Birefringence Of Liquid Crystal Research Articles

Size-dependent optical and dielectric anisotropy of CdS nanowire doped high-birefringent nematic liquid crystalline compound

Highly birefringent liquid crystals have grabbed ample attention because of their potential applications as an active medium for devices in visible, infrared and other spectral regions. Here, we have attempted to study the effect of the size and concentration of CdS nanowires on a nematic liquid crystal. Anisotropy in liquid crystals is one of those distinguishing traits that gain their superiority over other materials. We hereby have analyzed how optical and dielectric anisotropy respond to variations in the size and concentration of the dopant. The study is of significant scientific value because there is very little literature available on how dopant size can alter liquid crystalline properties and also introduces a method to tune the properties of pure liquid crystals (LCs) by varying the size and concentration of the dopant. The study explains how the size of the nanowires changes the analyzed parameters and up to what extent. The obtained results yield a significant increase in both optical and dielectric anisotropy by doping CdS nanowires making them an excellent choice for dopant selection. CdS-doped nematic LC mixtures have proved to be highly birefringent and possess a high positive dielectric anisotropy as well. Such a compound can be of great application in device fabrications such as laser beam steerers, electronic lenses, attenuators and light shutters, etc.

Electrical polarization switching of perovskite polariton laser

Abstract Optoelectronic and spinoptronic technologies benefit from flexible and tunable coherent light sources combining the best properties of nano- and material-engineering to achieve favorable properties such as chiral lasing and low threshold nonlinearities. In this work we demonstrate an electrically wavelength- and polarization-tunable room temperature polariton laser due to emerging photonic spin–orbit coupling. For this purpose, we design an optical cavity filled with both birefringent nematic liquid crystal and an inorganic perovskite. Our versatile growth method of single CsPbBr3 inorganic perovskite crystals in polymer templates allows us to reach strong light–matter coupling and pump-induced condensation of exciton–polaritons resulting in coherent emission of light. The sensitivity of the liquid crystal to external voltage permits electrical tuning of the condensate energy across 7 nm; its threshold power, allowing us to electrically switch it on and off; and its state of polarization sweeping from linear to locally tilted circularly polarized emission.

Discrimination of lipid composition and cellular localization in human liver tissues by stimulated Raman scattering microscopy.

The molecular mechanisms driving the progression from nonalcoholic fatty liver (NAFL) to fibrosing steatohepatitis (NASH) are insufficiently understood. Techniques enabling the characterization of different lipid species with both chemical and spatial information can provide valuable insights into their contributions to the disease progression. We extend the utility of stimulated Raman scattering (SRS) microscopy to characterize and quantify lipid species in liver tissue sections from patients with NAFL and NASH. We applied a dual-band hyperspectral SRS microscopy system for imaging tissue sections in both the C-H stretching and fingerprint regions. The same sections were imaged with polarization microscopy for detecting birefringent liquid crystals in the tissues. Our imaging and analysis pipeline provides accurate classification and quantification of free cholesterol, saturated cholesteryl esters (CEs), unsaturated CE, and triglycerides in liver tissue sections. The subcellular resolution enables investigations of the heterogeneous distribution of saturated CE, which has been under-examined in previous studies. We also discovered that the birefringent crystals, previously found to be associated with NASH development, are predominantly composed of saturated CE. Our method allows for a detailed characterization of lipid composition in human liver tissues and enables further investigation into the potential mechanism of NASH progression.

A High Throughput Tunable Filter Module for Multispectral Imaging

AbstractTunable filters are vital not only for dynamic display but also in the miniaturized spectral imaging system. Yet, the present tunable devices possess low optical throughput, bulky volume, slow response, or complex configuration. In this paper, the study proposed and experimentally demonstrated a compact tunable filter module based on metagratings combined with liquid crystals (LC). By the synergetic effect between the birefringent LC and the dichroic metagratings, the module possesses distinct transmissive features by electrically tuning the LC material, resulting from the polarization interference effect. A simple nanoimprinting method, which is suitable for the mass production of nanostructures is adopted for low‐cost fabrication. These unique features endow the present tunable filter module with low cost, high optical throughput, and compact volume. As a proof of concept, a multispectral imager is built with this filter based on a commercial microscope. Experimental results show the system response is five times higher in optical throughput across the whole visible region than the traditional liquid crystal tunable filter and the signal‐to‐noise ratio can be improved by 7 dB. Considering the feasibility of nanoimprinting, this module paves the way for spectral imaging in low‐light conditions and on‐site scenarios such as tongue diagnosis and food quality inspection.

