bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of electric field on the phenomenological coefficient and electrocaloric strength in ferroelectrics
Authors: Lu Sheng-Guo , Li Dan-Dan , Lin Xiong-Wei , Jian Xiao-Dong , Zhao Xiao-Bo , Yao Ying-Bang , Tao Tao , Liang Bo ,   Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, Guangdong Provincial Research Center on Smart Materials and Energy Conversion Devices, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China,   Dongguan South China Design Innovation Institute, Dongguan 523808, China
Published: December 2020

As the electric field can affect the polarization and dielectric constant, the phenomenological coefficient a0 is an implicit function of electric field. The phenomenological coefficient a0 is determined by the polarization and the reciprocal of permittivity, and a nonlinear function of electric field in the ferroelectric phase regime. In the paraelectric phase regime, however, a0 is merely subjected to the reciprocal of permittivity, and also a nonlinear function of electric field. In this paper, we investigate the electric field dependence of phenomenological coefficient in ferroelectric copolymers, terpolymers and Ba0.85Ca0.05Sr0.1TiO3 ceramics. It is indicated that the phenomenological coefficient increases with the increasing electric field, the maximum value is obtained to be about 2 times the original value. Moreover, the electrocaloric strength is used to measure the magnitude of electrocaloric effect of electrocaloric materials in an external electric field. It can be used to find out novel and efficient electrocaloric materials through studying the electrocaloric strength. Based on the thermodynamic theory, the analytical expression of electrocaloric strength is deduced. It is found that the phenomenological coefficient, phase transition, specific heat capacity, and permittivity versus temperature peak value at the phase transition temperature have a clear influence on the electrocaloric strength. The expression can be applied to 1st order, 2nd order phase transition materials and relaxor ferroelectrics.
Research progress of photonics devices on lithium-niobate-on-insulator thin films
Authors: Li Geng-Lin , Jia Yue-Chen , Chen Feng ,   State Key Laboratory of Crystal Materials, School of Physics, Shandong University, Jinan 250100, China
Published: December 2020

 Lithium niobate (LiNbO3, LN) crystals have excellent electro-optical and nonlinear optical properties, and they have been regarded as one of the most promising materials for constructing the multifunctional photonic integrated systems. Due to the excellent optical properties of LN crystal, the emerging LN thin film technology has received great attention in the research of integrated photonics in recent years. With the help of advanced micro-nano fabrication technologies, many high-performance lithium niobate integrated photonic devices have been realized.

 Integrated photonic platform can incorporate high-density, multi-functional optical components, micro-nano photonics structures, and optical materials on a monolithic substrate, which can flexibly implement a variety of photonic functions. At the same time, it also provides a low-cost, small-size, and scalable solution for miniaturizing and integrating the free-space optical systems. Photonic chips based on LN have been widely used in fast electro-optic modulation, nonlinear optical frequency conversion and frequency comb generation. In particular, periodically poled lithium niobate (PPLN) based on quasi-phase matching has gradually become a mature integrated photonic platform and has been widely used in the field of nonlinear optics.

 As wafer bonding technology is matured, the lithium-niobate-on-insulator (LNOI) thin films made by the “smart-cut” process have been commercialized. The thickness of the LN film on a Si or SiO2 substrate can reach several hundred nanometers, and good uniformity in film thickness at a larger size (3 inches) can be ensured. With the development of micro-nano fabrication technologies, the quality and functions of photonic devices on LNOI chips have been significantly improved in recent years, and research on integrated photonic devices based on LNOI has also been developed rapidly in recent years.

 In this article we briefly review the development of LNOI technology, introducing the applications of several advanced micro-nano fabrication techniques and summarizing their applications in the micro-/nano-fabrication of on-chip photonic devices based on LNOI wafers. In addition, in this article we also summarize the latest advances in the functionality of LNOI on-chip photonic devices and give a short prospective on their future applications in integrated photonics.
GaN based micro-light-emitting diode size effect and array display
Authors: Tai Jian-Peng , Guo Wei-Ling , Li Meng-Mei , Deng Jie , Chen Jia-Xin ,   Optoelectronics Technology Lab, Ministry of Education, Beijing University of Technology, Beijing 100124, China
Published: December 2020

