Adaptive focal lengths in white light focusing Fresnel lenses enabled by reflective-type and phase-only spatial light modulator.

Fresnel zone plates (FZPs) are widely used in integrated optical systems to meet new cutting-edge demands for photonic integration and device miniaturizing. However, their use in applications of cross-scale fabrication still faces several obstacles, such as low efficiency, fixed focal length, single wavelength, large size, and complicated fabrication. Here, we first examine a novel adaptive focal length in white light focusing by using reflective-type and phase-only spatial light modulator (RLC-SLM) based on a liquid crystal on silicon. The device achieves a maximum diffraction efficiency of approximately 38% at primary focal points of binary phase-type FZPs throughout the visible range (red, green, and blue wavelengths). The RLC-SLM focuses the light of the desired wavelength while other sources are defocused. White light focusing and color separation are demonstrated by sequentially and additively switching different FZPs. These recent advances show that optically tunable FRZs are promising potential candidates to enhance adaptive camera systems, microscopes, holograms, and portable and wearable devices, thereby opening up novel possibilities in optical communications and sensing.

Fabrication of azimuthally/radially symmetric liquid crystal plates using two-step photoalignments.

Fabrication of azimuthally/radially symmetric liquid crystal plates (A/RSLCPs) using two-step photoalignments with the two dichroic dyes methyl red (MR) and brilliant yellow (BY) is demonstrated. BY molecules coated onto a substrate and MR molecules doped in LCs in a cell can azimuthally and radially align LCs by illuminating radially and azimuthally symmetric polarized light with specific wavelengths, respectively. In contrast to the previous fabrication methods, the fabrication method proposed here can prevent the contamination/damage of photoalignment films on substrates. A method to improve the proposed fabrication to avoid generating undesirable patterns is also elucidated.

Hiding private information in private information protection liquid crystal displays using periodical waveplates and pixel quaternity.

A method to hide private information (PI) in private information protection liquid crystal displays (PIPLCDs) using periodical waveplates and pixel quaternity is reported in this study. The PIPLCD structure and its operation mechanism are conceptually elucidated. Users employing wide-view broadband circular polarizers (WVBCPs) can see PI hidden in images displayed on the PIPLCDs in PIP mode. However, people who do not use WVBCPs can only see the displayed images but not the PI at almost any viewing angles. The issues of the PIPLCD in PIP mode and possible fabrications of the periodical waveplates are also discussed.

Multiple-Color Reflectors Using Bichiral Liquid Crystal Polymer Films and Their Applications in Liquid Crystal Displays.

Multiple-color reflectors using bichiral liquid crystal polymer films (BLCPFs) are investigated. The BLCPFs consist of alternate layers of two different single-pitch cholesteric liquid crystal (CLC) layers, named CLC#A and CLC#B. The thickness of each CLC layer equals its single pitch length. The optical properties in terms of reflections, reflection-wavelength ranges, and distributions of reflection spectra of the BLCPFs that result from the fixed pitch length of CLC#A along with the decrease of the pitch length of CLC#B are qualitatively simulated and investigated. The results indicate that the above optical properties of the BLCPFs depend on the LC birefringence and pitch lengths of CLC#A and CLC#B layers. The concept of fabrication method of the BLCPFs by using polymerizable CLCs and thin films of poly(vinylalcohol) or photoalignment materials is discussed. They have potential practical applications in functional color filters, asymmetrical transmission systems, etc., owing to the multiple reflection bands of BLCPFs. Moreover, the BLCPFs, which can enhance the color gamut and light-utilization efficiency of light sources/LC displays, are reported herein.

High-Contrast and Scattering-Type Transflective Liquid Crystal Displays Based on Polymer-Network Liquid Crystals.

The methods to enhance contrast ratios (CRs) in scattering-type transflective liquid crystal displays (ST-TRLCDs) based on polymer-network liquid crystal (PNLC) cells are investigated. Two configurations of ST-TRLCDs are studied and are compared with the common ST-TRLCDs. According to the comparisons, CRs are effectively enhanced by assembling a linear polarizer at the suitable position to achieve better dark states in the transmissive and reflective modes of the reported ST-TRLCDs with the optimized configuration, and its main trade-off is the loss of brightness in the reflective modes. The PNLC cell, which works as an electrically switchable polarizer herein, can be a PN-90° twisted nematic LC (PN-90° TNLC) cell or a homogeneous PNLC (H-PNLC) cell. The optoelectric properties of PN-90° TNLC and those of H-PNLC cells are compared in detail, and the results determine that the ST-TRLCD with the optimized configuration using an H-PNLC cell can achieve the highest CR. Moreover, no quarter-wave plate is used in the ST-TRLCD with the optimized configuration, so a parallax problem caused by QWPs can be solved. Other methods for enhancing the CRs of the ST-TRLCDs are also discussed.

