A polarisation-independent blue-phase liquid crystal lens array using gradient electrodes

A polarisation-independent blue-phase liquid crystal lens array using gradient electrodes is proposed. A high dielectric constant layer helps to smoothen out the horizontal electric field and reduce the operating voltage. With gradient electrodes and a planar top electrode, gradient electric fields are generated and lens-like phase profile is obtained. When the applied voltage is changed, the focal length of the lens can be tuned from ∞ to 5.94 mm. Besides, the simulation results show that the lens is insensitive to polarisation while keeping parabolic-like profile.     

Polarisation-independent blue-phase liquid crystal microlens array with different dielectric layer

A fast response (sub-milliseconds) and polarisation-independent blue-phase liquid crystal (BPLC) microlens array with periodical double layer electrodes using different dielectric layers is proposed. The bottom double layer electrodes are coated with transparent and different dielectric layers to generate linearly varying electric potential from the centre to the edge, while the top planar electrode iridium tin oxide (ITO) electrode has a constant potential. As a result, gradient vertical electric fields are generated, and a gradient refractive index profile is obtained. When the applied voltage is changed, the focal length of the BPLC microlens array can be tuned from ∞ to 12.05 mm while keep a low operating voltage (~35V_rms). Besides, the driving mode (simplification driving) and fabrication process (using printing method or mold-pressing method) of the BPLC microlens array is very simple. The simulation results show that the BPLC microlens array is insensitive to the polarisation of incident light while keeping parabolic-like phase profile.     

Relationship between lens properties and director orientation in a liquid crystal lens

Lenses with a homogeneously aligned liquid crystal having a Fresnel structure have been prepared by using a nematic with a positive dielectric anisotropy. Their focal length can be varied continuously from the value f_e for an extraordinary ray to f _o for an ordinary ray by applying an electric field across the lens cell. The effective refractive index of the lens where the director is aligned perpendicular to the grooves of the Fresnel structure becomes smaller than when the director is aligned parallel to the grooves. Then the liquid crystal lens has a characteristic aberration which could not be observed in a conventional glass lens; that is, the focal length of the lens becomes different according to the incidence of rays on the different parts of the lens. The properties of the liquid crystal lens can be improved by making the director orientation axially symmetric, in the form of a concentric circle, but the polarization component rotated 90° from the incident extraordinary ray appears when the voltage is applied across the lens cell. This phenomenon is discussed in relation to the optical properties and the director orientation in a liquid crystal prism cell.     

Adaptive nematic liquid crystal lens array with resistive layer

ABSTRACT

We propose an adaptive nematic liquid crystal (LC) lens array using a dielectric layer with low dielectric constant as resistive layer. With the resistive layer and periodic-arranged iridium tin oxide (ITO) electrodes, the vertical electric field across the LC layer varies linearly over the lens aperture is obtained in the voltage-on state. As a result, a centrosymmetric gradient refractive index profile within the LC layer is generated, which causes the focusing behaviour. As a result of the optimisation, a thin cell gap which greatly reduces the switching time of the LC lens array can be achieved in our design. The main advantages of the proposed LC lens array are in the comparatively low operating voltage, the flat substrate surface, the simple electrodes, and the uniform LC cell gap. The simulation results show that the focal length of the LC lens array can be tuned continuously from infinity to 3.99 mm by changing the applied voltage.     

Relationship between lens properties and director orientation in a liquid crystal lens

Lenses with a homogeneously aligned liquid crystal having a Fresnel structure have been prepared by using a nematic with a positive dielectric anisotropy. Their focal length can be varied continuously from the value f_e for an extraordinary ray to f_o for an ordinary ray by applying an electric field across the lens cell. The effective refractive index of the lens where the director is aligned perpendicular to the grooves of the Fresnel structure becomes smaller than when the director is aligned parallel to the grooves. Then the liquid crystal lens has a characteristic aberration which could not be observed in a conventional glass lens; that is, the focal length of the lens becomes different according to the incidence of rays on the different parts of the lens. The properties of the liquid crystal lens can be improved by making the director orientation axially symmetric, in the form of a concentric circle, but the polarization component rotated 90° from the incident extraordinary ray appears when the voltage is applied across the lens cell. This phenomenon is discussed in relation to the optical properties and the director orientation in a liquid crystal prism cell.     