Single micro-light emitting diodes(LEDs) with different sizes and array micro-LED are designed and prepared, where the sizes of the single micro-LEDs are in a range of 40−100 μm, their electrodes are all co-N electrodes, P electrode is drawn out alone; the number of array pixels is \begin{document}$ 8\times8 $\end{document}, which is a passively driving structure with a pixel size of 60 μm. In the process of device preparation, N electrode and P electrode are fabricated by the sputtering & stripping method. The electrode thickness is 2.4 μm. Thick photoresist 5120 is used as a mask, and N GaN is etched to the substrate by using the ICP dry etching to form an isolation trench. The PECVD technique is used to deposit an SiO2 insulating layer with a thickness of 10000 Å. By optimizing the electrode structure and thickness, the reliability of the P electrode at the slope of the isolation trench is improved, and the SiO2 insulating layer has good encapsulation; field programmable gate array (FPGA) is used to drive and display the micro-LED passive array. The single micro-LEDs of different sizes are tested and analyzed in the aspects of electrics, photics and thermotics and the results of which show that the current density corresponding to the peak radiation flux of 80 μm micro-LED is 1869.2 A/cm2, which is 57.1% higher than that of 100 μm micro-LED, indicating that the current density corresponding to the peak radiation flux of micro-LED increases as the size decreases; compared with the ordinary blue LED, the micro-LED has a large k factor, and with the size decreases, the value of the k factor increases, indicating that the micro-LED series resistance is larger, and the thermal stability is not so good as the traditional blue LED. Finally, the field programmable gate array (FPGA) can achieve a good drive for the micro-LED passive array. The driving principle is passive scanning driving, which is carried out in a row-by-row lighting mode. The FPGA clock is 50 MHz, and 320 ns is required for the circuit to scan all rows.
Optimization of electrical and photovoltaic properties of Au-BiFeO3 nanocomposite films
Authors: Zhang Ya-Ju , Xie Zhong-Shuai , Zheng Hai-Wu , Yuan Guo-Liang ,   National Demonstration Center for Experimental Physics and Electronics Education, School of Physics and Electronics, Henan University, Kaifeng 475004, China,   School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
Published: December 2020

Ferroelectric films, are an important class of photoelectric functional material, which possess the following characteristics: the breaking of their symmetry can lead to self-polarization and this polarization state can be regulated by external stimuli. The photovoltaic properties of ferroelectric films have been extensively investigated due to their potential applications in the field of photodetection, energy conversion harvesting and nonvolatile storage. In view of the small photocurrent density and the degradation of photovoltaic property caused by the depolarization effect in ferroelectric films, it is necessary to explore an approach to improving the self-polarization phenomenon and regulating the conduction mechanism to further optimize their photovoltaic properties. Here in this work, BiFeO3 (BFO) films dispersed with Au nanoparticles are deposited on FTO glass substrates by the sol-gel method to obtain the Au-BFO nanocomposite films. Moreover, the relationships between Au content (0 mol%, 0.25 mol%, 0.5 mol%, 1 mol% and 3 mol%) and microstructure, electrical and photovoltaic properties of Au-BFO nanocomposite films are investigated to determine the optimal Au content. Piezoresponse force microscopy studies show that the Au-BFO nanocomposite film with 0.5 mol% Au has the strong self-polarization phenomenon. With the increase of Au content, the conduction mechanism of the Au-BFO nanocomposite films is described by the space-charge limited current theory but not the Schottky emission model any more. The photovoltaic properties of the Au-BFO nanocomposite films first increase and then decrease. When Au content is 0.5 mol%, the Au-BFO nanocomposite film has the best photovoltaic property. The open-circuit voltage and short-circuit photocurrent density of the Au-BFO nanocomposite film with 0.5 mol% Au increase nearly 3 and 5 times counterparts of the BFO film, respectively. The photovoltaic effects of Au-BFO nanocomposite films are improved mainly by regulating the self-polarization phenomenon and conduction mechanism. This study demonstrates the merits of BFO films dispersed with Au nanoparticles, specifically, the photovoltaic properties of Au-BFO nanocomposite films are further optimized. In this work, we propose a simple and effective method to regulate the electrical and photovoltaic properties of ferroelectric films, which provides a new perspective for further understanding the photovoltaic effects of ferroelectric films.
Two-dimensional nanomaterials and their derivatives for laser protection
Authors: Liu Zhi-Wei , Zhang Bin , Chen Yu ,   Key Laboratory for Advanced Materials, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China
Published: December 2020

To achieve simultaneous protection against both pulsed and continuous wave (CW) or quasi-CW lasers, significant research effort has been devoted to the state-of-the-art optical limiting (OL) materials and processes in an attempt to achieve some measures of protection against such laser beams in the past decades. Two-dimensional (2D) nanomaterials with a lot of unique properties, including graphene, transition metal dichalcogenides, black phosphorus and others, have aroused the extensive research interest of many researchers. In this review paper, we describe systematically the OL mechanisms and the recent achievements in the 2D nanomaterials and their organic/polymeric derivatives for laser protection. In an effort to sustain the advantage of 2D nanomaterials, one can not only introduce the functional molecules or polymers to blend with them to form a complex multi-phase material system, but also embed the soluble 2D nanosheets covalently functionalized with organic/polymeric materials in a polymer host to form host-guest composite materials that are expected to improve the OL performance of the whole system. All in all, an optimized complex multi-component nanomaterial system enormously enhances the performance and applicability of OL devices. In addition, the fundamental studies of the photophysical and photonic properties of 2D nanomaterials and their derivatives in various solid hosts are of significance for modifying the nanomaterials at a molecular level.
Magnetic field propagation model of low frequency/very low communication based on mechanical antenna of electret
Authors: Wang Chen , Cui Yong , Song Xiao , Yuan Hai-Wen ,   School of Automation Science and Electrical Engineering, Beihang University, Beijing 100191, China
Published: December 2020