Achromatic linear polarization rotators by tandem twisted nematic liquid crystal cells.

An achromatic linear polarization rotator based on a tandem-2ϕ-twisted nematic liquid crystal cell (tandem-2ϕ-TNLC cell, where 2ϕ represents the total twisted angle) is theoretically analyzed and experimentally demonstrated. The tandem-2ϕ-TNLC cell comprises two conventional ϕ-TNLC cells with the required arrangement that the LC director close to the last layer of the first ϕ-TNLC cell should be perpendicular to that close to the first layer of the second ϕ-TNLC cell. With such a simple combination, the TNLC performances are considerably improved. According to the experimental results and theoretical analyses by Jones Calculus, the tandem-2ϕ-TNLC polarization rotator with suitable parameters is achromatic and insensitive to the polarization plane of incident light. Such properties provide these polarization rotators with potential for practical applications.

General theory of asymmetrical polarization-dependent optics in functional material-doped 90° twisted nematic liquid crystals.

General theory, which can completely describe the asymmetrical optics in a functional material (FM)-doped 90° twisted nematic liquid crystals (TNLCs), is proposed using Cayley-Hamilton theorem and Jones calculus. The FMs, whose shape and size are similar to those of the adopted NLCs, can be aligned along the long axes of the NLCs. The FMs discussed herein are dichroic dye (DD) and polymer. The experimental results of asymmetrical transmission in DD-doped 90° TNLCs are consistent with the theoretical calculation. Such asymmetrical characterization can be further used in the current applications based on 90° TNLCs in all fields to obtain new potential functions.

Low-Threshold-Voltage and Electrically Switchable Polarization-Selective Scattering Mode Liquid Crystal Light Shutters.

Low-threshold-voltage (Vth) and electrically switchable, polarization-selective scattering mode light shutters (PSMLSs) using polymer-dispersed liquid crystals (PDLCs) are demonstrated in this work. The optimized weight ratio of the nematic liquid crystals (LCs) to the adopted monomer (NBA107, Norland Optics) in the low-Vth PDLCs based on NBA107 is 7:3, [7:3]-PDLCsNBA107. The properties of the low-Vth PDLCsNBA107, such as light-scattering performance, initial transmission, Vth, and droplet size were investigated. Experiment results show that the surface anchoring (threshold-voltage) of NBA107 is weaker (lower) than or equal to that of the common NOA65. The cost is that the response time of the proposed PDLCsNBA107 is relatively long. A method to reduce the decay time, which can be applied to all other PDLC devices, will be elucidated. In addition to the low Vth of the proposed PDLCsNBA107, the operation voltage (~6 Vrms) to approach the maximum transmission is relatively low in a 7 µm-thick PDLCsNBA107 cell. Moreover, the polarization-selective light-scattering performances of the proposed PSMLSs based on the [7:3]-PDLCsNBA107, mainly driven by in-plane and vertical fields, are also demonstrated.

Optically Controllable Linear-Polarization Rotator Using Chiral-Azobenzene-Doped Liquid Crystals.

A linear-polarization rotator based on the optically tunable pitch of chiral-azobenzene-doped liquid crystals (CAdLCs) has been investigated. It is shown that the orientation of linearly polarized (LP) light can be optically tuned using CAdLCs and that the transmitted light possesses a good degree of linear polarization (DoLP). Experimental and simulation (4 × 4 Berreman matrix) results show that the rotation angle is dependent on the pitch as well as the number of turns of the cholesteric LC helix. Some causes to affect the DoLP of the output LP lights during photoisomerization are also discussed. Moreover, a calibration term, ß(t), is also introduced to elucidate the behavior of the discontinuous change of the CAdLC pitch in a fixed cell thickness.

Asymmetrical polarization-dependent scattering and reflection in a sole cell of polymer network-90° twisted nematic liquid crystals.

