Microwave Liquid Crystal Enabling Technology for Electronically Steerable Antennas in SATCOM and 5G Millimeter-Wave Systems

An Ultrathin, Fast-Response, Large-Scale Liquid-Crystal-Facilitated Multi-Functional Reconfigurable Metasurface for Comprehensive Wavefront Modulation

The rapid advancement of prevailing communication/sensing technologies necessitates cost-effective millimeter-wave arrays equipped with a massive number of phase-shifting cells to perform complicated beamforming tasks. Conventional approaches employing semiconductor switch/varactor components or tunable materials encounter obstacles such as quantization loss, high cost, high complexity, and limited adaptability for realizing large-scale arrays. Here, a low-cost, ultrathin, fast-response, and large-scale solution relying on metasurface concepts combined together with liquid crystal (LC) materials requiring a layer thickness of only 5 µm is reported. Rather than immersing resonant structures in LCs, a joint material-circuit-based strategy is devised, via integrating deep-subwavelength-thick LCs into slow-wave structures, to achieve constitutive metacells with continuous phase shifting and stable reflectivity. An LC-facilitated reconfigurable metasurface sub-system containing more than 2300 metacells is realized with its unprecedented comprehensive wavefront manipulation capacity validated through various beamforming functions, including beam focusing/steering, reconfigurable vortex beams, and tunable holograms, demonstrating a milli-second-level function-switching speed. The proposed methodology offers a paradigm shift for modulating electromagnetic waves in a non-resonating broadband fashion with fast-response and low-cost properties by exploiting functionalized LC-enabled metasurfaces. Moreover, this extremely agile metasurface-enabled antenna technology will facilitate a transformative impact on communication/sensing systems and empower new possibilities for wavefront engineering and diffractive wave calculation/inference.

Liquid Crystals for Advanced Smart Devices with Microwave and Millimeter-Wave Applications: Recent Progress for Next-Generation Communications

Liquid crystals (LCs) technology have a well-established history of applications in visible light, particularly in the display industry. However, with the rapid growth in communication technology, LCs have become a topic of current interest for high-frequency microwave (MW) and millimeter-wave (mmWave) applications due to promising characteristics such as tunability, continuous tuning, low losses, and price compatibility. To improve the performance of future communication technology using LCs, it is not sufficient only with the perspective of radio-frequency (RF) technology. Therefore, it is imperative to understand not only the novel structural designs and optimization of MW engineering but also the perspective of materials engineering when implementing advanced RF devices with maximum performance for next-generation satellite and terrestrial communication. Herein, based on advanced nematic LCs, polymer-modified LCs, dual-frequency LCs, and photo-reactive LCs, this article summarizes and examines the modulation principles and key research directions for the design strategies of LCs for advanced smart RF devices with improved driving performance and novel functionality. Furthermore, the challenges in development of state-of-the-art smart RF devices that use LCs are discussed.

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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pi$$\end{document}π-electron coupling, providing high birefringence (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta n = 0.3375$$\end{document}Δn=0.3375 at 636 nm and 23 °C) and low viscosity (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\eta$$\end{document}η = 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.

A Mesoporous Silica Nanoparticle-Doped Photo-Alignment Layer and Liquid Crystal Layer for Optimizing the Rewriting Speed and the Response Time of Optically Driving Liquid Crystal Displays

Optically driving liquid crystal displays (ODLCDs) are widely applied in display and optical devices due to their long axis of liquid crystal (LC) molecules that can be tuned by a photo-alignment layer under exposure polarized light. However, their use remains challenging due to their long rewriting time and response time. In this work, the rewriting time and the response time of an ODLCD depending on mesoporous silica nanoparticles (MSNs) doped in azo-dye (SD1) and LC 5CB were studied. Among the different concentration ratios of SD1-MSNs (1-0 to 1-0.1), a ratio of 1-0.07 was optimal, decreasing the rewriting time by 40 s (from 69.1 to 29.6 s). Meanwhile, the response time was improved 10 times with MSNs doped into 5CB.

