Optical Phase Retardation Research Articles

Tunable all-optical liquid crystal lens based on the thermo-plasmonic effect

Surface plasmon resonance (SPR) thermal effects have been the focus of researchers lately and are commonly used in sensors, micro heaters in integrated circuits, and other applications. In this study, a new type of tunable all-optical liquid crystal (LC) lens is fabricated by employing the SPR thermal effects. Due to the absorption of pump light by gold nanoparticles, the generated heat is transferred to the surrounding LC layer, leading a radial gradient distribution of LC molecules. Passing a probe light through the center of the fabricated cell, a lens shaped optical phase retardation observed. The temperature increase with intensification of the pump light power has been numerically calculated. In addition, the focal length of the lens reduces from 157.8 to 13.1 cm during this process. The tunable all-optical LC lens is a fascinating new concept that could open up new horizons and has numerous applications. It has a simpler and cheaper structure compared to electronic lenses, provides greater stability than other all-optical counterparts, and allows for optical control of the focal length.

High phase retardation polarization-independent liquid crystal devices

A polarization-independent single-layer liquid crystal (LC) device is demonstrated for high optical phase retardation. Using a single LC layer with twist angles of an integer multiple of 180°, the LC device shows good polarization independence. As the phase retardation increases with the decrease of the twist angle, the LC device with a single LC layer of 180°twist angle may get the highest polarization-independent phase retardation when the polarization dependence is less than 15%. Compared with OCB and ECB modes, the LC device with a twist angle of 180°has a larger field of view (FoV). The simple structure LC device shows a great application for display, optical communication and imaging systems.

Black phosphorus phase retarder based on anisotropic refractive index dispersion

Black phosphorus (BP) has gained wide interest as a promising layered material for its unique physical properties. In particular, the anisotropic optical property of BP can act as a retarder or a polarizer in nano-optoelectronic devices, for which quantitative qualification of the phase retardation and the anisotropic refractive index dispersion are essential. Here, we report the anisotropic refractive index and extinction coefficient dispersions of BP in the visible and near infrared range of 540–1500 nm, and then characterize optical phase retardation in a BP flake. Cauchy absorbent equations are provided for the refractive index dispersions along both armchair and zigzag directions, which well reproduce the experimentally measured reflectance and transmittance contrast spectra of BP flakes. Furthermore, we demonstrate that a linear polarized light through BP becomes elliptical, a finding that agrees well with simulation results using the obtained anisotropic refractive index dispersions. The two-dimensional phase retarder in this work is expected to find various applications in novel polarization-sensitive nano-optoelectronic devices.

Identification of thermo optical parameters of 8ocb pure and nano dispersed liquid crystalline compound using image processing based IME method

In the present paper synthesis, characterization and image enhancement studies have been carried out on 8ocb LC with dispersed ZnO nanoparticles in conjunction with identification of thermo optical parameters at various concentrations. The microscopic textures are analyzed with MATLAB tool to obtain optical parameters. Image processing methods are used to identify the behavior of optical transmittance, absorption coefficient and phase retardation calculated using proposed Illumination Map Estimation (IME) technique and compared with the available literature. The proposed image processing-based method is a simple experimentation procedure to identify the thermo optical parameters of liquid crystal compounds for various electro optic applications.

Identification of thermo optical parameters in 4l-hexyloxy-4-cyanobyphenyl with dispersed ZnO nano particles

Abstract In this present article, synthesis, characterization, and study of optical parameters through image enhancement methods have been carried out on 4l-hexyloxy-4-cyanobyphenyl (6OCB) pure liquid crystal (LC) and 6OCB with dispersion of 0.5 wt% ZnO nanoparticles. Textural determinations of the synthesized compounds are recorded by using SDTECHS POM connected with a hot stage and camera. differential scanning calorimetry (DSC) is used to measure enthalpy and transition temperature values. The results show that the dispersion of ZnO in 6OCB exhibits nematic phase as same as the pure 6OCB with slightly reduced clearing temperature as expected. Further characterization is carried out by various spectroscopic techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet visible (UV–Vis) spectroscopy. To evaluate and identify the behavior of optical parameters viz optical transmittance (OT), absorption coefficient (AC), and phase retardation (PR) as a function of temperature, an image processing method has been proposed, i.e. illumination enhancement algorithm (IEA) using MATLAB software. The proposed enhancement algorithm is one of the simplest and efficient algorithms to evaluate the thermo optical parameters for various electro optical applications. The results are compared with the body of the data available.