Dielectric studies of the paracetamol-lenticular tissue interactions

This paper reports on the effect of paracetamol on the dielectric behavior of the rabbit lens. Measurements were performed over the frequency range of 100 Hz-100 kHz in air and at the temperature of 35°C. The frequency dependencies of the relative permittivity and dielectric loss for the control and paracetamol-control lenses are described in terms of a power-low, Debye and Cole-Cole relations. The effect of paracetamol on the dielectric properties of the lens is visible in the lower values of the relative permittivity than those for the control sample at the same frequency. In addition, the relaxations around 18 and 46 kHz for the paracetamol-control lens are shifted to lower frequencies compared with the control lens. The results of this work indicate that the present method is useful in detection of the lens toxicity elicited by overdoses of paracetamol in animal.     

Double-layer liquid crystal lens array with composited dielectric layer

ABSTRACT

A double-layer liquid crystal (LC) lens array with composited dielectric layer is proposed. In our design, a spatially non-uniform electric field is generated between the strip electrodes, resulting in a gradient refractive index distribution in the LC layer. Since the upper and lower parts of the LC lens array both adopt a composite dielectric layer, the operation voltage of the LC lens array is effectively reduced. In terms of LC lenslet, the double-layer design doubles the phase difference between the centre and the periphery of the LC layer, thereby reducing the focal length of the LC lens array. In addition, the shortest focal length (~1.78 mm) of the LC lens array is obtained at V = 3.3 V, and the LC lens array has a large focusing range.     

Nearest-neighbour-interaction model in the coupled-optical-phonon-mode theory of the infrared dispersion in monoclinic crystals: Application to Tutton salt single crystal

The coupled-optical-phonon-mode theory of Barker and Hopfield of two mode interaction in isotropic crystals has been extended to monoclinic crystals. The analytical expressions for the calculation of the dielectric tensor elements in the ac crystal plane have been derived. It has been shown that the interaction dielectric model is a generalized expression of the dielectric tensor for monoclinic case when no interaction between modes is present. Also, the results of Barker and Hopfield are obtained from this more general theory, when an isotropic case is considered. In order to be able to investigate real crystals, meaning extending the interactions to a large number of pairs of modes, but at the same time to make the fitting procedure possible, a model dielectric function taking into account the interaction between modes with closest frequencies has been derived. The validity of the model obtained has been tested on a Tutton salt single crystal of K₂Co(SO₄)₂·6H₂O. The recorded spectra from the ac crystal plane were fitted in order to obtain best fit parameters. The comparison between the experimentally recorded spectra and the model reflectance function give good results and verify this model to be applicable.     

Functional Groups Assisted Tunable Dielectric Permittivity of Guest-Free Zn-Based Coordination Polymers for Gate Dielectrics

The dielectric properties of coordination polymers has been a topic of recent interest, but the role of different functional groups on the dielectric properties of these polymers has not yet been fully addressed. Herein, the effects of electron-donating (R=NH₂) and electron-withdrawing (R=NO₂) groups on the dielectric behavior of such materials were investigated for two thermally stable and guest-free Zn-based coordination polymers, [Zn(L₁)(L₂)]_n ( 1 ) and [Zn(L₁)(L₃)]_n ( 2 ) [L₁=2-(2-pyridyl) benzimidazole (Pbim), L₂=5-aminoisophthalate (Aip), and L₃=5-nitroisophthalate (Nip)]. The results of dielectric studies of 1 revealed that it possesses a high dielectric constant (κ=65.5 at 1 kHz), while compound 2 displayed an even higher dielectric constant (κ=110.3 at 1 kHz). The electron donating and withdrawing effects of the NH₂ and NO₂ substituents induce changes in the polarity of the polymers, which is due to the inductive effect from the aryl ring for both NO₂ and NH₂. Theoretical results from density functional theory (DFT) calculations, which also support the experimental findings, show that both compounds have a distinct electronic behavior with diverse wide bandgaps. The significance of the current work is to provide information about the structure-dielectric property relationships. So, this study promises to pave the way for further research on the effects of different functional groups on coordination polymers on their dielectric properties.     

