Thickness Of Liquid Crystal Layer Research Articles

Liquid Crystal Metasurface for On‐Demand Terahertz Beam Forming Over 110° Field‐Of‐View

AbstractThe terahertz spectral region, which bridges between electronics and optics, is poised to play an important role in the development of transformative wireless communication and imaging systems with unprecedented functionality. Currently, a major challenge in terahertz technology is to develop high‐performance terahertz beam‐forming devices that can dynamically shape the terahertz radiation in a flexible manner. Existing terahertz beam‐forming devices have limited coding bits, field‐of‐view, and beam gain. Here, a reconfigurable liquid crystal‐integrated terahertz metasurface is experimentally demonstrated, with each metasurface unit cell being independently addressable. The metasurface has a 260° continuous phase tuning range with a liquid crystal layer thickness of only 1% of the free‐space wavelength. The terahertz wave diffracted from the metasurface can be steered toward a wide range of directions is shown, covering a record‐large 110° field‐of‐view with a peak gain of 25 dBi. The metasurface also features a low power consumption and sub‐second switching time. Furthermore, the formation of multiple terahertz beams is demonstrated, with the direction of each beam and the power ratio between beams adjustable on demand. The proposed liquid crystal metasurface possesses compelling prospects for future terahertz communication and imaging applications.

Liquid Crystalline Electro-Optical Modulator of Submegahertz Range

A liquid crystalline (LC) electro-optical modulator providing an electro-optical response time up to 400 nanoseconds with a modulation frequency up to 500 kHz has been developed. The ferroelectric liquid crystal FLC-576 obtained by us was used as the electro-optical medium of the modulator. Its helix pitch р0 is much smaller than wavelengths of visible light (р0 < 100 nm). The liquid crystal operates in the deformed helix ferroelectric mode (DHF-effect) under the action of electric fields. The controlling electric fields are less than a critical field of helix unwinding. At high frequencies of the control voltage (over 10 kHz), the heating of electro-optical cell by repolarization currents of FLC can occur up to the temperature of phase transition from ferroelectric to paraelectric phase, which leads to the cessation of electro-optical modulation. The work experimentally and theoretically examines the self-heating issues of electro-optical modulators depending on the liquid crystal layer thickness, frequency and strength of applied electric field. The heat removal conditions from the liquid crystal cell have been determined and experimentally implemented. Under these conditions the LC is in the smectic C* ferroelectric phase temperature range, which is necessary to provide electro-optical modulation in submegaherts diapason.

Influence of Design Parameters of Liquid Crystal Cell on the Optical Threshold Voltage Value of Electro-Optical Effect

The influence of liquid-crystal (LC) cell design parameters on the voltage of the optical threshold of the interference effect of polarized beams controlled by an electric field has been studied by computer simulation. It is shown that under strict boundary conditions (both symmetrical and antisymmetrical), the voltage value of the optical threshold decreases rapidly depending on the cell thickness in an exponential manner and reaches a certain constant value. Under non-strict boundary conditions (both symmetrical and antisymmetrical), the voltage dependence of the optical threshold on the LC layer thickness follows a hyperbolic pattern. At the same time, under strict symmetric boundary conditions, the voltage magnitude of the optical threshold effect decreases exponentially with an increase in the pretilt angle of LC molecules on the orienting surfaces. If the boundary conditions are rigid and antisymmetric, an increase in the pretilt angle on the orienting surfaces results in a linear decrease of the optical threshold voltage of the effect. The voltage magnitude of the optical threshold effect increases linearly with an increase in the twist angle of the LC structure in the cell, for both symmetric and antisymmetric boundary conditions. A decrease in the anchoring energy of LC molecules on a substrate leads to a decrease in the voltage of the optical threshold of the effect, and this regularity has an exponential character depending on the reciprocal value of the anchoring energy of LC molecules on orienting substrates.

Three-dimensional instrument polarization analysis and optimization of liquid-crystal-based Stokes polarimeter.

