Tunable Liquid Crystal Research Articles

Progress and Prospect of Liquid Crystal Droplets

Liquid crystal (LC) droplets are highly attractive for applications in privacy windows, optical switches, optical vortices, optical microresonators, microlenses, and biosensors due to their ease of fabrication and easy alignment at surfaces. This review presents the latest advancements in LC droplets, which have nematic, chiral nematic, and twist–bend nematic and ferroelectric nematic phases, or blue phases. Finally, it discusses the challenges and opportunities for applications based on LC droplets. The main challenges encompass the precise control of internal structures and defects to meet diverse application requirements, enhancing stability and durability across various environments, reducing large-scale production costs to improve commercial feasibility, increasing response speeds to external stimuli to adapt to rapidly changing scenarios, and developing tunable LC droplets to achieve broader functionalities.

Electrically Tunable Liquid Crystal Lens with Varifocal Capability

Electrically Tunable Liquid Crystal Lens with Varifocal Capability

Design of a tunable microlens based on hybrid single-wall carbon nanotube and liquid crystal

: Tunable liquid crystal (LC) lenses have garnered significant interest in recent years for their lightweight, cost-effectiveness, and versatility in applications like augmented reality, ophthalmic devices, and astronomy. Despite numerous proposed structures to enhance LC lens performance, achieving a minimal focal length while preserving image quality and balancing cost and complexity remains a daunting challenge. Adaptive microlenses encounter these obstacles due to physical constraints and diffraction effects. One promising solution lies in using adaptive LC-carbon nanotubes (CNTs) lenses. By adjusting external stimuli like electric or magnetic fields, it is possible to modify the refractive index distribution of the LC medium, thus changing the focal length of the microlens. When paired with CNTs, the LC molecules' orientation and control of focal length can be even more precise. Our research findings suggest that to achieve the shortest focal length in a microlens with a thickness of 22.5 μm, it is recommended that the diameter of the CNT should not exceed 10 nm, and the height of CNT should fall within the range of 10 to 20 μm considering the presence of screening effect. Conversely, for a microlens with low full width at half maximum (FWHM), narrower light spot, it is advisable to use CNT with a diameter larger than 100 nm and a height greater than 13 μm. Additionally, for a microlens with an efficient Fresnel number in the range of 1 to 10, the microlens should have CNT with widths from 100 to 200 nm. Therefore, based on the importance of each factor (focal length, FWHM, and Fresnel number), the dimensions of the carbon nanotube can be selected to suit the desired application.

Design, modeling, fabrication, and characterization of 50 mm diameter focus tunable liquid crystal lens with enhanced optical performance

Design, modeling, fabrication, and characterization of 50 mm diameter focus tunable liquid crystal lens with enhanced optical performance

Efficient preparation of cellulose nanocrystals and regulation of cholesteric liquid crystals for advanced anti‐counterfeiting

AbstractCellulose nanocrystals (CNCs) can self‐assemble to form cholesteric liquid crystal, which make it one of the most promising optical materials at present. CNCs can be produced by sulfuric acid hydrolysis. It is found that hydrolysis conditions influence the existing form of reaction components and their reaction activities, which leads to different reaction routes and mechanisms under varied reaction conditions, with CNCs of varied quality produced. High‐quality CNC with more uniform particle size could be prepared at a critical acid concentration of 64 wt%, which was used to study the formation process of CNC liquid crystal and the structure regulation of cholesteric liquid crystal. By adjusting the drying temperature and adding a small molecule of water‐soluble chitosan (WSC) can well regulate cholesteric liquid crystal and change the pitch of a film, and the reflection wavelength can be redshifted, thus enriching texture patterns and creating distinctive color. Suitable addition of WSC can enhance the multi‐domain effect during liquid crystal formation process and finally creates more colorful film with fine texture. The rich and tunable liquid crystal characteristics endowed by WSC addition is irreplicable and artistic, which helps to realize the great potential application of the composite film in advanced anti‐counterfeiting.

