An ultrawide field-of-view pinhole compound eye using hemispherical nanowire array for robot vision.

Garnering inspiration from biological compound eyes, artificial vision systems boasting a vivid range of diverse visual functional traits have come to the fore recently. However, most of these artificial systems rely on transformable electronics, which suffer from the complexity and constrained geometry of global deformation, as well as potential mismatches between optical and detector units. Here, we present a unique pinhole compound eye that combines a three-dimensionally printed honeycomb optical structure with a hemispherical, all-solid-state, high-density perovskite nanowire photodetector array. The lens-free pinhole structure can be designed and fabricated with an arbitrary layout to match the underlying image sensor. Optical simulations and imaging results matched well with each other and substantiated the key characteristics and capabilities of our system, which include an ultrawide field of view, accurate target positioning, and motion tracking function. We further demonstrate the potential of our unique compound eye for advanced robotic vision by successfully completing a moving target tracking mission.

Identifying the role of carrier overflow and injection current efficiency in a GaN-based micro-LED efficiency droop model.

In this paper, we investigate the efficiency droop phenomenon in green and blue GaN-based micro-LEDs of various sizes. We discuss the distinct carrier overflow performance in green and blue devices by examining the doping profile extracted from capacitance-voltage characterization. By combining the size-dependent external quantum efficiency with the ABC model, we demonstrate the injection current efficiency droop. Furthermore, we observe that the efficiency droop is induced by injection current efficiency droop, with green micro-LEDs exhibiting a more pronounced droop due to more severe carrier overflow compared to blue micro-LEDs.

Design of a high-speed circular polarization converter with a large field of view and wavelength range.

A high-speed circular polarization converter (CPC) with a wide field of view (FOV) and wavelength range is designed and fabricated in this paper. The multi-waveplate combined structure is applied to constitute the basic configuration of the CPC for broadening the wavelength range. An electrically suppressed helix ferroelectric liquid crystal (ESHFLC) material with fast response is used as a medium for dynamic polarization operation. The compensation films are used to expand the FOV by attaching to the configuration. The simulation results demonstrate that the optimized CPC structure can achieve over 97% orthogonal circular polarization conversion efficiency in 300 nm bandwidth at a 90° viewing cone for both working states. Finally, we have experiments and the results show well consistency with the theoretical results.

Flexible and wide-range tuning of liquid crystal polarization grating period based on single and interference-free exposure.

We propose a period control method of liquid crystal polarization grating (LCPG) based on an nterference-free and single exposure process. By adjusting three parameters of exposure setup, including incident angle of exposure beam, wedge angle of birefringent prism and tilt angle of the sample, polarization distribution of the exposure beam is changed. The spatially variant polarization of the exposure beam is transferred to liquid crystal (LC) molecules by an azo-dye photo-sensitive layer. Consequently, the LCPG with the target period is obtained. The proposed method has high flexibility and a wide range of period adjustment covering several microns to more than thousands of microns according to calculated results. Experimental results fit well with calculations. The LCPGs with different values of period from 4.5µm to more than 200µm have been realized experimentally. The proposed interference-free method would accelerate the application of LCPGs with a robust and simple fabrication process.

Lattice strain modulation toward efficient blue perovskite light-emitting diodes.

The successful implementation of perovskite light-emitting diodes (PeLEDs) in advanced displays and lighting has proven to be challenging because of the inferior performance of blue devices. Here, we point out that a strained system would lead to the quasi-degenerate energy state to enhance the excited-state transition due to the formation of double-polarized transition channel. The tensile strained structure also brings about a synergetic control of the carrier dynamics in virtue of lattice structure deformation and reduced dimensional phase regulation to promote carrier population in large bandgap domains and to realize near-unit energy transfer from the large bandgap phases to the emitter phases. Accordingly, high external quantum efficiencies of 14.71 and 10.11% are achieved for the 488- and 483-nanometer PeLEDs. This work represents a versatile strategy using a strained system to achieve enhanced radiative emission for the development of efficient PeLEDs.

Ionic liquid multistate resistive switching characteristics in two terminal soft and flexible discrete channels for neuromorphic computing.

By exploiting ion transport phenomena in a soft and flexible discrete channel, liquid material conductance can be controlled by using an electrical input signal, which results in analog neuromorphic behavior. This paper proposes an ionic liquid (IL) multistate resistive switching device capable of mimicking synapse analog behavior by using IL BMIM FeCL4 and H2O into the two ends of a discrete polydimethylsiloxane (PDMS) channel. The spike rate-dependent plasticity (SRDP) and spike-timing-dependent plasticity (STDP) behavior are highly stable by modulating the input signal. Furthermore, the discrete channel device presents highly durable performance under mechanical bending and stretching. Using the obtained parameters from the proposed ionic liquid-based synaptic device, convolutional neural network simulation runs to an image recognition task, reaching an accuracy of 84%. The bending test of a device opens a new gateway for the future of soft and flexible brain-inspired neuromorphic computing systems for various shaped artificial intelligence applications.

