Nematic Liquid Crystal Layer Research Articles

Carbon dots induced homeotropic alignment in a negative dielectric nematic liquid crystal material

Abstract Recently, doping guest materials such as quantum dots (QDs) into liquid crystals (LCs) has been of great interest since their addition substantially enhances the properties of LC and opens new avenues for scientific advancement. Here, we report the induction of homeotropic alignment in cells without alignment layers of the negative dielectric nematic liquid crystal, N-(4-Methoxybenzylidene)-4-butylaniline (MBBA) by doping with carbon dots (CDs ~2.8±0.72 nm). The CDs-MBBA composites (CDs concentration: 0.002, 0.01, 0.03, 0.1 and 0.3 wt%) were investigated using optical polarising microscopy, electro-optical and dielectric techniques. Polarizing optical micrographs and voltage dependent optical transmission revealed the induced homeotropic alignment for all the composites under investigation. Interestingly, the least concentrated sample, 0.002 wt% exhibited partial homeotropic alignment. However, due to light leakage, the optical transmission value below threshold voltage was relatively higher than the rest of the composites. MBBA is a negative dielectric material, hence the application of a voltage across the cell was able to switch the alignment from a dark to a bright state for all composites. However, above a certain voltage (> threshold voltage), the bright state produced some instabilities. The value of dielectric permittivity was observed to decrease with increasing concentration, confirming the effect of CDs in producing homeotropic alignment in MBBA. Measurements as a function of temperature were conducted to examine the thermal stability of the induced alignment. The alignment was found to be stable throughout the nematic phase of MBBA. The induction of such alignment without conventional alignment (i.e., rubbing of polyimides) technique can be helpful in addressing the evolving display demands by making liquid crystal displays (LCDs) and other display devices cost effective.

Liquid Crystal Enables Extraordinarily Precise Tunability for a High‐Q Ultra‐Narrowband Filter Based on a Quasi‐BIC Metasurface

AbstractNotch filters usually involve high costs, great difficulties in processing, and very limited tunability. By coating a nematic liquid crystal (LC) layer onto a well‐designed quasi‐bound states in the continuum (quasi‐BICs) metasurface, this work designs and demonstrates a high‐Q tunable filtering system with precise tunability in the near‐infrared spectral range. Optimal structural parameters and the filtering performance are first determined by numerical simulations and then confirmed in experiments. The precise tunability is enabled by modifying the LC molecules’ principal axes with an applied voltage, where the least distinguishable central wavelength interval is smaller than 0.3 nm, and the largest Q factor extracted from experiments can be ≈256.1. Compared to most of the commercial notch filters that work in the visible region and have no tunability, this work achieves an ultra‐narrowband filtering system that features low manufacture difficulty and cost, and great control of light propagation. The proposed design also opens new avenues for developing BIC‐based high‐Q devices with enhanced signal‐to‐noise ratio and multifunctional properties.

Modeling of laser generation in a Fabry–Pérot-Tamm structure with a nematic liquid crystal layer

In the presented work, the possibility of controlling laser generation using a nematic liquid crystal (NLC) in a hybrid layered structure consisting of a thin metal layer (Ag), a layer of NLC doped with a light-absorbing dye, and a distributed Bragg reflector (DBR) with a rectangular refractive index profile is theoretically studied. Spectral dependencies of the reflection, transmission, and absorption coefficients of light as well as the localization coefficient of the light field in NLC within the photonic bandgap of the DBR are obtained. Narrow dips in the reflection coefficient and peaks in the transmission coefficient are achieved due to the excitation of plasmons at the Ag-NLC interface. The dependence of the spectral position and magnitude of the plasmonic dips/peaks and the enhancement of the light field in the NLC medium on the thickness and orientation of the NLC layer as well as the impact of a light-absorbing dye doping are investigated. Theoretical calculations of the temporal dependencies of luminescence pulses for pumping pulses of different power settings (below, above, and at the threshold of laser generation) and different values of light absorption in the dye-doped NLC medium are performed, taking into account the peculiarities of the optical properties of the dye-doped NLC.

