Tuning the high-entropy perovskite as efficient and reliable electrocatalysts for oxygen evolution reaction.

Due to their unique electronic structure, atomic arrangement and synergistic effect, high-entropy materials are being actively pursued as electrocatalysts for oxygen evolution reaction (OER) in water splitting. However, a relevant strategy to improve high-entropy materials is still lacking. Herein, substitutional doping on the La-site in high-entropy perovskite La1-x Sr x (CrMnFeCoNi)0.2O3 is reported as an efficient OER catalyst. Sr doping is found to be crucial to enhancing the OER activity. The overpotential for the best catalyst La0.3Sr0.7(CrMnFeCoNi)0.2O3 is only 330 mV at 10 mA cm-2, achieving a reduction of 120 mV in overpotential compared to La(CrMnFeCoNi)0.2O3, which is attributed to the enhancement in intrinsic catalytic activity. Experimental evidences including in situ electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) indicate Sr doping induces the formation of high-valence Cr6+, Mn4+, Fe4+, Co4+ and Ni3+ species, which can accelerate the faster charge transfer at the interface, thereby increasing the intrinsic catalytic activity. The assembled two-electrode overall water splitting system operates stably at 10 mA cm-2 for 200 h without attenuation. This work offers an important method for developing a high-performance, high-entropy perovskite OER catalyst for hydrogen production by electrochemical water splitting.

Co-phase errors sensing method for Golay3 telescope system via a transfer network.

Optical sparse-aperture systems face severe challenges, including detecting and correcting co-phase errors. In this study, a search framework based on fine tuning a pre-trained network is proposed to analyze the co-phase errors of a Golay3 telescope system. Based on this, an error compensation control system is established. First, a hash-like binary code is created by fine-tuning the pre-trained model. Secondly, a pre-trained network is used to extract the deep features of the image, and an index database is built between the image features and the corresponding co-phase error values. Finally, the Top 1-ranked features and corresponding co-phase error values are returned through the hash-like binary code hierarchical deep search database to provide driving data for the error correction system. Numerical simulations and experimental results verify the method's validity. The experimental results show that the correction system works well when the dynamic piston is [-5,5]λ, and the tilt error range is [-15,15]µr a d. Compared with existing detection methods, this method does not require additional optical components, has a high correction accuracy, and requires a short training time. Furthermore, it can be used to detect piston and tilt errors simultaneously.

Efficient online real-time video stabilization with a novel least squares formulation and parallel AC-RANSAC.

A novel online real-time video stabilization algorithm (LSstab) that suppresses unwanted motion jitters based on cinematography principles is presented. LSstab features a parallel realization of the a-contrario RANSAC (AC-RANSAC) algorithm to estimate the inter-frame camera motion parameters. A novel least squares based smoothing cost function is then proposed to mitigate undesirable camera jitters according to cinematography principles. A recursive least square solver is derived to minimize the smoothing cost function with a linear computation complexity. LSstab is evaluated using a suite of publicly available videos against state-of-the-art video stabilization methods. Results show that LSstab achieves comparable or better performance, which attains real-time processing speed when a GPU is used.

Application of torque sensitivity in the alignment of a Cassegrain system.

In order to improve the alignment accuracy of a Cassegrain system, to the best of our knowledge, a novel computer-aided alignment method based on torque sensitivity is proposed. Different from the traditional position sensitivity curve guiding scheme, the accurate position of the secondary mirror is not necessary while the torque sensitivity curve is generated. By establishing the relationship between the torque of the secondary mirror setting screw and the Zernike coefficients of the system, a practical quantitative alignment scheme for the Cassegrain system can be realized. For a two-mirror Cassegrain optical-mechanical system, an alignment scheme based on torque sensitivity is designed. The results show that the wavefront aberrations of three Cassegrain systems reach 0.0479λ,0.0537λ, and 0.0698λ respectively. It proves that the torque sensitivity curves can well guide the real alignment process.

