Topologically Engineered Strain Redistribution in Elastomeric Substrates for Dually Tunable Anisotropic Plasmomechanical Responses.

The prompt visual response is considered to be a highly intuitive tenet among sensors. Therefore, plasmomechanical strain sensors, which exhibit dynamic structural color changes, have recently been developed by using mechanical stimulus-based elastomeric substrates for wearable sensors. However, the reported plasmomechanical strain sensors either lack directional sensitivity or require complex signal processing and device design strategies to ensure anisotropic optical responses. To the best of our knowledge, there have been no reports on utilizing anisotropic mechanical substrates to obtain directional optical responses. Herein, we propose an anisotropic plasmomechanical sensor to distinguish between the applied force direction and the force magnitude. We employ a simple strain-engineered topological elastomer to mechanically transform closely packed metallic nanoparticles (NPs) into anisotropic directional rearrangements depending on the applied force direction. The proposed structure consists of a heterogeneous-modulus elastomer that exhibits a highly direction-dependent Poisson effect owing to the periodically line-patterned local strain redistribution occurring due to the same magnitude of applied external force. Consequently, the reorientation of the self-assembled gold (Au)-NP array manifests dual anisotropy, i.e., force- and polarization-direction-dependent plasmonic coupling. The cost-effectiveness and simple design of our proposed heterogeneous-modulus platform pave the way for numerous optical applications based on dynamic transformation and topological inhomogeneities.

Antibiotic-Powered Energy Harvesting: Introducing Benzylpenicillin as an Efficient Tribopositive Material for Triboelectric Nanogenerators.

The pursuit of enhancing the performance of triboelectric nanogenerators (TENGs) has led to the exploration of new materials with efficient charge-generating capabilities. Herein, we propose benzylpenicillin sodium salt (b-PEN) as a candidate biomaterial for the tribopositive layer owing to its superior electron-donating capability via the lone pairs of electrons on its sulfur atom, carbonyl, and amino functional groups. The proposed b-PEN TENG device exhibits promising electrical performance with an open-circuit voltage of 185 V, a short-circuit current of 4.52 µA, and a maximum power density of 72 µW/cm2 under force applied by a pneumatic air cylinder at 5 Hz. The biomechanical energy-harvesting capabilities of the b-PEN TENG device are demonstrated by actuating it with finger, hand, and foot movements. Moreover, the proposed TENG device is utilized to charge capacitors and power light-emitting diodes by scavenging the externally applied mechanical energy. This outstanding electrical performance makes b-PEN a promising tribopositive material.

Steerable and Agile Light-Fueled Rolling Locomotors by Curvature-Engineered Torsional Torque.

On-demand photo-steerable amphibious rolling motions are generated by the structural engineering of monolithic soft locomotors. Photo-morphogenesis of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs) is designed from spiral ribbon to helicoid helices, employing a 270° super-twisted nematic molecular geometry with aspect ratio variations of azo-LCN strips. Unlike the intermittent and biased rolling of spiral ribbon azo-LCNs with center-of-mass shifting, the axial torsional torque of helicoid azo-LCNs enables continuous and straight rolling at high rotation rates (≈720 rpm). Furthermore, center-tapered helicoid structures with wide edges are introduced for effectively accelerating photo-motilities while maintaining directional controllability. Irrespective of surface conditions, the photo-induced rotational torque of center-tapered helicoid azo-LCNs can be transferred to interacting surfaces, as manifested by steep slope climbing and paddle-like swimming multimodal motilities. Finally, the authors demonstrate continuous curvilinear guidance of soft locomotors, bypassing obstacles and reaching desired destinations through real-time on-demand photo-steering.

NIR-Triggered High-Efficiency Self-Healable Protective Optical Coating for Vision Systems.

Recently, self-healing materials have evolved to recover specific functions such as electronic, magnetic, acoustic, structural or hierarchical, and biological properties. In particular, the development of self-healing protection coatings that can be applied to lens components in vision systems such as augmented reality glasses, actuators, and image and time-of-flight sensors has received intensive attention from the industry. In the present study, we designed polythiourethane dynamic networks containing a photothermal N-butyl-substituted diimmonium borate dye to demonstrate their potential applications in self-healing protection coatings for the optical components of vision systems. The optimized self-healing coating exhibited a high transmittance (∼95% in the visible-light region), tunable refractive index (up to 1.6), a moderate Abbe number (∼35), and high surface hardness (>200 MPa). When subjected to near-infrared (NIR) radiation (1064 nm), the surface temperature of the coating increased to 75 °C via the photothermal effect and self-healing of the scratched coatings occurred via a dynamic thiourethane exchange reaction. The coating was applied to a lens protector, and its self-healing performance was demonstrated. The light signal distorted by the scratched surface of the coating was perfectly restored after NIR-induced self-healing. The photoinduced self-healing process can also autonomously occur under sunlight with low energy consumption.

