bluuebunny/crossref_metadata_2025 huggingface dataset:
Absorption enhancement of terahertz wave in microbolometers by titanium disk array
Authors: Niu Qing-Chen , Gou Jun , Wang Jun , Jiang Ya-Dong ,   State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
Published: December 2020

In this paper, a two-dimensional subwavelength periodic titanium (Ti) disk array integrated in micro-bridge structure is proposed to enhance the absorption of terahertz (THz) microbolometer. Based on the rigorous coupled wave analysis (RCWA) method, THz absorption characteristics of Ti disk arrays with different structure parameters in micro-bridge structure arrays are studied. Periodic disk array structure reduces the surface plasmon frequency of Ti, excites the spoof surface plasmons in the THz band and leads to resonance enhanced absorption. The resonance absorption frequency is determined by the structural parameters of Ti disk array including period and diameter while the absorption rate of THz wave is greatly affected by the thickness of Ti disks. The resonant cavity in micro-bridge structure can reduce the resonance frequency and enhance the coupling efficiency. The micro-bridge structure designed in this paper breaks the diffraction limit and traps the THz wave with a small period (37 μm). An absorption of nearly 90% is achieved at 3.5 THz. The structure meets the requirements of small size, high absorption and good process compatibility of the THz microbolometer.
Rapid solidification mechanism of liquid quinary Ni-Zr-Ti-Al-Cu alloy investigated by high-speed cinematography
Authors: Xu Shan-Sen , Chang Jian , Wu Yu-Hao , Sha Sha , Wei Bing-Bo ,   MOE Key Laboratory of Space Applied Physics and Chemistry, Northwestern Polytechnical University, Xi’an 710072, China
Published: December 2020

 The ability to undercool and solidification mechanism of liquid quinary Ni40Zr28.5Ti16.5Al10Cu5 alloy are investigated by electromagnetic levitation (EML) and drop tube (DT) technique. Under the EML condition, the maximum undercooling of levitated alloy can reach up to 290 K (0.21TL). Under the DT condition, the alloy achieves higher undercooling than EML, and solidifies finally into metallic glass. At lower undercooling, the solidification structure of the alloy is composed of primary Ni3Ti phase, secondary Ni10Zr7 phase and eutectic (Ni10Zr7+Ni21Zr8) phase. With the rise of undercooling, the solidification structure displays the following evolution events: phase morphology refinement, primary phase inhibition, phase number reduction, and amorphous phase formation.

 By using the high-speed cinematography technique, three nucleation modes are distinctly observed on the levitated alloy melt surface at the beginning of solidification, that is, single-point nucleation, multi-point nucleation and annular nucleation. The levitation state corresponding to single-point mode nucleation is relatively stable, and the alloy undercooling is also relatively low. The annular nucleation only occursin the case with high rotation speed, and the undercooling is greater than 208 K. The discrepancy between nucleation modes is due to the He gas flow for forced cooling.

 The theoretical calculations indicate that the alloy droplets achieve high undercoolingand large cooling rate under the DT condition. The experimental results show that when the droplet diameter decreases to 498 μm, the amorphous phase begins to appear in the alloy particles. It is noteworthy that the amorphous phase is preferentially formed inside the droplet, but not on the outer surface. The morphology of solidification structure reveals that different regions of the droplet have various local undercoolings, which result in the distribution characteristics of amorphous phase. The volume fraction of amorphous phase increases linearly with the decrease of particle diameter. When the droplet diameter decreases to 275 μm, the alloy droplets are completely frozen into glassy particles.

 The average eutecticspacing values are also measured at different alloy undercoolings. Compared with the classical binary eutectic growth model, the experimental eutectic growth law exhibits a large deviation in index. This indicates that the eutectic growth in multicomponent alloys displays more complex kinetic characteristics.
Numerical study on phase change behavior of liquid nitrogen droplets impinging on solid surface
Authors: Zhao Ke , She Yang-Zi , Jiang Yan-Long , Qin Jing , Zhang Zhen-Hao ,   Key Laboratory of Aircraft Environment Control and Life Support, Ministry of Industry and Information Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210000, China
Published: December 2020

The distinct physical properties of liquid nitrogen make liquid nitrogen spray cooling a promising technique in aerospace engineering, the electronic industry, superconductor cooling, cryobiology, etc. In-depth study of the dynamics and thermodynamic behavior of liquid nitrogen droplets impinging on the wall surface is helpful to understand the heat transfer mechanism of spray cooling technology with liquid nitrogen. Therefore, the mathematical model of single-liquid nitrogen droplet impacted solid surface is developed by Level Set-VOF method. The effects of wall wettability (30°－150°), initial velocity (0.1, 1.6 m/s) and wall temperature (300－500 K) on the phase change behavior during the evolution of droplets are investigated, and the mathematical model of film thickness is established. The results show that enhancing the wall wettability and increasing the impact speed facilitate the spreading of the droplets in the radial direction, thereby increasing the heat exchange area and reducing the thermal resistance. Ultimately, the heat exchange performance is significantly improved. Increasing the wall temperature results in an increase in the difference between temperatures of the solid surface and the liquid, thereby significantly increasing the wall heat flux density. The lower thermal resistance at the three-phase contact line results in a higher heat flux density at the edge than in the center; the difference among the heat flux distributions on different wetted walls decreases due to the increase of initial velocity, showing a significant velocity effect. In the film boiling region, the heat transfer process is mainly concentrated in the initial stage of impact, and the gas film is the main heat transfer resistance. Based on conservation of mass and energy, a numerical model of film thickness is developed in this paper. The model predictions are in good agreement with the simulation results of this paper and others.
Topological acoustic transports in chiral sonic crystals
Authors: Geng Zhi-Guo , Peng Yu-Gui , Shen Ya-Xi , Zhao De-Gang , Zhu Xue-Feng ,   School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China,   Innovation Institute, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

