Fringing-field effect in liquid-crystal beam-steering devices: an approximate analytical model

An approximate analytical model was developed that links the fringing-field broadening of the phase profile of a liquid-crystal (LC) beam-steering device, and the resulting diffraction efficiency, to the physical parameters of the device including the cell thickness as well as the dielectric, optical, and geometrical constants of the device. The analysis includes a full solution of the Laplace equation for the LC device in which the broadening of the initial voltage profile into an effective voltage-drop profile, due to the fringing-field effect, is derived. It is shown that within the linear approximation used, the broadening of the phase profile is identical to the broadening of the effective voltage profile in the presence of the fringing field. On the basis of this model, the resulting broadening kernel of the phase profile is found to be proportional to the LC cell thickness. These results are found to be in an excellent agreement with high-precision computer simulations performed on the LC beam-steering structure, thereby validating this approximate linear model.     

Diffraction patterns of binary liquid crystal gratings in homeotropic and hybrid geometries

We report on the diffraction properties of the binary liquid crystal (LC) gratings consisting of alternating homeotropic and hybrid domains. The measured diffraction patterns agree well with theoretical results in a comprehensive model with a linearly approximated phase profile around the domain boundary. It is found that the linearly distorted length is independent of the grating period and depends only on the cell thickness, the surface anchoring energy, and the elastic constants of the LC material. For the LC cell thickness of 5.9 μm, the linearly distorted length is determined as 2.12 μm. The binary LC grating devices are expected to play an important role in the area of optical data storage and optical information processing.     

Experimental study of phase-step broadening by fringing fields in a three-electrode liquid-crystal cell

The fringing-field broadening of a phase-step profile and its dependence on the thickness of a liquid-crystal (LC) cell were studied in a simple, three-electrode LC cell structure consisting of two lateral electrodes biased with a differential voltage and a third, grounded, electrode placed on the opposite substrate. The results were compared both with an approximate analytical model developed earlier for a fringe-field-broadening kernel and with computer simulations. Good agreement between the experiment and the theoretical as well as the simulation results is shown.     

Numerical analysis of modal dispersion in graded-index plastic optical fibers

Modal dispersion properties of a fabricated plastic optical fiber are numerically calculated through a finite-element method. The modal index, group delay, impulse response, and output pulse shape are compared with those for the power-law profile plastic optical fiber; the influence of index profile deviations from the power-law profile is described. It is shown that index profile fluctuations in the actual index profile strongly affect the group delays, even though they are relatively small. On the other hand, they have little effect on the modal indices.     

Refractive-index profiling of embedded microstructures in optical materials

We describe use of a phase-sensitive low-coherence reflectometer to measure spatial variation of refractive index in optical materials. The described interferometric technique is demonstrated to be a valuable tool to profile the refractive index of optical elements such as integrated waveguides and photowritten optical microstructures. As an example, a refractive-index profile is mapped of a microstructure written in a microscope glass slide with an ultrashort-pulse laser.     

Midinfrared modulation through the use of field-induced scattering in ferroelectric liquid crystals

The feasibility of the use of modulation devices based on field-induced transient scattering in ferroelectric liquid crystals (LC) to replace mechanical choppers used in uncooled infrared-imaging systems was investigated. Devices fabricated with ITO-coated ZnSe substrates and a ferroelectric LC path length of 25 μm were able to modulate optical radiation by transient forward scattering at rates approaching 20 kHz. Through the use of a commercial arbitrary waveform generator and associated PC-based software, drive waveforms were developed that produced a variable, square-wave optical-modulation pattern by the extension of the duration of the scattering state to periods ranging from hundreds of microseconds to milliseconds. The ability of these extended-scattering-mode (ESM) devices to modulate radiation in both the visible and midinfrared regions was verified in a simple experiment through the use of a Fourier-transform infrared spectrometer, in which an unoptimized ESM device displayed a 40% modulation dep th for IR radiation in the 8-12-μm region.     

Liquid-Crystal-Mediated Geometric Phase: From Transmissive to Broadband Reflective Planar Optics

Planar optical elements that can manipulate the multidimensional physical parameters of light efficiently and compactly are highly sought after in modern optics and nanophotonics. In recent years, the geometric phase, induced by the photonic spin–orbit interaction, has attracted extensive attention for planar optics due to its powerful beam shaping capability. The geometric phase can usually be generated via inhomogeneous anisotropic materials, among which liquid crystals (LCs) have been a focus. Their pronounced optical properties and controllable and stimuli-responsive self-assembly behavior introduce new possibilities for LCs beyond traditional panel displays. Recent advances in LC-mediated geometric phase planar optics are briefly reviewed. First, several recently developed photopatterning techniques are presented, enabling the accurate fabrication of complicated LC microstructures. Subsequently, nematic LC-based transmissive planar optical elements and chiral LC-based broadband reflective elements are reviewed systematically. Versatile functionalities are revealed, from conventional beam steering and focusing, to advanced structuring. Combining the geometric phase with structured LC materials offers a satisfactory platform for planar optics with desired functionalities and drastically extends exceptional applications of ordered soft matter. Some prospects on this rapidly advancing field are also provided.     

