Polarimetric calibrated robust dual-SLM complex-amplitude computer-generated holography.

Liquid crystal on silicon (LCoS) is a widely used spatial light modulator (SLM) in computer-generated holography (CGH). However, the phase-modulating profile of LCoS is often not ideally uniform in application, bringing about undesired intensity fringes. In this study, we overcome this problem by proposing a highly robust dual-SLM complex-amplitude CGH technique, which incorporates a polarimetric mode and a diffractive mode. The polarimetric mode linearizes the general phase modulations of the two SLMs separately, while the diffractive mode uses camera-in-the-loop optimization to achieve improved holographic display. Experimental results show the effectiveness of our proposal in improving reconstructing accuracy by 21.12% in peak signal-to-noise ratio (PSNR) and 50.74% in structure similarity index measure (SSIM), using LCoS SLMs with originally non-uniform phase-modulating profiles.

Increasing steering angle of LCoS in a WSS system through integration with a metasurface.

Liquid crystal on silicon (LCoS) has been the dominant choice for wavelength selective switches (WSSs) in telecommunication industry due to its high spatial resolution and compatibility with software defined flexible grid feature. Current LCoS devices generally have a limited steering angle, which also limits the minimum footprint of the WSS system. The steering angle of LCoS devices is fundamentally determined by the pixel pitch, which is highly challenging to be optimized without resorting to other techniques. In this paper, we present an approach to increase the steering angle of LCoS devices through the integration with dielectric metasurfaces. Here a dielectric Huygens-type metasurface is integrated with an LCoS device to increase its steering angle by 10°. This approach can effectively minimize the overall size of the WSS system while maintaining a small form factor of the LCoS device.

Phase flicker-induced sharpness deterioration on 2D holographic displays with digitally driven phase-only LCoS devices.

Phase flicker in liquid crystal on silicon (LCoS) devices can decrease the effective phase modulation resolution by introducing overlapped phase oscillations between adjacent modulated gray levels, thus degrading the performance of LCoS devices in various applications. However, the effect of phase flicker on a holographic display is often overlooked. From an application angle, this paper investigates the quality of the holographic reconstructed image, especially sharpness, under the static and dynamic effects of different flicker magnitudes. Both the simulation and experimental results reveal that the increment in the magnitude of phase flicker causes an equal sharpness deterioration with the reduction of the numbers of hologram phase modulation levels.

Electrically tuned active metasurface towards metasurface-integrated liquid crystal on silicon (meta-LCoS) devices.

Active metasurfaces add a new dimension to static metasurfaces by introducing tunability, and this has received enormous attention from industry. Although various mechanisms have been proposed over the past few years in literature, solutions with good practicality are limited. Liquid crystal (LC)-based active metasurface is one of the most promising approaches due to the well-established LC industry. In this paper, an electrically tunable active metasurface was proposed and experimentally demonstrated using photoaligned nematic LC. The good quality of the LC photoalignment on the metasurface was demonstrated. Tunable transmission was obtained for telecommunication C band and the modulation depth in transmission amplitude of 94% was realized for 1530 nm. Sub-millisecond response time was achieved at operating a temperature of 60°C. The progress made here presents the potential of LC-based active metasurfaces for fast-switching photonic devices at optical communication wavelengths. More importantly, this work lays the foundations for the next-generation liquid crystal on silicon (LCoS) devices that are integrated with metasurfaces (meta-LCoS).

Effects of phase flicker in digitally driven phase-only LCOS devices on holographic reconstructed images.

Phase flicker can degrade the performance of holographic applications at the device and application levels. On the device side, the meaningful phase modulation resolution is proved to be limited by the overlapping between adjacent phase levels caused by flicker. Here, the tolerance of the overlapping for different modulation levels is provided. The frame rate of the device is also constrained by the phase flicker. The balance between low flicker and fast LC response for fast frame rate is quantitatively analyzed. On the application side, the effects of real phase flicker on the performance of blazed gratings and image holograms are investigated using the temporal phase flicker profiles measured from a phase-only liquid crystal on silicon (LCOS) device; they are shown to be comparable with that introduced by quantization level and amplitude noise, respectively.

Sub-millisecond switching of multi-level liquid crystal on silicon spatial light modulators for increased information bandwidth.

