Liquid Crystal Infiltration Research Articles

Mesoporous chochleate particles doped liquid crystal as normally transparent smart window

A composite layer of homogeneously aligned liquid crystal (LC) doped with nano or mesoporous microparticles (NMPs) has been shown to act as normally opaque smart window (SW) in what is called NMP-LC mode. The porosity of the particle plays two important roles: (i) When the LC infiltrates the particle, it brings its refractive index to be closer to that of its LC surrounding at a certain orientation of the LC molecules and creates a large index mismatch at the orthogonal orientation; (ii) the LC infiltration brings the porous particle density to be closer to the surrounding LC, thus preventing precipitation and floating. Here we demonstrate that LC doped with cochleate cigar-shaped hollow particles and using DMOAP as an alignment layer acts as a normally transparent smart window. Under voltage, the best scenario that explains the polarization-independent scattering is to have the cochleate particles inclined at an angle to balance the dielectric with the elastic forces effectively. The emergence of the scattering mode results from the symbiotic interplay of two distinct phenomena – the NMP effect and the electro-hydrodynamic instability effect. The synergy between these two effects manifests in lower operating voltage and frequency requirements for the device, hence less energy consumption. The scattering increases between 30 °C and 50 °C, thus it acts as a self-adjustable window giving more shade as the outside temperature rises. The threshold voltage is found to decrease with temperature. The SW exhibits high transparency in the OFF state, and high haze in the ON state, and the durability test shows that the SW switches for at least 6 days without degradation of optical contrast.

Compact and tunable broadband polarization splitter based on nematic liquid crystal infiltrated dual-core photonic crystal fiber

A compact and broadband polarization splitter is proposed based on nematic liquid crystal infiltrated dual-core photonic crystal fiber having a simpler hexagonal platform. The infiltration of the nematic liquid crystal in the properly optimized structure largely increases the mode birefringence (0.013491), which helps the splitter to attain a compact length of 71.35 µm. Also, the proposed splitter has a broad bandwidth of 356 nm (1285 nm to 1641 nm) keeping a better extinction ratio than -20 dB, covering all the O-E-S-C-L-U optical communication bands, and the minimum extinction ratio is -83.34 dB at the wavelength of 1550 nm. Moreover, nematic liquid crystal of E7 material enables temperature tuning of the device within the wavelengths of 1200 nm to 1650 nm; hence, bandwidth is extended to 450 nm. The compact design along with broadband and temperature tuning properties make the device interesting and it may find diverse applications in optical communication.

Liquid Crystal-Embedded Hollow Core Fiber Temperature Sensor in Fiber Ring Laser

An optical fiber temperature sensor based on Mach–Zehnder interferometer and thermo-optic effect of the liquid crystal (LC) in fiber ring laser (FRL) system is proposed and experimentally demonstrated. The LC is infiltrated into the core of hollow core fiber, and the resonant wavelength is more sensitive to temperature variation due to the interaction between the incident light and the cavity infiltrating liquid crystal with high thermal light coefficient. Meanwhile, the FRL system was further used to make the sensor have good performance in the case of high signal-to-noise ratio (∼35 dB), narrow half-height width (FWHM = 0.15 nm), and high sensitivity in the temperature range from 20 °C to 50 °C, with the maximum sensitivity of 1.318 nm/°C. As far as we know, in the FRL system, the liquid crystal material has a better temperature sensing performance than the previous fiber. Nevertheless, the system has the advantages of good repeatability, low cost, simple production, small volume, high sensitivity. In marine microbial culture and detection, it is necessary to carry out high sensitivity measurement within a small temperature variation range. This reliable and excellent temperature performance has a potential application prospect.

Optical properties of metasurfaces infiltrated with liquid crystals

Optical metasurfaces allow the ability to precisely manipulate the wavefront of light, creating many interesting and exotic optical phenomena. However, they generally lack dynamic control over their optical properties and are limited to passive optical elements. In this work, we report the nontrivial infiltration of nanostructured metalenses with three respective nematic liquid crystals of different refractive index and birefringence. The optical properties of the metalens are evaluated after liquid-crystal infiltration to quantify its effect on the intended optical design. We observe a significant modification of the metalens focus after infiltration for each liquid crystal. These optical changes result from modification of local refractive index surrounding the metalens structure after infiltration. We report qualitative agreement of the optical experiments with finite-difference time-domain solver (FDTD) simulation results. By harnessing the tunability inherent in the orientation dependent refractive index of the infiltrated liquid crystal, the metalens system considered here has the potential to enable dynamic reconfigurability in metasurfaces.