Chiral dopant effects on the birefringence and transmittance of nematic and polymer-stabilized liquid crystals for future applications

Chiral dopant effects on the birefringence and transmittance of nematic and polymer-stabilized liquid crystals for future applications

Liquid crystal-embedded fiber optic fabry perot temperature sensor based on Vernier effect

An optical temperature sensing method with ultra-high sensitivity and a wide measurement range is proposed. The performance enhancements come from the high thermos-optical coefficient of liquid crystal (LC) material, and the Vernier effect produced by LC birefringence characteristics. This paper proposes and demonstrates a method to enhance the measurement range of the LC temperature sensor. A miniature LC temperature sensor was made by using an optical fiber Fabry-Perot interferometer (FPI). To compensate for and attenuation of LC and improve the contrast of the FPI, silver films are coated on one end of the optical fiber. Due to the high birefringence of the LC in the interferometer, a Vernier spectrum can be observed in the reflection. The temperature measurement range of 20–32 °C and the temperature sensitivity of the Vernier peaks of a sensor sample is about 42.18 nm/°C, which is much higher than the sensitivity of the cascaded FBG in structure. The limited measurement range is due to the free spectral range (FSR) limitation. In this context, the FBG peak can be used as a temperature reference while switching the adjacent Vernier peaks. Hence, this LC temperature sensor exhibits high sensitivity as well as a wide measuring range, which would broaden the application scope of the LC temperature sensors.

Method for increasing the utilization rate of birefringence in liquid crystal lenses.

We propose a method to increase the utilization rate of birefringence in liquid crystal (LC) lenses. The method involves designing electrode patterns based on the phase response curve of the LC material, enabling the lenses to operate across a wide range of driving voltages while generating the desired phase profile. The underlying principle of this technique is discussed in detail. Electrode patterns have been successfully designed for positive and negative cylindrical LC lenses. The experimental results demonstrate that the designed lenses generate a parabolic phase profile even when the driving voltage exceeds the linear response region. The utilization rate of LC birefringence for the positive lens has increased from 41.3% to 69.7%, indicating a 68.8% increase from the original. For the negative lens, the utilization rate has risen from 41.8% to 68.7%, representing a 64.4% increase from the original.

Synthesis and properties of biphenyl liquid crystal diluters terminated by 2,2-difluorovinyloxyl for high birefringence liquid crystals

ABSTRACT A series of liquid crystal (LC) diluters containing a rigid biphenyl core with a lateral fluorine substituent, a 2,2-difluorovinyloxyl terminal group and a flexible n-alkyl chain were designed and synthesised through classical organic synthetic reactions. Their chemical structures were characterised and confirmed by traditional methods. Meanwhile, the effects of alkyl-chain length, terminal group and lateral fluorine substituent on the melting point (T m), birefringence (Δn), dielectric anisotropy (Δε) and rotational viscosity (γ 1) of LC diluters were investigated. Density functional theory (DFT) calculations were employed to obtain the molecular polarisability, dipole moment, frontier orbitals, biphenyl dihedral angle and aspect ratio. Furthermore, their comprehensive properties in low-Δn LC mixture 002 and high-Δn LC mixture P01-F were further evaluated, and it was found that the 2,2-difluorovinyloxyl LC diluter 3FV have a significant advantage in reducing the T m and γ 1 of LC mixtures, and increasing the values of Δn and Δε. The research results provide technical support and theoretical guidance for the molecular design of new LC diluters and the development of fast response liquid crystals.