Sub-millisecond response time with a refresh rate higher than 2000 frames per second (fps) and no degradation of the contrast ratio or diffraction efficiency is demonstrated in working liquid crystal on silicon (LCOS) spatial light modulators (SLMs) with 8-bit grey levels of amplitude and phase modulations. This makes possible to achieve an information bandwidth of about 190 Gb s-1 with a 4k LCOS operating at 10-bit phase modulation levels. The normalised contrast stays at almost the unit level for a frame rate up to 1700 fps and at higher than 0.9 for 2500 fps. The diffraction efficiency stays above -1.0 dB for a frame rate up to 2400 fps. Such a fast response allows us to eliminate image blurring in replaying a fast movie.

Variety of Ordered Patterns in Donor-Acceptor Polymer Semiconductor Films Crystallized from Solution.

A huge challenge is to control the nucleation of crystallites/aggregates in the solution during polymer film formation to generate desired structures. In this work, we investigate crystallization of P(NDI2OD-T2), a donor-acceptor polymer semiconductor, with controlled solution flow along the contact line between the drying film and solution through a seesaw-like pivoting of samples during polymer drying. By controlling the pivoting frequency/amplitude, various types of line patterns can be observed: (I) an array of fishbone-like stripes oriented in the film-growth direction; (II) the pinning-depinning of contact line (PDCL)-mechanism-defined patterned wires along the contact line; and (III) periodic twined crystalline line pattern oriented in the direction of the contact line. The rich variety of pattern formation observed is attributed to the distinctiveness of the donor-acceptor conjugated polymer structure. The result measured from thin-film transistors made of the generated films/structures showed that the charge mobility of P(NDI2OD-T2) does not change much with the film morphology, which supports recent controversy over the charge-transportation mechanism of some donor-acceptor polymer semiconductors.

Improvements of phase linearity and phase flicker of phase-only LCoS devices for holographic applications.

Significant phase distortion corrections were achieved by optimizing the digital driving patterns of phase-only liquid crystal on silicon devices for digital holographic applications. Nearly perfect phase linearity and phase flicker of 0.09% over 256 addressed phase levels in respect to the total modulation range of 2π were realized, enabling a meaningful increase of phase levels from 8 bits (256 levels) to 9 bits (512 levels). Tests were carried out to evaluate the qualities of optically reconstructed holographic images with reduced phase flicker and optimized phase linearity, and an increase of 17.7% in the root-mean-square contrast was demonstrated.

High-resolution patterning of solution-processable materials via externally engineered pinning of capillary bridges.

Electronics based on solution-processable materials are promising for applications in many fields which stimulated enormous research interest in liquid-drying and pattern formation. However, assembling of structure with submicrometre/nanometre resolution through liquid process is very challenging. We show a simple method to rapidly generate polymer structures with deep-submicrometre-sized features over large areas. In this method, a solution film is dried on a substrate under a suspended flexible template with groove/ridge surface topography. Upon solvent evaporation, the solution splits in the grooves and forms capillary bridges between the template and substrate, which are firmly pinned by the edges of the template grooves. This groove pinning stabilizes the contact lines, thereby allowing the formation of fine patterned structures with high aspect ratios which were used to fabricate various functional materials and electronic devices. We also produced secondary self-assembled nano-stripe patterns with resolutions of about 50 nm on the primary lines.

Fast bistable intensive light scattering in helix-free ferroelectric liquid crystals.

A new type of ferroelectric liquid crystal (FLC) is considered, where the reorientation of the director (main optical axes) at the interaction of an electric field with the FLC's spontaneous polarization is due to the movement of spatially localized waves with a stationary profile: solitons arise at the transition due to the Maxwellian mechanism of energy dissipation. Under certain conditions, the appearance of such waves leads to the formation of a structure of transient domains, and as a consequence, to the scattering of light. The Maxwellian mechanism of energy dissipation allows one to reduce the electric field strength at which the maximum efficiency of light scattering is achieved down to 2-3 V/µm and to increase the frequency of light modulation up to 3-5 kHz. Intensive bistable light scattering in an electro-optical cell filled with a specially designed helix-free FLC was studied, and a stable scattering state can be switched on and off for a few tens of microseconds and memorized for a few tens of seconds.

Pixel-level fringing-effect model to describe the phase profile and diffraction efficiency of a liquid crystal on silicon device.