Towards real-time 3D visualization with multiview RGB camera array.

A real-time 3D visualization (RT3DV) system using a multiview RGB camera array is presented. RT3DV can process multiple synchronized video streams to produce a stereo video of a dynamic scene from a chosen view angle. Its design objective is to facilitate 3D visualization at the video frame rate with good viewing quality. To facilitate 3D vision, RT3DV estimates and updates a surface mesh model formed directly from a set of sparse key points. The 3D coordinates of these key points are estimated from matching 2D key points across multiview video streams with the aid of epipolar geometry and trifocal tensor. To capture the scene dynamics, 2D key points in individual video streams are tracked between successive frames. We implemented a proof of concept RT3DV system tasked to process five synchronous video streams acquired by an RGB camera array. It achieves a processing speed of 44 milliseconds per frame and a peak signal to noise ratio (PSNR) of 15.9 dB from a viewpoint coinciding with a reference view. As a comparison, an image-based MVS algorithm utilizing a dense point cloud model and frame by frame feature detection and matching will require 7 seconds to render a frame and yield a reference view PSNR of 16.3 dB.

Large-Field-of-View Visualization with Small Blind Spots Utilizing Tilted Micro-Camera Array for Laparoscopic Surgery.

Existing laparoscopic surgery systems use a single laparoscope to visualize the surgical area with a limited field of view (FoV), necessitating maneuvering the laparoscope to search a target region. In some cases, the laparoscope needs to be moved from one surgical port to another one to detect target organs. These maneuvers would cause longer surgical time and degrade the efficiency of operation. We hypothesize that if an array of cameras can be deployed to provide a stitched video with an expanded FoV and small blind spots, the time required to perform multiple tasks at different sites can be significantly reduced. We developed a micro-camera array that can enlarge the FoV and reduce blind spots between the cameras by optimizing the angle of cameras. The video stream of this micro-camera array was designed to be processed in real-time to provide a stitched video with the expanded FoV. We mounted this micro-camera array to a Fundamentals of Laparoscopic Surgery (FLS) laparoscopic trainer box and designed an experiment to validate the hypothesis above. Surgeons, residents, and a medical student were recruited to perform a modified bean drop task, and the completion time was compared against that measured using a traditional single-camera laparoscope. It was observed that utilizing the micro-camera array, the completion time of the modified bean drop task was 203 ± 55 s while using the laparoscope, the completion time was 245 ± 114 s, with a p-value of 0.00097. It is also observed that the benefit of using an FoV-expanded camera array does not diminish for subjects who are more experienced. This test provides convincing evidence and validates the hypothesis that expanded FoV with small blind spots can reduce the operation time for laparoscopic surgical tasks.

Biologically inspired artificial eyes and photonics.

Natural visual systems have inspired scientists and engineers to mimic their intriguing features for the development of advanced photonic devices that can provide better solutions than conventional ones. Among various kinds of natural eyes, researchers have had intensive interest in mammal eyes and compound eyes due to their advantages in optical properties such as focal length tunability, high-resolution imaging, light intensity modulation, wide field of view, high light sensitivity, and efficient light management. A variety of different approaches in the broad field of science and technology have been tried and succeeded to duplicate the functions of natural eyes and develop bioinspired photonic devices for various applications. In this review, we present a comprehensive overview of bioinspired artificial eyes and photonic devices that mimic functions of natural eyes. After we briefly introduce visual systems in nature, we discuss optical components inspired by the mammal eyes, including tunable lenses actuated with different mechanisms, curved image sensors with low aberration, and light intensity modulators. Next, compound eye inspired photonic devices are presented, such as microlenses and micromirror arrays, imaging sensor arrays on curved surfaces, self-written waveguides with microlens arrays, and antireflective nanostructures (ARS). Subsequently, compound eyes with focal length tunability, photosensitivity enhancers, and polarization imaging sensors are described.