Flicker-free dual-volume augmented reality display using a pixelated interwoven integral floating technique with a geometric phase lens.

A geometric phase (GP) integral floating display can provide multifocal three-dimensional (3D) augmented reality (AR) images with enhanced depth expression by switching the focal modes of the GP lens via polarization control. However, using temporal multiplexing to switch between the focal modes of GP optics causes flickering as each 3D AR image is fully presented in different frames and their temporal luminance profile becomes easily recognizable, particularly as the number of available focal modes increases. Here, we propose a novel integral floating technique to generate pixelated interwoven 3D AR images; a half of each image is spatially mixed with another and presented in both focal modes simultaneously to resolve the flickering issue. The principle was verified via experimental demonstration and optically measured data.

Asymmetric voltage waveform for enhanced exciton generation in alternative-current field-induced electroluminescence device.

We propose an alternating current (AC) field operation scheme by using an asymmetric voltage waveform to improve the electroluminescence property of AC field-induced electroluminescence (AC-FIEL) devices. Hole injection and transport can be improved by carbon nanotubes (CNT) doping into the emission layer of an AC-FIEL structure operated by a single electrode for AC-responsive alternating carrier injections. However, under an AC operation, highly unbalanced charge transports are inevitably present in CNT-doped AC-FIEL devices due to faster carrier paths through CNTs. Compared with symmetric waveform, asymmetric waveform can be adjusted to allow longer relative duty time for faster carriers in which the luminance level of CNT-doped AC-FIEL devices can be improved by 1.4 times at the same device structure and operation frequency condition.

Compensation of color breaking in bi-focal depth-switchable integral floating augmented reality display with a geometrical phase lens.

A bi-focal integral floating system using a geometrical phase (GP) lens can provide switchable integrated spaces with enhanced three-dimensional (3D) augmented reality (AR) depth expression. However, due to the chromatic aberration properties of the GP lens implemented for the switchable depth-floating 3D images, the floated 3D AR images with the red/green/blue (R/G/B) colors are formed at different depth locations with different magnification effects, which causes color breaking. In this paper, we propose a novel technique to resolve the color breaking problem by integrating the R/G/B elemental images with compensated depths and sizes along with experiments to demonstrate the improved results. When we evaluated the color differences of the floated 3D AR images based on CIEDE2000, the experimental results of the depth-switchable integral floating 3D AR images showed that the color accuracies were greatly improved after applying a pre-compensation scheme to the R/G/B sub-images in both concave and convex lens operation modes of the bi-focal switching GP floating lens.

Light-driven complex 3D shape morphing of glassy polymers by resolving spatio-temporal stress confliction.

Programmable 3D shape morphing of hot-drawn polymeric sheets has been demonstrated using photothermal local shrinkage of patterned hinges. However, the hinge designs have been limited to simple linear hinges used to generate in-plane local folding or global curvature. Herein, we report an unprecedented design strategy to realize localized curvature engineering in 3D structures employing radial hinges and stress-releasing facets on 2D polymeric sheets. The shape and height of the 3D structures are readily controlled by varying the number of radial patterns. Moreover, they are numerically predictable by finite elemental modeling simulation with consideration of the spatio-temporal stress distribution, as well as of stress competition effects. Localized curvature engineering provides programming capabilities for various designs including soft-turtle-shell, sea-shell shapes, and saddle architectures with the desired chirality. The results of local curvilinear actuation with quantifiable stress implies options to advance the applicability of self-folded architectures embodying coexisting curved and linear geometric surfaces.

Highly enhanced voltage holding property for low-frequency-driven fringe-field switching liquid crystal mode by charge-trapping effect of carbon-nanotube-doped surface.

We herein propose a multiwalled carbon nanotube (MWCNT) doping method into liquid crystal (LC) alignment polyimides (PIs) with low resistivity for resolving both issues of voltage holding and image sticking in low-frequency-driven fringe-field switching (FFS) LC modes using negative dielectric LCs (n-LCs). By utilizing strong ion trapping ability of MWCNTs, the FFS n-LC cell aligned by low resistivity PIs with 0.05 wt% MWCNT doping exhibited an excellent voltage holding ratio of 99% under an extremely low operation frequency of 0.5 Hz and approximately 23.6 times better surface discharging property than that aligned by high resistivity PIs.

Fast-switching laterally virtual-moving microlens array for enhancing spatial resolution in light-field imaging system without degradation of angular sampling resolution.