The Stokes polarimeter based on liquid crystal variable retarders (LCVRs) is a space polarization measurement technology widely used. However, due to the tilt of the optic axis of the LCVR with the driving voltage in the direction of light propagation and the interference in LCVR, the LCVRs-based Stokes polarimeter produces a large instrument polarization, which affects the accurate polarization measurement. In this paper, we combine polarization ray tracing with multi-beam interference, and establish a general three-dimensional polarization analysis model of the LCVRs-based Stokes polarimeter. The simulation results of adjusting the LCVR voltage to reduce the instrument polarization are analyzed, and the variation of polarization measurement accuracy with the field of view before and after optimization of the LCVRs-based Stokes polarimeter is simulated and analyzed. A LCVR structure with additional films for matching the refractive index is proposed. According to the simulation results, this structure can significantly reduce the interference effects and reduce the impact of variations in liquid crystal layer thickness on the interference effects.

Tunable spectral manifestation of Tamm plasmon-polaritons in a hybrid structure with 2d black phosphorus in the terahertz range

ABSTRACT A structure consisting of the 2d black phosphorus (BP) film and a rugate filter (RF) divided by a liquid crystal (LC) layer is theoretically studied as a system in which Tamm plasmon-polaritons (TPP) can be excited. The spectral distribution of the reflection coefficient of the system in the RF band gap region is calculated for the cases when the band gap is in the regions of 1 and 10 THz. It is shown that, in both cases, the reflection coefficient dip associated with TPP excitation near the 2d BP film appears in the RF band gap region. The spectral position and magnitude of the reflection coefficient dip depend on the harmonic profile parameters of the RF refractive index, concentration and effective mass of the 2d BP charge carriers, and the LC layer thickness and refractive index. It was found that the TPP wavelength is a periodic function of the LC layer thickness and its refractive index, and with their increase, two or more TPP can appear at wavelengths within the RF band gap. The possibility of controlling the TPP spectral position using an external electric field is noted.

Structural and Optical Characteristics of Flexible Optically Rewritable Electronic Paper

A comprehensive theory of light-reflective characteristics and experimental technique of liquid crystal layer thickness control for flexible optically rewritable electronic paper is presented. Cylindrical pillars were used to control the gap between flexible substrates. The introduced prototype of optically rewritable electronic paper has shown very promising performance. In this regard, we report theoretical results of structural photosensitive alignment of nematic liquid crystals on flexible substrate. The focus of theoretical study is on understanding the self-assembled complex structure, governed by the interplay between surface anchoring and liquid crystal elasticity. Mueller matrix spectroscopic ellipsometry was used to study light-reflecting characteristics and polarization properties of the twisted nematic film.

Development of Liquid Crystal Layer Thickness and Refractive Index Measurement Methods for Scattering Type Liquid Crystal Displays

Abstract We report the measuring method of scattering type display liquid crystal layer thickness based on capacitance values suitable for inline production process control. The method is selected for its effectiveness and simplicity over spectroscopic methods as conventional methods for scattering type displays are not applicable. During the method approbation process, a novel diffuser liquid crystal mixture refractive index was determined based on liquid crystal layer thickness measurement data.

Switchable 2D/3D display based on a liquid crystal lens array and the rotating specimen shooting method.

A liquid crystal (LC) lenticular lens array with auxiliary electrodes is proposed. The introduction of the auxiliary electrodes helps to obtain an LC lens array (LCLA) with relatively large aperture without complex structures. When the LCLA is in the focusing state, the voltage of auxiliary electrodes is less than that of edge electrodes, and the generated electric field in the LCLA can penetrate into the LC layer. Therefore, the ideal phase profile is obtained with a relatively thin LC layer thickness. Experimental results show that the LCLA has the characteristics of high optical power and low operation voltage. Based on the proposed LCLA, a multi-view 2D/3D switchable display is realized. In the experiment, a series of parallax images is obtained by rotating the sample to replace the convergence shooting method for 3D imaging. Compared with other 2D/3D switchable display devices, the multi-view 2D/3D switchable display based on the LCLA is characterized by being thin and compact, and displaying no moiré pattern.