Because of its stable propagation characteristics and small attenuation in the medium, low-frequency (LF) electromagnetic wave can penetrate into the sea and underground with small loss. Although its transmission bandwidth is narrow, which limits its application range, it has irreplaceable wide applications in long-distance navigation, communication and frequency release, especially in underwater communication. Therefore, the study of low frequency/very low frequency (LF/VLF) propagation is of great theoretical and military value. In the LF/VLF communication systems, the transmitting antenna is an extremely important part, and its performance has an important influence on the whole system. However, the wavelength of the LF electromagnetic wave is very long. In order to obtain the ideal radiation effect, the traditional method needs a huge transmitting antenna system, which is too large in size and power consumption. Therefore, it will be a disruptive innovation in the field to realize a technology that can significantly reduce the size the existing LF/VLF information network communication system.

In view of this, in this paper we propose a kind of LF/VLF signal transmitting antenna in which an excitation device is used to drive the polarization charge of the electret to move mechanically. By accelerating the charge to form a conductive alternating electromagnetic field which can generate and radiate electromagnetic wave, under the excitation of the wave source, it carries the energy and information in the form of energy flow and propagates in a certain medium. Then, through using the magnetic field receiving system to measure the magnetic field vector in the electromagnetic wave, the effective LF/VLF signal can be obtained, thus achieving the high electromagnetic wave effective radiation which overturns the restriction that the antenna size needs to be comparable to the wave length of the radiation signal in the traditional LF navigation communication system. At the same time, an analytical model of magnetic field propagation is established based on this structure, and the influence of antenna size, shape and other relevant parameters on the performance of antenna communication are studied as well. In order to reduce the loss of accuracy and improve the calculation speed, it is necessary to choose the correct analytical model and the appropriate parameters of magnetic field generated by the mechanical antenna according to the actual situation. The research work is of great significance for designing and optimizing mechanical antennas.
Triple wavelength-switchable lasing in yellow-green based on frequency mixing of self-Raman operation
Authors: Sun Ying-Lu , Duan Yan-Min , Cheng Meng-Yao , Yuan Xian-Zhang , Zhang Li , Zhang Dong , Zhu Hai-Yong ,   Institute of Laser and Optoelectronic Technology, Wenzhou University, Wenzhou 325035, China
Published: December 2020

An efficient Nd:YVO4 crystal self-Raman laser combined with second-order nonlinear frequency conversion is demonstrated to achieve an switchable output of three wavelengths in the yellow-green band. In order to improve the thermal effect and increase the length of Raman medium, a three-stage diffusion-bonded YVO4/Nd:YVO4/YVO4 crystal is designed for high power and efficient self-Raman laser operation. Selective frequency mixing mechanisms between the fundamental wave and the first Stokes wave using the LiB3O5 (LBO) and BaB2O4 (BBO) crystals are comparatively studied by temperature tuning and angle tuning, respectively. Considering the frequency mixing conversion efficiency and a relatively fast wavelength switching, the BBO crystal with critical phase matching is selected as the second order nonlinear optical crystal for frequency conversion. It only needs to fine-tune the phase match angle of BBO crystal within 1.4°, and thus successfully realizing all second harmonic and sum frequency generation between the fundamental wave and the first Stokes wave. Therefore the efficient-switchable output of the three wavelengths of 532 nm green light, 559 nm lime light and 588 nm yellow light is obtained. Under the incident pump power of 19.5 W and the pulse repetition rate of 60 kHz, maximum average output power of 4.37 W at 532 nm, 2.03 W at 559 nm, 3.43 W at 588 nm are achieved. The conversion efficiency values of the corresponding pump light to visible light are 22.4%, 10.4% and 17.6%, respectively. The corresponding pulse widths are 36 ns, 12.2 ns and 12.7 ns, respectively. The results show that the selective frequency mixing of self-Raman operation is an efficient approach to achieving the wavelength-switchable emission in visible waveband. This wavelength-switchable laser source has important applications in the areas of laser therapy, visual display, spectral imaging and biological medicine.
Effect of spin levels broadening in electronic localized states of oxygen-doped nanosilocon localized state
Authors: Li Xin , Huang Zhong-Mei , Liu Shi-Rong , Peng Hong-Yan , Huang Wei-Qi ,   College of Materials and Metallurgy, Guizhou University, Guiyang 550025, China,   College of Physics & Electronic Engineering, Hainan Normal University, Haikou 571158, China,   State Key Laboratory of Environment Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550003, China
Published: December 2020

