Thin Liquid Crystal Layer Research Articles

18‐2: A Novel Design for Reconfigurable Intelligent Surfaces (RIS) with Thin Liquid Crystal Layer for Wireless Communications

This paper proposes a proof‐of‐concept design for RIS made with liquid crystals (LC) and Corning glass for wireless communication applications. The proposed RIS concept consists of the specially designed copper‐coated glass substrates and liquid crystal materials that were filled in between two glass substrates. The LC layer exists between the upper and lower Corning glass sheets, and it is 20 μm thick. Although the 20 μm cell‐gap is very thin compared to the operating frequency (it is more than 100 μm in previous research), more than the 180° phase difference was achieved by applying 10 V in the designed frequency band. Our proposed concept can improve wireless communication coverage by reflecting incident electromagnetic waves in the desired direction to secure a propagation path by simply changing the alignment of LC molecules under different driving voltages. Also, conventional LCD manufacturing technology can significantly reduce costs and support larger sized capabilities, compared to incumbent semiconductor‐based electronics and printed circuit board (PCB) processing.

Electromagnetic guided waves in composite liquid crystal-based interfaces

We study an air–crown glass planar interface that includes a thin layer of a cholesteric liquid crystal doped with silver spheres of nanometer size. We propose a new theoretical model for the propagation of electromagnetic waves through the liquid crystal part and use the Marcuvitz–Schwinger form of the Maxwell equations to compute guided surface wave profiles. The results suggest the presence of anisotropic surface modes with negligible attenuation. The dependence of the surface wave parameters on the liquid crystal layer parameters can be used in liquid crystal-based sensors.

Electrically Tunable Reflective Metasurfaces with Continuous and Full-Phase Modulation for High-Efficiency Wavefront Control at Visible Frequencies.

All-dielectric optical metasurfaces can locally control the amplitude and phase of light at the nanoscale, enabling arbitrary wavefront shaping. However, lack of postfabrication tunability has limited the true potential of metasurfaces for many applications. Here, we utilize a thin liquid crystal (LC) layer as a tunable medium surrounding the metasurface to achieve a phase-only spatial light modulator (SLM) with high reflection in the visible frequency, exhibiting active and continuous resonance tuning with associated 2π phase control and uncoupled amplitude. Dynamic wavefront shaping is demonstrated by programming 96 individually addressable electrodes with a small pixel pitch of ∼1 μm. The small pixel size is facilitated by the reduced LC thickness, strongly suppressing cross-talk among pixels. This device is used to demonstrate dynamic beam steering with a wide field-of-view and high absolute diffraction efficiencies. We believe that our demonstration may help realize next-generation, high-resolution SLMs, with wide applications in dynamic holography, tunable optics, and light detection and ranging (LiDAR), to mention a few.

A Low-Loss Defected-Ground-Structure Based Phase Shifter and Antenna on a 4-μm Thin Liquid Crystal Layer

In this paper, a liquid crystal (LC) phase shifter is proposed using a defected-ground-structure (DGS) transmission line on a 4-μm-thin LC layer. The DGS transmission line is a modified micro-strip line that improves phase shift and loss in a 4-μm-thin LC layer by partially removing the ground plane. The phase shift is achieved through the variation of the LC dielectric constant caused by the bias voltage of the DGS transmission line. The proposed DGS phase shifter exhibits the highest figure-of-merit (FoM) of 61.5°/dB at 30 GHz. Furthermore, the phase shifter exhibits an rms phase error of less than 20° at 26-30 GHz and a 1-dB compression point of 30 dBm with 2.5° phase error at 35 dBm. To demonstrate the feasibility of the LC-based antenna array systems, the DGS phase shifter is combined with a patch antenna, resulting in a gain of 0 dBi and 360° phase shift at 18 GHz. The thin LC layer and high FoM make the proposed DGS phase shifter a suitable solution for use in large phased array systems and reconfigurable intelligent surfaces.

Photo-reconfigurable and electrically switchable spatial terahertz wave modulator [Invited

Spatial terahertz wave modulators that can arbitrarily tailor the electromagnetic wavefront are in high demand in nondestructive inspections and high-capacity wireless communications. Here, we propose a liquid crystal integrated metadevice. It modulates the terahertz wave based on the adjustable electromagnetically induced transparency analog when spatially changing the environmental refractive index. The functions of the device can be arbitrarily programmed via photo-reorienting the directors of liquid crystals with a digital micromirror device-based exposing system. The thin liquid crystal layer can be further driven by an electric field, and thus the function can be rapidly switched. Amplitude modulation and the lens effect are demonstrated with modulation depths over 50% at 0.94 THz.