Two-Dimensional Hybrid Photonic Crystal With Graded Low-Index Using a Nonuniform Voltage

Abstract We proposed a new method for designing graded index lens using liquid crystal infiltration into annular photonic crystals. Applying an external nonuniform voltage in the transverse direction perpendicular to the direction of light propagation yields different orientation of liquid crystal molecules inside the photonic crystal unit cells. As a result, a gradient refractive index was modulated. We numerically investigate focusing properties of the designed graded index structure using plane-wave expansion and finite-difference time-domain methods. The gradient refractive index profile was adjusted by varying the nonuniform voltage excitations, which consequently altered the focal distance of the graded index structure. A wide tuning range of 1856 nm was achieved for focal distance by the proposed graded index structure. This feature can be implemented for planning a flat lens with tunable focal distance based on electro-optic effect. These achievements may have future applications in some optical devices such as near-field imaging and scanning.

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.

Active beam steering and afocal zooming by nematic liquid crystal-infiltrated graded index photonic structures

This study presents active beam steering and afocal zooming of light by incorporating liquid crystals (LCs) with graded index photonic crystals (GRIN PCs). The GRIN PC structures are composed of low refractive index polymer annular rods with holes of gradually varying radii. To actively manipulate incident light, the annular rods are infiltrated with nematic LCs. By applying an external voltage to the infiltrated LCs, the effective index profile of the low-index GRIN PC structure is modulated without introducing any mechanical movement. The incident beam deflection and corresponding focal distance modulation are tuned only by controlling the applied bias voltage. In the present work, the hyperbolic secant refractive index profile is chosen to design GRIN PC structures. To design a GRIN PC structure with annular PCs, the Maxwell–Garnett effective medium approximation is employed. We analytically express the relation between infiltrated LCs and the gradient parameter to show the physical background of the tuning ability of the proposed devices. Beam steering and afocal zooming devices are analytically investigated via geometrical optics, and numerically realized with the help of a finite-difference time-domain method. A beam deflection with an angle change of Δθout = 44° and a light magnification with maximum ×2.15 are obtained within operating frequencies of a/λ = [0.10–0.15] and a/λ = [0.15–0.25], respectively, where ‘a’ is the lattice constant and λ is the incident wavelength. The corresponding operating frequency bandwidths are calculated as 40% and 50% for the beam steering and afocal zooming applications, respectively. LCs are inexpensive materials and work under low voltage/power conditions. This feature can be used for designing an electro–optic GRIN PC device that has the potential for use in a wide variety of optical applications.

Electrically switchable photonic crystals based on liquid-crystal-infiltrated TiO2-inverse opals.

Electrically switchable photonic crystals are demonstrated based on TiO2 inverse opals infiltrated with liquid crystals. Macroporous anatase TiO2 inverse opals are fabricated from polystyrene opal templates through a sandwich vacuum backfilled method and followed by calcination. Upon liquid crystal infiltration, the optical properties of the hybrid organic/inorganic structure are characterized by reflectance measurements of the Bragg peak, the position of which can be switched using an external electric field. The physical mechanism underlying this switchable behavior is the reorientation of the liquid crystal molecules inside the spherical voids by the applied electric field, resulting in a significant change of the refractive index contrast between the liquid crystal and the TiO2 inverse opal. With advantageous features of cost-effective fabrication, easy integration, and electric control, such TiO2 inverse opals infiltrated with liquid crystals could play an important role in future development of active photonic devices.

Hybrid photonic crystal fiber for highly sensitive temperature measurement

A hybrid photonic crystal fiber (PCF) is proposed and demonstrated for highly sensitive temperature measurement. The hybrid PCF is formed by selectively infiltrating liquid crystal 5CB into the first ring of air holes around one silica core of a twin-core PCF (TCPCF). In the modified TCPCF, one silica core guides light with a total internal reflection (TIR) mechanism, while the other silica core guides light through a photonic bandgap (PBG) mechanism, due to the infiltration of high refractive index 5CB. The co-existing of TIR and PBG waveguiding mechanisms in the TCPCF leads to a hybrid light guiding mechanism PCF, i.e. hybrid PCF. Theoretical analysis reveals that the TIR-guided core mode, PBG-guided core modes and 5CB channel modes in the hybrid PCF can concurrently propagate and interfere with each other, which is further verified by experimental results. Besides, experimental investigation on the temperature response of the hybrid PCF shows that the temperature sensitivities can be up to 4.91 nm °C−1 for nematic phase of 5CB, and −3.68 nm °C−1 for isotropic phase of 5CB, respectively. The proposed hybrid PCF is promising for applications of temperature-tunable optical filtering in optical fiber communication, sensing and laser systems.