Synthesis and performance evaluation of fluorinated biphenyl diluters for high birefringence liquid crystals

ABSTRACT Liquid crystal (LC) mixtures with high birefringence (∆n) and low rotational viscosity play an essential role for fast-response LC devices. However, high birefringence and low rotational viscosity are a pair of contradictory performance parameters. In this work, two high-∆n LC diluters are proposed to achieve a fine balance between them. These two terminal olefin diluters containing a rigid biphenyl core with a lateral fluorine substituent, a buty-3-enyl terminal group and a flexible n-alkyl chain were designed and synthesised through classical organic synthetic reactions. Meanwhile, their comprehensive properties were further evaluated in a LC mixture P02-F containing fluoro-tolane and a LC mixture P12-ISO containing isothiocyano-tolane. The results show that the biphenyl diluters 2BBF2V and 3BFB2V could effectively decrease the T m of LC mixture P12-ISO to below −25°C, while maintain high ∆n and large Δε, which are suitable for the high-∆n LC mixtures containing isothiocyano-tolane. Furthermore, DFT calculations of molecular conformation and polarisability components were utilised to correlate the experimental findings.

47.2: An Essential Role of Diluters on the Fast Response Time for High Birefringence Liquid Crystals

A new liquid crystal diluter was designed and synthesized to balance trade‐offs between low viscosity, high Δn, and large Δε in the LC mixture. Research results indicated that this diluter had excellent properties of low melting point, high Δn, and large Δε. Meanwhile, in a high‐Δn LC mixture, it could effectively reduce the viscoelastic coefficient (γ1/K11), and increase the As while maintaining an equivalent LC phase temperature range and Δn. It means that the LC diluter plays an essential role to improve the response time of LCs.

P‐12.12: Development and Application of High Birefringence Liquid Crystal Materials

The development of new liquid crystal optoelectronic devices, including laser phase modulation, microwave and terahertz devices, requires liquid crystal materials with high birefringence. In this paper, three new molecular design strategies are proposed, including the introduction of olefin chain, lateral fluorine substitution and new ring core. Four new series of isothiocyanate liquid crystal compounds with high birefringence, low melting point and wide nematic temperature range were synthesized. Based on these new liquid crystal compounds, a liquid crystal mixture with nematic temperature range of ‐40~146 ºC and birefringence of 0.42 was developed. At 19 GHz, the new liquid crystal mixture shows excellent dielectric properties, with dielectric anisotropy of 1.28 and dielectric loss of 0.0087. The new liquid crystal mixture also has very low viscoelastic coefficient. The results at 1550nm and 60 ºC show that the response speed can reach 3.6 milliseconds with 2π phase modulation in the reflection mode.

Fluorination Improves the Electro-Optical Properties of Benzoxazole-Terminated Liquid Crystals in High Birefringence Liquid Crystal Mixtures: Experimental and Theoretical Investigations.

Aromatic heterocyclic liquid crystal (LC) materials have received much attention from LC chemists for their high birefringence and large dielectric anisotropy, yet few have reported their properties in LC mixtures. In this work, a series of fluorinated benzoxazole liquid crystal compounds were synthesized to evaluate their electro-optical properties in high birefringence LC mixtures, with the expectation of further establishing the theoretical basis and experimental evidence for their applications in LC photonics. Firstly, the effects of the lateral fluorine substituent positions on the molecular synthetic yield, mesomorphic and solubility properties were comparatively investigated. Afterwards, we focused on the fluorination effects on the core electro-optical properties, including birefringence, dielectric anisotropy and further investigation of the viscoelastic coefficient of high birefringence LC mixtures. Research results showed that the benzoxazole liquid crystal compounds possess low melting points, wide nematic phase intervals and good solubility by appropriate lateral fluorine substitution, which is beneficial to further improve the electro-optical properties of high birefringence LC mixtures. Meanwhile, the theoretical and experimental results corroborate each other to well reveal the structure-property relationship. This study demonstrates that fluorination would promote promising applications of benzoxazole-terminated liquid crystals in emerging LC optical devices.