Topological sound has enriched the way of implementing the sound manipulation, which can effectively suppress the backscattering due to topological protection. As an inherent longitudinal wave, sound wave has no " spin” and only supports longitudinal vibration. Creating the " pseudospin” degree of freedom is crucial to topological state for acoustic wave. In previous studies, a circulating fluid flow in the background field is introduced to break the reciprocity of wave propagation in an acoustic system, which still faces technically a challenge. On the other hand, acoustic analogues of quantum spin Hall state and valley Hall state are realized by relying on the Kramers doublet in the lattices with C6 symmetry and the broken mirror symmetry or inversion symmetry, respectively. In these cases, the distributions of acoustic energy flux in the unit cells emulate the pseudospins. Based on the band inversion, the topological sound carrying pseudospin is implemented at the interface between topologically trivial and non-trivial sonic crystal. Because of the close relevance to the lattice symmetry, these pseudospin-based topological state in the time-reversal invariant system is sensitive to structural defects.

In this work, we investigate the topological sound in chiral sonic crystal consisting of resonant air tubes. The counterclockwise and clockwise length variation of air tube correspond to different topological phases. A defect meta-molecule is created at the symmetric interface, which supports resonant state in the band gap. The interface state occurs at the boundary between two opposite chiral sonic crystals. Owing to the resonant structure, we realize subwavelength topological sound transport with a subwavelength-transverse confinement. For the state carrying monopolar-mode symmetry, it is expected to preserve the mode symmetry under randomly introduced defects. As anticipated, the numerical results show that the topological sound has very strong robustness against various defects, such as the variation of positions and length of air tube. Finally, we utilize the field symmetry of topological sound in chiral sonic crystal to realize robust edge transport along soft or rigid boundary. Through the mirror symmetry operation of soft or rigid boundary, we construct an interface between the real lattice and its virtual image. The approach greatly reduces the dimension of sonic crystal device. Our work may conduce to the advances in topological acoustics, since the subwavelength-scale topological state promotes the applications of miniaturized acoustic devices.
Fabrication of tellurite glass microdisks with thermal pressing method
Authors: Hu Bo , Wu Yue-Hao , Zheng Yu-Lu , Dai Shi-Xun ,   Faculty of Electrical Engineering and Computer Science, Ningbo University, Ningbo 315211, China,   Advanced Technology Research Institute, Laboratory of Infrared Materials and Devices, Ningbo University, Ningbo 315211, China,   Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province, Ningbo 315211, China
Published: December 2020

In this work, we report a two-step thermal pressing method of fabricating microdisk lasers that are suitable for processing thermally stable glass materials, and we utilize a tellurite glass (TeO2-ZnO-Na2O) to demonstrate the feasibility of this method. Compared with the conventional microdisk fabricating methods that rely on a series of complicated procedures, such as lithography, etching, thermal reflow, and ion implantation, this thermal pressing method involves only two heating procedures and can be accomplished with simple laboratory resistive heating furnaces. In the first heating procedure, we crush bulk tellurite glass into powders and drop those powders through a vertical heating furnace. Glass powders are transformed into microspheres by surface tension in the furnace. In the second heating procedure, microspheres are placed between two flat/smooth surfaces and are thermally treated when being pressed with suitable weights. With this procedure, the “flattened” microspheres turn into the microdisks. In this work, we demonstrate that our fabricated tellurite glass microdisks possess diameters of 100－400 μm, thinnest thickness of ～ 8 μm, and typical quality-factor (Q-factor) of ～ 105. We also dope tellurite glass with active dopants such as Nd3+ and Tm3+ to fabricate the active microdisk resonators. We couple those active microdisk resonators with fiber tapers and demonstrate that with appropriate pump power, apparent fluorescence whispering gallery mode and laser mode can be obtained. Taking a 105.74-μm-diameter, 10.4-μm-thickness, and Nd3+-doped tellurite glass microdisk for example, we show that as the pump power increases above a threshold of 1.364 mW, a lasing peak near 1.06 μm can be obtained. We also show that lasing peaks near 1.9 μm can be obtained by coupling/pumping Tm3+ doped microdisks.
Structure stability, magneto-electronic properties, and modulation effects of Fe3GeTe2 nanoribbons
Authors: Han Jia-Ning , Fan Zhi-Qiang , Zhang Zhen-Hua ,   Hunan Provincial Key Laboratory of Flexible Electronic Materials Genome Engineering, Changsha University of Science and Technology, Changsha 410114, China
Published: December 2020

