Showing papers in "Acta Physica Sinica in 2018"

Review of high temperature piezoelectric materials, devices, and applications

Abstract: Piezoelectric functional materials have been extensively studied and employed in numerous devices. With the rapid development of modern industries, such as power plants, aerospace, automotive, renewable energy and material processing industries, the high temperature piezoelectric materials that can work in extreme environments are in great demand. Piezoelectric materials including piezoelectric single crystals, ceramics and films, are at the heart of electromechanical actuation and sensing devices. A variety of applications where piezoelectric actuators and sensors operate at elevated temperatures (T 200℃) would be extremely desired. The actuators need to work efficiently with high strokes, torques, and forces while operating under relatively harsh conditions. These include high-temperature fans and turbines, motors for valves or natural gas industries, kiln automation, and actuators for automotive engines such as fuel injectors and cooling system elements. Yet, the majority of industrial actuator applications are at or below the 250℃ temperature limit. In addition to the increase in operational temperatures of piezoelectric motors and actuators, a future area of interest is high-temperature MEMS research, which can be used for high-temperature valving. On the other hand, the piezoelectric sensors have been widely used for structural health monitoring applications. This is due to their wide bandwidth, versatility, simplicity, high rigidity, high stability, high reproducibility, fast response time, wide operating temperature range, insensitivity to electric and magnetic fields, the capacity for miniaturization and minimal dependence on moving parts and low power consumption, and wide piezoelectric materials and mechanisms selections, which will greatly benefit the sensing applications. In addition to the temperature usage range, the piezoelectric sensors must withstand the harsh environments encountered in space, engine, power plants, and also need to possess high sensitivity, resistivity, reliability, stability and robustness. In order to use the piezoelectric materials for a specific high temperature application, many aspects need to be considered together with piezoelectric properties, such as phase transition, thermal aging, thermal expansion, chemical stability, electrical resistivity, and the stability of properties at elevated temperature. In this paper, ferroelectric materials with high Curie point, including perovskite-type ferroelectrics, bismuth layer structured ferroelectrics, tungsten-bronze structured ferroelectrics, together with non-ferroelectric piezoelectric single crystals, are surveyed. The crystal structure characteristics, high temperature piezoelectric properties, and recent research progress are discussed. A series of high temperature piezoelectric devices and their applications are reviewed, including high temperature piezoelectric detectors, sensors, transducers, actuators, etc. Finally, recent important research topics, the future development of high temperature piezoelectric materials and the potential new applications are summarized.

Refractive index sensor and filter of metal-insulator-metal waveguide based on ring resonator embedded by cross structure

Abstract: Continuous improvement in nanofabrication and nano-characterization capabilities have changed projections about the role that metals could play in developing the new optical devices. Surface plasmon polaritons are evanescent waves that propagate along a metal-dielectric interface. They can be laterally confined below the diffraction limit by using subwavelength metal structures, rendering them attractive to the development of miniaturized optical devices. A surface plasmon polariton refractive index sensor and filter which consist of two metal-insulator-metal (MIM) waveguides coupled to each other by a ring resonator embedded by cross structure are proposed. And the transmission characteristics of surface plasmon polaritons are studied in our proposed structure. The transmission properties of such a structure are simulated by the finite element method, and the eigenvalue wavelengths of the ring resonator are calculated theoretically. The sensing characteristics of such a structure are systematically analyzed by investigating the transmission spectrum. The results show that there are three resonance peaks in the transmission spectrum, that is, three resonance modes corresponding to the eigenvalue solutions of the first, second and third-order Bessel eigen-function equations, and each of which has a linear relationship with the refractive index of the material under sensing. Through the optimization of structural parameters, we achieve a theoretical value of the refractive index sensitivity (S) as high as 1500 nm/RIU, and the corresponding sensing resolution is 1.3310-4 RIU. More importantly, it is sensitive to none of the parameters of our proposed structure, which means that the sensitivity of the sensor is immune to the fabrication deviation. In addition, by the resonant theory of ring resonator, we find a linear relationship between the resonance wavelength and the radius of ring resonator. So the resonance wavelength can be easily manipulated by adjusting the radius and refractive index. In addition, the positions of transmission peaks can be easily modulated by changing the radius of the ring, which can be used to design band-pass filter for a large wavelength range. Moreover, the transmission intensity and the transmission bandwidth decrease as spacing distance between the MIM waveguide and ring cavity increases. These results would be helpful in designing the refractive index sensor of high-sensitivity and band-pass filters, and have guiding significance for biological sensor applications.