Relationship between liquid crystal layer thickness and variable-focusing characteristics of an ultrasound liquid crystal lens

The effect of the liquid crystal (LC) layer thickness on the optical characteristics of an ultrasound LC lens was explored. Three LC lenses with differing LC layer thicknesses (100, 200, and 300 μm) were fabricated, and the optical focal lengths were measured by an optical microscope with a varying driving voltage. For the lens with a 200 μm thick LC layer, a larger change in the focal length was observed for a smaller driving voltage compared with that of the other two lenses, indicating that the LC layer thickness is appropriate for a variable-focus lens.

Liquid-crystal-based floating-electrode-free coplanar waveguide phase shifter with an additional liquid-crystal layer for 28-GHz applications

A liquid-crystal (LC)-based floating electrode-free (FE-free) coplanar waveguide (CPW) phase shifter with an additional LC layer is demonstrated for the first time. An LC layer is overlain on the electrodes of the original model; this change increases the amount of electric flux that the proposed structure can confine in the tunable region, and thereby greatly increases the figure-of-merit (FoM) while maintaining the benefits of the simple coplanar structure. We simulated the variations in the phase shifter’s FoM, characteristic impedance, and driving voltage while sweeping the additional LC layer thickness up to 300 μm with each electrode condition at 28 GHz. In the case of electrode thickness variation, the FoM increased as electrode thickness increased, regardless of the presence of the additional LC layer. However, in the case of the signal electrode width variation, we obtained an opposite FoM tendency depending on the presence of the additional LC layer. This work shows the possibility of an efficient LC-based FE-free CPW phase shifter design for a given LC layer and electrode conditions.

Continuous modulation of geometric phase via electrically tuning the in-plane orientation of nematic liquid crystal

ABSTRACT Holographic display requires thinner liquid crystal cell for smaller pixels and fast response. In this paper, a continuous geometric phase modulation method realised by electrically tuning a dual in-plane-switching (IPS) LCD is proposed. Firstly, the function of geometric phase modulation and the driving voltage is obtained via numerical simulation. Secondly, a dual IPS liquid crystal cells is designed and fabricated, and by using it, the experimental data is measured. Lastly, the simulation and test results are matched, which show good consistence with each other, and a phase modulation range of near is achieved. By using the method, the phase modulation quantity is not only independent of the thickness of liquid crystal layer, but can be achieved almost 360° continuous modulation of the geometric phase. This method paves an avenue towards dynamic geometric phase modulation of light, which is meaningful for large field of view and colourful holographic display.

Study of the vertically aligned in-plane switching liquid crystal mode in microscale periodic electric fields.

The ongoing interest in fast liquid crystal (LC) modes stimulated by display technology and new applications has motivated us to study in detail the in-plane switching (IPS) vertically aligned (VA) mode. We have studied how the decrease of the period of the interdigitated electrodes (down to sub-micrometer scale) influences the switching speed, especially the LC relaxation to the initial homeotropic state. We have found that there are two types of the relaxation: a fast relaxation caused by the surface LC sub-layer deformed in the vicinity of the electrodes and the slower relaxation of the bulk LC. The speed of the fast (surface) mode is defined by half of a period of the electrode grating, while the relaxation time of the bulk depends on the LC layer thickness and the length of the driving electric pulses. Thus, the use of the surface mode and the reduction of the electrode grating period can result in significant increase of switching speed compared to the traditional LC modes, where the bulk relaxation dominates in electrooptical response. We have studied thoroughly the conditions defining the surface mode applicability. The numerical simulations are in good agreement with experimental measurements.

Laser-induced erasable patterns in a N* liquid crystal on an iron doped lithium niobate surface.