We propose a fringing-effect model based on the experimentally measured phase response across the phase transition region of a liquid crystal on silicon (LCOS) device. The measured phase profile in the phase transition region is characterized by a scaled error function of the flyback width. The flyback width can be determined by a cubic function of the phase depth between neighboring pixels. This dependence of the flyback width on the phase depth is explained by a linear rotation model of the liquid crystal director. The simulated diffraction efficiency based on the fringing-effect model shows a close agreement with the experimental measurement.

Colorimetric-based detection of TNT explosives using functionalized silica nanoparticles.

This proof-of-concept study proposes a novel sensing mechanism for selective and label-free detection of 2,4,6-trinitrotoluene (TNT). It is realized by surface chemistry functionalization of silica nanoparticles (NPs) with 3-aminopropyl-triethoxysilane (APTES). The primary amine anchored to the surface of the silica nanoparticles (SiO2-NH2) acts as a capturing probe for TNT target binding to form Meisenheimer amine-TNT complexes. A colorimetric change of the self-assembled (SAM) NP samples from the initial green of a SiO2-NH2 nanoparticle film towards red was observed after successful attachment of TNT, which was confirmed as a result of the increased separation between the nanoparticles. The shift in the peak wavelength of the reflected light normal to the film surface and the associated change of the peak width were measured, and a merit function taking into account their combined effect was proposed for the detection of TNT concentrations from 10-12 to 10-4 molar. The selectivity of our sensing approach is confirmed by using TNT-bound nanoparticles incubated in AptamerX, with 2,4-dinitrotoluene (DNT) and toluene used as control and baseline, respectively. Our results show the repeatable systematic color change with the TNT concentration and the possibility to develop a robust, easy-to-use, and low-cost TNT detection method for performing a sensitive, reliable, and semi-quantitative detection in a wide detection range.

A compensation method for the full phase retardance nonuniformity in phase-only liquid crystal on silicon spatial light modulators.

A simple and efficient compensation method for the full correction of both the anisotropic and isotropic nonuniformity of the light phase retardance in a liquid crystal (LC) layer is presented. This is achieved by accurate measurement of the spatial variation of the LC layer's thickness with the help of a calibrated liquid crystal wedge, rather than solely relying on the light intensity profile recorded using two crossed polarizers. Local phase retardance as a function of the applied voltage is calculated with its LC thickness and a set of reference data measured from the intensity of the reflected light using two crossed polarizers. Compensation of the corresponding phase nonuniformity is realized by applying adjusted local voltage signals for different grey levels. To demonstrate its effectiveness, the proposed method is applied to improve the performance of a phase-only liquid crystal on silicon (LCOS) spatial light modulator (SLM). The power of the first diffraction order measured with the binary phase gratings compensated by this method is compared with that compensated by the conventional crossed-polarizer method. The results show that the phase compensation method proposed here can increase the dynamic range of the first order diffraction power significantly from 15~21 dB to over 38 dB, while the crossed-polarizer method can only increase it to 23 dB.

High quality micro liquid crystal phase lenses for full resolution image steering in auto-stereoscopic displays.

The use of pixel-level tunable liquid crystal (LC) lenses to steer the images shown on a flat panel display in full resolution for auto-stereoscopic applications was proposed. Micro lenticular LC lenses of different full widths ranging from 40 to 140 µm were designed and fabricated with laser patterned transparent ITO electrodes as narrow as 10 µm in width and two LC materials of high birefringence. Optical characterization of the lenses showed consistent parabolic phase profiles closely matched to that of ideal lenses. A proof-of-concept device with an array of tunable micro LC lenses each covers two sub-pixels of different colors was fabricated and applied on a standard computer monitor to confirm its capability of sub-pixel-level image steering.

Diffraction based phase compensation method for phase-only liquid crystal on silicon devices in operation.

A method to measure the optical response across the surface of a phase-only liquid crystal on silicon device using binary phase gratings is described together with a procedure to compensate its spatial optical phase variation. As a result, the residual power between zero and the minima of the first diffraction order for a binary grating can be reduced by more than 10 dB, from -15.98 dB to -26.29 dB. This phase compensation method is also shown to be useful in nonbinary cases. A reduction in the worst crosstalk by 5.32 dB can be achieved when quantized blazed gratings are used.