Bifurcation mechanism of not increase but decrease of spike number within a neural burst induced by excitatory effect

Abstract: Nonlinear dynamics is identified to play very important roles in identifying the complex phenomenon, dynamical mechanism, and physiological functions of neural electronic activities. In the present paper, a novel viewpoint that the excitatory stimulus cannot enhance but reduce the number of the spikes within a burst, the novel viewpoint which is different from the traditional viewpoint, is proposed and is explained with the nonlinear dynamics. When the impulse current or the autaptic current with suitable strength is used in the suitable phase within the quiescent state of the bursting pattern of the Rulkov model, a novel firing pattern with reduced number of spikes within a burst is evoked. The earlier the application phase of the current within the quiescent state, the higher the threshold of the current strength to evoke the novel firing pattern is and the less the number of the spikes within a burst of the novel firing pattern. Moreover, such a novel phenomenon can be explained by the intrinsic nonlinear dynamics of the bursting combined with the characteristics of the current. The nonlinear behaviors of the fast subsystem of the Rulkov model are acquired by the fast and slow variable dissection method, respectively. For the fast subsystem, there exist a stable node with lower membrane potential, a stable limit cycle with higher membrane potential, a saddle serving as the border between the stable node and limit cycle, a saddle-node bifurcation, and a homoclinic orbit bifurcation. When external simulation is not received, the bursting pattern of the Rulkov model exhibits behavior alternating between the spikes corresponding to the limit cycle of the fast subsystem and quiescent state of the fast subsystem, which is located within the parameter region between the saddle-node bifurcation point and the homoclinic orbit bifurcation point of the fast subsystem. The spikes begin with the saddle-node bifurcation and end with the homoclinic orbit bifurcation. As the bifurcation parameter turns close to the homoclinic orbit bifurcation, the disturbation or stimulus that can induce the transition from the quiescent state to the spikes becomes strong. Therefore, as the application phase of the current within the quiescent state becomes earlier, the strength threshold of the current that can induce the transition from the quiescent state to the spikes becomes stronger, and the initial phase of the spikes becomes closer to the homoclinic orbit bifurcation, which leads the parameter region of the spikes to become shorter and then leads the number of spikes within a burst to turn less. It is the dynamical mechanism of the decrease of the spike number induced by the excitatory currents. The results enrich the nonlinear phenomenon and dynamical mechanism, present a novel viewpoint for the excitatory effect, and provide a new approach to modulating the neural bursting patterns.

Research progress and application prospect of Fe-based soft magnetic amorphous/nanocrystalline alloys

Abstract: Amorphous alloy is a kind of metallic materials prepared by rapidly cooling the alloy melt through hindering crystallization in cooling process. Due to the unique structure of atomic random packing, Fe-based amorphous alloys exhibit not only structural and property isotropy, but also small structural correlation length, small magnetic anisotropic constant, and then small coercivity Hc. Like crystalline Fe-based alloys, Fe-based amorphous alloys also possess high saturation induction Bs. As a result, research on engineering applications of Fe-based amorphous alloys has been promoted by their excellent soft magnetic properties. Now Fe-based soft magnetic amorphous/nanocrystalline alloys have been produced and applied to various areas on a large scale. Here in this paper, the processes of discovery, development and application of Fe-based soft magnetic amorphous alloys are reviewed, and the effects of chemical composition, structure and preparation technology on the soft magnetic properties are introduced and discussed. The obtained theoretic results and the technological innovation show that the great contributions have been made to the development and application of Fe-based soft magnetic amorphous/crystalline alloys. Based on the progress of structure and soft magnetic property and our understanding, the development process of the fundamental research and the application progress of Fe-based soft magnetic amorphous alloys could be divided into three periods. In addition, the present challenge topics in their researches and applications are proposed.