Tunable liquid crystal double-layer frequency selective surface with wide transmission band and small insertion loss

ABSTRACT In this paper, a tunable Frequency Selective Surface (FSS) based on liquid crystal (LC) with a wide transmission band, large tuning range, high tuning rate, and miniaturization is designed. First, a single-layer FSS is developed with two frequency selective layers and one LC tunable layer. The simulation and experiment results show that the single-layer FSS has a transmission bandwidth greater than 4.25 GHz, and the LC change can make the center frequency of the transmitted band shift to the lower frequency by 576 MHz, with a tuning rate of 4.2%. Subsequently, a double-layer FSS is designed by adding a coupling layer to the single-layer FSS, achieving a flat-top passband and enhancing the tuning rate. Simulation and experimental validation show that altering the LC state enables continuous tuning of the center frequency of the double-layer FSS from 14.95 GHz to 13.86 GHz, with a tuning rate of 7.2%. This structure exhibits a flat-top passband wider than 5.7 GHz, a relative bandwidth of 41%, and a rectangular coefficient below 1.877. Moreover, the double-layer FSS demonstrates favorable polarization stabilization and wide incidence angle stabilisation properties in various states.

Responsive refractive index sensor based on actively tuning liquid crystal topological edge states

In this paper, using the electric field regulation and low loss properties of liquid crystal materials, a tunable polarization-separated liquid crystal (LC) topological edge state is proposed, whose potential in responsive sensors (RSs) is explored. Adjustment of the measuring range and sensitivity of the RS can be realized by controlling the orientation angle of LC and the analyte proportion. In the case of a low ratio of analytes, as the LC orientation angle changes from 18° to 0°, the measurement range will also vary from 1–1.8 RIU (refractive index unit) to 1.8–2.3 RIU. When adding the proportion of analytes and the number of periods, the normalized sensitivity will be increased from 0.0759 c/d/RIU (c is the propagation speed of light in vacuum, and d is the normalized thickness) to 0.299 c/d/RIU, leading to a reduction in the detection limit from 2.75 × 10−4 to 5 × 10−6 RIU, so biological indicators such as bacteria Leptospira in rodent urine can be detected.

Tunable liquid crystal lens with symmetric bipolar operation.

We describe an electrically tunable liquid crystal lens that can dynamically generate symmetric wavefront profiles. The curvature of these profiles may be inversed, enabling a bipolar response (focusing and defocusing). Different wavefronts, including non-monotonic, are predicted theoretically and demonstrated experimentally. The optical performance of the devices is characterized experimentally in an imaging scheme.

Dynamic control of defocus, astigmatism, and tilt aberrations with a large area foveal liquid crystal lens

We have recently reported the dynamic adjustment of the focal length in an electrically tunable liquid crystal “foveal” lens, the center of which can be shifted over a large working area. In the present work, we show that this design allows also the independent generation of astigmatism with arbitrary axis and tilt of the light wavefront by simply changing the phase and the voltage differences between 4 control electrodes. Furthermore, we also demonstrate the capability of generating highly localized negative (defocusing) lenses with the same device by using a dual frequency liquid crystal.

Optically Tunable Multistable Liquid Crystal Grating for Anti‐Counterfeiting through Multilayer Continuous Phase Analysis

AbstractOptically tunable liquid‐crystal (LC) gratings have remarkable applications in the field of image analysis for anti‐counterfeiting. However, existing tunable LC gratings suffer from limited adjustable ranges and weak stability. The spacing variability and accuracy of the grating cannot satisfy the demands of continuous analysis. This study presents a novel approach to anti‐counterfeiting through multilayer continuous phase analysis (MCPA) based on the continuous multistable states of the LC grating. This optically tunable LC grating featuring continuous multistable states is successfully prepared using a novel optically tunable chiral dopant, tetra(n‐hex‐azo), which exhibits an exceptionally slow conformational relaxation rate (half‐life: 78.02 h). By exposing the LC grating to ultraviolet irradiation with an intensity of 0.1054 mW cm−2, the fringe spacing can be continuously and widely tuned between 0 and 250 µm, which enhances the ultraslow conformational relaxation of tetra(n‐hex‐azo). Four representative spacings (50, 55, 60, and 65 µm) are selected to construct four‐layer MCPA gratings, enabling simultaneous decoding of a four‐layer latent image for the first time. The mechanism of the MCPA grating is investigated through control experiments and theoretical simulations. This study demonstrates a facile, precise, and efficient approach for complex information encryption and anti‐counterfeiting.

