Highly sensitive plasmonic sensor based on eccentric-core photonic crystal fibers.

To further reduce the fabrication difficulty of optical fiber sensors and improve the sensing performance, this study introduced the surface plasmon resonance (SPR) effect into optical fiber sensing technology and designed an eccentric-core photonic crystal fiber (EC-PCF). We investigated the characteristics of the two fundamental modes in the fiber core and the surface plasmon polariton (SPP) modes on the surface of the gold film. We also investigated the influence of the structural parameters, such as gold film coating area and thickness, air hole diameter, and eccentricity, on the confinement loss and achieved a refractive index (RI) sensitivity of 31.25 µm RIU-1 in the RI range of 1.29-1.43, corresponding to a figure of merit (FOM) of 521.6 per RIU. When the resolution of the optical spectrum analyzer was 0.1 nm, the EC-PCF could achieve a refractive index resolution of 3.2 × 10-6 RIU. Moreover, we performed tests with two typical sensing types, one in which the sensor was directly in contact with adulterated gasoline to achieve kerosene-concentration detection, and another in which the sensor was coated with a layer of polydimethylsiloxane (PDMS), whose RI is sensitive to the temperature field, to achieve temperature sensing. The EC-PCF demonstrated excellent sensing performance and offers obvious manufacturing advantages, providing a new and easily fabricated structural design idea for optical fiber sensing.

Dynamically changeable terahertz metamaterial absorbers with intelligent switch and high sensitivity and wide and narrow band perfect absorption.

In this work, we theoretically designed a dynamically changeable terahertz metamaterial absorber with intelligent switch and high sensitivity, wide band and narrow band perfect absorption based on the combination of Dirac semimetal (BDS) and vanadium dioxide (VO2). It features two methods for absorption adjustment: altering the Fermi energy level of BDS to modify the resonant frequency of the absorption peaks and utilizing the phase change of VO2 to regulate the absorption rate of the peaks. In addition, its rotational symmetric design ensures strong polarization-insensitivity. The simulation results demonstrate the presence of two narrowband absorption peaks and one mini-broadband absorption peak within the frequency range of 6.0-9.5 THz, all with absorption rates exceeding 90%. We provide an explanation of the absorption mechanism of the device, employing the relative impedance theory and localized surface plasmon resonance to analyze its electric field distribution. We also defined the refractive index sensitivity (S), which is SI = 378 GHz per RIU and SIII = 204 GHz per RIU. Our device possesses high sensitivity and two methods of adjusting absorption modes, which endow it with advantages in the fields of metamaterial absorbers, intelligent switch, and optical sensors.

Learning for a Robot: Deep Reinforcement Learning, Imitation Learning, Transfer Learning.

Dexterous manipulation of the robot is an important part of realizing intelligence, but manipulators can only perform simple tasks such as sorting and packing in a structured environment. In view of the existing problem, this paper presents a state-of-the-art survey on an intelligent robot with the capability of autonomous deciding and learning. The paper first reviews the main achievements and research of the robot, which were mainly based on the breakthrough of automatic control and hardware in mechanics. With the evolution of artificial intelligence, many pieces of research have made further progresses in adaptive and robust control. The survey reveals that the latest research in deep learning and reinforcement learning has paved the way for highly complex tasks to be performed by robots. Furthermore, deep reinforcement learning, imitation learning, and transfer learning in robot control are discussed in detail. Finally, major achievements based on these methods are summarized and analyzed thoroughly, and future research challenges are proposed.

Illumination-Invariant Feature Point Detection Based on Neighborhood Information.

Feature point detection is the basis of computer vision, and the detection methods with geometric invariance and illumination invariance are the key and difficult problem in the field of feature detection. This paper proposes an illumination-invariant feature point detection method based on neighborhood information. The method can be summarized into two steps. Firstly, the feature points are divided into eight types according to the number of connected neighbors. Secondly, each type of feature points is classified again according to the position distribution of neighboring pixels. The theoretical deduction proves that the proposed method has lower computational complexity than other methods. The experimental results indicate that, when the photometric variation of the two images is very large, the feature-based detection methods are usually inferior, while the learning-based detection methods performs better. However, our method performs better than the learning-based detection method in terms of the number of feature points, the number of matching points, and the repeatability rate stability. The experimental results demonstrate that the proposed method has the best illumination robustness among state-of-the-art feature detection methods.

Tunable bandwidth terahertz perfect absorption device based on vanadium dioxide phase transition control.

Utilizing the phase transition principle of VO2, this paper presents a tunable ultra-wideband terahertz perfect absorption device with simple structure and tunability. The proposed broadband terahertz perfect absorption device is a three-layer structure with a metal reflective layer, a silicon dioxide dielectric layer and a VO2 layer from bottom to top. It was found that the terahertz perfect absorption device's absorption could be dynamically adjusted from 1.2% to 99.9% when changing from an insulated to a metallic state. With the VO2 in the metallic state, the terahertz perfect absorption device has an absorption efficiency of more than 90% in 4.00 to 10.08 THz's ultra-broadband range and near-perfect absorption is achieved in the ranges of 4.71 THz to 5.16 THz and 7.74 THz to 8.06 THz. To explain the working principle of this terahertz perfect absorption device, this paper utilizes wave interference's principle, theory of impedance matching and electric field analysis. Compared to previously reported terahertz metamaterial devices, the vanadium dioxide device proposed in this paper is significantly optimized in terms of tunable range and absorption bandwidth. In addition, the terahertz perfect absorption device is polarization insensitive and maintains good absorptivity over a wide-angle incidence range. This tunable ultra-wideband terahertz perfect absorption device could have applications in the fields of modulation, stealth devices, and thermal emission devices.