Generation of Bessel-Gaussian vortex beam by combining technology

Abstract: Bessel beam is an important member of the family of non-diffracting beams and has some unique properties which can be used in many areas, such as micro particle manipulating, material processing and optical communication. However, the source of Bessel beam generated by the existing methods can be used only in a short distance due to its low power. In this paper, according to the coherent combining technology, we propose a method to generate a second-order Bessel-Gaussian (BG) beam by loading discrete vortex phase on specific spatially distributed Gaussian beam array. The coherent combining technology can enhance the output power by increasing the number of beams and use the phase-locking technique to maintain the beam quality. The experimental scheme is described as follows. The expanded Gaussian beam is first split by an amplitude-based spatial light modulator, then the Gaussian beam array is incident on a phase-only spatial light modulator to load the discrete vortex phase, and finally the Gaussian beam array loaded with phase can synthesize BG beam in free space. Due to the diffraction effect of the sub-beams, the optical field distribution between the adjacent sub-beams which are loaded with phase differences, are superimposed. As a result, the optical field distribution of the approximate beam can be obtained by coherent synthesis in free space. After that, the degree of similarity between simulated results and theoretical data is analyzed by correlation coefficient, including the comparison of light intensity between experiment and simulation, and the power-in-the-bucket is used to evaluate beam quality. In addition, the topological charge of the synthesized BG beams is verified by the interference method. By studying the number of beams, the waist radius and the radius of the ring, we find some interesting results which are summarized as follows. Firstly, the closed arrangement of Gaussian beam arrays can improve the quality of the synthesized BG beam. Secondly, the smaller the phase difference between the sub-beams, the more easily the discontinuous piston phase approaches to the vortex phase. Therefore, increasing the number of sub-beams can significantly improve the beam quality of the synthesized BG beam and obtain a higher order synthetic BG beam. Finally, we define the parameter k to represent the tightness of a circular array of Gaussian beams. The present study shows that when the parameter k is close to 1, the best experimental results can be obtained. Therefore, the proposed method has important guidance in generating various vortex beams or enhancing the vortex beam power.

Piezo-electrochemical coupling of AgNbO3 piezoelectric nanomaterials

Abstract: 采用水热法合成了AgNbO3压电纳米材料，表征了其压-电-化学耦合用于机械催化的物理机理.该耦合是压电效应和电化学氧化还原效应的乘积效应.经历60 min的机械振动后，AgNbO3纳米材料机械催化振动降解罗丹明B（~5 mg/L）的降解率达70%以上.压-电-化学耦合效应的中间产物强氧化的羟基自由基也被检测到，这表明压-电-化学耦合效应在实现机械催化过程中的关键作用.经过5次回收再利用，AgNbO3纳米材料的机械催化活性无明显降低.AgNbO3压电纳米材料具有高的压-电-化学耦合、高的机械催化降解率、可多次重复使用等优点，在振动降解有机染料方面具有重要的应用前景.

Experimental study on the stimulated saturation of terahertz free electron laser

Abstract: China Academy of Engineering Physics terahertz free electron laser (CAEP THz FEL,CTFEL) is the first THz FEL oscillator in China,which is jointly built by CAEP,Peking University and Tsinghua University.It is designed as a high-repetition-rate and high-duty-cycle linac-based FEL facility. This THz FEL mainly consists of a gallium arsenide (GaAs) photocathode high-voltage direct current (DC) gun,a superconducting radio frequency (RF) linac,a planar undulator,and a quasi-concentric optical resonator. The DC gun provides a high-brightness electron beam with the bunch charge of about 100 pC and the repetition rate of 54.167~MHz.The normalized emittance of the electron beam is less than 10m,and the energy spread is less than 0.75%.A 24-cell superconducting RF accelerator provides an effective field gradient of about 10 MV/m and energizes the electron beam to 6-8~MeV.The beam then goes through the undulator and generates the spontaneous radiation,which is reflected back and forth in the optical resonator and then stimulated by the electron beam. The first stimulated saturation of CTFEL in the macro-pulse mode was obtained in August,2017.In this paper,the THz spectrum is measured by a Fourier spectrometer (Bruker VERTEX 80 V).The macro-pulse energy is measured by an absolute energy meter from Thomas Keating Instruments.The longitudinal beam length is preliminarily calculated by the auto-correlation curve from the time-domain signal of the spectrometer.The macro-pulse duration is captured by a GeGa cryogenic detector from QMC Instrument.The measurement results indicate that the terahertz laser frequency is continuously adjustable from 2 THz to 3 THz.The macro-pulse average power is more than 10 W and the micro-pulse power is more than 0.3 MW.The single-pass gain is larger than 2.5%. This facility is now working in macro-pulse mode in the first step,also called step one.The minimum macro-pulse duration is about 50s and the maximum is about 2 ms.The macro-pulse repetition is 1 Hz or 5 Hz.The typical pulse duration and repetition rate are 1 ms and 1 Hz,respectively.In the middle of 2018,the duty cycle will upgrade to more than 10% as step two.And the continuous wave (CW) operation will be obtained in step three by the end of 2018.The spectrum adjustment range will also be expanded to cover from 1 THz to 4 THz by then. Some application experiments have been carried out on the platform of CTFEL.This facility will greatly promote the development of THz science and its applications in material science,chemistry science,biomedicine science and many other cutting-edge areas in general.