We present an electrically controllable fast-switching virtual-moving microlens array (MLA) consisting of a stacked structure of two polarization-dependent microlens arrays (PDMLAs) with optical orthogonality, where the position of the two stacked PDMLAs is shifted by half the elemental pitch in the diagonal direction. By controlling the polarization of the incident light without the physical movement of the molecules comprising the virtual-moving MLA, the periodic sampling position of the MLA can be switched fast using a polarization-switching layer based on a fast-switching liquid crystal cell. Using the fast-switching virtual-moving MLA, the spatial-resolution-enhanced light-field (LF) imaging system was demonstrated without a decrease in the angular sampling resolution as compared to the conventional LF imaging system comprising a passive MLA; two sets of elemental image arrays were captured quickly owing to the short switching time of the virtual-moving MLA of 450 µs. From the two captured sets of the elemental array image, four-times resolution-enhanced reconstruction images of the directional-view and depth-slice images could be obtained.

Compact self-interference incoherent digital holographic camera system with real-time operation.

The video recording-capable compact incoherent digital holographic camera system is proposed. The system consists of the linear polarizer, convex lens, geometric phase lens, and the polarized image sensor. The Fresnel hologram is recorded by this simple configuration in real time. The system parameters are analyzed and evaluated to record a better-quality hologram in a compact form-factor. The real-time holographic recording and its digitally reconstructed video playback are demonstrated with the proposed system.

Polarization-dependent liquid crystalline polymeric lens array with aberration-improved aspherical curvature for low 3D crosstalk in 2D/3D switchable mobile multi-view display.

We demonstrated a 2D/3D switchable mobile display using a polarization-dependent switching liquid crystalline polymeric (LCP) lens array film. In spite of short viewing distance and enough viewing window conditions provided by a small f-number lens for mobile displays, the 3D images when switched to the multi-view 3D mode showed a low crosstalk property owing to the improved lens aberration, as applying an aspherical lens curvature interface between the planar-convex LCP layer and the concave-planar isotropic polymer layer. Both 2D and 3D images were demonstrated based on a 5.5-inch quad high definition mobile display panel, where the binocular crosstalk of the 3D mode was 3.3%.

Physical model for temperature-dependent dielectric properties of anisotropic nematic liquid crystals.

We derived a new analytic physical model for describing the temperature-dependent dielectric permittivities εe(T) and εo(T) in anisotropic mesophase molecules of nematic liquid crystals (NLCs). These temperature-dependent dielectric properties of NLCs could be explained by a six-parameter dielectric permittivity model, where the analytic dielectric permittivity curves of εe(T), εo(T), and Δε(T) from the model using the six parameters ε*, Aε, Bε, (Δε)o, λε, and T* showed excellent agreement with experimental data. The six-parameter dielectric permittivity model was compared to the conventional four-parameter refractive index model. To systematically investigate the temperature-dependent properties of the refractive indices (ne(T) and no(T)) and dielectric permittivities according to the molecular structure of the NLCs, four similar types of fluorinated phenyl bicyclohexane NLCs were selected. Using the presented models, the temperature-dependent behaviour of these four fluorinated NLCs was discussed, according to the molecular length of the alkyl chains and the positions of the fluorine substituents. In particular, two fitting equations for the temperature-dependent properties of threshold voltage and splay elastic LC constant could also be developed using the physical coefficients extracted from the six-parameter dielectric permittivity model, and these equations coincided well with experimental results.

Fast-switching optically isotropic liquid crystal nano-droplets with improved depolarization and Kerr effect by doping high k nanoparticles.

We proposed and analyzed an optically isotropic nano-droplet liquid crystal (LC) doped with high k nanoparticles (NPs), exhibiting enhanced Kerr effects, which could be operated with reduced driving voltages. For enhancing the contrast ratio together with the light efficiencies, the LC droplet sizes were adjusted to be shorter than the wavelength of visible light to reduce depolarization effects by optical scattering of the LC droplets. Based on the optical analysis of the depolarization effects, the influence of the relationship between the LC droplet size and the NP doping ratio on the Kerr effect change was investigated.

Enhancement of the depth-of-field of integral imaging microscope by using switchable bifocal liquid-crystalline polymer micro lens array.

An integral imaging microscopy (IIM) system with improved depth-of-field (DoF) using a custom-designed bifocal polarization-dependent liquid-crystalline polymer micro lens array (LCP-MLA) is proposed. The implemented MLA has improved electro-optical properties such as a small focal ratio, high fill factor, low driving voltage, and fast switching speed, utilizing a well-aligned reactive mesogen on the imprinted reverse shape of the lens and a polarization switching layer. A bifocal MLA switches its focal length according to the polarization angle and acquires different DoF information of the specimen. After two elemental image arrays are captured, the depth-slices are reconstructed and combined to provide a widened DoF. The fabricated bifocal MLA consists of two identical polarization-dependent LCP-MLAs with 1.6 mm and f/16 focal ratio. Our experimental results confirmed that the proposed system improves the DoF of IIM without the need for mechanical manipulation.

