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The current development of new liquid crystal devices often requires the use of thin cells and new experimental materials. Characterizing these devices and materials with optical methods can be challenging if (1) the total phase lag is small ("thin cells") or (2) the liquid crystal optical and dielectric properties are only partially known. We explore the limitations of these two challenges for efficient characterization and assessment of new, to the best of our knowledge, liquid crystal devices. We show that it is possible to extract a wealth of liquid crystal parameters even for cells with a phase lag of ΔΦ≈π, such as E7 liquid crystal in a 1.5 µm cell, using cross-polarized intensity measurements. The reliability of the optical method is also demonstrated for liquid crystals without precise values of dielectric or refractive index coefficients.
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We report the first integrated implementation of a polarizer based on the use of 45° tilted gratings in planar waveguides. The waveguides and gratings are fabricated by direct UV writing in a hydrogenated germanium-doped silica-on-silicon chip. We experimentally demonstrate a polarization extinction ratio per unit length of 0.25 dB mm -1 with a modelled wavelength dependence smaller than 0.3 dB for a 20 mm device over the C band from 1530-1570 nm. We also present a novel numerical study and analytical description of the architecture that are in good agreement with each other and the experimental data.
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We track the non-uniformity of a wide area liquid crystal device using multiple cross-polarized intensity measurements. They give us not only accurate estimates of the core physical liquid crystal parameters, such as elastic constants, but also spatial maps of the device properties, including the liquid crystal thickness and pretilt angle. A bootstrapping statistical analysis, coupled with the multiple measurements, gives us reliable error bars on all the measured parameters.
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An optical fiber with nano-electromechanical functionality is presented. The fiber exhibits a suspended dual-core structure that allows for control of the optical properties via nanometer-range mechanical movements. We investigate electrostatic actuation achieved by applying a voltage to specially designed electrodes integrated in the cladding. Numerical and analytical calculations are preformed to optimize the fiber and electrode design. Based on this geometry an all-fiber optical switch is investigated; we find that optical switching of light between the two cores can be achieved in a 10 cm fiber with an operating voltage of 35 V.
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We present a reliable optical method for measuring the twist elastic constant K 2 and for assessing the total twist angle in a standard nematic twist cell. The method relies on the use of a non-standard configuration of crossed polarisers and a twist cell, which allows us to measure accurately the twist-cell parameters by reducing the degeneracy between them. Grid patching and an efficient beam propagation method are utilised in the numerical models used for fitting the experimental data. The modelling shows that the polarisation dynamics in a twist cell is non-trivial and much more complex than in a planar cell. The twist elastic constant of three commonly used liquid crystals (5CB, 6CHBT and E7) was successfully extracted from cross-polarised intensity measurements.
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Structural disorder can improve the optical properties of metasurfaces, whether it is emerging from some large-scale fabrication methods or explicitly designed and built lithographically. For example, correlated disorder, induced by a minimum inter-nanostructure distance or by hyperuniformity properties, is particularly beneficial for light extraction. Inspired by topology, we introduce numerical descriptors to provide quantitative measures of disorder with universal properties, suitable to treat both uncorrelated and correlated disorder at all length scales. The accuracy of these topological descriptors is illustrated both theoretically and experimentally by using them to design plasmonic metasurfaces with controlled disorder that we then correlate to the strength of their surface lattice resonances. These descriptors are an example of topological tools that can be used for the fast and accurate design of disordered structures or as aid in improving their fabrication methods.
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Doping liquid crystals with nanoparticles is a widely accepted method to enhance liquid crystal's intrinsic properties. In this study, a quick and reliable method to characterise such colloidal suspensions using an optical multi-parameter analyser, a cross-polarised intensity measurement-based device, is presented. Suspensions characterised in this work are either plasmonic (azo-thiol gold AzoGNPs) or ferroelectric Sn2P2S6 (SPS) nanoparticles in nematic liquid crystals. The elastic constants and rotational viscosity showed nonlinear dependence on the concentration of AzoGNPs, initially increasing at lower concentrations and then decreasing at higher concentrations, indicating some degree of particle aggregation. For the SPS suspension, the elastic constant decreased with doping, while the rotational viscosity increased, in agreement with previous findings. Through viscosity measurements, the stability of SPS suspension over ten years is also highlighted.