Multielement Polychromatic 2D Liquid Crystal Dammann Gratings

AbstractMultielement and multiwavelength switchable Dammann gratings are demonstrated in polymerizable nematic liquid crystal (LC) devices fabricated using two‐photon polymerization direct laser writing. To begin with, a single element 2D Dammann grating, with a diffraction efficiency of around 60%, is fabricated in a thin polymerizable LC layer such that the grating could be activated and deactivated with the application and removal of an external applied voltage. The impact of the fabrication parameters on the resulting intensity distribution in the far‐field diffraction pattern is presented and discussed, along with results for the response times, which are found to be less than that recorded for the regions of the LC device that did not consist of the laser‐written grating. A multielement device is then presented that exhibits two different Dammann gratings that are activated by the application of two different voltage amplitudes. Results for the corresponding far‐field intensity profiles and response times are presented, which demonstrate that high quality laser written switchable Dammann gratings can be fabricated in polymerizable LCs. These switchable elements have potential in programmable beam shaping, microscopy and optical communication devices.

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.

Dember photovoltaic effect as method for structural characterization of phospholipidic membranes

Dember photovoltaic effect (Dember effect) was used to study permeability of a thin layer of lyotropic liquid crystal (dipalmitoylphosphatidylcholine) at various temperatures. By photo dissociation of a photosensitive ionic dye dissolved in a lyotropic liquid crystal, an inhomogeneous distribution of charge carriers was induced in a thin liquid crystal layer. The diffusion relaxation of these charge distributions depends on the structure of lyotropic liquid crystal and show up itself in the temperature dependence of pulsed photoelectric response. The data obtained are in a good agreement with the results of dielectric measurements. In perspective, above method can be applied to examine permeability of natural and synthetic lipid vesicles (cells, exosomes, liposomes and other vesicles) relevant in medicine and to characterize the structural phase transitions in lipid membranes when modified by molecules of interest (e.g. cholesterol in liposomes to increase rigidity and stability).

Fully Electronically Phase Modulation of Millimeter-Wave via Comb Electrodes and Liquid Crystal

This letter presented a nematic liquid crystal (LC) based metal-dielectric-metal structure to fully electronically modulate the reflection phase of electromagnetic waves. The demonstrated structure is composed of a thin LC layer sandwiched between copper comb electrodes and a single slotted reflection array. Based on a proper application of the external electric field, the whole process of nematic LC molecular orientation is controlled. The innovative feeding method can actively prealign and deflect the orientation of LC molecules. Experimental results show that the maximum reflection phase shift is 189.2° at 114.9 GHz. This letter provides a practical way of regulating LC-based terahertz devices. The proposed structure is a potential candidate for the reconfigurable reflectarray antennas.

Исследование диэлектрических параметров нематических жидких кристаллов в приграничных слоях

This article presents the results of a study of the dielectric properties of thin liquid crystal layer systems. To study the dielectric parameters of nematic liquid crystals in the boundary layers, the frequency and temperature dependences of the real (ε') and imaginary (ε'') parts of the dielectric constant were determined. We studied samples that are in macroscopic layers and samples located in flat capillary layers (d ~ 0.1 μm). In both cases, the measurement is performed by the same method, namely, the method of temporary dielectric spectroscopy. The entire procedure for measuring, recording, accumulating and processing data was carried out automatically. In this case, the results of dielectric measurements could be presented both in the frequency and time domains. For measurements, a plane-parallel measuring cell made of brass and titanium electrodes was used. It should be noted that the surface of a solid body has a strong influence on the phase state of liquids and liquid crystals in the boundary layer. For example, in such a layer, liquid molecules acquire a mesophase ordering. In the case of NLC, the nematic potential increases and, as a result, the temperature of the nematic-isotropic phase transition increases. However, a shift in the bleaching temperature in the NLC mixture studied by us did not appear due to the smallness of the effect. The substance enclosed in the microspace between the surfaces of the mica has a higher dielectric constant than the corresponding bulk sample. Moreover, such a difference is observed both in the isotropic and in the nematic phases. This can be explained as follows. In a thick layer, a dipole-dipole intermolecular interaction takes place, which reduces the effective dipole moment of the molecules.

Theoretical Prediction of Umbilics Creation in Nematic Liquid Crystals with Positive Dielectric Anisotropy.