Bulk chirality effect for symmetric bistable switching of liquid crystals on topologically self-patterned degenerate anchoring surface.

We demonstrate a bistable switching liquid crystal (LC) mode utilizing a topologically self-structured dual-groove surface for degenerated easy axes of LC anchoring. In our study, the effect of the bulk elastic distortion of the LC directors on the bistable anchoring surface is theoretically analyzed for balanced bistable states based on a free energy diagram. By adjusting bulk LC chirality, we developed ideally symmetric and stable bistable anchoring and switching properties, which can be driven by a low in-plane pulsed field of about 0.7 V/µm. The fabricated device has a contrast ratio of 196:1.

A 3D Optical Surface Profilometer Using a Dual-Frequency Liquid Crystal-Based Dynamic Fringe Pattern Generator.

We propose a liquid crystal (LC)-based 3D optical surface profilometer that can utilize multiple fringe patterns to extract an enhanced 3D surface depth profile. To avoid the optical phase ambiguity and enhance the 3D depth extraction, 16 interference patterns were generated by the LC-based dynamic fringe pattern generator (DFPG) using four-step phase shifting and four-step spatial frequency varying schemes. The DFPG had one common slit with an electrically controllable birefringence (ECB) LC mode and four switching slits with a twisted nematic LC mode. The spatial frequency of the projected fringe pattern could be controlled by selecting one of the switching slits. In addition, moving fringe patterns were obtainable by applying voltages to the ECB LC layer, which varied the phase difference between the common and the selected switching slits. Notably, the DFPG switching time required to project 16 fringe patterns was minimized by utilizing the dual-frequency modulation of the driving waveform to switch the LC layers. We calculated the phase modulation of the DFPG and reconstructed the depth profile of 3D objects using a discrete Fourier transform method and geometric optical parameters.

Efficient exciton generation in atomic passivated CdSe/ZnS quantum dots light-emitting devices.

We demonstrate the first-ever surface modification of green CdSe/ZnS quantum dots (QDs) using bromide anions (Br-) in cetyl trimethylammonium bromide (CTAB). The Br- ions reduced the interparticle spacing between the QDs and induced an effective charge balance in QD light-emitting devices (QLEDs). The fabricated QLEDs exhibited efficient charge injection because of the reduced emission quenching effect and their enhanced thin film morphology. As a result, they exhibited a maximum luminance of 71,000 cd/m2 and an external current efficiency of 6.4 cd/A, both significantly better than those of their counterparts with oleic acid surface ligands. In addition, the lifetime of the Br- treated QD based QLEDs is significantly improved due to ionic passivation at the QDs surface.

Implementation of active-type Lamina 3D display system.

Lamina 3D display is a new type of multi-layer 3D display, which utilizes the polarization state as a new dimension of depth information. Lamina 3D display system has advanced properties - to reduce the data amount representing 3D image, to be easily made using the conventional projectors, and to have a potential being applied to the many applications. However, the system might have some limitations in depth range and viewing angle due to the properties of the expressive volume components. In this paper, we propose the volume using the layers of switchable diffusers to implement the active-type Lamina 3D display system. Because the diffusing rate of the layers has no relation with the polarization state, the polarizer wheel is applied to the proposed system in purpose of making the sectioned image synchronized with the diffusing layer at the designated location. The imaging volume of the proposed system consists of five layers of polymer dispersed liquid crystal and the total size of the implemented volume is 24x18x12 mm3(3). The proposed system can achieve the improvements of viewing qualities such as enhanced depth expression and widened viewing angle.

Transient flickering behavior in fringe-field switching liquid crystal mode analyzed by positional asymmetric flexoelectric dynamics.

We analyzed a transient blinking phenomenon in a fringe-field switching liquid crystal (LC) mode that occurred at the moment of frame change even in the optimized DC offset condition for minimum image flicker. Based on the positional dynamic behaviors of LCs by using a high-speed camera, we found that the transient blink is highly related to the asymmetric responses of the splay-bend transitions caused by the flexoelectric (FE) effect. To remove the transient blink, the elastic property adjustment of LCs was an effective solution because the FE switching dynamics between the splay-enhanced and bend-enhanced deformations are highly dependent on the elastic constants of LCs, which is the cause of momentary brightness drop.