Liquid crystal lens with tunable conical lens properties

A three-dimensional numerical calculation for analyzing reorientation in liquid crystal (LC) directors and optical phase retardation in an LC lens, which exhibits a tunable conical lens property, is developed. The proposed LC lens design employs a circular electrode, ring electrodes, and a circularly hole-patterned electrode employing a highly-resistive layer in a flat nematic cell. The three-dimensional tilt and azimuthal angle distributions are calculated. Subsequently, the conical phase retardation in the lens region can be estimated. The experimental results obtained for the optical phase profile agree well with the numerically calculated results.

Bideposited silver nanocolloid arrays with strong plasmon-induced birefringence for SERS application

Silver nano-rod, nano-zigzag, nano-saw, and nano-particle arrays are fabricated with glancing angle bideposition. The structure-dependent anisotropic optical properties of those bideposited nanostructured arrays are measured and investigated. The equivalent birefringence values of nano-rod and nano-zigzag arrays are much larger than crystals found in nature and liquid crystal used in display products. The fact that induced localized plasmon-magnetic field between nanorods dominates the strong phase retardation between p-polarized and s-polarized transmitted wave. For the nano-saw, the strong localized electric field induced between the saw teeth leads to strong SERS signals. Although the bideposited nanoparticles own weak morphological anisotropy, strong optical phase retardation is still detected at wavelengths near 400 nm.

High-Definition Optophysical Image Construction Using Mosaics of Pixelated Wrinkles.

Despite many efforts in structuring surfaces using mechanical instabilities, the practical application of these structures to advanced devices remains a challenging task due to the limited capability to control the local morphology. A platform that programs the orientation of mechanically anisotropic molecules is demonstrated; thus, the surface wrinkles, promoted by such instabilities, can be patterned in the desired manner. The optics based on a spatial light modulator assembles wrinkle pixels of a notably small dimension over a large area at fast fabrication speed. Furthermore, these pixelated wrinkles can be formed on curved geometries. The pixelated wrinkles can record images, which are naturally invisible, by mapping the gray level to the orientation of wrinkles. They can retrieve those images using the patterned optical phase retardation generated under the crossed polarizers. As a result, it is shown that the pixelated wrinkles enable new applications in optics such as image storage, informative labeling, and anti‐counterfeiting.

Optical polarization measurement for measuring deflection radius of the optically anisotropic flexible-polymeric substrate

This study proposes an optical methodology to measure the deflection radius of flexible polymeric substrates; to our best knowledge, this is the first report taking into consideration the optical anisotropy effect of these substrates for such measurements. The relation between the deflection-induced optical phase retardation of two distinct refraction beams and the deflection radius was derived. Full-field phase maps were obtained via a polarization measurement system and the phase-stepping technique. The measurement results for three substrates having nominal deflection radii of 50, 55, and 60 mm were, correspondingly, 51.39 ± 0.37, 57.56 ± 1.00, and 58.71 ± 0.85 mm (error = 2.78%, 4.65%, and 2.15%, respectively); thus, the measurement precision was confirmed.

A single‐cell‐gap transflective blue‐phase liquid crystal display based on fringe in‐plane switching electrodes

AbstractA transflective blue‐phase liquid crystal display (TRBP‐LCD) based on fringe in‐plane switching (FIS) electrodes is proposed. The proposed structure generates combined fringe and in‐plane electric fields that cause more liquid crystal (LC) molecules to reorient almost in plane above and between the pixel electrodes. The fringe field is mainly generated in the transmissive (T) region, and the horizontal electric field is mainly generated in the reflective (R) region. By optimizing the width of the pixel electrodes and the gap between two adjacent pixel electrodes, the different electric field intensity in the T and R regions contribute to balance the optical phase retardation between the T and R regions. As a result, the proposed TRBP‐LCD exhibits a low operating voltage and high optical efficiency, while it preserves a relatively simple fabrication process.

Simulation study of single‐cell‐gap transflective liquid crystal display with nonuniform potential

A single‐cell‐gap transflective liquid crystal display with a nonuniform electric potential is demonstrated. The top substrate has a top planar common electrode, a transparent dielectric layer with a general dielectric constant is coated on the bottom substrate, and two planar pixel electrodes with the same size are coated on the dielectric layer and the bottom substrate, respectively. With the different gaps between the two planar pixel electrodes and top planar pixel electrode, the nonuniform electric potential from the transmissive region (T region) to the reflective region (R region) is generated, while a bumpy reflector is coated under the bottom substrate. In this device, with the dielectric layer, the pixel and common electrodes generate a strong electric potential in the T region and a relatively weak electric potential in the R region. Consequently, the T and R regions accumulate the same optical phase retardation. The simulation results show that the display exhibits reasonably low operating voltage, high optical efficiency, and well‐matched voltage‐dependent transmittance and reflectance curves. Besides, the fabrication process and the driving mode of the transflective liquid crystal display are relatively simple, and it is suitable for mobile applications.