Compact Integration of Hydrogen-Resistant a-InGaZnO and Poly-Si Thin-Film Transistors.

The low-temperature poly-Si oxide (LTPO) backplane is realized by monolithically integrating low-temperature poly-Si (LTPS) and amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) in the same display backplane. The LTPO-enabled dynamic refreshing rate can significantly reduce the display's power consumption. However, the essential hydrogenation of LTPS would seriously deteriorate AOS TFTs by increasing the population of channel defects and carriers. Hydrogen (H) diffusion barriers were comparatively investigated to reduce the H content in amorphous indium-gallium-zinc oxide (a-IGZO). Moreover, the intrinsic H-resistance of a-IGZO was impressively enhanced by plasma treatments, such as fluorine and nitrous oxide. Enabled by the suppressed H conflict, a novel AOS/LTPS integration structure was tested by directly stacking the H-resistant a-IGZO on poly-Si TFT, dubbed metal-oxide-on-Si (MOOS). The noticeably shrunken layout footprint could support much higher resolution and pixel density for next-generation displays, especially AR and VR displays. Compared to the conventional LTPO circuits, the more compact MOOS circuits exhibited similar characteristics.

Stokes Shift in Inorganic Lead Halide Perovskites: Current Status and Perspective.

Inorganic metal halide perovskite system is considered as a promising candidate for applications from display to biomedical industry. Intrinsic inorganic lead halides possess small Stokes shift or self-absorption, providing negative impact for both photo voltaic and biomedical applications. Therefore, the development of an inorganic halide perovskite system with large Stokes shift is a significant venture. This review aims to provide an updated survey of the Stokes shift phenomena in the inorganic lead halide perovskites. The first section focuses about the mechanism, the second section gives different approaches in preparing inorganic perovskites with distinct Stokes shift, while the third section highlights the potential applications in both photovoltaic and biomedical areas. This review provides deep insight about the importance and usefulness of such phenomena in inorganic lead halides, essential for various applications.

Investigation of Enhanced Ambient Contrast Ratio in Novel Micro/Mini-LED Displays.

In recent years, ambient contrast ratio (ACR) has become very critical for advanced outdoor displays, including transparent displays, portable displays, and so on. In this work, the ACR of typical flat panel displays was introduced, while LED-based displays showed distinctive advantages. Micro-LED displays with a different pitch of 10 µm, 15 µm, 30 µm, and 60 µm were fabricated and characterized. Various mini-LED and micro-LED panels were systematically investigated in the aspect of brightness, reflection phenomenon, and ACR to reveal their enormous potential for outdoor applications. Through a series of experiments and comparisons, three methods were proposed to further improve the ACR of LED-based panels, including optical method, antireflection coating, and structure optimization.

Ferroelectric liquid crystal Pancharatnam-Berry lens with a fast control of output light's polarization-handedness.

We report the ferroelectric liquid crystal (FLC) Pancharatnam-Berry lenses (PBLs) with rapid transmittance tunability. The FLC PBLs were fabricated using a single-step holographic exposure system based on a spatial light modulator working as numerous polarization retarders, providing a simple way to fabricate FLC continuous aligning structures. A state-selection sector containing a binary FLC switch was utilized for fast changing input light's polarization handedness. Thus, when light passes through a FLC PBL, the output light's polarization handedness can be switched accordingly. In this case, FLC PBLs can function as concave/convex lenses with rapidly switching speed. Photo sensitive azo-dye material was used as the aligning layer for both FLC PBLs and FLC switches. The fabricated FLC PBLs and the FLC switches show fast switching-on times of 150µs and 50µs respectively. The FLC PBLs combining with the state-selection sector can have potential applications on varies displays and augmented reality.

Humidity sensor based on Gallium Nitride for real time monitoring applications.

Gallium Nitride (GaN) remarkably shows high electron mobility, wide energy band gap, biocompatibility, and chemical stability. Wurtzite structure makes topmost Gallium atoms electropositive, hence high ligand binding ability especially to anions, making it usable as humidity sensor due to water self-ionization phenomenon. In this work, thin-film GaN based humidity sensor is fabricated through pulse modulated DC magnetron sputtering. Interdigitated electrodes (IDEs) with 100 µm width and spacing were inkjet printed on top of GaN sensing layer to further enhance sensor sensitivity. Impedance, capacitance, and current response were recorded for humidity and bio-sensing applications. The sensor shows approximate linear impedance response between 0 and 100% humidity range, the sensitivity of 8.53 nF/RH% and 79 kΩ/RH% for capacitance and impedance, and fast response (Tres) and recovery (Trec) time of 3.5 s and 9 s, respectively. The sensor shows little hysteresis of < 3.53% with stable and wide variations for accurate measurements. Especially, it demonstrates temperature invariance for thermal stability. Experimental results demonstrate fabricated sensor effectively evaluates plant transpiration cycle through water level monitoring by direct attachment onto leaves without causing any damage as well as freshness level of meat loaf. These properties of the proposed sensor make it a suitable candidate for future electronics providing a low-cost platform for real time monitoring applications.