Fe3GeTe2 monolayer is one of the currently fabricated 2-dimensional (2D) ferromagnetic materials. Based on the first principle of density functional theory, we here study the structural stability and magneto-electronic properties of nanoribbons NR(n) obtained by cutting 2D Fe3GeTe2. The calculated binding energy and molecular dynamics simulation results identify that nanoribbons are rather stable. The large magnetic moment and magnetized energy prove the extremely high magnetism stability for NR(n). Moreover, with the increase of the width, the magnetic moment of the nanoribbons generally increases, and gradually tends to a stable value. In particular, the nanoribbons possess a high spin polarization efficiency at the Fermi level (SPF). For example, the SPF for NR(5) is up to 100%. With the width variation of the nanoribbons, the SPF has a significant odd-even oscillating effect, that is, the spin-polarization of the odd nanoribbons is higher than that of the adjacent even nanoribbons, especially when the width is in the range of n ≤ 12. This means that the α-spin and β-spin are quite different in the density of states at the Fermi level when the width is odd or even. This may be caused by the difference of the quantum confinement effect for the odd or even nanoribbons, respectively. Meanwhile, when the width of the nanoribbons is wide enough, the odd-even oscillation effect of the spin polarizability is stabilized in a relatively small range, and the nanoribbons finally tend to be 2D Fe3GeTe2 monolayer. The nanoribbons have an obvious advantage on SPF over the 2D Fe3GeTe2 monolayer. In addition, the calculation of the strain effect demonstrates that the strain can flexibly tune the SPF varying from approximately zero to 85.6%, and the SPF reaches a maximum of 85.6% at a stretch of 4%, which is a fairly high value; then reaches a minimum at a stretch of 8%, almost being zero, which means that a mechanical switch can be designed to control the low-bias spin transition, allowing it work between high spin polarization and spin unpolarization.
Topological properties of graphene moiré superlattice systems and recent optical studies
Authors: Lü Xin-Yu , Li Zhi-Qiang ,   College of Physics, Sichuan University, Chengdu 610065, China
Published: December 2020

When 2D materials with different lattice constants or lattice rotation angles are stacked together, a periodic moiré pattern will appear. Such moiré superlattice introduces a new two dimensional periodic potential, which can greatly change the physical properties of the original systems. Recent experimental studies of moiré superlattices formed by graphene on graphene and graphene on hexagonal boron nitride have revealed very rich strong correlation effects and topological effects due to novel states in superlattice minibands. It has been shown that flat bands in graphene-based moiré superlattice systems can host both topological states and strongly correlated states, which can be controlled by an external electric field. In bilayer graphene, ABC stacked trilayer graphene and twisted bilayer-bilayer graphene, the number of valence and conduction bands near the Dirac point and even the band topology and bandwidth can be changed by varying the stacking angle between graphene layers or the applied bias voltage. Moreover, the competition between kinetic energy and coulomb interaction depends on the bandwidth and the external electric field, and at the so-called magic angle mott insulator states and superconductivity were observed. Twisted bilayer-bilayer graphene has also been predicted to show similar intriguing properties, including electrically tunable strongly correlated insulators, superconductivity and many rich topological states. In graphene-based moiré systems, the combination of topological states and strong correlations is expected to lead to a broad range of novel phenomena that are not achievable in other material systems. Therefore, graphene moiré systems is likely to bring substantial progress to the study of topological materials. In this paper, we review theoretical and experimental investigations of the topological properties of graphene moiré superlattices, including topological domain wall states in bilayer graphene and topological effects in twisted bilayer graphene, ABC trilayer graphene and twisted double bilayer graphene. The origins of topological properties of these systems are discussed as well as topological phenomena observed in various experiments. Finally, recent near-field optical studies of the band structure and novel topological properties of graphene moiré superlattices are discussed.
Topological states in electric circuit
Authors: Luo Kai-Fa , Yu Rui ,   School of Physics and Technology, Wuhan University, Wuhan 430072, China
Published: December 2020

Based on the correspondence between tight-binding Hamiltonian in condensed matter physics and the Kirchhoff’s current equations in lumped parameters circuits, profuse topological states can be mapped from the former to the latter. In this article, the electric-circuit realizations of 1D SSH model, 3D nodal-line and Weyl semimetals are devised and elaborated, in which the edge states, surface drum-head and Fermi-arc states are appearing on the surface of the circuit lattice. Of these circuits, the effective hopping terms in Hamiltonian have high degree of freedom. The hopping strength, distance and dimension are easy to tune, and therefore our design is convenient to be extended to non-Hermitian and four or higher dimensional cases, making the fancy states that hard to reach in conventional condensed matter now at our fingertips. Besides, the electric circuit has the advantage of plentiful functional elements and mature manufacture techniques, thus being a promising platform to explore exotic states of matter.
Microwave experimental platform to demonstrate topology physics based on tight-binding model
Authors: Mei Yu-Han , Shao Yue , Hang Zhi-Hong ,   School of Physical Science and Technology, Soochow University, Suzhou 215006, China,   College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China
Published: December 2020