The anisotropically intrinsic scattering and reflection of a sole cell of polymer network-90° twisted nematic liquid crystals (PN-90° TNLCs) without any polarizer are proposed. Light with specifically linear polarizations, incident from one direction, can penetrate the PN-90° TNLCs with applied voltage. The polarization direction of the output beam will be rotated 90°. The same linearly polarized light, incident from the other direction, will be scattered because it encounters the refractive indices mismatch of various LC domains. The reflection, resulting from the boundaries of LCs and polymers, also shows optical anisotropy. Such LC devices can be applied as scattering-type linear polarizers.

Expanding color gamut of reflective liquid crystal displays from filtering undesirable wavelengths of a light source by an embedded etalon.

This work demonstrates a method to reduce the intensity of the undesirable wavelengths of blue (B-) and green (G-) ambient lights to expand the color gamut of reflective liquid crystal displays (LCDs) by an embedded etalon. The built-in reflector of the reflective LCDs is replaced by the blue-green overlapping wavelengths filtering etalon, which is used to reduce the intensity of undesirable B- and G-primaries, thereby decreasing the color cross talk of B- and G-color filters. After etalon adoption, the color gamut can be expanded from 105.96% to 121.81% of National Television System Committee (NTSC) in International Commission on Illumination (CIE) 1976 color space. Compared with the color gamut of the display without the blue-green overlapping wavelength etalon, the maximum expansion of color gamut is ∼15.85%. Moreover, the balance between light loss and color gamut expansion should be taken into consideration.

Electro-opto-thermal addressing bistable and re-addressable display device based on gelator-doped liquid crystals in a poly(N-vinylcarbazole) film-coated liquid crystal cell.

This paper reports an electro-opto-thermal addressing bistable and re-addressable display device based on gelator-doped liquid crystals (LCs) in a poly(N-vinylcarbazole) film-coated LC cell. The bistability and re-addressability of the devices were achieved through the formation of a rubbery LC/gel mixture at room temperature. The desired patterns were addressed, erased, and re-addressed by controlling the temperature, applied voltage, and UV light illumination. Moreover, grayscales were obtained by adjusting UV light intensity. The initiation, relaxation, rise, and fall times of photoconductive poly(N-vinylcarbazole) via UV light illumination of various intensities were also examined.

Optical simulation of single-cell-gap transflective liquid crystal displays based on surface anchoring energy of periodical nano-grooved structures.

The single-cell-gap transflective liquid crystal display (TR-LCD) based on periodic distribution of surface anchoring energy (SAE) of periodical nano-grooved structures is reported in this study. Different SAEs of planar-aligned nematic LC cells are associated with the threshold and operation voltages of the adopted LCs. Thus, according to the transmittance versus applied voltage curves, the regions with strong and weak SAEs in LC cells can be the transmission and reflection regions of a TR-LCD, respectively. According to the simulation results using 1D-DIMOS software, the phase retardation of a strong SAE region is approximately twice as large as that of a weak SAE region when a specific voltage is applied, and a suitable difference in SAE exists between these two regions. Moreover, various SAEs based on periodical nano-grooved structures can be fabricated to demonstrate the TR-LCD.

Electrically Switchable and Permanently Stable Light Scattering Modes by Dynamic Fingerprint Chiral Textures.

Negative dielectric nematic liquid crystals (LCs) doped with two azobenzene materials provide electrically switchable and permanently stable scattering mode light modulators based on dynamic fingerprint chiral textures (DFCT) with inhomogeneously helical axes. These light modulators can be switched between transparent (stable large domains of DFCT) states and scattering (stable small domains of DFCT) states by applying electric fields with different frequencies. The generation of DFCT results from the long flexible side chains of the doped chiral dopant. That is, if the DFCT can be obtained, then the large domains of DFCT reflect an intrinsically stable state. Moreover, the stabilization of the small domains of DFCT are caused by the terminal rigid restricted side chains of the other doped chiral dopant. Experimentally, the required amplitude to switch the light modulator from a scattering (transparent) state to a transparent (scattering) state decreases as the frequency of the applied electric field increases (decreases) within the set limits. This study is the first report on the advantages of the light scattering mode of DFCT, including low operating voltage, permanently stable transmission, wide viewing angle, high contrast, and polarization-independent scattering and transparency.