Fast-switchable, high diffraction-efficiency ferroelectric liquid crystal Fibonacci grating.

Low-voltage fast switchable 1D and 2D Fibonacci grating (FbG), using an electrically suppressed helix ferroelectric liquid crystal (ESHFLC), with high diffraction efficiency for a super-resolution imaging system in far-field are disclosed in this paper. Specifically, the polarization-independent two-domain (0, π) structure is well designed based on photoalignment technology to maximize the total diffraction efficiency that can reach 97.4% (1st order:8.5%, 2nd order: 30%). Apart from that, the FLC gratings offer two tunable states: non-diffractive and diffractive states. Derived from the fast-response property of ferroelectric liquid crystal material, the switching speed of the 1D and 2D ESHFLC-FbG is 103µs at 4 V of the driving voltage. Furthermore, this system achieves the high-resolving power of (λ/2.25) for object detection based upon the intensity map received behind 1D ESHFLC-FbG at far-field. Contribution from the quasi-periodic FbG's special ability to translate the super-resolution information (including at evanescent wave) into the detectable far-field region. Concisely, the proposed ESHFLC-FbG can be a promising candidate for a super-resolution imaging system, superstructure fibre sensor, and other photonic applications.

Fast refocusing lens based on ferroelectric liquid crystals.

Optical devices like virtual reality (VR) headsets present challenges in terms of vergence-accommodation conflict that leads to visual fatigue for the user over time. Lenses available to meet these challenges include liquid crystal (LC) lenses, which possess a response time in the millisecond range. This response time is slow, while accessing multiple focal lengths. A ferroelectric liquid crystal (FLC) has a response time in the microsecond range. In this article, we disclose a switchable lens device having a combination of the fast FLC-based polarization rotation unit and a passive polarization-dependent LC lens. A cascaded combination of three such lens units allows access to eight different focal points quite rapidly and can be a convenient device for VR applications.

Tribo-Induced Smart Reflector for Ultrasensitive Self-Powered Wireless Sensing of Air Flow.

Air-flow sensing is essential in broad applications of weather forecasting, ocean monitoring, gas leakage alarming, and health monitoring. However, in severe environments where electrical power supply and cable connection are not available, the sensing of air flow in a self-powered way is a challenging issue. In this work, we reported a tribo-induced smart reflector to achieve the self-powered wireless sensing of the air flow by combining an aerodynamics-driven triboelectric nanogenerator (TENG) and a silver-coated polymer network liquid crystal. Upon being driven by the air flow, the developed reflector performed specular and diffused reflectance without and with charging by the TENG, respectively, enabling wireless sensing through mechanical-electrical-optical signal conversion. In the developed sensing paradigm, the sensing module can be fully self-powered without the need of signal pre-amplification, which is electrically separated from the light source and detection modules without cable connections. The applications of self-powered wireless wind speed sensing and breath monitoring were performed to demonstrate the effectiveness of the developed paradigm toward self-powered wireless sensing nodes in the internet of things.

Voltage-controlled liquid crystal Pancharatnam-Berry phase lens with broadband operation and high photo-stability.

Pancharatnam-Berry phase optical elements (PBOEs) have received much attention due to their ability to generate complex structured light or to manipulate the shape of a light beam. This work demonstrates a tunable liquid crystal (LC) Pancharatnam-Berry (LCPB) lens using a simple and cost-effective PB phase hologram optical setup and thermal polymerization to form an irreversible photo-patterning alignment layer. The LCPB lens with high photo-stability supports ultra-broadband operation and provides a diffraction efficiency of ∼90% throughout the visible spectral range, achieved by applying the appropriate voltages. The LCPB lens functions as a convex or a concave lens, depending on the handedness of the circularly polarized incident light, so its image reduction and magnification functions are demonstrated, and its photo-stability is characterized. The fabrication of the proposed LC PBOEs is simpler and more cost-effective than previous methods, and the irreversible photo-patterning alignment layer that is formed by thermal polymerization allows larger operational bandwidths, supporting new applications.

Introducing Ion Migration and Light-Induced Secondary Ion Redistribution for Phase-Stable and High-Efficiency Inorganic Perovskite Solar Cells.