A chiral nematic (N*) liquid crystal (LC) was hybridized with a z-cut iron doped lithium niobate (Fe:LN) substrate and exposed with a focused continuous wave diode laser beam. The N* LC layer was confined with a cover glass to provide a homogeneous LC layer thickness. Two distinct kinds of test cells were investigated, one with an uncoated glass covering slip and one with an indium tin oxide (ITO) coated cover glass. Photo generated electric fields (generated in the Fe:LN) resulted in a localized defect formation and textural transitions in the N* LC. Due to field confinement, the field induced responses were more localized in samples with ITO coated cover glasses. By scanning the laser beam on programmed trajectories, formation of persistent patterns could be achieved in the N* LC layer. Polarized optical microscopy of the exposed samples revealed that these patterns consisted of adjacent circular Frank-Pryce defects. Exposure with a slightly defocused laser beam could be applied selectively to erase these patterns. Thus, a promising method is reported to generate reconfigurable patterns, photonic motives, and touch sensitive devices in a hybridized N* LC with micron accuracy.

Bistable liquid crystal device fabricated via microscale liquid crystal alignment

Bistable liquid crystal (LC) molecular orientation properties in micropatterned LC cells were investigated experimentally and theoretically. When an LC cell was heated to the phase-transition temperature and then cooled, an LC orientation with ±π/2-twist domains (±π/2-twist mode) was obtained. Furthermore, a different LC orientation with ±π-twist domains (±π-twist mode) was observed when a 10-V potential was applied across a sample LC cell. Both orientation states were stably retained over a long period. Herein, cross-sectional LC orientation models in the ±π/2- and ±π-twist modes are proposed to explain the generation and behavior of two different disclination lines. The total energies within one period in the ±π/2- and ±π-twist modes (F±π/2 and F±π, respectively) were estimated theoretically. These energies were found to depend on the LC layer thickness and to cross over at a certain thickness; this indicates that F±π is equal to F±π/2 at this equilibrium thickness. The best temporal stability is likely attained at this equilibrium thickness. We demonstrated a bistable color-switching device by combining a full-wave plate and crossed polarizers. When these optical components were configured properly, stable bistable switching between two colors was achieved.

Bulk heterojunction solar cells made from carbazole copolymer and fullerene derivative with an inserted layer of discotic material with improved efficiency

ABSTRACTBulk heterojunction photovoltaic cells based on composites of copolymer poly [N-90-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] and the fullerene derivative [6,6]-phenyl C71-butyric acid methyl ester with an inserted layer of discotic liquid crystalline material (2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene) between the interface of active layer and hole transporting layer has been reported. Different hole transporting layers deposited on indium tin oxide substrates such as poly (3,4-ethylenedioxythiophene)-poly (styrenesulphonate) or molybdenum trioxide has been used in these devices. All the devices with inserted discotic liquid crystal layer showed better performance than the reference cells. Power conversion efficiency of 5.14% was achieved for these photovoltaic solar cells containing self-organised discotic liquid crystal layer of 30 nm thickness under one sun condition which is substantial jump as compared to earlier reports. The mobility of holes in the discotic liquid crystal inserted devices was found to be of the order of 10–6 cm2 V–1 s–1 due to which high values of current density was achieved. The influence of varying the thickness of liquid crystal layer and annealing on the photovoltaic parameters of these devices was also studied.

Incorporation of liquid crystalline triphenylene derivative in bulk heterojunction solar cell with molybdenum oxide as buffer layer for improved efficiency

ABSTRACTWe report efficient bulk heterojunction solar cells fabricated by inserting a discotic triphenylene derivative into poly (3-hexylthiophene): [6, 6]- phenyl-C61-butyric acid methyl ester. A layer of molybdenum oxide was inserted between anode and active layer. Power conversion efficiency of 2.0% was achieved for these photovoltaic solar cells containing self-organised discotic liquid crystals in the active layer under one sun condition. The influence of varying the thickness of liquid crystal layer and annealing on these solar cells was also studied. Post annealing the bulk heterojunction devices with discotic liquid crystal layer of thickness 20 nm in them yielded an open circuit voltage of 0.41 V, short circuit current density of 17.0 mA cm−2, a Fill factor of 0.35 and power conversion efficiency of 2.5%.