Topology photonic, a combination of topology physics and optics provides novel visions to the demonstration of theoretical physics and designs principles to new optical devices. Being a key tool to condensed matter physics, tight-binding model helps the development of topology physics. We found that by changing the background material from vacuum to an effective medium with negative permittivity in traditional photonic crystals, a one-to-one correspondence to tight-binding model can be found for this new type of photonic crystal. We show by numerical simulations the existence of edge states located at both the zigzag and bearded boundaries of a honeycomb-lattice photonic crystal imbedded in negative permittivity material. Two experimental realizations are proposed that it is possible to build up a demonstration platform working at microwave frequencies to verify corresponding topology physics theories using simple photonic crystal structures. We hope that the successful verification of new topology physics can further trigger applications in optics.
Nonlinear optical effect and nonlinear optical mass sensor based on graphene optomechanical system
Authors: Chen Hua-Jun ,   School of Mechanics and Photoelectric Physics, Anhui University of Science and Technology, Huainan 232001, China
Published: December 2020

Graphene, atomically thin two-dimensional (2D) nanomaterial consisting of a single layer of carbon atoms, has received tremendous attention in the past few decades. Graphene may be considered as an excellent nanomaterial for fabricating nanomechanical resonator systems to investigate the quantum behavior of the motion of micromechanical resonators because of its unique properties of low mass density, high frequency, high quality-factor, and intrinsically small size. Additionally, graphene optomechanics based on a bilayer graphene resonator coupled to a microwave on-chip cavity, where light and micromechanical motion interact via the radiation pressure, has been demonstrtated experimentally recently. In this work, we demonstrate theoretically the nonlinear optical effect including optical bistability and four-wave mixing under the regimes woth different parameters and detunings in a graphene resonator-microwave cavity system. When the graphene optomechanics is driven by one strong pump laser beam, we find that the optical bistability can be controlled by tuning the power and the frequency of the pump beam. The four-wave mixing (FWM) phenomenon is also investigated and we find that sharp peaks in the FWM spectrum exactly are located at the resonant frequency of graphene resonator. Therefore, a straight nonlinear optical means for determining the resonant frequency of the graphene resonator is presented. Setting the cavity field resonating with pump field, and then scanning the probe frequency across the cavity frequency, one can easily and exactly obtain the resonant frequency of the resonator from the FWM spectrum. We further theoretically propose a mass sensor based on the graphene optomechanical system. The mass of external nanoparticles deposited onto the graphene resonator can be measured conveniently by tracking the shift of resonant frequency due to mass changing in the FWM spectrum. Compared with optomechanical mass sensors in linear regime, the nonlinear optical mass sensor may be immune to the detection noise. The system may have potential applications in communication networks for frequency conversion and provide a new platform for high sensitive sensing devices.
Experimental study on supersonic plate boundary layer transition under pulsed arc plasma excitation control
Authors: Tang Bing-Liang , Guo Shan-Guang , Song Guo-Zheng , Luo Yan-Hao ,   Science and Technology on Plasma Dynamics Laboratory, College of Aeronautical Engineering, Air Force Engineering University, Xi’an 710038, China
Published: December 2020

Pulsed arc plasma excitation is characterized by strong local heating effect and wide disturbance range, and it has a broad application prospect in supersonic flow control. In this paper, by using electrical parameter measurement system and high speed schlieren technique, we study the electrical and flow field characteristics of pulsed arc plasma excitation under the condition of Ma = 3 incoming flow. The nano-particle planar laser scattering (NPLS) is used to investigate the flow structure of the supersonic flat boundary layer, and the transition characteristics of the boundary layer at different plasma excitation frequencies are studied. The experimental results show that in the flow field with Ma = 3 and the total incoming pressure P0 = 1 atm (1 atm = 1.01 × 105 Pa), the peak voltage of the pulsed arc plasma actuator discharge is 6 kV, the peak current is 70 A, the time scale of the discharge is about 300 ns, the single discharge energy is 70 mJ; the pulsed arc discharge will produce the precursor shock wave with higher velocity and the thermal deposition zone with higher temperature, which will exert continuously disturbance on the boundary layer. The pulsed arc plasma excitation with perturbation can promote the transition of supersonic plate boundary layer. Moreover, the high-frequency impact effect of pulsed discharge can promote the transition to occur ahead of time, and the higher the frequency, the better the effect is. As the excitation frequency increases, the transition position of the boundary layer of the supersonic flat plate moves forward, and the length of the transition area of the boundary layer becomes shorter as the excitation frequency increases. When the excitation frequency is 60 kHz, the length of transition zone is 0 and the thickness of turbulent boundary layer is 25 mm. When a high frequency is applied (f = 40, 60 kHz), the transition path of the boundary layer is that the shock wave generated by the plasma excitation triggers the unstable wave, and the development of unstable waves directly skips the linear growth stage, passes through the bypass and transitions into turbulent flow. The pulsed arc plasma excitation can be used to promote supersonic boundary layer transition.
Polar optical phonon scattering of two-dimensional electron gas in AlGaN/GaN high electron mobility transistor
Authors: Zhang Xue-Bing , Liu Nai-Zhang , Yao Ruo-He ,   School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510640, China
Published: December 2020