Synthesis of the novel perovskite-type oxyfluoride PbScO2F under high pressure and high temperature

We synthesized a novel perovskite-type oxyfluoride, PbScO₂F, and investigated its crystal structure, thermal stability and dielectric properties. PbScO₂F has a cubic perovskite-type structure with Pb ions displaced from the ideal A-site positions along the 〈110〉 direction. By thermal gravity and differential thermal analyses, we found that this compound is stable up to 963 K (690 °C) under ambient atmosphere. The dielectric permittivity of PbScO₂F is approximately 80, at room temperature, which is almost the same as KTiO₂F and smaller than that of PbFeO₂F. Furthermore, an anomaly in the dielectric permittivity was observed in the vicinity of 100 K that seems to be related to the displacement of the Pb ions.     

Core–Shell Structured Polystyrene/BaTiO3 Hybrid Nanodielectrics Prepared by In Situ RAFT Polymerization: A Route to High Dielectric Constant and Low Loss Materials with Weak Frequency Dependence

A novel route to prepare core–shell structured nanocomposites with excellent dielectric performance is reported. This approach involves the grafting of polystyrene (PS) from the surface of BaTiO₃ by an in situ RAFT polymerization. The core–shell structured PS/BaTiO₃ nanocomposites not only show significantly increased dielectric constant and very low dielectric loss, but also have a weak frequency dependence of dielectric properties over a wide range of frequencies. In addition, the dielectric constant of the nanocomposites can also be easily tuned by varying the thickness of the PS shell. Our method is very promising for preparing high‐performance nanocomposites used in energy‐storage devices.     

Switchable Dielectric Phase Transition Induced by a Twisting Transformation in Diglycine Methanesulfonate

A new glycine‐based reversible phase transition compound diglycine methanesulfonate ( 1 ) has been successfully synthesized. Differential scanning calorimetry (DSC) measurements of 1 showed a pair of broad peaks around 134 K (T_c=phase transition temperature) with a slight thermal hysteresis during the heating/cooling cycle, thereby indicating that this compound undergoes a reversible second‐order phase transition. Dielectric measurements further confirmed the phase transition and revealed a switchable response to the ambient temperature change for the dielectric constants of 1 , namely, the dielectric constants have a distinctive step‐like anomaly switching between a high dielectric state in the room temperature phase (RTP) and a low state in the low temperature phase (LTP). Variable‐temperature single‐crystal X‐ray diffraction analyses show that 1 undergoes an atypical transition from the space group P2₁/m in the RTP to P2₁/c in the LTP. The origin of the switchable dielectric phase transition is ascribed to the movement of the moieties in 1 from the equilibrium position, and this stems from the twisting of the molecules in the compound. We believe that these findings will be useful in exploring switchable dielectric phase transition materials.     

Influence of floating-ring-electrode on large-aperture liquid crystal lens

Large-aperture liquid crystal (LALC) lens with hole-patterned electrodes possesses small lens power and high addressing voltage because of the thick dielectric layer inserted between the hole-patterned electrode and LC layer. With an embedded narrow floating ring electrode (FRE), the lens power and addressing voltage of the LALC lens could be effectively increased and decreased, respectively. In this study, we analyse the electro-optic performance of LALC lens upon variation of the diameter of the embedded FRE. Results reveal that the FRE diameter determines the electric-field distribution and hence the electro-optic behaviour of the LALC lens. The LALC lens with embedded 2-mm-diameter FRE has excellent lens properties, such as low aberration, high focal quality and modulation transfer function performance comparable with solid glass lens.     

Dielectric spectroscopy of an antiferroelectric liquid crystal showing an antiferroelectric–ferrielectric transition

We report the dielectric relaxation behaviour in the antiferroelectric SmC_A* and ferrielectric SmC_γ* phases of the antiferroelectric liquid crystal 4-[5-(4-octloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(methoxyphenyl)butanoate which shows an antiferroelectric transition at around 88±0.1°C. In the SmC_A* phase, two dielectric relaxation modes have been found, namely the usual antiferroelectric Goldstone mode and another arising from molecular rotation around its short axis. In the SmC_γ* phase, one dielectric relaxation mode has been observed due to the ferrielectric Goldstone mode. Dielectric increments and relaxation frequencies of the antiferroelectric and ferrielectric phases are estimated from the fits of the Cole–Cole function of the dielectric spectrum. The dependence of the bias field in the ferrielectric phase is also discussed.     