Polar Self-Organization of Ferroelectric Nematic-Liquid-Crystal Molecules on Atomically Flat Au(111) Surface.

Understanding nanoscale mechanisms responsible for the recently discovered ferroelectric nematics can be helped by direct visualization of self-assembly of strongly polar molecules. Here, we report on scanning tunneling microscopy studies of monomolecular layers of a ferroelectric nematic liquid crystal on a reconstructed Au(111) surface. The monolayers are obtained by deposition from a solution at ambient conditions. The adsorbed ferroelectric nematic molecules self-assemble into regular rows with tilted orientation, resembling a layered structure of a smectic C. Remarkably, each molecular dipole in this architecture is oriented along the same direction giving rise to polar ferroelectric ordering.

Condensed state physics in biology: Liquid crystal - semiconductor system

The possibility of application of condensed state physics methods for the study of biological objects is considered. The object of use is a cell with a liquid crystal as a model of a biological system - a living cell. The system of liquid crystal - semiconductor was experimentally investigated by methods of modulation spectroscopy. By determining the value of electric reflection, it is possible to judge about the state of surface polarization of liquid crystal. Exposure to an electric field changes the energy of the semiconductor’s zone structure, which leads to changes in the optical properties of both crystals. To investigate the optical modulation changes in our case, we used the method with the use of electrolyte KOH, which allowed us to measure the modulation of the spatial charge. The study of thin layers (up to 120 μm) of various nematic liquid crystals has shown that at values of electric fields comparable to the Fredericks transition threshold, the orientation angle of liquid crystal molecules changes due to the interaction of surface polarization with the external field, and an odd electro-optic effect is observed. If the value of the electric field is much larger than the threshold of the Fredericks transition, the electric moment, which depends nonlinearly on the field, predominates and leads to an even electro-optic effect. The results of the study are proposed to be used for the development of new drugs and materials with specified parameters for medicine.

Thermoplasmonic Controlled Optical Absorber Based on a Liquid Crystal Metasurface.

Metasurfaces can be realized by organizing subwavelength elements (e.g., plasmonic nanoparticles) on a reflective surface covered with a dielectric layer. Such an array of resonators, acting collectively, can completely absorb the resulting resonant wavelength. Unfortunately, despite the excellent optical properties of metasurfaces, they lack the tunability to perform as adaptive optical components. To boost the utilization of metasurfaces and realize a new generation of dynamically controlled optical components, we report our recent finding based on the powerful combination of an innovative metasurface-optical absorber and nematic liquid crystals (NLCs). The metasurface consists of self-assembled silver nanocubes (AgNCs) immobilized on a 50 nm thick gold layer by using a polyelectrolyte multilayer as a dielectric spacer. The resulting optical absorbers show a well-defined reflection band centered in the near-infrared of the electromagnetic spectrum (750-770 nm), a very high absorption efficiency (∼60%) at the resonant wavelength, and an elevated photothermal efficiency estimated from the time constant value (34 s). Such a metasurface-based optical absorber, combined with an NLC layer, planarly aligned via a photoaligned top cover glass substrate, shows homogeneous NLC alignment and an absorption band photothermally tunable over approximately 46 nm. Detailed thermographic studies and spectroscopic investigations highlight the extraordinary capability of the active metasurface to be used as a light-controllable optical absorber.

Liquid crystal resonator as a nonlinear reflector for passive Q-switching and mode locking

We carry out a theoretical study of a Fabry–Perot resonator with a central layer of nematic liquid crystal as a nonlinear polarization rotation (NPR) device for passive Q-switching and mode locking. We develop a self-consistent procedure for calculating the device dynamical response as a function of the input light intensity and polarization. We show how a positive feedback effect for the optical reorientation of the liquid crystal director makes the resonator abruptly switch to high-reflectivity state. We demonstrate that the time-delay between the pulse initiation and the resonator switch can be avoided by tilted liquid crystal molequles at the resonator boundaries. We show that such a NPR optical device is suitable for Q-switching operation with μs pulses and hundreds of kHz repetition rates. Further decrease of pulse duration and increase of the pulse repetition rate is possible by either increasing the pulse power or by increasing the Fabry–Perot resonator finesse. Finally, we show that the same Fabry–Perot resonator can be used for Q-switching and mode locking at tens of MHz repetition rates and ns pulse durations by using an electro-optic uniaxial–biaxial transition as an alternative to NPR in order to reduce pulse duration and increase repetition rate.