Towards a mini-endoscope design with spatially selective excitation and imaging.

We describe a mini-endoscope design that uses a new type of electrically tunable liquid crystal lens array enabling the dynamic increase of spatial resolution by adjusting the working distance in various zones of interest over a relatively large field of view (FoV) without mechanical movement. The characterization of the system is performed by using uniform fluorescent films, fluorescent micro spheres and a tissue sample expressing the fluorescent calcium indicator GCaMP6s. Lateral resolution of up to 2 µm over the FoV between 300 µm - 400 µm is experimentally demonstrated.

Electrically-switched differential microscopy based on computing liquid-crystal platforms

Abstract Detection of transparent phase specimens especially biological cells with desired contrasts is of great importance in visual display and medical diagnosis. Due to the pure-phase nature, conventional detection approaches may damage samples or require complex operations. Computing liquid crystal (LC) is a thin and flat optical element with excellent capability in optical field modulation, which gives a feasible way to this issue from the perspective of analog optical computing. We here propose and experimentally implement an electrically switched two-dimensional (2D) differential microscopy based on computing LC platforms. The Pancharatnam–Berry phase LC polarization grating induces light’s spin separation to promote the 2D differential operation. Using the electrically tunable LC plate as the system phase retardance provider, the detecting mode can be flexibly switched from bright-field images to edge-enhanced images with desired contrasts. Remarkably, owing to the wavelength-independent feature closely related to the geometric phases, our proposed scheme is demonstrated to be applicable to the multi-wavelength microscopy imaging. These results open avenues to form real-time all-optical image processing and may facilitate multifunctional differential microscopy.

Bioplastic tunable liquid crystal lenses for dynamic focus in virtual and augmented reality

Bioplastic tunable liquid crystal lenses for dynamic focus in virtual and augmented reality

Optimizing Tunable LC Devices with Twisted Light

Tunable circular devices made of liquid crystals or other materials, like lenses, axicons, or phase plates, are often constrained by limitations in size, tunability, power, and other parameters. These constraints restrict their use and limit their applicability. In this review, a thorough study of the use of light’s orbital angular momentum in the manufacturing of liquid crystal (LC) devices is presented. Twisted light fosters the simultaneous optimization of most critical parameters. Experimental demonstrations of the unmatched performance of tunable LC lenses, axicons, and other elements in parameters such as lens diameter (>1″), power and tunability (>±6 diopters), fill factor (>98%), and time response have been achieved by reversible vortex generation created by azimuthal phase delay. This phase delay can eventually be removed within the optical system so that lens performance is not affected.

Large diameter electrically tunable lens for ophthalmic distance accommodation.

Electrically tunable liquid crystal lens with 30 mm diameter is presented based on the refractive Fresnel concept. Relatively large optical power variation range (from - 0.74 to +0.71 Diopters) is demonstrated along with very low root mean square aberrations (≤0.15 µm). Optical characterizations, including with Snellen chart, show that good vision recovery may be obtained with fast response time (under 500 msec) and relatively low haze. The proposed design is very simple and may be fabricated by using single step lithography. Perspectives on its applications are discussed.