Large-Field-of-View Visualization Utilizing Multiple Miniaturized Cameras for Laparoscopic Surgery.

The quality and the extent of intra-abdominal visualization are critical to a laparoscopic procedure. Currently, a single laparoscope is inserted into one of the laparoscopic ports to provide intra-abdominal visualization. The extent of this field of view (FoV) is rather restricted and may limit efficiency and the range of operations. Here we report a trocar-camera assembly (TCA) that promises a large FoV, and improved efficiency and range of operations. A video stitching program processes video data from multiple miniature cameras and combines these videos in real-time. This stitched video is then displayed on an operating monitor with a much larger FoV than that of a single camera. In addition, we successfully performed a standard and a modified bean drop task, without any distortion, in a simulator box by using the TCA and taking advantage of its FoV which is larger than that of the current laparoscopic cameras. We successfully demonstrated its improved efficiency and range of operations. The TCA frees up a surgical port and potentially eliminates the need of physical maneuvering of the laparoscopic camera, operated by an assistant.

Optimal Camera Pose and Placement Configuration for Maximum Field-of-View Video Stitching.

An optimal camera placement problem is investigated. The objective is to maximize the area of the field of view (FoV) of a stitched video obtained by stitching video streams from an array of cameras. The positions and poses of these cameras are restricted to a given set of selections. The camera array is designed to be placed inside the abdomen to support minimally invasive laparoscopic surgery. Hence, a few non-traditional requirements/constraints are imposed: Adjacent views are required to overlap to support image registration for seamless video stitching. The resulting effective FoV should be a contiguous region without any holes and should be a convex polygon. With these requirements, traditional camera placement algorithms cannot be directly applied to solve this problem. In this work, we show the complexity of this problem grows exponentially as a function of the problem size, and then present a greedy polynomial time heuristic solution that approximates well to the globally optimal solution. We present a new approach to directly evaluate the combined coverage area (area of FoV) as the union of a set of quadrilaterals. We also propose a graph-based approach to ensure the stitching requirement (overlap between adjacent views) is satisfied. We present a method to find a convex polygon with maximum area from a given polygon. Several design examples show that the proposed algorithm can achieve larger FoV area while using much less computing time.

Artificial eye for scotopic vision with bioinspired all-optical photosensitivity enhancer.

The ability to acquire images under low-light conditions is critical for many applications. However, to date, strategies toward improving low-light imaging primarily focus on developing electronic image sensors. Inspired by natural scotopic visual systems, we adopt an all-optical method to significantly improve the overall photosensitivity of imaging systems. Such optical approach is independent of, and can effectively circumvent the physical and material limitations of, the electronics imagers used. We demonstrate an artificial eye inspired by superposition compound eyes and the retinal structure of elephantnose fish. The bioinspired photosensitivity enhancer (BPE) that we have developed enhances the image intensity without consuming power, which is achieved by three-dimensional, omnidirectionally aligned microphotocollectors with parabolic reflective sidewalls. Our work opens up a previously unidentified direction toward achieving high photosensitivity in imaging systems.

High Performance All-Solid-State Flexible Micro-Pseudocapacitor Based on Hierarchically Nanostructured Tungsten Trioxide Composite.

Microsupercapacitors (MSCs) are promising energy storage devices to power miniaturized portable electronics and microelectromechanical systems. With the increasing attention on all-solid-state flexible supercapacitors, new strategies for high-performance flexible MSCs are highly desired. Here, we demonstrate all-solid-state, flexible micropseudocapacitors via direct laser patterning on crack-free, flexible WO3/polyvinylidene fluoride (PVDF)/multiwalled carbon nanotubes (MWCNTs) composites containing high levels of porous hierarchically structured WO3 nanomaterials (up to 50 wt %) and limited binder (PVDF, <25 wt %). The work leads to an areal capacitance of 62.4 mF·cm(-2) and a volumetric capacitance of 10.4 F·cm(-3), exceeding that of graphene based flexible MSCs by a factor of 26 and 3, respectively. As a noncarbon based flexible MSC, hierarchically nanostructured WO3 in the narrow finger electrode is essential to such enhancement in energy density due to its pseudocapacitive property. The effects of WO3/PVDF/MWCNTs composite composition and the dimensions of interdigital structure on the performance of the flexible MSCs are investigated.