It is interesting that the electronic spin gap is opened in the localized states of nanosilicon doped with oxygen, where spin splitting of the individual two-level ±1/2 states isolated in the localized states increases by 1−2 order of magnitude (on the order of 100 meV). The opening spin level effect in the localized states is observed in experiment, which originates from the twin states of quantum vibration measured in the photovaltaic system consisting of the quantum dots and the quantum layers of silicon prepared by using a pulsed laser in an oxygen environment. The opening spin level effect in the localized states is investigated by using density functional theory (DFT) in the simulation models of the quantum dots and the quantum layers of silicon with Si=O bond or Si—O—Si bond on surface. The detailed simulating calculations show that the broader splitting gaps of the electronic spin polarization confined at the individual impurity atoms occur in the localized states, which are consistent with experimental results. A physical model is built to explain the opening spin levels effect, in which the opening spin level effect mechanism in the localized states originates from the quantum confinement at doping atom. The opening spin level effect will improve the fidelity of information stored and processed within such a spin qubit.
Hot injection synthesis of core-shell upconversion nanoparticles for bioimaging application
Authors: Liu Bei , Lu Xi-Jian , Liu Xiao-Ning , Wu Yi-Pin , Zou Bin ,   School of Science, Minzu University of China, Beijing 100081, China
Published: December 2020

In recent years, lanthanide-containing upconversion nanoparticles (UCNPs) have aroused the extensive interest in bioimaging due to their unique upconversion fluorescent properties, such as the high tissue penetration depth, good biocompatibility, low auto-fluorescence, and high imaging sensitivity. In this work, we synthesize a series of NaYF4:Yb, Tm@NaYF4 core-shell structured nanoparticles with various shell thicknesses. A “hot injection” strategy is introduced to fabricate the core-shell UCNPs through using high boiling-point mixtures (sodium/rare-earth trifluoroacetates dissolved in oleic acid and octadecene at 150 °C) as shell precursor solutions. The as-synthesized UCNPs are characterized by transmission electron microscope, particle size analysis and fluorescence spectra. The experimental results show that the shell thickness of UCNPs can be well controlled within a range from 4.2 nm to 32.6 nm by simply tuning the added quantity of the shell precursors. Meanwhile, the upconversion luminescence intensity of NaYF4:Yb, Tm@NaYF4 shows tens times higher than that of NaYF4:Yb, Tm owing to the effective suppression of surface quenching. The optimized thickness of the shell is determined to be 22.7 nm. An ultrathick inert shell (>22.7 nm) is not beneficial to upconversion luminescence mainly due to a strong scattering effect. In addition, the in vitro upconversion luminescent bioimaging application is demonstrated by using the as-synthesized core-shell structured UCNPs. Typically, the prepared OA capped UCNPs are dispersed in HCl solution to obtain hydrophilic ones, followed by polyethylene glycol (PEG) modification to improve their biological compatibility. The hydrophilic NaYF4:Yb, Tm@NaYF4@PEG nanostructures (denoted as UCNP@PEG) show a good biocompatibility with HeLa cells, as the viability of HeLa cells do not decrease obviously when the concentration of UCNP@PEG increases to 0.2 mg/mL. Then, we evaluate the upconversion luminescent signals of UCNP@PEG in HeLa cells under the excitation of 980 nm laser. An obviously increasing upconversion luminescent signal can be observed in HeLa cells with the incubation time increasing from 0.5 h to 6.0 h, indicating that the UCNP@PEG can be used as an excellent luminescence probe for cell imaging and monitoring the cell endocytosis process. All in all, we offer an efficient “hot injection” strategy of fabricating the core-shell structured UCNPs with various shell thickness for improving the upconversion efficiency of UCNPs, which will pave the way for new bioimaging and medical applications.
Mechanism of longitudinal magnetic field suppressed Richtmyer-Meshkov instability
Authors: Sha Sha , Zhang Huan-Hao , Chen Zhi-Hua , Zheng Chun , Wu Wei-Tao , Shi Qi-Chen ,   Key Laboratory of Transient Physics, Nanjing University of Science and Technology, Nanjing 210094, China,   Beijing Institute of Electronic System Engineering, Beijing 100854, China,   School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
Published: December 2020