Three-dimensional-two-dimensional mixed display system using integral imaging with an active pinhole array on a liquid crystal panel

A display system that simultaneously displays two-dimensional (2D) and three-dimensional (3D) images using a pinhole array on a liquid crystal (LC) panel is proposed. Using the transparent structure of the LC panel, the system can generate or eliminate pinholes electrically and can display a 3D image in a selectable specific area of the display panel, while 2D images are displayed on the rest of the screen. An analysis showing the advantages and limitations of the proposed system is provided. Finally, the proposed principle is proven by experimental results.     

Discharge characteristics of a plasma display panel with hump-shape electrodes

The long gap discharge in a plasma display panel has an advantage of high luminous efficacy but has a deficiency of high firing voltage as well. A hump-shape electrode is suggested to reduce the firing voltage of a long gap discharge while maintaining high luminous efficacy. The effects of hump electrodes inserted between two sustain electrodes are investigated by experiment and with a three-dimensional fluid simulation. The experimental results show that hump structure has lower firing and sustain voltage than the conventional long gap structure. A three-dimensional fluid simulation shows that the hump electrode enhances discharge ignition while maintaining the same optical characteristics of the long gap discharge. Also, the hump structure guarantees the addressing reliability with a reduced discharge time lag.     

Electrically controllable liquid-crystal rotatable wave plate with variable phase retardation

Liquid-crystal- (LC-) based rotatable wave plates exhibiting phase retardation that is electrically controllable through more than 2pi and rotatable azimuthal orientation of the optical axis have been achieved. One outer surface of the LC cell is coated with a transparent electrode; this controls the phase retardation. A single wave plate of this type is shown to be capable of converting an arbitrary input polarization to any desired polarization, and its applicability to feedback polarization control is demonstrated.     

Synthesis of symmetric and asymmetric planar optical waveguides

A novel and accurate refractive index profile synthesis method for planar optical waveguides is presented and demonstrated using the transmitted near-electric-field-data. This method is based on the inverse transmission-line (TL) technique. From Maxwell's equations, a TL equivalent circuit (electric T-circuit) for the refractive index profile of a planar optical waveguide is derived. The authors demonstrate how to use this model to carry out the inverse problem and synthesise the exact refractive index profile numerically from near-field-data. The TL method can reconstruct arbitrary refractive index profiles for planar optical waveguides that support singlemode or multi- modes. The cases of both symmetric and asymmetric arbitrary refractive index profile planar waveguides are discussed. The accuracy of the reconstructed waveguides is examined numerically.     

Image Perturbations and Optical Flow in Time-dependent Refraction

Abstract

The ray formulation for light propagation is applied to the problem of image perturbations (‘shimmering’) resulting from perturbations in the refractive index profile of the medium between an object and an observer. Expressions are derived relating the image diagram perturbation and the instantaneous image motion (optical flow) to the change in the refractive index profile, in the paraxial ray approximation. In special circumstances the image perturbation and optical flow can be estimated directly. These results are applied to two simple examples.     

Characterization of inhomogeneous transparent thin films on transparent substrates by spectroscopic ellipsometry: refractive indices n(λ) of some fluoride coating materials

By incorporation of an achromatic three-reflection quarterwave retarder to a spectroscopic ellipsometer and application of appropriate calibration and error correction procedures, it has been possible to characterize real thin-film fluoride optical coatings that are inhomogeneous. The refractive index and its dispersion with wavelengths greater than 300-700 nm as well as the depth profile of voids in the film have been determined for AlF(3), CeF(3), HfF(4), LaF(3), ScF(3) and YF(3) films on vitreous silica substrates.     