Quasi-Isotropically Thermal Conductive, Highly Transparent, Insulating and Super-Flexible Polymer Films Achieved by Cross Linked 2D Hexagonal Boron Nitride Nanosheets

Polymer-based thermal management materials (TIMs) show great potentials as TIMs due to their excellent properties, such as high insulation, easy processing, and good flexibility. However, the limited thermal conductivity seriously hinders their practical applications in high heat generation devices. Herein, highly transparent, insulating, and super-flexible cellulose reinforced polyvinyl alcohol/nylon12 modified hexagonal boron nitride nanosheet (PVA/(CNC/PA-BNNS)) films with quasi-isotropic thermal conductivity are successfully fabricated through a vacuum filtration and subsequent self-assembly process. A special structure composed of horizontal stacked hexagonal boron nitride nanosheets (h-BNNSs) connected by their warping edges in longitudinal direction, which is strengthened by cellulose nanocrystals, is formed in PVA matrix during self-assembly process. This special structure makes the PVA/(CNC/PA-BNNS) films show excellent thermal conductivity with an in-plane thermal conductivity of 14.21 W m^-1 K^-1 and a through-plane thermal conductivity of 7.29 W m^-1 K^-1 . Additionally, the thermal conductive anisotropic constants of the as-obtained PVA/(CNC/PA-BNNS) films are in the range of 1 to 4 when the h-BNNS contents change from 0 to 60 wt%, exhibiting quasi-isotropic thermal conductivity. More importantly, the PVA/(CNC/PA-BNNS) films exhibit excellent transparency, super flexibility, outstanding mechanical strength, and electric insulation, making them very promising as TIMs for highly efficient heat dissipation of diverse electronic devices.

How Far Can We Push the Rigid Oligomers/Polymers toward Ferroelectric Nematic Liquid Crystals?

The emerging ferroelectric nematic (N_F) liquid crystal is a novel 3D-ordered liquid exhibiting macroscopic electric polarization. The combination of the ultrahigh dielectric constant, strong nonlinear optical signal, and high sensitivity to the electric field makes N_F materials promising for the development of advanced liquid crystal electroopic devices. Previously, all studies focused on the rod-shaped small molecules with limited length (l) range and dipole moment (μ) values. Here, through the precision synthesis, we extend the aromatic rod-shaped mesogen to oligomer/polymer (repeat unit up to 12 with monodisperse molecular-weight dispersion) and increase the μ value over 30 Debye (D). The N_F phase has a widespread existence far beyond our expectation and could be observed in all the oligomer/polymer length range. Notably, the N_F phase experiences a nontrivial evolution pathway with the traditional apolar nematic phase completely suppressed, i.e., the N_F phase nucleates directly from the isotropic liquid phase. The discovery of thte ferroelectric packing of oligomer/polymer rods not only offers the concept of extending the N_F state to oligomers/polymers but also provides some previously overlooked insights in oxybenzoate-based liquid crystal polymer materials.

Mirror Symmetry Breaking and Network Formation in Achiral Polycatenars with Thioether Tail

Mirror symmetry breaking in systems composed of achiral molecules is of importance for the design of functional materials for technological applications as well as for the understanding of the mechanisms of spontaneous emergence of chirality. Herein, we report the design and molecular self-assembly of two series of rod-like achiral polycatenar molecules derived from a π-conjugated 5,5'-diphenyl-2,2'-bithiophene core with a fork-like triple alkoxylated end and a variable single alkylthio chain at the other end. In both series of liquid crystalline materials, differing in the chain length at the trialkoxylated end, helical self-assembly of the π-conjugated rods in networks occurs, leading to wide temperature ranges (>200 K) of bicontinuous cubic network phases, in some cases being stable even around ambient temperatures. The achiral bicontinuous cubic Ia 3 ‾ d phase (gyroid) is replaced upon alkylthio chain elongation by a spontaneous mirror symmetry broken bicontinuous cubic phase (I23) and a chiral isotropic liquid phase (Iso1 [ *] ). Further chain elongation results in removing the I23 phase and the re-appearance of the Ia 3 ‾ d phase with different pitch lengths. In the second series an additional tetragonal phase separates the two cubic phase types.