Active thermally tunable and highly sensitive terahertz smart windows based on the combination of a metamaterial and phase change material.

A thermally tunable terahertz window based on the combination of a metamaterial and the phase change material VO2 is proposed. The window is composed of two vanadium oxide films with a SiO2 layer sandwiched between them. The thermochromic phase change properties of VO2 are the key to the functionality of the window. By controlling the temperature around the room temperature of 300 K, our material can be used as a smart window and it is able to regulate both the absorption and transmission of external terahertz waves in response to changes in temperature. The absorbance can be regulated by more than 90% and the transmittance by more than 80%. The switching characteristics of the window are explained by the insulator-metal transition that vanadium oxide undergoes during the heating process, while localized surface plasmon resonance explains the perfect absorption. In addition, the designed window is not only insensitive to polarised waves, but is thermally flexible and maintains excellent performance over a wide angular range of 0° to 40°. This design will have significant potential for applications in stealth technologies, thermal sensing and switching, and terahertz energy harvesting.

Comparative Study on the Generation and Characteristics of Debris Induced by Fretting and Sliding.

Objectives: The aim of the present work was to comparatively investigate the generation and characteristics of fretting and sliding wear debris produced by CuNiAl against 42CrMo4. Methods: Tribological tests were conducted employing a self-developed tribometer. Most experimental conditions were set the same except for the amplitudes and number of cycles. Morphological, chemical, microstructural and dimensional features of the worn area and debris were investigated using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and a laser particle sizer. Outcomes: Not only wear scar profiles but also the wear debris color, distribution and generated amount under fretting and sliding wear modes were quite different, which can be attributed to the significant difference in wear mechanisms. Particle size analysis indicates that the fretting debris has a smaller size distribution range; the biggest detected fretting and sliding wear debris sizes were 141 µm and 355 µm, respectively. Both fretting and sliding debris are mainly composed of copper and its oxides, but the former shows a higher oxidation degree.

Numerical and Experimental Investigations of Micro Thermal Performance in a Tube with Delta Winglet Pairs.

In this research, a novel vortex generator (VG) is presented. The experimental and numerical investigations were carried out to study the micro thermal-hydraulic performance in a heated tube. The numerical results showed that the fluid in the core flow region and the near-wall region was fully mixed because of the longitudinal vortices created by the vortex generators. In addition, the experimental results showed that the heat transfer coefficient (h) decreased with the increasing pitch ratio (PR) value, while the friction coefficient exhibited the opposite trend. With the increasing ration angle (RA) numbers, the h values decreased while the f numbers increased. In addition, the maximum and minimum values of the fraction ratio were 1.66 and 4.27, while these values of the Nusselt number ratio were 1.24 and 1.83. The maximum thermal enhancement factor (TEF) was 1.21 when PR = 0.5, RA = 0° and Re = 9090. The heat transfer enhancement mechanism of the vortex generator is explained from the microscopic point of view.

Simulation and Validation of Droplet Generation Process for Revealing Three Design Constraints in Electrohydrodynamic Jet Printing.

Droplet generation process can directly affect process regulation and output performance of electrohydrodynamic jet (E-jet) printing in fabricating micro-to-nano scale functional structures. This paper proposes a numerical simulation model for whole process of droplet generation of E-jet printing based on the Taylor-Melcher leaky-dielectric model. The whole process of droplet generation is successfully simulated in one whole cycle, including Taylor cone generation, jet onset, jet break, and jet retraction. The feasibility and accuracy of the numerical simulation model is validated by a 30G stainless nozzle with inner diameter ~160 µm by E-jet printing experiments. Comparing numerical simulations and experimental results, period, velocity magnitude, four steps in an injection cycle, and shape of jet in each step are in good agreement. Further simulations are performed to reveal three design constraints against applied voltage, flow rate, and nozzle diameter, respectively. The established cone-jet numerical simulation model paves the way to investigate influences of process parameters and guide design of printheads for E-jet printing system with high performance in the future.

Hazy Image Decolorization with Color Contrast Restoration.

It is challenging to convert a hazy color image into a gray-scale image because the color contrast field of a hazy image is distorted. In this paper, a novel decolorization algorithm is proposed to transfer a hazy image into a distortionrecovered gray-scale image. To recover the color contrast field, the relationship between the restored color contrast and its distorted input is presented in CIELab color space. Based on this restoration, a nonlinear optimization problem is formulated to construct the resultant gray-scale image. A new differentiable approximation solution is introduced to solve this problem with an extension of the Huber loss function. Experimental results show that the proposed algorithm effectively preserves the global luminance consistency while represents the original color contrast in gray-scales, which is very close to the corresponding ground truth gray-scale one.