With the increasing demand for high-frequency, high-power and high-temperature microwave applications, AlGaN/GaN high electron mobility transistors have received much attention due to their promising material features such as wide band gaps, high-concentration two-dimensional electron gas (2DEG), strong electric field, at which the electron velocity is saturated, and high operating temperature. The 2DEG mobility at AlGaN/GaN interface is a key parameter to describe characteristics of high electron mobility transistor, and the mobility of 2DEG in AlGaN/GaN high electron mobility transistor is determined by a variety of scattering mechanisms in which the polar optical phonon scattering caused by electrostatic field between uneven polar positive and negative charges is responsible for mobility limitation in a 2DEG at high temperature.

Calculation of polar optical phonon scattering is carried out by the analytical model in which Fang-Howard variational wave function and Fermi’s golden rule are used. The interaction between 2DEG and phonon is described by scattering matrix element for the transition, in which phonon occupation number is given by Bose-Einstein statistics. The scattering time is derived by neglecting the in-scattering, and the numerically calculated energy-dependent scattering time is averaged according to Fermi statistics.

At temperatures in a range of 200–400 K and two-dimensional electron gas concentration in a range of 6 × 1011–1 × 1013 cm–2, the mobility varying with temperature is analyzed. It is found that the mobility limited by polar phonon scattering decreases monotonically with the temperature rising and their dependence is well approximated by a function of \begin{document}$\mu_{\rm PO} = AT^{-\alpha} ~ (\alpha = 3.5)$\end{document} as the phonon occupation number increases with temperature rising. Furthermore, the polar optical phonon scattering is enhanced by greater electron concentration as a result of increased interaction between phonon and 2DEG. The mobility limit is calculated separately by emission phonon and absorption phonon, the results indicate that absorption phonon is predominant, which is attributed to high optical phonon energy in GaN. The mobility of polar optical phonon scattering is further studied by changing the optical phonon energy, which shows that the room temperature mobility of 2DEG can be improved by increasing the energy of polar optical phonon.
Dual-wavelength self-starting mode-locking Er-doped fiber laser with MnPS3 saturable absorber
Authors: Yu Qiang , Guo Kun , Chen Jie , Wang Tao , Wang Jin , Shi Xin-Yao , Wu Jian , Zhang Kai , Zhou Pu ,   i-lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China,   College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China
Published: December 2020

As a member of the metal phosphorus trichalcogenide family, MPS3 is widely used in nonlinear optics and devices, which can be regarded as a significant benefit for the excellent photonic and optoelectronic properties. In this work, the MnPS3 nanosheet is prepared by the chemical vapor transport method and the MnPS3 saturable absorber is demonstrated by modifying mechanical exfoliation. To the best of our knowledge, the dual-wavelength self-starting mode-locking erbium-doped fiber laser with MnPS3 saturable absorber is demonstrated for the first time. The dual wavelength mode-locked laser with a pulse repetition rate of 5.102 MHz at 1565.19 nm and 1565.63 nm is proposed. Its maximum output power at the dual-wavelength is 27.2 MW. The mode-locked laser can self-start and stably run for more than 280 h.
Research progress of electromagnetic metamaterial absorbers
Authors: Wang Yan-Zhao , Xu He-Xiu , Wang Chao-Hui , Wang Ming-Zhao , Wang Shao-Jie ,   Air and Missile Defense College, Air Force Engineering University, Xi’an 710051, China
Published: December 2020

