of frequency and voltage; thus, allowing for tuning the diffracted signals. Cells doped with MR and CNTs had a maximum relative diffraction efficiency of 67% while cells doped only with MR exhibited a maximum of 28%. The maximum diffraction efficiency appears to be associated with the Freedericksz threshold voltage. The permanent gratings are robust and have remained stable for over two years.

Inside Front Cover: Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters (Adv. Funct. Mater. 2/2008)

AbstractOn p. 212, Torsten Hegmann and co‐workers describe nematic liquid crystals (N‐LCs) confined in planar liquid crystal cells after doping with small quantities of gold nanoclusters. These give rise to a dual alignment mode and electro‐optic response (Freedericksz transition). By fine‐tuning of experimental conditions, N‐LCs doped with gold nanoclusters can be electrically reoriented and aligned either like N‐LCs with a positive dielectric anisotropy (used in twisted nematic displays) in a planar cell or alternatively as N‐LCs with a negative dielectric anisotropy (used in large LCD TVs based on the vertical alignment mode).We demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.

Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters

AbstractWe demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.

Optical properties of permanent gratings in liquid crystal doped with dye and carbon nanotube

Permanent gratings were written in planar nematic liquid crystal cells containing E7 doped with the azo dye Methyl Red and single-wall carbon nanotubes. An order parameter of S = 0.605 shows that the dye molecules align themselves very well with the liquid crystal’s director axis. A maximum absolute diffraction efficiency of 0.39% was found for cells doped only with 0.6% MR while a maximum of 2.6% was observed for cells doped with 0.6% MR and 0.002% CNTs. The data are consistent with permanent grating formation based on trans-cis photoisomerism of the dye molecules, followed by surface adsorption of the cis-isomer that leads to homeotropic alignment in the bright regions of the light illumination pattern. The robust permanent gratings were written without the aid of an external electric field and have remained stable for over 2 years.

Light-field-induced spatially periodic freedericksz transition in a planar nematic cell

It is shown that a threshold spatially periodic reorientation of the director by a light field is possible in a planar nematic liquid crystal cell if the ratio of the Frank elastic constants, K2/K1, exceeds a critical value. The periodic director structure arising in the cell leads to a self-diffraction of the incident light wave. The dependences obtained for this phenomenon make it possible to determine the values of the elastic constants K1 and K2, the director reorientation threshold, and the period of the director structure from the experimental values of the self-diffraction angle.

Inhomogeneous Threshold Director Reorientation in a Planar Nematic Flexoelectric Cell with Finite Anchoring Energy

The dependence of the threshold parameters and the period of the electric-field-induced spatially periodic reorientation of the director in a flexoelectric nematic liquid crystal (NLC) on the anchoring conditions at the surface of a planar NLC cell has been studied. The threshold electric field and the corresponding wave-number of the periodic structure of the director field have been numerically calculated for arbitrary values of the anchoring energy. In the case of strong anchoring, the corresponding analytical expressions are obtained in a single-constant approximation. A decrease in the azimuthal anchoring energy leads to an increase in the intervals of possible values of the flexoelectric parameter ν and the ratio K2/K1 of the Frank elastic constants. A decrease in the polar anchoring energy leads to narrowing of these intervals as compared to the case of infinitely strong anchoring at the NLC cell surface.

Mode-hop-free fine tuning of an external-cavity diode laser with an intracavity liquid crystal cell.

A planar nematic liquid crystal (NLC) cell is incorporated into a Littman-type external cavity as the wavelength-tuning device for a semiconductor laser diode. By varying the driving voltage of the NLC cell and laser diode bias current simultaneously, we demonstrate single-mode oscillation and mode-hop-free tuning over 19.2 GHz at 775 nm. The result is in good agreement with the theoretical predictions.

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.

Phase-conjugate reflection and self-starting optical phase-conjugate oscillation in planar nematic liquid-crystal cells

A theoretical model for phase-conjugate reflection of planar nematic liquid-crystal cells is presented based on the continuum theory of liquid crystals and the optical theory of degenerate four-wave mixing. An analytical solution for the phase-conjugate reflectivity R is derived under the undepleted-pump approximation, and the necessary conditions for self-starting optical phase-conjugate oscillation are obtained. The dependence of these conditions on the related physical parameters is also discussed. The reconstruction phenomenon of phase-conjugate light and the self-starting optical phase-conjugate oscillation have been observed experimentally. The intensities of the phase-conjugate light versus the biased voltages for various incident optical intensities have been measured, and the results agree very well with the prediction of our theoretical model.

Electrically assisted self-confinement and waveguiding in planar nematic liquid crystal cells

We report on spatial soliton formation and self/cross waveguiding in planar cells containing a nematic liquid crystal in the presence of an externally applied voltage. Self-confinement and cross-induced guidance are demonstrated with an Argon ion laser (514 nm) and a helium–neon probe (633 nm), respectively, over millimeter lengths and with milliwatt pump powers.

Twist molecular orientation transition in a nematic liquid crystal cell

We theoretically demonstrate that the application of a transverse next-to-static electric field to a planar nematic liquid crystal cell - obtained by strong planar anchoring of the liquid crystal on the first surface and negligible interaction with the second surface - causes an electro-reorientational transition of the Freedericksz kind. We show that, above the transition the cell behaves as a twisted nematic liquid crystal cell with total twist depending on the applied voltage. Preliminary experimental results confirming the predicted effect are presented.