Based on the ideal magneto-hydro-dynamic equations (MHD) and adopting the CTU (Corner Transport Upwind) + CT (Constrained Transport) algorithm, the Richtmyer-Meshkov (RM) instability of the Gaussian distribution R22 gas column generated by the interaction of a planar shock wave with the gas column in the presence of magnetic field is investigated numerically. The numerical results show that the evolution of shock wave structure and interface instability during the interaction between shock wave and gas column are consistent with the experimental results of Haas and Sturtevant (Hass J F, Sturtevant B 1987 J. Fluid Mech. 181 41) in the absence of a magnetic field. In the presence of a longitudinal magnetic field, the evolution of the shock structures is almost unaffected, while the density interface appears very smooth and interfacial vortex sequences vanish, which is a clear indication of the RMI suppresion. Moreover, as the shock wave impinges on the interface, the magnetic field lines refract on the interface, resulting in the generation of magnetic field gradient in the flow field, which is not consistent with the scenario in the above case. As a result, a stronger magnetic tension is produced on the inner and outer vorticity layer. Simultaneously, the magnetic tension produces a moment on the interface fluid, which is opposite to the rolling direction of the velocity shear. Therefore, the interface instability and the main vortex rolling-up are effectively suppressed. In addition, the non-uniform distribution of magnetic tensions along the interface changes the aggregation degree of the magnetic lines, amplifying the magnetic strength, and finally enhancing the suppression effect of the magnetic field on the interface instability.
Surface plasmon induced transparency in coupled microcavities assisted by slits
Authors: Chu Pei-Xin , Zhang Yu-Bin , Chen Jun-Xue ,   School of Science, Southwest University Of Science and Technology, Mianyang 621010, China
Published: December 2020

The coupled waveguide-microcavity structure has a wide range of applications in optical filters and optical modulators. The optical transmission properties of structure are mostly determined by the coupling strength of the modes. In the conventional waveguide-microcavity structure, the mode coupling is finished by the form of evanescent field, which is usually achieved by controlling the geometric spacing between waveguide and microcavity. Surface plasmon polaritons are the excitations of the electromagnetic waves coupled to collective oscillations of free electrons in metal. Since the electromagnetic waves are attenuated sharply in the metal, this requires precise control of the spacing between the waveguide and the metal microcavity, and poses a great challenge for controlling the coupling of modes in the metal waveguide-cavity structure. In this paper, we proposed a scheme of using a metal-dielectric-metal waveguide side coupling metal microcavities to overcome this limit. Based on the resonant characteristics of the Fabry–Pérot mode in the metal microcavity, a slit is introduced to connect the waveguide and microcavities. By adjusting the width and the offset location of slits, the leakage rate and coupling strength of the mode in metal microcavity can be controlled. The finite difference frequency domain (FDFD) method was used to numerically simulate the electromagnetic properties of structure. First, we have studied the transmission behaviors of surface plasmon polaritons in the system consisted by metal waveguide and single microcavity. As other microcavity is introduced to the structure and connected the original microcavity by slit, the electromagnetically induced transparency phenomena based on surface plasmon polaritons are demonstrated in the coupled metal waveguide and double microcavities structure. As the width of slit connected the microcavity is increased, the transmission peak of structure and the full width at half maximum of the transparency window also increase accordingly. The change of the geometric parameters of slit will modulate the resonance characteristics of structure, and the corresponding physical mechanism is explained by the temporal coupled mode theory. In our works, the metal waveguide and microcavities are coupled by the energy leakage of microcavities assisted by slits, which breaks the limit of separation distance between metal waveguide and microcavity, and contributes to the manufacture of devices. The results of the paper will have applications in designing the compact photonic devices based on surface plasmon polaritons.
Bursting oscillation analysis and synergetic control of permanent magnet synchronous motor
Authors: Zhang Shao-Hua , Wang Cong , Zhang Hong-Li ,   School of Electrical Engineering, Xinjiang University, Urumqi 830047, China
Published: December 2020

The main purpose of this paper is to reveal the evolution mechanism of the bursting oscillation and suppress the bursting oscillation. The permanent magnet synchronous motor (PMSM) system is taken as a research object, and the case of the PMSM with periodic external load perturbation is considered. The first part in this paper is for the analysis of bursting oscillation. First, a mathematical model of the non-autonomous PMSM system with external load perturbation is established, and the frequency of the external load perturbation is set to be far less than the natural frequency of the PMSM system, so that the PMSM system has a fast-slow coupling effect. Then, the non-autonomous PMSM system with external load perturbation is transformed into a generalized autonomous PMSM system by taking the external load perturbation as a slow-varying parameter of the PMSM system. In order to obtain the bifurcation behaviors and different equilibrium types of the PMSM system, the time series diagram, the equilibrium point distribution curve that changes with slow-varying parameter, and the transformed phase portrait are analyzed. Finally, the evolution mechanism of bursting oscillation is revealed by analyzing the overlay of the equilibrium point distribution curve and the transformed phase portrait, and it is found that the change of the equilibrium type and the corresponding bifurcation behavior will cause the PMSM system to exhibit “periodic symmetrical subcritical Hopf bursting oscillation”. The second part focuses on the control of the bursting oscillation. First, a macro-variable is defined by using the synergetic control strategy, which is a linear combination of all state variables of the PMSM system. Then, the synergetic controller is designed based on the constraint that the macro-variable converges to the invariant manifold. When the macro-variable converges to the invariant manifold, the PMSM system is also stabilized to the equilibrium. In addition, in order to explore the influence of controller parameters, a large number of simulation experiments are carried out, and the relationship between the control parameters with the response speed of the PMSM system is obtained. Finally, the effectiveness of the synergetic control strategy is verified by changing the amplitude of the external load perturbation. The simulation results show that the synergetic control strategy has a continuous control law when the system has external load perturbations, and can effectively suppress the bursting oscillation phenomenon of the PMSM system, so that the PMSM system runs stably.
Topological phase transition based on structure reversal of two-dimensional photonic crystals and construction of topological edge states
Authors: Fang Yun-Tuan , Wang Zhang-Xin , Fan Er-Pan , Li Xiao-Xue , Wang Hong-Jin ,   School of Computer Science and Telecommunication Engineering, Jiangsu University, Zhenjiang 212013, China
Published: December 2020