Electromagnetic absorbing technology can effectively suppress the radiation of electromagnetic waves, and has been widely used in military and civilian fields. However, traditional absorbing technology cannot meet the new requirements for modern absorbing materials. The advent of metamaterials provides a solution for this problem Metamaterial absorber has the advantages of simple structure, light weight, high absorption rate, and can realize the flexible control of electromagnetic waves, which has led the electromagnetic absorption research to rapidly develop. In this paper, the research and development of using metamaterials to absorb electromagnetic wave is reviewed. Firstly, the principle, implementation, and presently existing bottlenecks of electromagnetic wave absorption in using metamaterials are outlined. Secondly, recent progress of the aforementioned key issues in three aspects is introduced, including multi-band and broadband, polarization and angle independence, and dynamic tunability. Several typical methods of making metamaterial absorbers are illustrated here. Generally speaking, the prerequisite of broadband metamaterial absorbers is to provide multiple resonances that are close enough to each other. The structure with multiple rotationally symmetric geometry is helpful in achieving polarization- and angle-insensitive properties. The flexible control of absorption performance can be realized by introducing lumped elements such as resistances, capacitances, and diodes. In addition, by means of composite traditional materials or new materials and other methods the dynamic adjustment of the absorption performance can be achieved. Although researchers have done a lot of work on the metamaterial absorbers, there remain many problems and challenges. For the future design, several promising directions are suggested from three perspectives: high performance, multifunctionality, and new structures. In terms of high performance, it is still a challenge to achieve ultra-thin broadband metamaterial absorber for low-frequency which can break through the limitation of wavelength. Integrated multifunctional metamaterials can adapt to the increasingly complex application scenarios and should gradually become the focus of attention. Since three-dimensional (3D) printing technology has proved to be applicable to the preparation of complex metamaterial structures, the new 3D metamerial absorbers will bring more vitality to the development of metamaterials. Finally, as regards the application of metamaterials in stealth, the future development of metamaterial absorbers is further summarized.
Progress in perovskite solar cells based on different buffer layer materials
Authors: Chen Yong-Liang , Tang Ya-Wen , Chen Pei-Run , Zhang Li , Liu Qi , Zhao Ying , Huang Qian , Zhang Xiao-Dan ,   Institute of Photo-Electronics Thin Film Devices and Technique, Nankai University, Tianjin 300071, China,   Key Laboratory of Photo-Electronics Thin Film Devices and Technique of Tianjin, Tianjin 300071, China,   Anhui Polytechnic University, Wuhu 241000, China
Published: December 2020

Based on the excellent optoelectronic properties of organic-inorganic hybrids perovskite materials, the power conversion efficiency of perovskite solar cells (PSCs) is rapidly increasing. However, factors that restrict the performance of PSCs still exist, such as interface and stability problems. Problems, such as band mismatching, carrier recombination and chemical reaction between interfaces, could be alleviated by introducing a buffer layer (BL) with a proper band structure between different layers. Moreover, stability as well as charge separation and collection could also be efficiently improved in PSCs. In this paper, an overview of the most contemporary strategies of BLs was provided. The passivation mechanism of BLs at different interfaces are highlighted and discussed in detail. Furthermore, the performances of recently developed BLs in PSCs are compared. Finally, we elaborate on the remaining challenges and future directions for the development of BLs to achieve high-efficiency and high-stability PSCs.
Home-made confined-doped fiber with 3-kW all-fiber laser oscillating output
Authors: Zhang Zhi-Lun , Zhang Fang-Fang , Lin Xian-Feng , Wang Shi-Jie , Cao Chi , Xing Ying-Bin , Liao Lei , Li Jin-Yan ,   Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

Ytterbium doped fiber lasers (YDFLs) with small volume, good beam quality, good heat dissipation performance and high conversion efficiency are widely used in industrial processing, military, medical and other fields. In past decades, with the development of high-performance double cladding gain fiber and fiber devices, the output power of YDFLs increases rapidly. However, nonlinear effects (NLEs), such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), are produced, which limits the further enhancement of the output power of fiber laser. Large mode area ytterbium-doped fiber (LMAYDF) can effectively increase the nonlinear effect threshold. However, increasing the core diameter will support more high-order modes (HOMs), which may lead the beam quality to deteriorate and induce the mode instability (MI) effect to occur in fiber lasers. Thus, MI and NLEs have become the main limiting factors for the further improving of output power and beam quality in fiber lasers. The confined-doped ytterbium-doped double-clad fiber (CDYDF), by reducing the doping diameter of gain ions in the fiber core, makes the fundamental mode (FM) dominate in mode competition and HOM suppressed to achieve LMAYDF gain control for different modes, thus improving the output power of the fiber laser and maintaining good beam quality. The 33/400 μm confined-doped ytterbium-doped double-clad fiber (CDYDF) is fabricated by modifying the chemical vapor deposition (MCVD) process with solution doping technology (SDT). The Yb3+ doping diameter ratio is 70% and refractive index profile is close to step-index. Utilizing the master oscillator power amplifier (MOPA) system the beam quality optimization effect of confined-doped fiber is verified and optimized to 1.43 as the power increases while the M2 of seed laser is 1.53. An all-fiber structure counter-pumped fiber oscillator is constructed to test the laser performance of home-made confined-doped fiber. When the pump power is ~4.99 kW, laser power of 3.14 kW with a central wavelength of 1081 nm and line width of 3.2 nm at 3 dB is obtained. Moreover, there is no MI nor SRS in the whole experiment. We demonstrate that it is the highest output power based on home-made confined-doped fiber. The above results indicate that confined-doped fibers have the potential to achieve high-power and high-beam-quality fiber laser output.
Study on the electronic structure and photocatalytic properties of a novel monolayer TiO2
Authors: Xiong Zi-Qian , Zhang Peng-Cheng , Kang Wen-Bin , Fang Wen-Yu ,   Public Health and Management School, Hubei University of Medicine, Shiyan 442000, China,   Hubei Biomedical Detection Sharing Platform in Water Source Area of South to North Water Diversion Project, Shiyan 442000, China
Published: December 2020