Optically assisted electrical generation of umbilic defects, arising in homeotropically aligned nematic liquid crystal cells and known as topological templates for the generation of optical vortices, are reported in nematic liquid crystals with positive dielectric anisotropy in detail. It is shown that nematic liquid crystals with positive dielectric anisotropy can serve as a stable and efficient medium for the optical vortex generation from both linearly and circularly polarized input Gaussian beams. Hybrid cells made from a thin layer of nematic liquid crystal confined between a photoresponsive slab of iron-doped lithium niobate and a glass plate coated with an active material, i.e., indium tin oxide, were studied. Exposure to a laser beam locally induces a photovoltaic field in the iron-doped lithium niobate substrate, which can penetrate into the liquid crystal film and induce realignment of molecules. The photovoltaic field drives charge carrier accumulation at the interface of indium tin oxide with the liquid crystal, which effectively modifies the shape and symmetry of the electric field. The photovoltaic field has a continuous radial distribution in the transverse xy-plane, weakening with increasing distance from the light irradiation center, where the electric field is normal to the cell plane. Umbilics are created as a result of the liquid crystal tendency to realign parallel to the electric field. Numerical studies of the transmitted intensity profiles in between linear polarizers reveal optical vortex pattern (of four and eight brushes) characteristics for the umbilical defects. The application of crossed circular polarizers results in annular-shaped intensity patterns as a result of spin-to-orbital angular momentum conversions, which give rise to the optical vortices.

On the Structural Instability of a Nematic in an Alternating Electric Field and Its Connection with Convection and the Flexoelectric Effect

The threshold structural instability arising in the thin layer of a nematic liquid crystal (nematic) along the surface of an electrode during the flow of a weak direct injection current is described. Local, limited in length Lу, nuclei (precursors) of electrohydrodynamic and flexoelectric instabilities are assumed to be in this thin layer. In the case of electrohydrodynamic instability, such precursors have been called “bullets” (solitons) because of their specific appearance, and their length Lу is a measure of local perturbation of the orientational structure of a nematic. In the case of flexoelectric instability, pieces Lу are formed by an irregular system of short polarized flexoelectric domains. Such an instability corresponds to a system consisting of groups of stripes, which are characterized by the opposite motion of “bullets” along these stripes and the average velocity of this movement.

Electrically driven flexible 2D plasmonic structure based on a nematic liquid crystal

A novel 2D active plasmonic grating based on liquid crystal (LC) infiltration is demonstrated and theoretically analyzed, by combining the plasmonic properties of the gold nanostructure and the optical properties of the liquid crystal. In this structure, a thin layer of E7 liquid crystal was typically injected onto a gold nanostructure, deposited on a polydimethyl siloxane substrate, using nanoimprint lithography method. The surface plasmon resonance of the fabricated plasmonic structure can be controlled by changing the refractive index of the LC, which was achieved with an external electric field. LC molecules confined between the gold nanostructure and an indium–tin–oxide glass are randomly aligned, and they can exhibit a reversible refractive index, depending on their orientation under the external voltage and the polarization of the incident light. Both theoretical and experimental results demonstrate that the wavelength of the resonance peak can be red shifted by the electric field-dependent refractive index of the LC. This experimental work provides us an active control of surface plasmon resonance using LC which can act as an ideal active medium for different applications such as a low voltage sensor with a sensitivity of 0.4375 nm V−1.

Uniqueness of director configuration states for liquid crystals in the case of weak anchoring

Abstract In a thin layer of liquid crystal the configuration of the director field depends on the interaction between the elastic properties of the material, the thickness $d$ of the layer, the boundary conditions and the external fields that may have been applied. Suitable combinations of these factors can give rise to distorted configurations (Freedericksz transitions). In this paper we assume the Oseen-Frank model for the energy and that the director field depends only on the direction orthogonal to the layer; we assume also weak anchoring conditions at the two bounding surfaces, and we mainly study the problem of uniqueness of such distorted configurations. More precisely, we first consider the nematic case in the presence of a magnetic field $\mathbf H$, and we prove the uniqueness of the stable configuration provided the magnitude of $\mathbf H$ is between two critical thresholds, simplifying some results already known in the literature, and calculating explicitly the critical thresholds. Then we study the case of a cholesteric liquid crystal without external field. In this case the director field tends to form a right-angle helicoid around a twist axis orthogonal to the layer, and we have distorted configurations (namely oblique helicoid) for suitable value of $d$. Also in this case, with suitable restrictions on the elastic constants in the Oseen-Frank energy, we find two critical thresholds for $d$, and we prove the existence of only one stable director configuration if $d$ is between them.