Two kinds of two-dimensional photonic crystal with hexagonal honeycomb lattices are constructed in which the scatterer and the matrix materials are reversed. Due to the symmetry of special point group, the lattices have p and d orbitals in the center of Brillouin region, which are similar to those in the electronic system. With the structure reversal, the p and d orbitals are also directly inverted. Quantitative analysis shows that the orbital inversion is due to the inversion of air band and medium band because of the local resonance effect in the low frequency bands. Based on the parity properties of p and d orbitals, the pseudo spin states are constructed by analogy to the quantum spin Hall effect in electronic systems. The analysis of the effective Hamiltonian at Γ point shows that the topological phase transition caused by orbital inversion is revealed. The pseudo spin edge states construct an optimal structure. The electromagnetic wave simulations and energy flow vector analysis show that the structure edge takes on the properties of quantum spin Hall effect, namely, the propagation direction is locked by the spin direction and the propagation is topologically protected. The results also show that the quantum spin Hall effect can be realized without undergoing the closing of gap. The comparison among similar researches indicates that the realization of the pseudo spin states does not need the deformation of lattice, and the structure proposed in this work possesses the characteristics of simple design, wide band gap and strong edge localization.
Temperature dependent characteristics of Ni/Au vertical Schottky diode based on mechanically exfoliated beta-Ga2O3 single crystal
Authors: Long Ze , Xia Xiao-Chuan , Shi Jian-Jun , Liu Jun , Geng Xin-Lei , Zhang He-Zhi , Liang Hong-Wei ,   School of Microelectronics, Dalian University of Technology, Dalian 116024, China
Published: December 2020

In this paper, a Ni/Au vertical structure Schottky diode based on mechanically exfoliated β-Ga2O3 is fabricated. The temperature dependent characteristics of I-V curves are measured. The device shows a good rectifying behavior. As the temperature increases from 300 K to 473 K, the barrier height increases from 1.08 eV to 1.35 eV, and the ideal factor decreases from 1.32 to 1.19. Both of them show strong temperature dependence, which indicates that the Schottky barrier of the device is inhomogeneous. The device has a double exponential forward I-V characteristic curve, which may be related to crystal defects, surface states, surface energy band bending and the effect of mechanical exfoliation from the crystal surface. Through Cheung's method and Norde's method, the series resistances and barrier heights of the device at different temperatures are extracted. It is found that the parameters extracted by the Norde's method are in good agreement with the values obtained from the forward I-V curve. The series resistance decreases with temperature increasing, which is mainly caused by the increase of the concentration of thermally excited carriers. In this paper, the temperature characteristics of the device are modified by the Gauss distribution of the barrier height. The corrected barrier height is 1.54 eV and Richardson's constant is 26.35 A·cm–2·K–2, which is closer to the theoretical value. It shows that the I-V temperature characteristics of Au/Ni/β-Ga2O3 Schottky diodes can be described by the thermionic emission model of the Gauss distribution barrier height accurately. There are a lot of surface states on the surface of Ga2O3 single crystal obtained by Mechanical exfoliation, which has a great influence on the Schottky contact of the device and may lead to the inhomogeneity of Schottky barriers. At the same time, due to mechanical exploiation, the surface of gallium oxide single crystal material is not completely continuous, and the single crystal surface has layered or island structure. This will also cause the inhomogeneous Schottky barrier height. Considering the influence of inhomogeneous barrier on Schottky diode, the method of measuring the temperature characteristics is more suitable to extracting the electrical parameters of β-Ga2O3 Schottky diodes than the method of fitting I-V forward curve by TE model.
Effects of ascorbic acid post-treatment on thermoelectric properties of poly (3, 4-ethylenedioxythiophene) thin films by a vapor phase polymerization
Authors: Wang Jiao , Liu Shao-Hui , Zhou Meng , Hao Hao-Shan ,   School of Science, Henan Institute of Engineering, Zhengzhou 451191, China,   School of Materials Science and Engineering, Tongji University, Shanghai 201804, China
Published: December 2020