Progress of converse magnetoelectric coupling effect in multiferroic heterostructures

Abstract: Electric-field control of magnetism has recently received much attention because of low-power consumption, which has potential applications in low-power multifunction devices. Ferromagnetic/ferroelectric multiferroic heterostructure is a useful way to realize the electric-field control of magnetism. Strain-mediated magnetoelectric coupling with large magnetoelectric coupling coefficient at room temperature is one of the current research hotspot. In this paper, we give an overview of recent progress of strain-mediated magnetoelectric coupling in multiferroic heterostructures.This review paper consists of five parts:introduction of multiferroics, electric-field control of magnetism in multiferroic heterostructures, electrical control of magnetization reversal, electric-field control of magnetic tunnel junctions, and the future prospects of multiferroic heterostructures. The basic concepts of multiferroics and background of magnetoelectric coupling effect are introduced in the first part.In the second part, a brief review of the recent work on the Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) based multiferroic heterostructures is given. The PMN-PT has a FE domain structure, which plays a vital role in electric-field control of magnetism, especially the 109 domain switching. For PMN-PT (001), the importance of 109 domain switching on the nonvolatile electrical control of magnetism is discussed. For PMN-PT (011), it is shown how to obtain nonvolatile strain which induces magnetic easy axis to be rotated by 90. The work on electric-field modulation of ferromagnetic material with perpendicular magnetic anisotropy is also mentioned.Electric-field control of magnetization reversal is still a challenge and remains elusive. Combination of strain-mediated magnetoelectric coupling and exchanging bias is a promising method to reverse magnetization by electric field, and the exchange-biased system/ferroelectric structures are given in the third part. There are also some theoretical attempts and proposals to realize the electrical control of 180 magnetization reversal. Then the method to manipulate magnetic tunnel junctions by electric field is given through integrating multiferroics and spintronics. Further outlook of the multiferroic heterostructures is also presented finally.

Evaluation methods of node importance in undirected weighted networks based on complex network dynamics models

Abstract: Identifying the most important nodes is significant for investigating the robustness and vulnerability of complex network. A lot of methods based on network structure have been proposed, such as degree, K-shell and betweenness, etc. In order to identify the important nodes in a more reasonable way, both the network topologies and the characteristics of nodes should be taken into account. Even at the same location, the nodes with different characteristics have different importance. The topological structures and the characteristics of the nodes are considered in the complex network dynamics model. However, such methods are rarely explored and their applications are restricted. In order to identify the important nodes in undirected weighted networks, in this paper we propose a method based on dynamics model. Firstly, we introduce a way to construct the corresponding dynamics model for any undirected weighted network, and the constructed model can be flexibly adjusted according to the actual situation. It is proved that the constructed model is globally asymptotic stable. To measure the changes of the dynamic model state, the mean deviation and the variance are presented, which are the criteria to evaluate the importance of the nodes. Finally, disturbance test and destructive test are proposed for identifying the most important nodes. Each node is tested in turn, and then the important nodes are identified. If the tested node can recover from the damaged state, the disturbance test is used. If the tested node is destroyed completely, the destructive test is used. The method proposed in this paper is based on the dynamics model. The node importance is influenced by the network topologies and the characteristics of nodes in these two methods. In addition, the disturbance test and destructive test are used in different situations, forming a complementary advantage. So the method can be used to analyze the node importance in a more comprehensive way. Experiments are performed on the advanced research project agency networks, the undirected networks with symmetric structures, the social network, the Dobbs-Watts-Sabel networks and the Barrat-Barthelemy-Vespignani networks. If the nodes in the network have the same dynamic model, the network is considered to be the homogeneous network; otherwise, the network is heterogeneous network. And experiments can be divided into four categories, namely, the disturbance test, the destructive test on the homogeneous network, the disturbance test and the destructive test on the heterogeneous network. The experimental results show that the methods proposed in this paper are effective and credible.