放电间隙和显示电极宽度对彩色AC-PDP光电特性的影响

为了提高彩色AC PDP的性能 ,以放电间隙和显示电极宽度对表面放电型AC PDP光电特性的影响进行实验研究和理论分析。结果表明 ,表面放电型AC PDP显示电极间的放电不符合巴邢定律 ,即着火电压Uf 不是气体压强p和放电间隙g1乘积的函数 ,并且在Uf-pg1曲线上 ,随着g1增加 ,最小着火电压Ufmin上升 ,(pg1) min向增大的方向偏移 ,同时放电间隙增加还引起维持电压余度增大 ,亮度和光效提高 ;随着显示电极宽度增加 ,平均放电电流增大 ,亮度提高 ,而光效保持不变。根据这些实验结果 ,总结出放电间隙和显示电极宽度的设计方法。     

Patterned Arrays of Functional Lateral Heterostructures via Sequential Template‐Directed Printing

The precise integration of microscale dots and lines with controllable interfacing connections is highly important for the fabrication of functional devices. To date, the solution‐processible methods are used to fabricate the heterogeneous micropatterns for different materials. However, for increasingly miniaturized and multifunctional devices, it is extremely challenging to engineer the uncertain kinetics of a solution on the microstructures surfaces, resulting in uncontrollable interface connections and poor device performance. Here, a sequential template‐directed printing process is demonstrated for the fabrication of arrayed microdots connected by microwires through the regulation of the Rayleigh–Taylor instability of material solution or suspension. Flexibility in the control of fluidic behaviors can realize precise interface connection between the micropatterns, including the microwires traversing, overlapping or connecting the microdots. Moreover, various morphologies such as circular, rhombic, or star‐shaped microdots as well as straight, broken or curved microwires can be achieved. The lateral heterostructure printed with two different quantum dots displays bright dichromatic photoluminescence. The ammonia gas sensor printed by polyaniline and silver nanoparticles exhibits a rapid response time. This strategy can construct heterostructures in a facile manner by eliminating the uncertainty of the multimaterials interface connection, which will be promising for the development of novel lateral functional devices.     

Low-cost spectroscopy with a variable multivariate optical element

Here, successful realization of a variable multivariate optical element (VMOE) based on a transmissive liquid crystal (LC) panel is reported. In contrast to conventional multivariate optical elements (MOEs), a single VMOE is a dynamic system, allowing measurement of numerous analytes in different mixtures. Furthermore, VMOE has superior spectral resolution in comparison to a conventional MOE. It is demonstrated here that the system implemented in a Raman spectrometer predicts the concentration of each individual component in toluene-acetonitrile-cyclohexane mixtures with a prediction error of <6% (mass percentage). With a dedicated optical design of the setup, a prediction error smaller than 2% is expected to be feasible for the current chemical system.     

Quantitative phase and refractive index analysis of optical fibers using differential interference contrast microscopy

A systematic and straightforward image processing method to extract quantitative phase and refractive index data from weak phase objects is presented, obtained using differential interference contrast (DIC) microscopy. The method is demonstrated on DIC images of optical fibers where a directional integration routine is applied to the DIC images to extract phase and refractive index information using the data obtained across the whole DIC image. By applying the inverse Abel transform to the resultant phase images, an accurate refractive index profile is obtained. The method presented here is compared to the refracted near-field technique, typically used to obtain the refractive index profile of optical fibers, and shows excellent agreement. It is concluded that through careful image processing procedures, DIC microscopy can be successfully implemented to obtain quantitative phase and refractive index information of optical fibers.     

Adjustable refractive index modulation for a waveguide with SU-8 photoresist by dual-UV exposure lithography

We present a new fabrication technique based on a two-step UV exposure lithographic process to marginally modulate the refractive index in commercial SU-8 photoresist. This technique achieves refractive index modulation as different regions undergo different thermal densification prior to UV-induced polymerization. A small refractive index contrast of 0.0008 or lower can be achieved, and this is especially useful for fabricating waveguides with a low level of propagation modes. This technique may be extended to other UV-curable epoxy photoresists and can easily be applied in the fabrication of optical elements such as optical interconnects and integrated optical sensors without the development process.     

Effect of electro-optic modulation on coupled quasi-phase-matched frequency conversion

The electro-optic effect can be employed to modulate the refractive index of an optical superlattice. In coupled quasi-phase matched processes, this modulation will introduce quasi-phase mismatches and result in energy redistribution among the optical waves. Numerical results indicate that an efficient third harmonic in a periodic or quasi-periodic superlattice can be achieved by varying the external dc electric field. This method provides a simple and convenient way to control the efficiencies of frequency conversion.     

Characteristic analysis of absorptive multiple-quantum-well optical waveguides

Abstract

By extending the transfer matrix technique from the real domain to the complex, the complex eigenvalue equation is presented for both transverse electric and transverse magnetic modes of absorptive multiple-quantumwell optical waveguides, and a computer program is developed to solve the eigenvalue equation in the complex plane. On the basis of the computed results, the effects of the structural parameters and the refractive index profile on the mode propagation and loss properties are analysed and discussed for In^x Ga^1?xAs/Al^yGa^1?yAs absorptive multiple-quantum-well optical waveguides. The results indicate that, if approximate guided structural parameters and an appropriate refractive index profile are selected, higher-order modes can be controlled and single-mode propagation can be realized in the multiple-quantum-well optical waveguide with lower mode loss.