Thermoelectric (TE) material is a kind of energy conversion material, which can be used for power generation and refrigeration. Until now, traditional inorganic TE materials have shown high dimensionless thermoelectric figure of merit (ZT) values. But their expensive raw material and high processing cost, heavy metal pollution and poor processability limit their broad applications. Poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymers possess some excellent features, such as high electrical conductivity, low thermal conductivity, flexibility, low cost, abundance, and light weight. More and more attention has recently been paid to the TE properties of PEDOT polymers and PEDOT polymer based nanocomposites. Ascorbic acid (VC) is used as a reducing agent to tune the PEDOT-Tos-PPP film. The PEDOT-Tos-PPP films via VPP technique are treated with VC solutions with different concentrations. The TE properties of the films before and after being treated with VC at different concentrations are measured. The effect of concentration of VC aqueous solution on the thermoelectric properties and stabilities of the film are studied. The results indicate that the power factor of the film after being treated with 20% VC is 55.6 μW·m–1·K–2, which is 1.7 times as high as that of the pristine PEDOT-Tos-PPP film (34.4 μW·m–1·K–2). The maximum ZT value at room temperature is 0.032. After the VC treatment, the conductivity and Seebeck coefficient of the PEDOT film show unstable characteristics in the air, which is mainly due to the further oxidation of the neutral state on the PEDOT film surface in the air.
Improving interface properties of Te based thermoelectric materials and composite electrodes
Authors: Guo Jing-Yun , Chen Shao-Ping , Fan Wen-Hao , Wang Ya-Ning , Wu Yu-Cheng ,   College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China,   College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China
Published: December 2020

Owing to their excellent performances, Te-based thermoelectric materials have been extensively concerned. However little attention has been paid to the bonding interfaces with electrodes, which play an important role in their practical applications. Excessive element mutual diffusion occurs across the bonding interfaces when Te is connected with metallic electrode, such as copper, aluminum, iron, etc, which will impair its transport performance and life especially when they serve in the higher temperature environments. Seeking proper barriers is the key to solving the interface problem. In this work, a gradient bonding structure of Te/FeTe/Fe is prepared in one step by the spark plasma sintering (SPS) method, in which a metallic layer of FeTe, referred to as β(FeTe) phase, is introduced as barrier. The interface microstructure, element distribution, and new phases are analyzed, and the joint properties including contact resistance and shearing strength after being aged are evaluated. The results show that the introduction of β(FeTe) phase can promote the boding of Fe/β(FeTe)/Te and thus inhibiting the excessive element diffusion across the interfaces, which is due to the formation of ε(FeTe2) phase between β(FeTe) phase and Te. The contact resistance of Fe/β(FeTe) and β(FeTe)/Te are 4.1 μΩ·cm2 and 7.54 μΩ·cm2, respectively, and the shearing strength are 16.11 MPa and 15.63 MPa, respectively. The annealing temperature has significant effect on the performance of the gradient bonding structure. It has been indicated that the whole joint still owns good performance after being annealed at 553 K for 15 days, while it decreases sharply when the temperature is increased to 573 K. Hence, the optimal service temperature of Te/β(FeTe)/Fe should not be higher than 553 K. The gradient bonding structure is successfully achieved, thus attaining the purposes of inhibiting interface elements from excessively diffuse, reducing interface residual stress, and improving interface working stability and service life. So the design ideas and research results in this work have great reference significance for the study on semiconductor devices.
Approximate method to evaluate the regional control efficacy of COVID-19
Authors: Li Ji-Peng , Hong Feng , Bai Wei , Liao Jing-Yi , Zhang Yan-Ru , Zhou Tao ,   Big Data Research Center, University of Electronic Science and Technology of China, Chengdu 611731, China,   Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China
Published: December 2020

The cumulative number of local confirmed cases was highly correlated with the total inflow population from Wuhan before the city closure, and the proportion of third-generation infections in a local region is tiny. Accordingly, we proposed an approximate method to quantitatively evaluate the regional control efficacy of COVID that takes into account effects of both imported cases and regional population. We applied this method to evaluate the control efficacy of the top-50 cities with maximum inflow population from Wuhan. The 10 cities with the most significant control efficacy are Shijiazhuang, Luoyang, Enshi, Zhoukou, Xiamen, Guiyang, Xianning, Anqing, Xinyang, and Nanning.
Research on negative curvature terahertz fiber based on nested triangle structure cladding
Authors: Meng Miao , Yan De-Xian , Li Jiu-Sheng , Sun Shuai ,   Key Laboratory of Electromagnetic Wave Information Technology and Metrology of Zhejiang Province, College of Information Engineering, China Jiliang University, Hangzhou 310018, China,   Centre for THz Research, China Jiliang University, Hangzhou 310018, China,   College of Precision Instrument and Optoelectronic Engineering, Tianjin University, Tianjin 300072, China
Published: December 2020