Inorganic halide perovskites have been demonstrated as a promising alternative for light absorption because of their improved thermal stability compared with organic-inorganic halide perovskites. However, low power conversion efficiency and phase instability are major hindrances to their application. Here, a straightforward approach, by adding a layer of CsBr on the top of CsPbI3, is reported for high-efficiency and phase-stable CsPbI3-based solar cells. Characterizations demonstrate that the bromide ion can migrate from the surface into the bulk of CsPbI3, mitigating the nonuniform depth distribution of iodide in the CsPbI3 absorber and passivating the bulk defects. Impressively, the light illumination can induce secondary-ion redistribution, which is identified as a crucial process to further enhance the carrier extraction efficiency, strengthen the lattice stability, and improve the film homogenization. Accordingly, a high efficiency of 17% is obtained for the CsPbI3-based solar cell. Moreover, the unencapsulated device exhibits remarkable phase stability, maintaining 93% of its initial efficiency under room temperature after being stored in the nitrogen glovebox for over 5000 h.

Low-driving-voltage, polarizer-free, scattering-controllable liquid crystal device based on randomly patterned photo-alignment.

In this work, a polarizer-free, electrically tunable liquid crystal device is presented. This device is based on randomly patterned photo-aligned boundaries generating light scattering in an adjacent liquid crystal film. Its transparent on state requires only 7.5Vrms driving voltage. Switching from a 49.5% hazy off state into the transparent on state occurs at 1.25 V, with only 1.2% residual haze. The effect exhibits fast rise and decay times of 0.3 ms and 7.2 ms, respectively. Thanks to its field-effect nature and the surface preparation process, zero ohmic low power consumption, fast response times, and steep transmission-voltage characteristics result. Applications are smart windows, light shutters, and/or transparent displays, as well as complementary metal-oxide-semiconductor driven multiplexed displays.

Investigation of Electrical Properties and Reliability of GaN-Based Micro-LEDs.

In this paper, we report high-performance Micro-LEDs on sapphire substrates, with pixel size scaling to 20 µm and an ultra-high current density of 9902 A/cm2. The forward voltages (VF) of the devices ranged from 2.32 V to 2.39 V under an injection current density of 10 A/cm2. The size and structure-dependent effects were subsequently investigated to optimize the device design. The reliability of Micro-LED devices was evaluated under long-aging, high-temperature, and high-humidity conditions. It was found that Micro-LED devices can maintain comparable performance with an emission wavelength of about 445 nm and a full width at half maximum (FWHM) of 22 nm under extreme environments. Following this, specific analysis with four detailed factors of forward voltage, forward current, slope, and leakage current was carried out in order to show the influence of the different environments on different aspects of the devices.

Normally Transparent Tribo-Induced Smart Window.

Self-powered smart windows are desirable with the expectations of their energy-saving, weather-independent, user-controllable, and miniature performance. Recently developed solar- or thermal-powered smart windows largely depend on the weather conditions and have an extremely slow response, and only a certain portion of the saved energy can be utilized by the external circuit for mode conversion. In this work, a self-powered normally transparent smart window was developed by the conjunction of a rotary freestanding sliding triboelectric nanogenerator (RFS-TENG) and a polymer network liquid crystal (PNLC) cell. To fabricate the PNLC cell, the alignment layer with randomly distributed microdomains was constructed to encapsulate a mixture of LC polymers and nematic LCs. The opacity of the smart window exposed to an alternating electric field was considerably improved owing to the embedded microdomains and a dense web of LC polymers. The ultrahigh haziness greatly alleviates the charge density required for the LC actuation and thus enables the driving by the TENG where the charge amount is usually limited. The RFS-TENG was elaborately designed with six periodic bent triboelectric films and Ag electrodes, which presented an ultralow friction wear and met the frequency requirement to achieve the steady opacity. By harvesting the mechanical energies from ambient environments, the tribo-induced smart window can benefit a wide variety of fields, such as self-powered sunroofs, wind-driven smart farming systems etc.

Interference-free and single exposure to generate continuous cycloidal alignment for large-area liquid crystal devices.

An approach for generating cycloidal pattern of liquid crystal (LC) molecules based on interference-free and single exposure is illustrated. The spatial manipulation of polarization state is achieved using birefringent prism and wave plates. And then, the spatially variant polarization of exposure beam is transferred to LC molecules by azo-dye photo-sensitive layer. Consequently, the LC samples fabricated shows periodically cycloidal texture and diffraction efficiency more than 99%. The measured period Λ and diffraction angle are in good consistency with theoretical results. Thus, this exposure method provides an effective and robust way for fabricating large-area LC elements, therefore paving the way for widespread applications of high-performance diffractive LC devices.