Hewei Liu

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.

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.

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

Significance Although biological eyes ingeniously adopt diverse optical approaches to improve their scotopic vision, enhancement of the photosensitivity in artificial imaging systems still clings to electronic methods. Here we present an all-optical strategy to significantly improve the low-light imaging capability of manmade sensors, which is inspired by the optical concept of superposition eyes and elephantnose fish eye. Besides showing an artificial eye whose scotopic vision is largely improved by a bioinspired photosensitivity enhancer, we also demonstrate a complete solution to acquire high-resolution images under low-light conditions with our device. More importantly, our purely optical approach can be used on top of other electronic technologies, which can boost the most state-of-the-art imaging sensor whose photosensitivity is gaining on the physical limitations. 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.

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.

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.

Fiber-Optic Biological/Chemical Sensing System Based on Degradable Hydrogel

To improve the sensitivity and shorten the testing time of hydrogel-based bio-toxin sensors, we introduce an optical fiber Fabry–Perot interferometer (FPI) to detect the changes in the optical properties induced by the reactions between the target agent and the hydrogel. The concept is demonstrated by a polyacrylamide hydrogel filled into a cavity of an optical fiber FPI with a length of 150

Sub-10-nanometer-scale laser ablation on hard materials from dielectric near-field nanophotonics

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Bioinspired broad-spectrum micro-photocollectors improve sensitivity of image sensors in low-light environment

. Dithiothreitol (DTT) solutions with different concentrations were used as target agent. The degradation of the hydrogel by the DTT solution leads to a long and indistinct change in optical properties, which is difficult to be observed by conventional microscopy methods, but which can be detected by measuring the unique shifting process of the interfering spectrum caused by the hydrogel cleavage in the FPI cavity. Compared to our previous hydrogel-based sensor based on the microscopy observation, the sensitivity of the optic fiber FPI is improved by 2000 times, and the testing time is shortened from hundreds of hours to a few hours. Our approach opens up an avenue for highly sensitive, high-speed in-field detection of bio-toxins in live samples.

Fabrication of through-wafer 3D microfluidics in silicon carbide using femtosecond laser

This paper demonstrates sub-10-nanometer (nm) laser ablation on hard materials. An 800-nm-wavelength femtosecond laser is focused down to sub-diffraction-limited scale by a silicon nanophotonic structure fabricated on a silicon-on-isolator (SOI) substrate, which is utilized to enable the ablation on the silicon dioxide (SiO2) layer of the SOI. Atomic force microscopy (AFM) results show a minimum ablation linewidth of about 8 nm with a depth-to-width ratio close to 1.