Efficient visible light modulation based on electrically tunable all dielectric metasurfaces embedded in thin-layer nematic liquid crystals

All-dielectric metasurfaces have attracted attention for highly efficient visible light manipulation. So far, however, they are mostly passive devices, while those allowing dynamic control remain a challenge. A highly efficient tuning mechanism is immersing the metasurface in a birefringent liquid crystal (LC), whose refractive index can be electrically controlled. Here, an all-dielectric tunable metasurface is demonstrated based on this concept, operating at visible frequencies and based on TiO2 nanodisks embedded in a thin LC layer. Small driving voltages from 3~5 V are sufficient to tune the metasurface resonances, with an associated transmission modulation of more than 65%. The metasurface optical responses, including the observed electric and magnetic dipole resonance shifts as well as the interfacial anchoring effect of the LC induced by the presence of the nanostructures, are systematically discussed. The dynamic tuning observed in the transmission spectra can pave the way to dynamically tunable metasurface devices for efficient visible light modulation applications.

Reversible LED controlled optical activity of a cholesteric liquid crystal layer

A thin layer of the photosensitive cholesteric liquid crystal possessing a high value of the optical activity is studied. Reversible change in the optical activity controlled by two light emission diodes (LEDs) with different emission wavelengths is applied for information recording. The behavior of this structure under exposure to UV and blue light is investigated. A smooth decrease in optical activity caused by the UV light resulting in a change in the color of the structure between crossed polarizers is obtained. The reverse process occurs under exposure to the blue light. On the basis of the obtained results, two methods of optical information recording in such a layer have been demonstrated.

Liquid-crystal-integrated metadevice: towards active multifunctional terahertz wave manipulations.

Active terahertz elements with multifunction are highly expected in security screening, nondestructive evaluation, and wireless communications. Here, we propose an innovative terahertz metadevice that exhibits distinguishing functions for transmitted and reflected waves. The device is composed of a thin liquid crystal layer sandwiched by Au comb electrodes and a dual-ring resonator array. For transmission mode, the metadevice manifests the electromagnetically induced transparency analog. For reflection mode, it works as a perfect absorber. The comb electrodes actuate the in-plane switching of liquid crystals, making the metadevice actively tuned. 60GHz frequency tuning of an electromagnetically induced transparency analog and 15% modulation depth of the absorption are demonstrated. Such modulations can be realized in the millisecond scale. The in-plane switching driving mode avoids the electrode connections among separate resonators, thus freeing the design of the metadevice. The proposed work may pave a bright road towards various active multifunctional terahertz apparatuses.

Spontaneous striped pattern formation in thin chiral nematic liquid crystal layers

Periodic modulations in chiral nematic liquid crystals (CLCs) are used in several optical and photonic devices. A remarkable way of fabricating and controlling such patterns is in a thin wetting layer that grows as temperature is decreased. Here, we study this stripe formation process experimentally and by means of Monte Carlo simulations. We report on samples of CLCs that present, after the wetting transition, a stripe/fringe formation occurring either through a nucleation process or homogeneously across the cell. These patterns depend on the ratio of the pitch to thickness of the layer, elastic anisotropy, and anchoring strength. Although such process is well documented for CLCs under applied field, we show here that similar textural transitions occur in thin layers mediated by the weak anchoring at the CLC-isotropic interface; it happens for values of pitch to thickness ratio much smaller than the ones reported in field driven situation. The Monte Carlo method takes action in simulating the experimentally observed textures by using a pairwise additive potential model for CLCs. A qualitative comparison with the experimental phenomena is possible to be made in which both the stripes formation and their behavior agree with the experiments.

Liquid-Crystal Active Tamm-Plasmon Devices

For the surface electronic states of a metal, the Tamm plasmonic (TP) mode can be excited without an additional phase-matching component, but in a typical device the TP resonance wavelength is fixed, which is quite limiting. This work develops an approach to tuning TP resonance by incorporating a thin layer of liquid crystal (LC) into the design. Here phase retardation by the LC layer is an ensemble effect, due to the entire layer, not just the molecules at the metal's surface, as in devices based on surface-plasmon polaritons (SPPs). Also unlike SPP systems, the proposed structure is highly amenable to mature thin-film processing, for scalability to lengths of several meters.

Topology-commanded optical properties of bistable electric-field-induced torons in cholesteric bubble domains

Nowadays, complicated topological defects enable many experimental manipulations and configurational simulations of active soft matter for optical and photonic applications. Investigation of topological defects in soft anisotropic materials enables one to better understand three-dimensional orientation fields in cholesteric liquid crystals. Here, we describe optical properties of bistable bubble domain (BD) texture torons in a thin layer of cholesteric liquid crystal (CLC), frustrated by homeotropic anchoring conditions, and reliably switchable by a random process. The control of macroscopic optical density and diffraction efficiency of the BD texture is demonstrated by a selection of a confinement ratio of the CLC. Experimentally reconstructed CLC director profile reveals the topology of BD torons allowing consideration of naturally occurring BD texture for applications in optical and photonic devices, which are bistably switchable between active and transparent optical states.