One-step femtosecond laser patterning of light-trapping structure on dye-sensitized solar cell photoelectrodes†.

Light-trapping patterns were constructed in TiO2 photoelectrodes for dye-sensitized solar cells (DSSCs) by a one-step femtosecond laser structuring method that utilized ablation to create patterns at the surface of nanostructured TiO2 films. As a result, much more light was trapped in the photoelectrodes. Grating and orthogonal-grid patterns were studied, and the light trapping performance was optimized through the adjustment of pattern spacing, which was easily realized in the laser ablation process. With a 5-µm-spacing orthogonal-grid pattern, DSSCs showed a highest photon-to-electron conversion efficiency of 9.32% under AM 1.5G, a 13.5% improvement compared to the same cell without laser ablation. This simple and universal laser ablation method could be used to process many kinds of nanomaterials, and could be applied for various devices with nanostructures.

Large-field-of-view wide-spectrum artificial reflecting superposition compound eyes.

In nature, reflecting superposition compound eyes (RSCEs) found in shrimps, lobsters and some other decapods are extraordinary imaging systems with numerous optical features such as minimum chromatic aberration, wide-angle field of view (FOV), high sensitivity to light and superb acuity to motion. Here, we present life-sized, large-FOV, wide-spectrum artificial RSCEs as optical imaging devices inspired by the unique designs of their natural counterparts. Our devices can form real, clear images based on reflection rather than refraction, hence avoiding chromatic aberration due to dispersion by the optical materials. Compared to imaging at visible wavelengths using conventional refractive lenses of comparable size, our artificial RSCEs demonstrate minimum chromatic aberration, exceptional FOV up to 165° without distortion, modest aberrations and comparable imaging quality without any post-image processing. Together with an augmenting cruciform pattern surrounding each focused image, our large-FOV, wide-spectrum artificial RSCEs possess enhanced motion-tracking capability ideal for diverse applications in military, security, medical imaging and astronomy.

Scalable shape-controlled fabrication of curved microstructures using a femtosecond laser wet-etching process.

Materials with curvilinear surface microstructures are highly desirable for micro-optical and biomedical devices. However, realization of such devices efficiently remains technically challenging. This paper demonstrates a facile and flexible method to fabricate curvilinear microstructures with controllable shapes and dimensions. The method composes of femtosecond laser exposures and chemical etching process with the hydrofluoric acid solutions. By fixed-point and step-in laser irradiations followed by the chemical treatments, concave microstructures with different profiles such as spherical, conical, bell-like and parabola were fabricated on silica glasses. The convex structures were replicated on polymers by the casting replication process. In this work, we used this technique to fabricate high-quality microlens arrays and high-aspect-ratio microwells which can be used in 3D cell culture. This approach offers several advantages such as high-efficient, scalable shape-controllable and easy manipulations.

A facile preparation route for netlike microstructures on a stainless steel using an ethanol-mediated femtosecond laser irradiation.

Netlike or porous microstructures are highly desirable in metal implants and biomedical monitoring applications. However, realization of such microstructures remains technically challenging. Here, we report a facile and environmentally friendly method to prepare netlike microstructures on a stainless steel by taking the full advantage of the liquid-mediated femtosecond laser ablation. An unordered netlike structure and a quasi-ordered array of holes can be fabricated on the surface of stainless steel via an ethanol-mediated femtosecond laser line-scan method. SEM analysis of the surface morphology indicates that the porous netlike structure is in the micrometer scale and the diameter of the quasi-ordered holes ranges from 280 nm to 320 nm. Besides, we find that the obtained structures are tunable by altering the laser processing parameters especially scanning speed.