Driving voltage properties sensitive to microscale liquid crystal orientation pattern in twisted nematic liquid crystal cells

We investigated the micropattern-sensitive driving voltage properties of twisted nematic liquid crystal (LC) cells and found that the threshold voltage for inducing the Fréedericksz transition strongly depends on the micropatterned LC molecular orientation state. We discuss the effects of various cell parameters such as the period of the micropattern Λ, the LC layer thickness d, and the twist angle Φ on the threshold voltage. By a computer simulation of the LC molecular orientation, we found that the threshold voltage Vth varies in response to the deformation factor Δ (= d2/Λ2 + Φ2/π2) of the spatially distributed LC molecular orientation. We confirm that is proportional to 1 − Δ from both theoretical and experimental standpoints.

Bulk heterojunction solar cells based on self-assembling disc-shaped liquid crystalline material

ABSTRACTIn this article, effect of addition of disc-shaped liquid crystalline material, namely 2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene, in poly (3-hexylthiophene): [6, 6]-phenyl-C61-butyric acid methyl ester containing bulk heterojunction (BHJ) solar cells has been investigated. These disc-shaped molecules organise into ordered columnar hexagonal structures through intermolecular π − π interactions as monitored by polarised light optical microscopy. Current–voltage characteristics of the device prepared with liquid crystal layer exhibited a short-circuit current of 10.5 mA cm−2 and a fill factor of 35%. The resultant power conversion efficiency (PCE) was 1.54%. The influence of varying the thickness of liquid crystal layer and annealing on these solar cells was also studied. The short circuit current and PCE of 12.9 mA cm−2 and 2.3% was achieved for these BHJ solar cells containing self-organised discotic liquid crystals in the active layer under one sun condition after annealing.

Impact of thickness of liquid crystal layer on response rate and exponential gain coefficient with assistance of ZnSe film.

A response time as short as 5.4 ms and an exponential gain coefficient as large as 1795.0 cm(-1) were obtained in C(60) doped 4,4'-n-pentylcyanobiphenyl liquid crystal cells sandwiched with two indium tin oxide glass plates coated with nanoscale photoconductive ZnSe films, which is believed to be facilitating charge-carrier generation and transportation and, hence, to be responsible for the fast response rate. The surface-mediated photorefractive effect and the ZnSe interlayers were both behind the high gain coefficients. The two-dimensional diffraction patterns observed in our system are also discussed.

A compensation method for the full phase retardance nonuniformity in phase-only liquid crystal on silicon spatial light modulators.

A simple and efficient compensation method for the full correction of both the anisotropic and isotropic nonuniformity of the light phase retardance in a liquid crystal (LC) layer is presented. This is achieved by accurate measurement of the spatial variation of the LC layer's thickness with the help of a calibrated liquid crystal wedge, rather than solely relying on the light intensity profile recorded using two crossed polarizers. Local phase retardance as a function of the applied voltage is calculated with its LC thickness and a set of reference data measured from the intensity of the reflected light using two crossed polarizers. Compensation of the corresponding phase nonuniformity is realized by applying adjusted local voltage signals for different grey levels. To demonstrate its effectiveness, the proposed method is applied to improve the performance of a phase-only liquid crystal on silicon (LCOS) spatial light modulator (SLM). The power of the first diffraction order measured with the binary phase gratings compensated by this method is compared with that compensated by the conventional crossed-polarizer method. The results show that the phase compensation method proposed here can increase the dynamic range of the first order diffraction power significantly from 15~21 dB to over 38 dB, while the crossed-polarizer method can only increase it to 23 dB.