Electrically tunable long period gratings temperature sensor based on liquid crystal infiltrated photonic crystal fibers

A novel high sensitivity temperature sensor has been proposed and theoretically investigated based on a photonic crystal fiber (PCF) infiltrated with liquid crystal (LC). The LC is infiltrated into the core of PCF, and the refractive index (RI) of LC will be periodically altered due to the electro-optic effect when external voltage is applied on the comb electrodes, which satisfies the formation principle of long period fiber gratings (LPFGs) and thus the attenuation bands can be observed in transmission spectrum. The resonance wavelength is more sensitive to the temperature variations due to incident light interacts with the internal infiltrated LC which has a high thermo-optic coefficient (TOC). Meanwhile, the simulation of theoretical optimization is further carried out in order that the sensor is good performance with narrow full width at half maximum (FWHM) and high sensitivity in the temperature change from 15 °C to 58 °C. To the best of our knowledge, the sensing performance of temperature variation is superior to previous optical fibers infiltrated by LC material. It is imperative to realize that there is non-linear relationship between the temperature variation and resonant wavelength shift. Nevertheless, the excellent temperature performance that is reliable could be expected to be utilized in the biochemical reaction and cell culture where the high sensitivity measurement within a small temperature change range is needed.

Switchable Plasmonic Film Using Nanoconfined Liquid Crystals.

Structural coloration using plasmonic particles has received substantial attention due to its robust, permanent, and scalable characteristics across the full color range. In this study, a plasmonic structure based on a porous anodic aluminum oxide (AAO) film coated with a metallic film was fabricated. Colors were varied by changing the refractive index, which was achieved with a convolution with nanopores of AAO film and an infiltrated liquid crystal (LC) material. LC molecules confined in the porous AAO film were uniformly aligned, and they exhibited pore-size-dependent colors because of the specific refractive index. The thermal phase transition of the LC material in the nanopores changed the effective refractive index, switching the reflected colors, and the LC-infiltrated AAO remained stable over a month. We believe LC materials can extend the use of rigid conventional plasmonic structures from simple sensor applications to multifunctional uses such as color printing, writing pens, and displays.

Enhanced dual-frequency operation of a polymerized liquid crystal microplate by liquid crystal infiltration

The electric-field-induced switching behavior of a polymer microplate is investigated. A microplate fabricated with a photopolymerizable dual-frequency liquid crystal was surrounded by an unpolymerized photopolymerizable dual-frequency liquid crystal in the isotropic phase. As an electric field was applied along the plane of the microplate, the microplate switched to set its interior molecular orientation to be either parallel or perpendicular to the field, depending on the frequency. Analysis of the rotational behavior, as well as numerical calculations, showed that the surrounding unpolymerized photopolymerizable dual-frequency liquid crystal infiltrated into the microplate, which enhanced the dielectric properties of the microplate. To the best of our knowledge, this is the first report of an enhanced dual-frequency dielectric response of a polymer microplate induced by liquid crystal infiltration.

Adaptive Graded Index Photonic Crystal Lens Design via Nematic Liquid Crystals

An adaptive graded index photonic crystal (GRIN PC) lens system is designed by using liquid crystal (LC) infiltration. LCs have the property to change their refractive indices when an external voltage is applied. This feature allows for the modulation on the effective index profile of the low-index contrast GRIN PC lens. Alongside this property, without changing the length of the designed 2D GRIN PC, the corresponding focal distance can be adapted from infinity to a certain positive or negative focal point by controlling the applied voltage. The effects of random perturbations and intentional line defects on the focal tuning capability of proposed 2D GRIN PC are also studied. Moreover, due to the scalability of PC’s dispersion relations, the sizes of the proposed devices can be adapted so that it may operate in either infrared or microwave regimes. Consequently, the proposed tunable polymeric GRIN PC can be implemented in various optical applications, such as near-field imaging and scanning systems, vision correction, and auto-focusing.

INFILTRATED SILICA PHOTONIC CRYSTAL WITH LIQUID CRYSTAL AND VARIOUS ORGANIC LIQUIDS

Tunable photonic crystal has been developed by infiltration of nematic liquid crystal (NLC) and different organic liquids in the void of silica microspheres. Optical properties were investigated by ultraviolet–visible (UV–Vis) spectroscopy. It has been observed that the position of photonic band gap (PBG) shifts from 336[Formula: see text]nm to 326[Formula: see text]nm with the increase of applied field from 0[Formula: see text]V to 9[Formula: see text]V and 316[Formula: see text]nm to 324[Formula: see text]nm after the infiltration of the organic liquids. The refractive index of infiltrated liquid crystal (LC) is calculated at different applied electric field. The present results could be suited for implementation of low cost and compact design tunable devices with low power consumption for high density integrated optics.