Fast copolymer network liquid crystals for tunable birefringence colors.

A fast-switching, tunable color filter was found in a copolymer network liquid crystal (LC), which was in situ generated in a conventional LC test cell with parallel aligned glass plates and investigated with polarized light. Polarization filters were used to convert the tunable optical phase retardance of the test cells to birefringence colors as is always possible in a LC test cell with carefully adjusted cell gap and effective birefringence. The cell gap of the samples could be adjusted to a value of 9 μm, which is not easily possible in a polymer LC composite without creating defects. In these samples, the typical pastel colors seen frequently in birefringent samples could be avoided. The transmittance spectra were recorded and converted to CIE 1931 color coordinates, which showed that the colors seen had a reasonable distance to the white point. The electro-optic switching times of the samples were investigated: Fast responses of t on +t off <5 ms were found, which is an impressive speed for tunable birefringence colors in LCs and LC composites. Upon increasing addressing voltages, a blueshift of the peak seen in the transmittance spectra was observed. The samples consisted of copolymer network LC, generated from a reactive mixture with mesogenic monomer and nonmesogenic comonomer. The tunable color was seen selectively in samples with dodecyl acrylate as comonomer. The experiments show how even a straightforward electro-optic experiment still can result in unexpended findings, which may expand the use of LC composites in nondisplay applications. The polymer morphology in samples with a larger cell gap was investigated with scanning electron microscopy, and interdefect distances of ≈40 μm were found. The appearance of defects in test cells with a cell gap of 9 μm could be avoided because the cell gap was much smaller than the measured interdefect distances in test cells with a larger cell gap.

Dynamic characteristics of nematic liquid crystal variable retarders investigated by a high-speed polarimetry

Liquid crystal variable retarders (LCVRs) have been widely used as a transmissive element with electrically tunable optical phase retardance in polarimetric optical systems. Since the responsive characteristics of the LCVRs depend on the modulation processes, accurately and rapidly characterizing LCVRs is of great significance for the performance improvement of the optical system with LCVRs. Firstly, we proposed a Mueller matrix based model to characterize an LCVRs with considerations of both linear birefringence (LB) and circular birefringence (CB), with which the LCVRs is described by several driving voltage-dependent optical parameters such as retardance, azimuth, relative transmittance and optical rotation angle. The experimental results show that the Mueller matrices of the LCVRs measured by a commercial Mueller matrix ellipsometer are consistently fitted by the proposed method, and the improvement of the proposed characterization method can be read from the 15 times reduced average residual errors of the reconstructed Muller matrix compared to the matrix reconstructed with the conventional characterization methods. Through several typical dynamic measurement experiments using a high-speed Stokes polarimeter with a temporal resolution of several nanoseconds, we demonstrated the driving-voltage-dependence of these optical parameters as well as the existence of both LB, CB and depolarization properties in the LCVRs modulation processes. Based on the analysis of the extracted optical parameters, we have obtained the continuous modulation characteristics and step response characteristics of LCVRs. Additionally, a comparison between the Mueller matrices of air measured with the proposed and conventional characterization method has been carried out to demonstrate the fidelity of the proposed method as well.

Transflective blue-phase liquid crystal display with dielectric protrusion

ABSTRACTA transflective blue-phase liquid crystal display (TRBP-LCD) with dielectric protrusions is proposed. The dielectric protrusions are fixed on the in-plane-switching (IPS) electrodes. The potential drops very slowly in the dielectric protrusions, thus the total field penetrating depth is significantly enlarged and the operating voltage is lowered, which helps suppressing electrostriction effect. The different heights of the dielectric protrusions in the transmissive (T) and reflective (R) regions contribute to balance the optical phase retardation between the T and R regions. The simulation results verify that the proposed TRBP-LCD can achieve low operating voltage and good sunlight readability.