Study of dynamic light scattering in nematic liquid crystal and its optical,electrical and switching characteristics

In the present study, the polarisation-independent dynamic light scattering (DLS) in a nematic liquid crystal (LC) with a negative dielectric anisotropy has been compared in two operating modes by applying DC voltage or AC voltage to LC cells with a vertical and hybrid alignment. The attenuation of light transmittance and the DLS optical threshold without polarizers as well as a phase retardation of LC bend–splay deformation and a Fredericks threshold voltage with polarizers have been determined. Diffuse scattering of a light beam and broadband transmittance spectra of LC cells have been examined using the DC voltage in the interval of 0–40 V. Multi-domain structures and turbulent flows in the LC cells have been observed by polarisation optical microscope. The results show that the enhancement of the diffuse scattering of light in the LC cells correlate with ion density increase. The largest attenuation of the light intensity in the LC cells with a vertical alignment was 16.4 dB at a wavelength of 650 nm with a minimum decay time equal to 2.5 ms at DC voltage of 60 V. Our study has been shown that switching from a diffuse light scattering state to a transparent state can be twice accelerated by applying AC voltage with high frequency to LC cells.     

A multifunctional blue phase liquid crystal lens based on multi-electrode structure

A blue phase liquid crystal (BPLC) lens with multifunction using multi-electrode structure and a dielectric layer with high dielectric constant is proposed. The refractive index of the BPLC can be changed flexibly in different regions. Some functional or technical requirements such as switch between positive and negative lenses can be achieved. The lens reveals a good parabolic refractive index distribution and focus adjustment capacity simultaneously. The applied voltage on the electrodes is regular and computable. To decrease the applied voltage of the proposed lens with a large diameter, a drive-type adopted Fresnel lens is introduced.     

Investigation on the growth of CaCu3Ti4O12 thin film and the origins of its dielectric relaxations

Using the radio frequency magnetron sputtering, CaCu₃Ti₄O₁₂ (CCTO) thin films were deposited on platinized silicon substrates. The influence of annealing temperature on structures and morphologies of the thin films was investigated. The high annealing temperature increased the crystallinity of the films. Temperature dependence of the dielectric constant revealed an amazing different characteristic of the dielectric relaxation at ∼10 MHz, whose characteristic frequency abnormally increased with the decrease of the measuring temperature unlike the relaxations due to extrinsic origins. Meanwhile, the dielectric constant at high frequencies was close to the value derived from the first principle calculation. All these gave the evidences to ascribe this relaxation to the intrinsic mechanism.     

High Dielectric Constant in the Charge‐ordered Manganese Oxide CaMn7O12

The dielectric properties of mixed‐oxide CaMn₇O₁₂ are reported. This compound exhibits a high dielectric constant at room temperature (ε′_r> 10⁴), for frequencies up to 10⁴ Hz, values that make it very attractive for potential applications provided that its rather high losses can be minimized. Complex plane analysis of the obtained data reveal an important extrinsic contribution to these high ε′_r values; and also that the intrinsic dielectric constant of this material is ε′_r,∞ ≈ 30, a value rather high for this type of compounds, that could be related to the electronic process of charge‐ordering present in this oxide below 440 K.     

Characterization of the electroclinic effect of smectic LCs in terms of the transitional dielectric constant in the SA phase

Abstract

The electric-field dependence of the dielectric constant of some materials which show a strong electroclinic effect in the S_A phase, has been studied by measurement of the transitional dielectric constant. We suggest that the observed variation of the dielectric constant may originate from the electric field induced dielectric biaxiality; this is inherent in a material that exhibits the electroclinic effect.     

Experimental characterization of hexatic smectic phases through electro-optic studies and dielectric relaxation spectroscopy

The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4′-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmC_A* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmC_A* and hexatic smectic phases were determined. The dielectric strength at the SmC_A* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmC_A* and hexatic smectic I* phases.