Field programmable topological edge array

Valley Hall topological photonic crystals, inspired by topological insulators in condensed matter physics, have provided a promising solution to control the flow of light. Recently, the dynamic manipulation property of topological photonic crystals has been widely studied. Here, we propose a novel solution for programmable valley photonic crystals, called field programmable topological edge array (FPTEA), based on the field reorientation property of nematic liquid crystals and robust valley-protected edge modes. FPTEA is composed of an array of graphene-like lattices with C3 symmetry, in which the birefringence of liquid crystal is larger than 0.5105. Due to the dielectric anisotropy of liquid crystals being sensitive to external fields such as light, heat, electric, and magnetic fields, each lattice is tunable, and the topological propagation routes and even the lattice parameters can be dynamically changed while changing the distribution of external fields. We numerically demonstrate three methods of composing an FPTEA device to design arbitrary passive optical devices by electric driving, thermal inducing, or UV writing. These results show the great application potential of liquid crystals in topological photonic crystals, and enrich the design of programmable integrated topological devices with broad working bandwidth ranging from microwave to visible light.

Movable Optical Frequency Ruler with Optical Activity

Optical frequency rulers (OFR) are suggested for use as optical wavelength or frequency references for spectra manipulation or unknown wavelength measurement. In the past, complicated mechanisms that are not easy to utilize were used to make OFR, such as a double-slits with a high-speed fluid or an external circuit to control the liquid crystal birefringence. This work introduces a simple structure to produce an OFR, which should be easier to implement. It utilizes quartz block optical activity and two polarizers. Because of the strong wavelength dependence of the rotatory power, each wavelength component in the spectrum experiences a different amount of polarization angle rotation. Some components whose angles are perpendicular to that of the analyzer are filtered out and naturally form the OFR’s ticks. The numerical results show that those spectral ticks can be moved to higher or lower wavelengths by rotating the analyzer’s angle. This scheme provides another possibility for creating movable OFR with the merit of easy usage.

Bis-tolane liquid crystals terminated by 2,2-difluorovinyloxyl with high birefringence and large electrical anisotropy

A series of liquid crystals (LCs) containing bis-tolane, lateral fluorine substituents and 2,2-difluorovinyloxyl group were designed and synthesized. Their LC phase behavior, birefringence, dielectric anisotropy and viscosity properties were discussed by comparing with previously reported LCs with high birefringence and large electrical anisotropy. Meanwhile, the effects of lateral fluorine numbers on the melting point, birefringence and dielectric anisotropy of these LCs were explored, and density functional theory (DFT) calculations of molecular conformation and polarizability were used to correlate the experimental results. Research results indicated that the 2,2-difluorovinyloxyl terminal group decreased the melting points of these LCs, and these LCs showed relatively low melting point (<113.7 ℃), broad nematic-phase temperature interval, high birefringence (>0.4), and large dielectric anisotropy (35.87). Furthermore, performance in high birefringence LC mixture demonstrated that they could effectively improve the birefringence and dielectric anisotropy, and maintain low viscosity, which is expected to possess widespread applications in fast response LC optical devices.

Liquid Crystal-Tuned Planar Optics in Terahertz Range

Recently, terahertz waves of higher frequencies compared to microwave and radio frequency have shown great potential in radar detection and high-speed wireless communication. To spatially control the wavefront of terahertz beams, various novel components, such as terahertz filters, polarization converters and lenses, have been investigated. Metamaterials and metasurfaces have become the most promising technique for the free manipulation of terahertz waves. Metadevices integrated with liquid crystals have been widely used in active terahertz devices. In this review, the birefringence of liquid crystals in the terahertz band and terahertz devices based on liquid crystals are summarized. By integrating liquid crystals with plasmonic metamaterials, the functions become dynamically adjustable and are reconstructed. Utilizing liquid crystals to change the resonance of metamaterials, tunable filters, absorbers, and programmable metasurfaces are realized. To solve the problem of low efficiency, terahertz wavefront shaping devices based on dielectric metasurfaces and liquid crystals, such as a variable deflection angle grating and zoom metalenses, are presented. Finally, we discuss and anticipate the future developments of liquid-crystal-integrated meta-devices, which will inspire broad applications in terahertz communication and imaging.