Direct optical switching of bistable cholesteric textures in chiral azobenzene-doped liquid crystals.

The direct optical switching of bistable cholesteric textures (i.e., planar and focal conic textures) in chiral azobenzene-doped liquid crystals (LCs) is demonstrated. Chiral azobenzene is a chiral dopant with optically tuned helical twisting power that results from the photo-isomerization between trans- and cis- isomers via exposure to UV or visible light. The pitch length of the material can be optically and repeatedly elongated and shortened. With regard to free energy, LCs tend to be stable at planar (focal conic) textures when pitch length is elongated (shortened) by exposure to UV (visible) light. Thus, direct optical switchable LC displays are investigated.

Dual liquid crystal alignment configuration based on nanoparticle-doped polymer films.

This paper proposes an approach for producing dual liquid crystal (LC) alignment configuration based on nanoparticle-doped polymer films. Experimental results indicate that illuminating a nanoparticle-doped pre-polymer film, coated onto a substrate with a homogeneous alignment layer, with unpolarized UV light through a photomask causes the polymerization of pre-polymer, ultimately generating homogeneous and vertical alignment layers in unpolymerized and polymerized regions, respectively. The dual LC alignment configuration of the homogeneous (vertical) and hybrid alignments can be achieved by combining the treated substrate with another substrate that has a homogeneous (vertical) alignment layer. Additionally, the applications of the dual LC alignment layer in phase gratings and transflective LC displays are demonstrated.

Polarization rotators fabricated by thermally-switched liquid crystal alignments based on rubbed poly(N-vinyl carbazole) films.

This paper demonstrates the thermally-switched liquid crystal (LC) alignments based on a rubbed poly(N-vinyl carbazole) (PVK) film, and their application for polarization rotators. The mechanically rubbed PVK layer can induce a planar alignment of LCs with their director axis perpendicular to the direction of rubbing. This direction can be switched toward the rubbing direction by thermal treatment. Experimentally, the angle of re-orientation of the director axis increases with the temperature in a specific range of temperatures. In this study, the optical properties of linear and concentric polarization rotators, fabricated using a rubbed PVK film with thermal treatment, are examined.

Polarization-independent holographic gratings based on azo-dye-doped polymer-dispersed liquid-crystal films.

We demonstrate polarization-independent holographic gratings (PIHGs) based on azo-dye-doped polymer-dispersed liquid crystal films. The PIHGs are recorded by irradiation under an intensity-modulated interference field, generated by two linearly polarized green optical beams, and by simultaneously applying a suitable AC voltage to the sample. The photoexcited azo dyes are adsorbed onto the UV-cured polymer film with their long axes parallel to the normal of the substrate. When the applied voltage is switched off, the PIHGs are generated with periodic modulation of liquid-crystal structures with transparently homeotropic and randomly scattered alignments. Additionally, the generated PIHGs can be completely switched off by an applied voltage.

Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals.

This work demonstrates the feasibility of a polarizer-free, electrically switchable and optically rewritable display based on dye-doped polymer-dispersed liquid crystals (DD-PDLCs). Experimental results indicate that the doped dyes are homeotropically adsorbed onto the polymer film when an appropriate AC voltage is applied during patterning. The adsorbed dyes in the illuminated region then align the liquid crystals homeotropically, and produce a transparent pattern in the scattering background without any polarizer. Notably, the adsorbed dyes can be erased and readsorbed using thermal and optical treatments, respectively. The switching time of the fabricated display is of the order of milliseconds, and the contrast ratio is approximately 30.

Electrically switchable and optically rewritable reflective Fresnel zone plate in dye-doped cholesteric liquid crystals.

This work demonstrates a reflective Fresnel zone plate based on dye-doped cholesteric liquid crystals (DDCLC) using the photo-induced realignment technique. Illumination of a DDCLC film with a laser beam through a Fresnel-zone-plate mask yields a reflective lens with binary-amplitude structures - planar and focal conic textures, which reflect and scatter probed light, respectively. The formed lens persists without any external disturbance, and its focusing efficiency, analyzed using circularly polarized light, is ~ 23.7%, which almost equals the measured diffraction efficiency of the used Fresnel-zone-plate mask (~ 25.6%). The lens is thermally erasable, rewritable and switchable between focusing and defocusing states, upon application of a voltage.