In the terahertz communication and imaging systems, terahertz fibers have aroused great interest in the past several years. Countering the terahertz wave applications ‘inefficient transmission’ calls for a rapid development in terahertz fibers that could achieve low confinement loss, chromatic dispersion and large refraction of power at the same time. In this paper, a new type of negative curvature terahertz fiber is designed, which consists of six cladding tubes evenly distributed in the cladding and nested with equilateral triangle structure. By using full vector finite element method and changing the thickness of cladding tube and triangle, the effective mode field area, core power ratio, confinement loss, dispersion and other parameters of negative curvature fiber are studied. Here, the thickness range of 70–100 μm is selected. It is found that the confinement loss of optical fiber can reach 0.005 dB/cm at 2.36 THz, the dispersion coefficient can float up and down at ±0.1 ps/(THz·cm) at the frequency range of 2.1–2.8 THz, the core power ratio can reach above 99% in the same frequency range. Compared with the known terahertz negative curvature fiber, the nested triangle negative curvature fiber has lower confinement loss and wide transmission bandwidth of 2.1–2.8 THz. After that, when the cladding tube and the triangle thickness are kept at 90 μm, the bending degree of the triangle edge is changed, and the above properties are further studied. When the triangle edge is bent in and out, the transmission performance of the fiber is analyzed. It is found that when the triangle edge is bent inward, the transmission characteristics of terahertz wave is much better than that when the triangle edge is bent outward. When the triangle edge is bent inwards, the confinement loss is obviously reduced, reaching 0.002 dB/cm at 2.36 THz. Compared with triangle straight edge, the confinement loss is reduced by 40% and still maintaining 99% core power ratio at certain frequency band. The designed terahertz fiber will have an important application value in the fields of sensing and imaging systems with low loss and wide bandwidth. This makes the Topas COC-based terahertz fiber very suitable for guiding terahertz wave over the desired frequency range.
Multiple aperture underwater imaging algorithm based on polarization information fusion
Authors: Liu Bin , Zhao Peng-Xiang , Zhao Xia , Luo Yue , Zhang Li-Chao ,   School of Information and Communication Engineering, North University of China, Taiyuan 030051, China
Published: December 2020

Underwater optical imaging is the key technology to explore the underwater mystery. However, due to the absorption and backscattering effects of the media in the underwater environment, the image acquired by the detector will be severely degraded. In order to obtain the effective underwater scene information, it is necessary to restore the acquired underwater image. The restoration technology based on differential polarization is one of the main methods of restoring the underwater images, which can suppress the background scattered light by the common-mode suppression between orthogonal polarization graphs, thus realizing the restoration of underwater image. However, the relevant research shows that the restoration effect of this method is general for the underwater non-uniform light field. The main reason is that the estimation errors of polarization degree and background scattering intensity under the condition of the non-uniform underwater light field are large. Out of the above problem, in this paper we present the multiple aperture underwater imaging technology of fused polarization information. The method uses the camera array to realize the large virtual aperture imaging system, thus obtaining the wide-angle light field information, and then to fuse the depth information of the scene to realize the accurate estimation of background scattering light intensity and polarization degree under the condition of underwater non-uniform light field. The estimated parameter value can better reflect the global characteristics of the scene. Through the imaging experiments on the targets with different polarization degrees in the turbid underwater environment, comparing with the current advanced restoration algorithm, the results show that the proposed method can effectively solve the problems of background scattering and polarization degree significant estimation error caused by non-uniform underwater light field, and obtain high-quality restoration results. Through the contrast imaging experiment of the target in the underwater environment with different turbidity concentrations, the results show that with the increase of turbidity concentration of the water, the image recovery effect of the method in this paper is gradually weakened. However, it still has a good restoration effect at a large concentration. At the same time, imaging experiments are conducted on targets in underwater environments with different sediment concentrations. The results show that the method proposed in this paper can also obtain a better restoration image in the turbid water environment containing sediment.
Two-dimensional material as a saturable absorber for mid-infrared ultrafast fiber laser
Authors: Zhang Qian , Jin Xin-Xin , Zhang Meng , Zheng Zheng ,   School of Electronic and Information Engineering, Beihang University, Beijing 100083, China
Published: December 2020

The two-dimensional (2D) nanomaterial saturable absorber represented by graphene is widely used in ultrafast fiber lasers due to its unique nonlinear optical properties. In this paper, we summarize the research and development of 2D nanomaterials as saturable absorbers in mid-infrared ultrafast mode-locked fiber lasers in recent years, and introduce the atomic structure and nonlinear optical characteristics of 2D nanomaterials, and saturable absorber device integration methods. The laser performance parameters such as center wavelength, repetition frequency and average output power of the laser are discussed, and the femtosecond fiber laser based on black phosphorus saturable absorber in the middle infrared band is highlighted. Finally, the developments and challenges of 2D materials in mid-infrared pulsed fiber laser are also addressed.