Energy landscapes on polymerized liquid crystal interfaces.

We measure and model monolayers of concentrated diffusing colloidal probes interacting with polymerized liquid crystal (PLC) planar surfaces. At topological defects in local nematic director profiles at PLC surfaces, we observe time-averaged two-dimensional particle density profiles of diffusing colloidal probes that closely correlate with spatial variations in PLC optical properties. An inverse Monte Carlo analysis of particle concentration profiles yields two-dimensional PLC interfacial energy landscapes on the kT-scale, which is the inherent scale of many interfacial phenomena (e.g., self-assembly, adsorption, diffusion). Energy landscapes are modelled as the superposition of macromolecular repulsion and van der Waals attraction based on an anisotropic dielectric function obtained from the liquid crystal birefringence. Modelled van der Waals landscapes capture most net energy landscape variations and correlate well with experimental PLC director profiles around defects. Some energy landscape variations near PLC defects indicate either additional local repulsive interactions or possibly the need for more rigorous van der Waals models with complete spectral data. These findings demonstrate direct, sensitive measurements of kT-scale van der Waals energy landscapes at PLC interfacial defects and suggest the ability to design interfacial anisotropic materials and van der Waals energy landscapes for colloidal assembly.

High birefringence liquid crystals with a wide temperature range and low melting point for augmented reality displays

Precise molecular tailoring was applied to obtain high performance liquid crystal materials for the broad temperature adaptability of LCoS devices in AR displays.

Electrically tunable dual-layer twisted nematic liquid crystal THz phase shifters with intermediate composite polymer thin film

We demonstrate electrically tunable twisted nematic (TN) aligned liquid crystal (LC) terahertz phase shifters with a novel structure, in which a single LC composite polymer thin film is inserted in the middle of the LC cell. For designing such types of phase shifters, we have applied the Mauguin formalism used in LC-based switches for applications in the visible. Experimental results are in good agreement with those predicted by the theoretical model. Using this design, the phase shift has been increased by 14.3° at 1.2 THz in comparison to the conventional design (98.1°). The threshold voltage has been lowered from 0.81 VRMS to 0.48 VRMS while the driving voltage for quarter-wave (π/2) phase shift operation has been reduced from 8.4 VRMS to 6.3 VRMS at 1.2 THz. The dynamic response of devices also enhances significantly. Such design has also successfully been adopted to demonstrate a 2π phase shifter with similar improvement. Full-wave operation is achieved at a bias of 25 VRMS at 1.2 THz using a highly birefringent LC material. Order parameter calculations show that the polymer film improves significantly LC molecular alignment in the thick LC cell for THz applications. We have also compared the figure of merits (FOMs) of the present devices with previously reported THz π/2 and 2π phase shifters.

A high birefringence liquid crystal for lenses with large aperture

This work presents the application of an experimental nematic liquid crystal (LC) mixture (1929) in a large aperture lens. The LC material is composed of terphenyl and biphenyl derivatives compounds with an isothiocyanate terminal group and fluorinated lateral substituents. The substitution with a strongly polar isothiocyanate group and an aromatic rigid core provides pi-electron coupling, providing high birefringence (Delta n = 0.3375 at 636 nm and 23 °C) and low viscosity (eta = 17.03 mPa s). In addition, it also shows high values of birefringence at near infrared (0.318 at 1550 nm). The synthesis process is simple when comparing materials with high melting temperatures. The excellent properties of this LC mixture are demonstrated in a large aperture LC-tunable lens based on a transmission electrode structure. Thanks to the particular characteristics of this mixture, the optical power is high. The high birefringence makes this LC of specific interest for lenses and optical phase modulators and devices, both in the visible and infrared regions.