Computational fluid dynamic investigation of the primary and secondary atomization of the free-fall atomizer in electrode induction melting gas atomization process

Abstract: Nickel-based superalloy is mainly used for fabricating the important high temperature parts including the turbine disk, turbine baffle, compressor disk, and other critical components. Ceramic inclusions in powder metallurgy (PM) superalloy could promote fatigue crack initiation, and thus accelerating the crack propagation under certain conditions. In this case, the ultra-clean nickel-based superalloy powder is critical for PM superalloy components. Generally, there are two well-known methods of fabricating superalloy powders, i.e., argon gas atomization (AA) and plasma rotating electrode process (PREP). Electrode induction melting gas atomization (EIGA) process is a newly developed method of preparing ultra-clean metal powders. The EIGA process is a completely crucible-free melting and atomization process developed by ALD vacuum technologies. In this process, a slowly rotating prealloyed bar is fed into a conical induction coil. The end of the bar is inductively heated and molten alloys falls into an atomizer where the liquid alloy is atomized with a high-pressure inert gas. The EIGA prepared powders possess the advantages of AA (more fine powders) and PREP (ultra-clean powders) processes. Generally, there are two key issues in EIGA process, and the free-fall gas atomizer design is one of the critical issues for the powder yield and quality. Free-fall gas atomizers are some of the first two fluid atomizer designs to be used for molten metal atomization. In a simple open (unconfined stream) design a melt stream falls from a tundish exit via gravity into the convergence of focused atomization gas jets where it is disintegrated. The gas-melt interaction is complex, and it is difficult to characterize the interaction process directly. To have a good understanding of the atomisation technology, the physical break-up process instead of correlating the gas dynamics with droplet fragmentation indirectly must be able to be examined. And it will be desirable, if we input the atomization parameters, we can obtain the particles' distributions directly. In this work, a computational fluid dynamic approach to simulating the primary and secondary atomization processes is developed by using the volume of fluid method and discrete phase model. By integrating the metal stream break-up (in primary atomization) with the flow field and particles distribution simulation (in second atomization), this numerical simulation method is able to provide the direct assessment for the atomisation process. To verify the method performance, the melt stream is initialized into a 4 mm-diameter stream, which is then injected into the gas flow field for further fragmentation. The experimental results show that the simulated particles' diameter distribution is consistent with the experimental results in the same conditions.