Composite color filters for wide color gamut and high optical efficiency liquid crystal displays

We propose a composite color filter to achieve the wide color gamut and high optical efficiency of liquid crystal display (LCD). The composite color filter comprises a multi-layer polarization interference filter (ML-PIF) and a color filter. Specifically, two crossed polarizers and four twisted nematic liquid crystal layers with alternating opposite twisted directions form the ML-PIF. The Jones matrix method provides the transmittance formula of the ML-PIF. In the experiment, we successfully fabricate the blue and green ML-PIFs samples using the temperature birefringence effect of liquid crystal. When composite color filters (ML-PIF with certain color filters) are used in LCD, this can successfully achieve a color gamut of 122.3% NTSC and an absolute optical efficiency improvement of 1.4%. Alternatively, the composite color filter can attain a color gamut similar to that of a high-performance color filter and an absolute optical efficiency improvement of 3.8%. This composite color filter presents a tremendous opportunity for achieving a wide color gamut and high optical efficiency.

Electrically and all-optically switchable nonlocal nonlinear metasurfaces.

Nonlocal effects on metasurfaces play an important role to achieve high-Q spectral selectivity, beneficial for development of multifunctional, multispectral integrated optics. In addition, they enhance the optical interaction and promote a variety of nonlinear effects, including frequency conversion and stimulated scattering. Active tuning of nonlocal nonlinearity is highly desirable for sensing and signal processing but was hardly explored until now. Here, we show drastic electric and all-optical tunability of nonlocal second-harmonic generation (SHG) from nonlinear metasurface, functionalized with a twisted nematic liquid-crystal (LC) layer. The addition of LC results in the emergence of strong nonlocal SHG, due to a surface lattice resonance of the system. We demonstrate a notable enhancement of SHG on resonance, more than 25 dB electrical switching amplitude, and all-optically induced phase transition imprinted on SHG. Our results on dynamic nonlocal effects introduce a very promising route for active nonlinear optical metadevices at the nanoscale.

Exploring the Core Parameters of CNC-Based Chiral Nematic Structures for Enhancing the Dissymmetry Factor of Right-Handed Circularly Polarized Luminescence.

The chiral nematic nanostructure formed from cellulose nanocrystal (CNC) self-assembly has shown great potential as a matrix for generating circularly polarized luminescent (CPL) light with a high dissymmetry factor. Exploring the relationship between the device composition and structure and the light dissymmetry factor is crucial to a common strategy for a strongly dissymmetric CPL light. In this study, we have compared the single-layered and double-layered CNC-based CPL devices with different luminophores, such as rhodamine 6G (R6G), methylene blue (MB), crystal violet (CV), and silicon quantum dots (Si QDs). We demonstrated that forming a double-layered structure of CNCs nanocomposites is a simple but effective pathway for enhancing the CPL dissymmetry factor for CNC-based CPL materials containing different luminophores. The |glum| values of double-layered CNC devices (dye@CNC5||CNC5) versus that of single-layered devices (dye@CNC5) are 3.25 times for Si QDs, 3.7 times for R6G, 3.1 times for MB, and 2.78 times for CV series. The different enhancement degrees of these CNC layers with a similar thickness may be due to the different pitch numbers in the chiral nematic liquid crystal layers whose photonic band gap (PBG) has been modified to match the emission wavelengths of dyes. Furthermore, the assembled CNC nanostructure has great tolerance to the addition of nanoparticles. Gold nanorods coated with the SiO2 layer (Au NR@SiO2) were added for enhancing the dissymmetry factor of MB in CNC composites (named MAS devices). When the strong longitudinal plasmonic band of the Au NR@SiO2 matched the emission wavelength of MB and the PBG of assembled CNC structures simultaneously, the increase in the glum factor and quantum yield of MAS composites was obtained. The good compatibility of the assembled CNC nanostructures makes it a universal platform for developing strong CPL light sources with a high dissymmetry factor.