Low voltage dual-period liquid crystal grating with tunable diffraction efficiency

A low voltage dual-period liquid crystal (LC) grating with tunable diffraction efficiency is demonstrated. Different periods can be achieved by controlling the driving scheme. Diffraction efficiencies of 25.9% for the small-period LC grating and 20.9% for the large-period LC grating have been realized. The diffraction angle can be alternatively tuned, as well as the diffraction efficiency by applying voltage to the top or bottom in-plane switching (IPS) electrodes. The driving voltage of the tunable LC grating is as low as 4.8 V, which shows great potential for diffractive optics components.

Stimuli-responsive grating with a tunable period and a diffraction order.

The next-generation diffraction grating not only requires a nonmechanical control and a remote control to generate a diffraction pattern but also requires a tunable period. Here, we propose a stimuli-responsive liquid crystal (LC) phase grating with a tunable period and a diffraction order. The stimuli-responsive LC diffraction grating is composed of periodically arranged electrodes on a double-sided glass substrate. By adjusting the driving scheme, the pitch and diffraction order of the LC grating can be switched between three different modes. The experimental results show that the LC grating has a lower driving voltage (∼5 V). In addition, the tunable LC grating can achieve more diffraction orders, which can be applied to a holographic display to achieve a wide-viewing angle and an enlarged size.

Wavefront control capability in a modal lens with segmented circular peripheral electrodes.

We report the detailed investigation of the capability of an electrically tunable liquid crystal lens (TLCL) to dynamically generate various wavefront shapes. The TLCL operates in the modal-control mode with a peripheral circular electrode divided into eight individually controlled segments. This segmentation allows producing a rather rich set of influence functions. We characterize these functions and the crosstalk between them by adjusting the voltage and the frequency of electrical signals applied to different electrode segments. Various wavefronts are produced in a closed-loop control mode and described using Zernike polynomials. The dynamical response of the lens is also briefly investigated. Obtained results may be used to design different adaptive optical systems where a dynamic wavefront control is required.

Multi-wavelength Q-plate arithmetic in an all-liquid-crystal modular setup

Vortex beams are a type of structured light characterized by phase rotation around the propagation axis, resulting in orbital angular momentum. Their properties make them useful in various applications such as high-resolution microscopy, optical tweezing, and telecommunications. This has led to a comprehensive development of methods for their generation, ranging from using single-purpose glass elements to utilizing computer-generated holograms using spatial light modulators. One of the most commonly used elements for vortex transformation is a vortex half-wave retarder called a q-plate, which can transform a Gaussian beam into a scalar vortex or vector beam depending on the input polarization. Although the commercially available ones are limited in the range of possible output topological charges, they can be stacked to perform arithmetic operations to expand them. However, changing the output or working wavelength requires rearranging the elements. We present an improvement to this method that solves these problems by introducing Q-modules, easy-to-fabricate, electrically tunable liquid crystal devices that combine the features of q-plates and half-wave plates and can be used as building blocks in modular assemblies. Electrical tuning makes it possible to change the working wavelength as well as the topological output charge or polarization order without the need to interact mechanically with the setup.

Electrically Tunable Liquid Crystal Phase Grating with Double Period Based on the VIS Mode

A tunable liquid crystal (LC) phase grating based on vertical-field in-plane electrical switching (VIS) is proposed. The tunable LC phase grating is composed of four parts: an LC layer, the top-plane and bottom-plane electrodes, the polyimide (PI) layer, and the top and bottom periodical strip electrodes. On the one hand, a large period LC phase grating can be obtained by applying voltage to the upper plane electrode and the lower periodic strip electrodes. On the other hand, a small period LC phase grating can be obtained by applying voltage to the lower plane electrode and the upper period electrodes. As a result, two kinds of LC phase grating with different periods can be realized by changing the driving scheme. Naturally, the diffraction angle can be adjusted by controlling different LC phase grating periods. The simulation results show that this tunable LC phase grating based on the VIS mode has a relatively short response time and low operating voltage (5.4 V). These characteristics make the tunable LC phase grating have good application prospects in holographic 3D or augmented reality (AR) display.