First-principles study on the optical properties of Fe-doped GaN

Abstract: Using hybrid density functional theory, we investigate the structural, electronic and optical properties of pristine GaN and Fe-doped GaN with a Fe concentration of 12.5%. Specifically, we first analyze the crystal lattice constant, band structure, and density of states, respectively. Then we predict the dielectric function, absorption coefficient, refractive index, reflectivity, energy-loss spectrum and extinction coefficient. Finally, we analyze the influences of the doping of Fe element on the photoelectric property of Fe doped systems. The calculated lattice constants for perfect GaN are a=b=3.19 A, c=5.18 A, which are in good agreement with the experimental values. Furthermore, we find that the doping of Fe element has little effect on the structural properties of GaN. The Band gap of pristine GaN is predicted to be 3.41 eV, very close to the experimental value of 3.39 eV. The band gap of Fe doped GaN (12.5%) significantly decreases to 3.06 eV. By comparing the densities of states of the systems with and without Fe doping, it is found that Fe-3 d state is mainly responsible for the decrease of band gap. The calculated static dielectric constant of perfect GaN is 5.74, and it increases to 6.20 after incorporating the Fe element. The results about the imaginary part of dielectric function show that two equal-strength perfect GaN peaks are observed to be at 6.81 eV and 10.85 eV. The first peak is closely related to the direction transition from the valence band top to the conduction band bottom. Furthermore, it is also observed that a peak is located at 4.04 eV in the low energy, which can be understood as resulting from the electron transition inside the valence band. The optical absorption edge of the intrinsic GaN is 3.25 eV, corresponding to the transition energy. The reason why this energy is smaller than the bandgap is because the electronic band gap equals the sum of optical bandgap and exciton energy. However, the maximum absorption coefficients of these two systems both occur at 13.80 eV in energy. The refractive index for intrinsic system is 2.39, and it increases to 2.48 after doping the Fe element. It is found from the energy-loss spectrum that the maximum energy-loss is at 20.02 eV for a perfect system, while it is at 18.96 eV for a doped system. Additionally, we obtain the reliable reflectivity and excitation coefficient. In conclusion, our calculated results provide a well theoretical basis for the theoretical research on the co-doping of Fe element and other elements. The analyses on the Fe-doped GaN high-voltage photoconductive switch materials and devices also provide a powerful theoretical basis and experimental support in the future research.

Multi-scale analysis method of underwater polarization imaging

Abstract: Underwater polarization imaging is a valuable technology for underwater detection and exploration, since it can provide abundant information about target scene via the removal of background light from raw images. However, in a conventional polarization imaging method, the reconstructed image has limited quality caused by the inaccurate estimation of degree of polarization (DoP) and noise amplification, which finally leads to the incomplete removal of background light. The situation becomes worse if the target and background light reach an almost equal DoP.To date, various approaches including acoustic imaging, photoacoustic imaging, and polarization imaging have been implemented to realize underwater imaging. Notably, underwater polarization imaging is of particular interest due to its simple system structure, low cost and excellent performance in recovering target information. It mainly involves the separation of the backscattered light denoted as background light from the target scattered light acting as the target light. Removal of the background light from the raw image gives rise to a clear target image, which has been the focus of polarization imaging for a long period. The most representative approach was presented by Schechner[Schechner Y Y, Karpel N 2005 IEEE Journal of Oceanic Engineering 30 570] who utilized the DoP of background light and target light to recover clear image. Further optimization of the approach was also conducted by researchers including Schechner[Tali T, Schechner Y Y 2009 IEEE Transactions on Pattern Analysis and Machine Intelligence 31 385], Huang[Huang B J, Liu T G, Hu H F, Han J H, Yu M X 2016 Optics Express 24 9826], et al. However, the influence of noise amplification in the process on the reconstruction results has always been ignored, which accounts for the results to some extent though the explanation is unsatisfactory.In this paper, we present a multi-scale polarization imaging strategy to suppress the noise amplification effect and its influence on the final results. It originates from the difference in polarization image between two diverse layers. Specifically, the image is divided into two layers, one of which is characterized by high contrast but remarkably difference between the target and background, known as base layer BTI; the other layer is low-contrast but contains the detailed information about the target, known as detail layer DTI. Special processes are applied to the two layers according to their characteristics, respectively. For the base layer BTI, combined bilateral filtering is used to suppress noise. As for the detail layer, it is first processed by wavelet transform with considering its multi-resolution characteristic. After the wavelet coefficient correction via adjusting the kernel function w(x, f), the details in target image is perfected with keeping iterations. During the updating procedure, the image noise can be further suppressed. Underwater experiments are conducted in the laboratory to demonstrate the validity of the proposed method. Besides, quantitative analyses also verify the improvement in final target image.Compared with conventional underwater polarization imaging methods, the proposed method is good at dealing with various target conditions, since it handles noise amplification without requiring any additional equipment. Furthermore, the proposed method is easy to incorporate in a conventional polarization imaging system to achieve underwater images with better quality and valid detail information. Therefore, the proposed method has more potential applications in underwater imaging.