By means of state-of-the-art density functional theory (DFT) computations, We designed a new two-dimensional material TiO2. We further investigated the stability, electronic structure, carrier mobility, and optical properties of monolayer TiO2. Our results show that monolayer TiO2 has good kinetic, thermodynamic and mechanical stability and can exist stably at room temperature. The results were demonstrated using the binding energy, phonon spectrum, molecular dynamics simulation, and elastic constant calculation. The band structure indicates that the monolayer TiO2 is an indirect bandgap semiconductor with energy gaps of 1.19 eV (GGA+PBE) and 2.76 eV (HSE06), respectively. The results of state density show that the Ti-3d state electrons constitute the top of the valence band and Ti-4s state electrons constitute the bottom of the conduction band. The electron states of O atoms contribute very little near the Fermi energy level and are mainly distributed in the deep energy level. In addition, the carrier mobility of monolayer TiO2 is smaller than monolayer MoS2, and the electron and hole mobility can reach 31.09 cm2·V–1·s–1 and 36.29 cm2·V–1·s–1, respectively. Due to the anisotropy of hole mobility and electron mobility, the composite rate of electrons and holes is relatively low. This ensures longer service life and better photocatalytic activity of monolayer TiO2. Furthermore, under the condition of uniaxial strain and biaxial strain, the energy gap of monolayer TiO2 has a clear response. The energy gap is more sensitive to biaxial strain than uniaxial strain, indicating that monolayer TiO2 can be applied to various semiconductor devices. The band-edge potential and optical properties of semiconductors indicate that two-dimensional TiO2 is capable of photo-splitting water production, H2 at –5~2% single/biaxial strain, and O2, H2O2, O3, etc. at –5~5% single/biaxial strain. Moreover, the monolayer TiO2 has a high absorption coefficient for visible and ultraviolet light. In conclusion, the monolayer TiO2 has a potential application prospect in the field of optoelectronic devices and photocatalytic materials in the future.
Research progress of enhancing perovskite light emitting diodes with light extraction
Authors: Chen Jia-Mei , Su Hang , Li Wan , Zhang Li-Lai , Suo Xin-Lei , Qin Jing , Zhu Kun , Li Guo-Long ,   Key Laboratory of Ningxia for Photovoltaic Materials, Ningxia University, Yinchuan 750021, China
Published: December 2020

Perovskite light emitting diodes (PeLEDs) have developed rapidly in recent years due to their advantages of tenability of band gap and high color purity. At present, the external quantum efficiency of PeLED has rised up to 20%. Like the scenario of organic light emitting diode, there exist various internal losses in PeLED with low light extraction efficiency. It arises from the absorption of substrates, waveguide transmission and surface plasmon resonance of metal electrode. To improve the luminescence performance of PeLED, a well-matched optical admittance between the thin-films inside the devices is required. In this paper, the strategies of enhancing the light extraction efficiency are adopted as the materials and structures in PeLED are concerned. The applications of alternative electrode in PeLED are discussed, such as graphene, silver nanowires, metal transparent electrode and some new-types of electrodes. In addition, the plasma effect is also introduced into the PeLED to deflect the emitting light. What is more, the nano-structure grating is inserted into device to reduce the optical losses due to the large refractive index difference between the interfaces in device. Therefore, the external quantum efficiency of PeLED rises up to 28.2%, and the current efficiency can reach 88.7 cd/A.
Effects of subsurface charge on surface defect and adsorbate of rutile TiO2 (110)
Authors: Wen Huan-Fei , Yasuhiro Sugawara , Li Yan-Jun ,   Key Laboratory of Instrumentation Science and Dynamic Measurement, North University of China, Taiyuan 030051, China,   Department of Applied Physics, Graduate School of Engineering, Osaka University, Osaka 5650871, Japan
Published: December 2020

Transition-metal-oxide as a typical model surface for investigating the catalytic mechanism has been widely studied. Over the past years, the TiO2 properties have been reported. It is commonly accepted that the catalytic activity of reduced TiO2 is related to its defects, with the accompanying excess electrons leading to n-type conductivity. It is realized that subsurface charge is of key importance for the redox chemistry of TiO2 (110).

Subsurface charge is explored by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). Subsurface charge exerts an additional attractive force on the scanning AFM tip, resulting in the relative retraction of tip motion in order to keep a constant frequency shift. As a result, the subsurface charged region is imaged as protrusion in an AFM topographic image. The height of bright hillock is mainly distributed in three different ranges, which means that the subsurface charges are at three different subsurface layers. The AFM results show such subsurface charges repel the electropositive oxygen vacancy, hydrogen atoms and step edges. It is obvious that there is not only an Ov depletion zone but also the subsurface charge free region in the proximity of the \begin{document}$\left\langle {001} \right\rangle $\end{document} and \begin{document}$\left\langle {1\bar 11} \right\rangle$\end{document} step edge.