Tuneable optical diffractive structures from liquid crystalline materials incorporated into periodic polymeric scaffolds

ABSTRACTDiffractive optical elements (DOEs) are micro-structured components used in various photonic devices to manipulate propagation direction and often also polarisation state of optical beams. The simplest type of DOEs are one-dimensional diffraction gratings. We perform theoretical investigation of electrical and magnetic tunability of such gratings composed as a periodic assembly of polymer and ferromagnetic nematic liquid crystal (LC) layers. We analyse how various grating parameters and boundary conditions influence the LC configuration in the assembly. For static response to external electric and/or magnetic fields, the distortion profile of the director field is determined numerically and analytically by minimisation of the Landau-de Gennes free energy, while for the dynamic response only numerical solutions are provided. The resulting optical diffraction properties are determined numerically via solving the Maxwell’s equations by using the rigorous coupled-wave analysis. We demonstrate that the described methodology provides an efficient tool for designing various new types of polarisation-sensitive LC-based DOEs, such as three-state polarisation switches for application in spectroscopic, optical communication, and remote sensing systems.

Dual controlled cross-shaped broadband terahertz absorber based on vanadium oxide—liquid crystal metamaterial

A low terahertz broadband tunable absorber was proposed, which composed of vanadium oxide (VO2) film and nematic liquid crystal layer. The simulation results show that the absorption is great than 90% in the frequency range of 0.458–1.1492 THz. The tuning absorber was realized by increasing the temperature to modulate the VO2 transited into metal state. Since the dielectric constant of the nematic liquid crystal can be adjusted by bias voltage, without changing the intensity of absorption, however, a shift in the bandwidth was observed. In addition, the designed structure is insensitive to incident light polarization and still remained high absorption at 60°. The tunable broadband metamaterial absorber can be used for attenuator and energy harvesting.

On‐Demand Polarization Controllable Liquid Crystal Laser

AbstractA polarization controllable band‐edge liquid crystal (LC) laser configuration is presented that provides on‐demand control of the polarization state of the laser using a four electrode in‐plane configuration to drive a nematic LC layer. By controlling the orientation of the electric field, and thus the orientation of the optic axis of the nematic LC, as well as the retardance, any laser polarization state (e.g., circular, elliptical) can, in principle, be created that is aligned along any desired direction in a plane perpendicular to the propagation direction. After calibrating the tuneable nematic device with a He‐Ne laser that has a well‐defined polarization state, control of the LC laser polarization is then demonstrated, where it is shown that different polarization states and orientations thereof can be obtained by changing the relative combination and amplitudes of the voltages applied to each of the four electrodes. It is found, however, that achieving a pure linear polarization state is challenging, which may indicate that the polarization from the LC laser is not a well‐defined circularly polarized state. These thin‐film LC lasers with full polarization control are potentially important for a variety of different applications ranging from the biomedical to the display industries.

Electrically switchable structural colors based on liquid-crystal-overlaid aluminum anisotropic nanoaperture arrays.

Actively tunable or reconfigurable structural colors are highly promising in future development for high resolution imaging and displaying applications. To this end, we demonstrate switchable structural colors covering the entire visible range by integrating aluminum nanoaperture arrays with nematic liquid crystals. The geometrically anisotropic design of the nanoapertures provides strong polarization-dependent coloration. By overlaying a nematic liquid crystal layer, we further demonstrate switchable ability of the structural colors by either changing the polarization of the incident light or applying an external voltage. The switchable structural colors have a fast response time of 28 ms at a driving voltage of 6.5 V. Furthermore, colorful patterns are demonstrated by coding the colors with various dimensions of nanoaperture arrays with dual switching modes. Our proposed technique in this work provides a dual-mode switchable structural colors, which is highly promising for polarimetric displays, imaging sensors, and visual cryptography.