Microwave electromagnetically induced transparency and Aulter-Townes spectrum of cesium Rydberg atom

Abstract: We present an electromagnetically induced transparency and Aulter-Townes (EIT-AT) spectrum of a Rydberg three-level atom that is dressed with a microwave field in a room-temperature cesium cell. The EIT is a quantum coherent effect produced by the interaction of atoms with electromagnetic waves, which leads to the decrease of the absorption for a weak resonant probe laser. AT splitting refers to the phenomenon, that the absorption line splits when an electromagnetic field that is in resonance or near resonance acts on the transition of atoms. Rydberg atoms are extremely sensitive to an external electric field due to their large polarizabilities and microwave transition dipole moments, which can be used to measure the external field. In this work, a Rydberg three-level EIT is used to detect Rydberg atom and AT splitting induced by the microwave field. Cesium levels 6S1/2, 6P3/2 and 50S1/2 constitute a Rydberg three-level system, in which a weak probe laser locking to the transition from 6S1/2 to 6P3/2 couples ground-state transition and the strong coupling laser resonates on the Rydberg transition from 6P2/3 to 50S1/2. The two Rydberg levels 50S1/2 and 50P1/2 are coupled with the microwave field at a frequency of 30.852 GHz, leading to the AT splitting of EIT line and forming an EIT-AT spectrum, which is used to measure the electric field amplitude of microwave. In order to further study the EIT-AT splitting characteristics of the Rydberg levels, we carry out a series of measurements by changing the microwave field. The experimental results show a broadened EIT-AT signal for the weak field range and the four-peak spectrum for the strong field, which is attributed to the inhomogeneity of the microwave field. The microwave in cesium cell, emitted by a function generator, shows inhomogeneous behavior such that the atoms interacting with the laser field experience the different fields, leading to the line broadened and multi-peak EIT-AT spectra. For the microwave transition of nS1/2-nP1/2 in this paper, a pair of EIT-AT lines should be obtained for an electric field value. The broadening of the EIT-AT spectrum and the multi-peak structure here are due to the inhomogeneity of the microwave field measurement. We propose a method to increase the spatial resolution by reducing the length of cesium cell. The result in this work provides a method of measuring the field amplitude and monitoring the distribution of microwave electric field, meanwhile the spatial resolution of the measurements can be improved by reducing the size of the cell.

Review of Parameter extraction methods for single-diode model of solar cell

Abstract: In recent years, the parameter extraction methods of solar cell have attracted a lot of research attention. The reason is that the matching solar cell parameters can effectively reduce the influences of internal and external factors on photovoltaic efficiencies. In this paper, the five-parameter extraction methods of solar cell single-diode model are discussed in detail. The five parameters are the photocurrent, the reverse diode saturation current, the ideality factor of diode, the series resistance, and the shunt resistance. In fact, the existing research methods are classified as four categories, namely, analytically extracting parameter methods, extracting parameter methods with the help of Lambert W function, constructing or using special functions to extract parameter methods, and using intelligent algorithm to extract parameter methods. In this article, we not only elaborate their main theories and approaches, but also discuss their advantages and disadvantages. The main conclusion is that the analytical method for the extraction of solar cell model parameters requires some assumptions. Therefore, this method is fast but less accurate due to various approximations. In addition, the parameter extraction using the analytical method needs a thorough calculation, and deducing the actual values of (dI/dV)|V=Voc and (dI/dV)|I=Isc and peak power point is also challenging. When the five parameters of solar cell are calculated using the Lambert W-function method, the results show that the extraction process is easier when using the consecrated software such as MATLAB, but the larger computational time is needed. Generally, the Lambert-W function provides the exact explicit expression for parameter extraction. As a result, the accuracy of approximate solution using Lambert-W function is much higher than that of the above method. It is obvious that the accuracy of using special functions to extract cell parameters is limited by those function characteristics. Of course, those special functions, such as Green's function, seem to be complex approaches. The accuracy of the extracting cell parameters by using intelligent algorithm strongly depends on the type of fitting algorithm, the fitting criterion, objective function and the starting values of the parameters. Finally, based on the conducted review, the future research trend of parameter extraction is also predicted