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In fiber based Fabry-Pérot Cavities (FFPCs), limited spatial mode matching between the cavity mode and input/output modes has been the main hindrance for many applications. We have demonstrated a versatile mode matching method for FFPCs. Our novel design employs an assembly of a graded-index and large core multimode fiber directly spliced to a single mode fiber. This all-fiber assembly transforms the propagating mode of the single mode fiber to match with the mode of a FFPC. As a result, we have measured a mode matching of 90% for a cavity length of ~400 µm. This is a significant improvement compared to conventional FFPCs coupled with just a single mode fiber, especially at long cavity lengths. Adjusting the parameters of the assembly, the fundamental cavity mode can be matched with the mode of almost any single mode fiber, making this approach highly versatile and integrable.
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Stiction is one of the major reliability issues limiting practical application of nano-electro-mechanical systems (NEMS), an emerging device technology that exploits mechanical movements on the scale of an integrated electronic circuit. We report on a discovery that stiction can be eliminated by infiltrating NEMS with nematic liquid crystals. We demonstrate this experimentally using a NEMS-based tunable photonic metamaterial, where reliable switching of optical response was achieved for the entire range of nanoscopic structural displacements admitted by the metamaterial design. Being a more straightforward and easy-to-implement alternative to the existing antistiction solutions, our approach also introduces an active mechanism of stiction control, which enables toggling between stiction-free and the usual (stiction-limited) regimes of NEMS operation. It is expected to greatly expand the functionality of electro-mechanical devices and enable the development of adaptive and smart nanosystems.
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Nanomechanical optical fibers with metal electrodes embedded in the jacket were fabricated by a multi-material co-draw technique. At the center of the fibers, two glass cores suspended by thin membranes and surrounded by air form a directional coupler that is highly temperature-dependent. We demonstrate optical switching between the two fiber cores by Joule heating of the electrodes with as little as 0.4 W electrical power, thereby demonstrating an electrically actuated all-fiber microelectromechanical system (MEMS). Simulations show that the main mechanism for optical switching is the transverse thermal expansion of the fiber structure.
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We have developed a facile method for preparing magnetic nanoparticles which couple strongly with a liquid crystal (LC) matrix, with the aim of preparing ferronematic liquid crystal colloids for use in magneto-optical devices. Magnetite nanoparticles were prepared by oxidising colloidal Fe(OH)(2) with air in aqueous media, and were then subject to alkaline hydrothermal treatment with 10 mol dm(-3) NaOH at 100°C, transforming them into a polydisperse set of domain magnetite nanorods with maximal length ~500 nm and typical diameter ~20 nm. The nanorods were coated with 4-n-octyloxybiphenyl-4-carboxylic acid (OBPh) and suspended in nematic liquid crystal E7. As compared to the conventional oleic acid coating, this coating stabilizes LC-magnetic nanorod suspensions. The suspension acts as a ferronematic system, using the colloidal particles as intermediaries to amplify magnetic field-LC director interactions. The effective Frederiks magnetic threshold field of the magnetite nanorod-liquid crystal composite is reduced by 20% as compared to the undoped liquid crystal. In contrast with some previous work in this field, the magneto-optical effects are reproducible on time scales of months. Prospects for magnetically switched liquid crystal devices using these materials are good, but a method is required to synthesize single magnetic domain nanorods.
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Most liquid crystals show low sensitivity to magnetic field. However, in this paper we show that a small bias magnetic field not only breaks the symmetry of the ground state, but also plays a crucial role in facilitating the reorientation induced by a large test magnetic field. In particular, a small bias field may alter significantly the strength of the test field needed to observe a given reorientation of the liquid crystal. Moreover, the bias field interacts with other symmetry breaking features of the cell, e.g., pretilt, to change also the qualitative features of the equilibrium state.