A single-cell-gap transflective liquid crystal display with a vertically aligned cell

ABSTRACTA single-cell-gap transflective liquid crystal display with a vertically aligned cell using square ring electrode is demonstrated. The top substrate has a top planar common electrode, a square ring pixel electrode is coated on the bottom substrate, while a bumpy reflector is coated under the bottom substrate. In this device, the planar common electrode and square ring pixel electrode generate a strong longitudinal electric field in the transmissive region (T region) and a weak fringe field in the reflective region (R region). As result, the T and R regions accumulate the same optical phase retardation. The simulation results show that the display exhibits reasonably low operating voltage, high transmittance and well-matched voltage-dependent transmittance and reflectance curves. Besides, fabrication process of the transflective liquid crystal display is very simple.

Liquid crystal lens with optimized wavefront across the entire clear aperture

Abstract Comparative theoretical analysis of performances of three electrically variable liquid crystal lens designs is conducted. Traditional hole patterned electrode, additional floating and inner-ring electrode configurations are considered within the framework of a modal control lens. The control mode of those electrodes and the choice of the voltage and frequency values of the driving electric signals are optimized to obtain spherical profile of the optical phase retardation across the entire clear aperture of the lens. This would allow the effective use of the tunable lens in combination with larger aperture fixed focus lenses. It is shown that we can improve significantly the acceptable (with low aberrations) dynamic variation range of the optical power of the lens by switching the control mode of electrodes.

Transflective blue-phase liquid crystal display with polar opposite electrodes

ABSTRACTA submillisecond response, wide view and single-cell-gap transflective (TR) display employing a blue-phase liquid crystal is proposed. The device employs polar opposite in-plane switching (IPS) electrodes. To balance the optical phase retardation between transmissive (T) and reflective (R) regions, the IPS electrodes are formed with unequal gaps in the two regions. This display exhibits reasonably high optical efficiency and well-matched voltage-dependent transmittance and reflectance curves.

A transflective polymer-stabilised blue-phase liquid display with partitioned wall-shaped electrodes

ABSTRACTA transflective polymer-stabilised blue-phase liquid crystal display (BP-LCD) with partitioned wall-shaped electrodes is proposed. The etched polymer layer contributes to balance the optical phase retardation between transmissive (T) and reflective (R) regions. The partitioned wall-shaped electrodes generate uniform and horizontal fields throughout the entire LC layer to induce isotropic-to-anisotropic transition in the blue-phase liquid crystal medium through Kerr effect. Consequently, the accumulated phase retardation along beam path is large, resulting in reasonable low operation voltage and high transmittance both in T and R regions. This approach enables the BP-LCD to be addressed by amorphous silicon thin-film transistors. Moreover, it exhibits wide viewing angle and a well-matched gamma curve.

Tunable plasmofluidic lens for subwavelength imaging

A tunable plasmofluidic lens consisting of nanoslit arrays on a metal film is proposed for subwavelength imaging in far field at different wavelengths. The nanoslit arrays with constant depths but varying widths could generate desired optical phase retardations based on the propagation property of the surface plasmon polaritons (SPPs) through the metal-dielectric-metal (MDM) nanoslit waveguide. We demonstrate the tunability of the plasmofluidic lens for subwavelength imaging by changing the surrounding dielectric fluid. This work provides a novel approach for developing integrative tunable plasmofluidic lens for a variety of lab-on-chip applications.

Flat polarization-controlled cylindrical lens based on the Pancharatnam–Berry geometric phase

The working principle of ordinary refractive lenses can be explained in terms of the space-variant optical phase retardations they introduce, which reshape the optical wavefront curvature and hence affect the subsequent light propagation. These phases, in turn, are due to the varying optical path length followed by light at different transverse positions relative to the lens center. A similar lensing behavior can, however, be obtained when the optical phases are introduced by an entirely different mechanism. Here, we consider the ‘geometric phases’ that arise from the polarization transformations occurring in anisotropic optical media, named after Pancharatnam and Berry. The medium anisotropy axis is taken to be space-variant in the transverse plane and the resulting varying geometric phases give rise to the wavefront reshaping and lensing effect, which however also depends on the input polarization. We describe the realization and characterization of a cylindrical geometric-phase lens that is converging for a given input circular-polarization state and diverging for the orthogonal one, which provides one of the simplest possible examples of optical elements based on geometric phases. The demonstrated lens is flat and only a few microns thick (not including the supporting substrates); moreover, its working wavelength can be tuned and the lensing can be switched on and off by the action of an external control electric field. Other kinds of lenses or more general phase elements inducing different wavefront distortions can be obtained by a similar approach. Besides their potential for optoelectronic technology, these devices offer good opportunities for introducing college-level students to an advanced topic of modern physics, such as the Berry phase, with the help of interesting optical demonstrations.