Rethinking Figure-of-Merits of Liquid Crystals Shielded Coplanar Waveguide Phase Shifters at 60 GHz

The demand for reconfigurable millimetre-wave (mm-Wave) components based on highly anisotropic liquid crystals (LC) is higher than ever before for the UK and worldwide. In this work, 60 GHz investigation on a bespoke shielded coplanar waveguide (SCPW) phase shifter structure filled with 16 types of microwave-enabled nematic LCs respectively indicates that the patterns of the device’s figure-of-merit (FoM, defined as the ratio of maximum differential phase shift to maximum insertion loss) reshuffle from those of the characterised LC materials’ FoM (defined as the ratio of tunability to maximum dissipation factor). To be more specific, GT7-29001- and MDA-03-2838-based phase shifters exhibit the highest FoM for devices, outperforming phase shifters based on GT5-28004 and TUD-566 with the highest FoM for materials. Such a mismatch between the device’s FoM and LC’s FoM implies a nonlinearly perturbed wave-occupied volume ratio effect. Furthermore, the relationship between insertion loss and the effective delay line length is nonlinear, as evidenced by measurement results of two phase shifters (0–π and 0–2π, respectively). Such nonlinearities complicate the established FoM metrics and potentially lead to a renewed interest in the selection and material synthesis of LCs to optimise reconfigurable mmWave devices, and promote their technological exploitation in phased array systems targeting demanding applications such as inter-satellite links and satellite internet.

Conventional and unconventional ionic phenomena in tunable soft materials made of liquid crystals and nanoparticles

A great variety of tunable multifunctional materials can be produced by combining nanoparticles and liquid crystals. Typically, the tunability of such soft nanocomposites is achieved via external electric fields resulting in the field-induced reorientation of liquid crystals. This reorientation can be altered by ions normally present in liquid crystals in small quantities. In addition, nanomaterials dispersed in liquid crystals can also affect the behavior of ions. Therefore, an understanding of ionic phenomena in liquid crystals doped with nanoparticles is essential for future advances in liquid crystal-aided nanoscience and nanotechnology. This paper provides an overview of the ionic effects observed in liquid crystals doped with nanomaterials. An introduction to liquid crystals is followed by a brief overview of nanomaterials in liquid crystals. After giving a basic description of ions in liquid crystals and experimental methods to measure them, a wide range of ionic phenomena in liquid crystals doped with different types of nanomaterials is discussed. After that, both existing and emerging applications of tunable soft materials made of liquid crystals and nanodopants are presented with an emphasis on the role of ionic effects in such systems. Finally, the discussion of unsolved problems and future research directions completes the review.

Characterization of Nematic Liquid Crystals at Microwave Frequencies

The use of nematic liquid crystal (LC) mixtures for microwave frequency applications presents a fundamental drawback: many of these mixtures have not been properly characterized at these frequencies, and researchers do not have an a priori clear idea of which behavior they can expect. This work is focused on developing a new procedure for the extraction of the main parameters of a nematic liquid crystal: dielectric permittivity and loss tangent at 11 GHz under different polarization voltages; splay elastic constant K11, which allows calculation of the threshold voltage (Vth); and rotational viscosity γ11, which allows calculating the response time of any arbitrary device. These properties will be calculated by using a resonator-based method, which is implemented with a new topology of substrate integrated transmission line. The LC molecules should be rotated (polarized) by applying an electric field in order to extract the characteristic parameters; thus, the transmission line needs to have two conductors and low electric losses in order to preserve the integrity of the measurements. This method was applied to a well-known liquid crystal mixture (GT3-23002 from MERCK) obtaining the permittivity and loss tangent versus bias voltage curves, the splay elastic constant, and the rotational viscosity of the mixture. The results validate the viability of the proposed method.

Microwave Liquid Crystal Technology

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