36‐1: Invited Paper: Liquid‐Crystal Lasers: Recent Advances and Future Opportunities

Liquid crystal (LC) lasers use a layer of chiral nematic liquid crystal to form a self‐organising resonant cavity. When doped with fluorescent dye and suitably excited, they form versatile micro‐lasers with high efficiencies, low thresholds, and selective wavelength emission across a continuous range of the visible spectrum. This, combined with their scalable fabrication, makes them attractive candidates for a wide variety of photonic applications, including laser displays. This paper includes a review of the principles of operation, of recent enhancements that are advancing the technology closer to market, and concludes with demonstrations of LC lasers used for medical imaging and low‐speckle holography.

Surface plasmon excitation on a grating assisted by a cholesteric liquid crystal layer.

While surface plasmon excitation assisted by nematic liquid crystal layers has been comprehensively studied in different aspects, application of cholesteric structures for surface plasmon excitation remains an unexplored area. Moreover, structures including cholesteric layers and metal grating for surface plasmon excitation have never been considered, to the best of our knowledge. We studied theoretically and experimentally such structures focusing on different regimes of propagation of normal modes in the cholesteric liquid crystal layer. The application of such structures for sensing is accordingly discussed.

Tunable triple-band millimeter-wave absorbing metasurface based on nematic liquid crystal

We propose and design a tunable triple-band millimeter-wave absorbing metasurface based on nematic liquid crystal (NLC). Numerical results based on both effective circuit model and full-wave simulations verify the triple-band absorption property. All triple absorption peaks can be shifted by changing the bias voltage applied to the nematic liquid crystal. We fabricate and measure a triple-band absorbing metasurface sample, which covers 100 × 100 mm2 and is composed of 20 × 20 unit cells. In experiments, when no bias voltage is applied to the NLC layer, the resonant frequencies of the absorbing metasurface are 25.4, 28.7, and 32.8 GHz, respectively. When a 5-V bias voltage is applied, all the absorbing peaks redshift, and a frequency tunability range of 9.5%, 9.1%, and 8.58% is observed in three resonant frequencies, respectively. The experimental results are in good agreement with the numerical simulations. The proposed tunable absorbing metasurface provides a way forward for the realization of tunable millimeter-wave metasurface devices that can be applied in multi-band detection and imaging.

Correction to: Efficient tunable plasmonic mode converters infiltrated with nematic liquid crystal layers

Correction to: Efficient tunable plasmonic mode converters infiltrated with nematic liquid crystal layers

Spatially periodic deformations in hybrid aligned flexoelectric nematic layers

ABSTRACT Electric field-induced deformations in hybrid aligned layers of nematic liquid crystals possessing flexoelectric properties were investigated numerically. The thresholdless one-dimensional deformations occurred at low voltages whereas the spatially periodic deformations, taking the form of parallel stripes, were found above sufficiently high threshold voltage. The threshold voltage decreased when the flexoelectric properties get stronger. The stripes were parallel to the direction of planar alignment. The director distributions in the stripes induced by positive voltages differed qualitatively from those arising at negative voltage. The evolution of deformations during increase of bias voltage was investigated. The role of flexoelectric properties and of dielectric anisotropy was studied. The spatial period of deformations shortened with increasing voltage and with increasing flexoelectric coefficients.

Efficient tunable plasmonic mode converters infiltrated with nematic liquid crystal layers

This paper presents two efficient tunable plasmonic mode converters in the infrared regime. The proposed configurations consist of silver layer with etched rectangular holes as metal insulator metal waveguides with central cavities. The holes are infiltrated by nematic liquid crystal (NLC) material to increase the transmission through the suggested designs. Additionally, the NLC is used to have tunable operation where the modes at the output port can be controlled. The simulations are carried out using full vectorial finite element method. The first design has a single output port which converts the TM mode into the TEM mode with high transmission conversion efficiency of 70%. Further, the second structure allows the generation of the two plasmonic modes simultaneously in two output waveguides. During the biased state, the s- mode transmission conversion efficiency reaches 50% while the transmission of a- mode at the unbiased state is equal to 49%. It is expected that the proposed tunable mode converters will play an important role in the development of the plasmonic-photonic circuits.