Direct fabrication of seamless roller molds with gapless and shaped-controlled concave microlens arrays.

This Letter demonstrates the direct fabrication of gapless concave microlenses on glass cylinders, which can be used as seamless roller molds for the continuous imprinting of large-area microlens arrays. The method involves femtosecond laser exposures followed by a chemical wet-etching process. A honeycomb-like concave microlens array was fabricated on a glass cylinder with a diameter of 3 mm. We demonstrated the flexibility of the method in tuning the shape and depth of the concave structures by the arrangements of the laser exposure spots and laser powers, and examined the replicating ability of the roller mold by the polymer castling method.

Fabrication of three-dimensional helical microchannels with arbitrary length and uniform diameter inside fused silica.

We demonstrate an improved femtosecond laser irradiation followed by chemical etching process to create complex three-dimensional (3D) microchannels with arbitrary length and uniform diameter inside fused silica. A segmented chemical etching method of introducing extra access ports and a secondary power compensation is presented, which enables the fabrication of uniform 3D helical microchannels with length of 1.140 cm and aspect-ratio of 522. Based on this method, a micromixer which consists of a long helical microchannel and a y-tape microchannel was created inside the fused silica. We measured the mixing properties of the micromixer by injecting the phenolphthalein and NaOH solution through the two inlets of the y-tape microchannel. A rapid and efficient mixing was achieved in the 3D micromixer at a low Reynolds number.

Versatile route to gapless microlens arrays using laser-tunable wet-etched curved surfaces.

This work reveals a cost-efficient and flexible approach to various microlens arrays on polymers, which is essential to micro-optics elements. An 800-nm femtosecond laser is employed to control the hydrofluoric (HF) acid etching process on silica glasses, and concave microstructures with smooth curved surfaces are produced by this method. Then, the micro-structured glass templates can serve as molds for replicating microlenses on polymers. In this paper, a high-ordered microlens array with over 16,000 hexagonal-shaped lenses is fabricated on poly (dimethyl siloxane) [PDMS], and its perfect light-gathering ability and imaging performance are demonstrated. The flexibility of this method is demonstrated by successful preparation of several concave molds with different patterns which are difficult to be obtained by other methods. This technique provides a new route to small-scaled, smooth and curved surfaces which is widely used in micro-optics, biochemical analysis and superhydrophobic interface.

A simple route to fabricate artificial compound eye structures.

A biologically inspired compound-eye structure, which composes of ~5,867 honeycomb-patterned microlenses, was fabricated on a hemispherical shell. The fabrication process was simple and low-cost, which involves a femtosecond laser-enhanced wet etching and casting process followed by a thermomechanical process to convert the film into a hemispherical surface. By optimizing the parameters of thermomechanical process to form the curvilinear surface, the experimental result shows that the microlenses are omnidirectionally aligned on the dome with lens diameters of ~85 µm and the angle between two lens of ~2°, and the individual microlenses have rudimentary focusing and imaging properties. The artificial compound-eye structure fabricated by this method has great potential applications in scale-invariant processing, robot vision, and fast motion detection.

Maskless fabrication of concave microlens arrays on silica glasses by a femtosecond-laser-enhanced local wet etching method.

A simple and efficient technique for large-area manufacturing of concave microlens arrays (MLAs) on silica glasses with femtosecond (fs)-laser-enhanced chemical wet etching is demonstrated. By means of fs laser in situ irradiations followed by the hydrofluoric acid etching process, large area close-packed rectangular and hexagonal concave MLAs with diameters less than a hundred of micrometers are fabricated within a few hours. The fabricated MLAs exhibit excellent surface quality and uniformity. In contrast to the classic thermal reflow process, the presented technique is a maskless process and allows the flexible control of the size, shape and the packing pattern of the MLAs by adjusting the parameters such as the pulse energy, the number of shots and etching time.