The KPFM image indicates that the subsurface charges are the positive charges. which is consistent with common sense. After oxygen exposure, it is found that the oxygen adatom is electronegative, but it is absent in the vicinity of positive subsurface charges. Irrespective of adsorbate being electropositive or electronegative, an adsorbate-free zone generally exists in the proximity of the charged region. Obviously, the present study is expected to provide some insights into clarifying the nature of subsurface charge and improving catalytic design.
Advances in improved photoluminescence properties of all inorganic perovskite nanocrystals via metal-ion doping
Authors: Yu Peng , Cao Sheng , Zeng Ruo-Sheng , Zou Bing-Suo , Zhao Jia-Long ,   School of Physical Science and Technology, Guangxi University, Nanning 530004, China,   Guangxi Key Laboratory of Processing for Non-Ferrous Metal and Featured Materials, Guangxi University, Nanning 530004, China
Published: December 2020

Metal halide perovskite nanocrystals (NCs) have attracted great attention in the fields of light-emitting diodes, lasers, X-ray imaging, solar cells and photoelectric detectors due to their excellent optoelectronic properties. Compared with organic-inorganic hybrid perovskite NCs, all inorganic perovskite CsPbX3 (X = Cl, Br, I) NCs have good photoelectric properties and high stability. To further improve the photoluminescence (PL) quantum yields (QYs) and stability of CsPbX3 NCs, researchers reduced the defects as nonradiative recombination centers in NCs by the following strategies: 1) surface treatment with different ligands; 2) control of synthesis conditions with halide rich compounds; 3) doping of metal ions. Among them, metal doping is considered as a universal and effective way to adjust the optoelectronic properties of semiconductors. It is found that the type and the concentration of metal ions have great influence on the electronic band structure and PL performance of NCs after the metal ions have been doped into CsPbX3 NCs. At the same time, compared with II-VI and III-V semiconductors, the unique structure of all inorganic perovskite NCs makes the doping of metal ions easier. Appropriate doping can not only enhance the intrinsic optical properties of the NCs without affecting their crystal structure, but also introduce new electronic energy levels into the NCs and new luminescent properties of doped metal ions. Based on metal ions doping strategy, the PLQYs of doped CsPbX3 NCs have been enhanced to nearly 100%. In this work, we summarize recent advances in metal doping of the four typical kinds of perovskite NCs, including CsPbCl3, CsPbBr3, CsPbI3, and Mn2+ doped CsPbX3, and discuss the physical mechanisms of the improved properties through doping metal ions. It should be pointed out that the doping of some metal ions such as Ni2+ and Cd2+ into the above four kinds of NC systems can effectively passivate NC defects, thus improving the PL QY and stability of NCs. In addition, we put forward some personal perspectives on the future research subjects of interest and directions of metal doping for enhanced PL of CsPbX3 NCs, which needs to be further explored in order to promote extensive application of perovskite NCs to various optoelectronic devices.
Optimal design and experimental verification of high-temperature and high-pressure assembly of neutron diffraction based on PE-type press
Authors: Jiang Ming-Quan , Li Xin , Fang Lei-Ming , Xie Lei , Chen Xi-Ping , Hu Qi-Wei , Li Qiang , Li Qing-Ze , Chen Bo , He Duan-Wei ,   Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China,   Key Laboratory for Neutron Physics, Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621999, China,   Guangdong Zhengxin Hard Material Technology R & D Co., Ltd, Heyuan 517000, China
Published: December 2020

High-pressure and high-temperature(high-P-T) in-situ neutron diffraction detection method is a field of growing interest, in particular, for its numerous applications in the field of condensed matter physics, crystal chemistry, geophysics, materials science and engineering. In this work, we design and optimize a set of assembly for high-P-T in-situ neutron diffraction experiment in neutron source of China by using Paris-Edinburgh(PE)-type press. The high-P-T experiment is carried out with a high-pressure neutron diffraction spectrometer (Phoenix) of China Mianyang Research Reactor (CMRR). A 1500 KN uniaxial loading system and a 1500 W constant current source provides extreme conditions of high-P-T for PE press. The toroidal anvil we use is made of tungsten carbide. We use two types of gaskets: one is machined from the null-scattering TiZr alloy and the other is made from the thermal insulation ceramic material of ZrO2. High-temperature furnace is formed by graphite. First, a simplified simulation analyses of the pressure change rates in different areas of the entire assembly are carried out, and it is concluded that the gasket I, II, III areas are designed with a gradient decreasing method. The compression ratio of the sample chamber is significantly improved. Then when the gasket reaches the same compression ratio, the cell pressure will be higher than the pressure before optimization. After that, we conduct experimental verification on the optimized design. Through a series of optimization experiments for assembly on the rheological control of gasket, the improvement of thermal insulation performance and the maximization of effective sample volume under high-P-T, the key technical indicators and design scheme of the high-P-T in-situ neutron diffraction platform are verified. The temperature and pressure in the sample cavity are calibrated by using the MgO's high-P-T in-situ neutron diffraction spectrum and equation of state. The in-situ neutron diffraction sample cavity environment of the designed platform can reach the conditions of 11.4 GPa and 1773 K. The successful development of this assembly greatly improves the experimental conditions of CMRR high-P-T neutron diffraction platform. At the same time, it has important reference significance for further improving the high-P-T loading conditions of the PE-type press and expanding the application scope of the PE-type press.