Unique Temperature Dependence of Selectively Liquid-Crystal-Filled Photonic Crystal Fibers

We demonstrate a unique temperature-dependent characteristic of the selectively liquid-crystal-filled photonic crystal fiber, which is realized by a selectively infiltrating liquid crystal into a single air hole located at the second ring near the core of the PCF. Three-resonance dips are observed in the transmission spectrum. Theoretical and experimental investigations reveal that the three-resonance dips all result from the coupling between the LP01 core mode and the rod modes, i.e., LP03 and LP51. Then, we find that the dip shift induced by temperature shows good agreements with the thermo-optic performance of the LC employed. Furthermore, the dips shift greatly with changes in temperature, providing a method to achieve temperature measurement in such a compact structure.

Ferroelectric Based Photonic Crystal Cavity by Liquid Crystal Infiltration

A novel type of two-dimensional photonic crystal is investigated for it optical properties as a core-shell-type ferroelectric nanorod infiltrated with nematic liquid crystals. Using the plane wave expansion method and finite-difference time-domain method, the photonic crystal structure, which is composed of a photonic crystal in a core-shell-type ferroelectric nanorod, is designed for the square lattice and the hexagonal lattice. It has been used 5CB as a photonic crystal core, and LiNbO3 as a ferroelectric material. The photonic crystal with a core-shell-type LiNbO3 nanorod infiltrated with nematic liquid crystals is compared with the photonic crystal with solid LiNbO3 rods and the photonic crystal with hollow LiNbO3 rods.

Ultralarge negative dispersion single-polarization photonic crystal fiber

The concept of employing a central air hole in the core is exploited to obtain an ultralarge negative dispersion photonic crystal fiber (PCF) over the wavelength range of 1350 to 1650 nm. The results show that the fiber may exhibit an average dispersion well over −500 ps/nm-km with a flattened dispersion profile. It is also found that the fiber shows a high birefringence in the order of 10−2 over the entire wavelength bands of interest. The endlessly single-mode behavior of PCFs is utilized to ensure the single modedness of the proposed fiber. Also, the technique of liquid crystal infiltration is exploited to suppress one of the two orthogonal modes of the fundamental mode. Along with the single-polarization behavior, the fiber shows an even more negative dispersion profile with less dispersion variation.

Optical properties of metamaterial-based devices modulated by a liquid crystal

Due to the fact that it is possible to manipulate light with photonic crystals (PCs), PCs hold a great potential for designing new optical devices. There has been an increase in research on tuning the optical properties of PCs to design devices. We presented a numerical study of optical properties of metamaterial-based devices by liquid crystal infiltration. The plane wave expansion method and finite-difference time-domain method for both TE and TM modes revealed optical properties in photonic crystal structures in an air background for a square lattice. E7 type has been used as a nematic liquid crystal and SrTiO3 as a ferroelectric material. We showed the possibility of the metamaterials for a two-dimensional photonic crystal cavity on a ferroelectric base infiltrated with a nematic liquid crystal.

Photoactive photonic liquid crystal fiber polarization switches

This study presents a light-controlled photonic liquid crystal fiber (PLCF) polarization switch. The solid-core PCF has an index-guiding effect that reduces the insertion loss produced by infiltrating liquid crystals (LCs) with a low refractive index (RI). The proposed approach achieves photoactive tuning through the trans–cis photoisomerization of doped azobenzene, which modulates the RI of infiltrated LCs. This design achieves an optically tunable extinction ratio of average 10dB and photonic bandgap in the wavelength range of 1527–1538nm under 30mW laser illumination. The repeatable and switchable phase change is nearly 60°, corresponding to a response time of 100ms, which is to date the fastest light-tunable PLCF polarization switch available.

Tunable photonic nanojet achieved using a core–shell microcylinder with nematic liquid crystal

A tunable photonic nanojet achieved using a core–shell microcylinder with nematic liquid crystal is reported. The core–shell microcylinder can be obtained by the infiltration of liquid crystal into the air core of a microcylinder. The refractive indices of the liquid crystals can be changed by rotating the directors of the liquid crystals. Therefore, we were able to control the flow direction of the photonic nanojet in two-dimensional core–shell microcylinder structures. Using high resolution finite-difference time-domain simulation, we demonstrate that the photonic nanojet can be continuously tuned in the core–shell microcylinder. The horizontal and vertical shifts of photonic nanojet depend strongly on the director of the liquid crystals. Such a mechanism of nanojet adjustment should open up a new application for using visible light to detect nanoparticles, optical gratings, and single molecules with subwavelength spatial resolution.