Research advances in acoustic metamaterials and metasurface

Abstract: Acoustic metamateiral (AM) is an artificially structured material with the unique properties that cannot be found in nature materials, such as negative refraction, slab focusing, super-resolution imaging, cloaking, inverse Doppler effect, etc. In this paper we first review the research advances in AM in recent 20 years and then mainly discuss the properties of the meta-atom AM (MAAM), meta-molecule AM (MMAM), meta-atom cluster AM, and meta-molecule cluster AM. The MAAM consists of local resonant meta-atoms, whose resonant frequency is related to the geometry size of the structure. The MAAM presents the transmission dip and inversed phase near the resonant frequency. The meta-atoms discussed in the paper contain the split hollow sphere and hollow tube (HT), which can be used to realize the AM with single negative modulus and AM with single negative mass density near the frequency, respectively. The effective parameter of the MAAM is calculated from the transmission and reflection data in experiment according to the homogeneous-medium theory. By combining the two kinds of meta-atoms together, the assembled two-layered composite AM presents a transmission peak similar to the electromagnetic metamaterial in the overlapping resonant frequency region. The effective parameters calculated by experimental data demonstrate that the composite AM could realize simultaneously negative modulus and negative mass density near the peak frequency. In the double-negative band, this kind of double-negative AM can faithfully distinguish the acoustic sub-wavelength details (/7). Furthermore, by coupling the two kinds of meta-atoms in a structure, we design a flute-like meta-molecule structure of perforated hollow tube, which can be used to fabricate double-negative AM in high or low frequency band. The experimental results also show that the double-negative AM has the properties of flat focusing and negative refraction effect. Based on the weak interaction of the meta-atoms, the meta-atom cluster AM can be fabricated by arraying different sized meta-atoms. The meta-atom cluster AM composed of different sized meta-atoms of SHSs can realize multi-band or broadband negative modulus, and the different sized meta-atoms of HTs can realize broadband negative mass density. Similarly, the meta-molecule cluster AMs are constructed with seven kinds of flute-like perforated hollow tubes, which can overcome the limitations of arbitrary broadband negative bulk modulus and mass density to provide a region of inverse Doppler effects. It is also shown that the inverse frequency shift values will be enhanced with the increase of frequency. As the resonant unit can realize the effect of discontinuous phase, it can be used to design acoustic metasurface (AMS) to control the acoustic wavefronts at will and realize the anomalous manipulation of acoustic waves. Finally, we introduce the research status and tendency of AMS in coming years.

Multispectral image enhancement based on illuminance-reflection imaging model and morphology operation

Abstract: In this paper we propose a multispectral image enhancement algorithm based on illuminance-reflection imaging model and morphology operation that enables us to solve the problem of improving the multispectral degraded images. Firstly, we transform the image from RGB space to HSV color space, and the hue remains unchanged. As for the saturation component, we use the adaptive nonlinear stretching to improve the image color saturation and brightness. Secondly, according to the illuminance-reflection imaging model, we adopt the guided image filtering method to decompose the brightness into illuminance component and reflection component. Usually, the illumination component mainly determines the dynamic range of the pixels in the image, corresponding to the low frequency part of the image, reflecting the global characteristics of the image and the edge detail information of the image; the reflected component represents the intrinsic essential characteristics of the image, corresponding to the high frequency part of the image, and contains most of the local detail information of the image as well as all noise. Thirdly, we present an improved adaptive gamma function, which can dynamically adjust the illuminance component by the local distribution characteristics, and use the contrast-limited adaptive histogram equalization to correct the illuminance component. Afterwards we propose a detail-feature weighted fusion strategy. The original illumination and the two corrected illuminations are fused to obtain the final illumination component. Fourthly, we propose an improved morphological operation to denoise and enhance the details of the reflection component. Finally, the corrected illumination component and the enhanced reflection component are combined to obtain the improved brightness component. In order to verify the efficiency of the algorithm proposed in the paper, we use both subjective visual effectiveness method and quantitative parameter analysis method to measure the enhancement performance in multispectral imaging scenarios, including low illumination image, underwater image, high-dynamic range image, sandstorm image, haze image and thermal infrared image. Then standard deviation, information entropy and average gradient are used as evaluation indices respectively, and qualitative and quantitative comparison with a variety of image enhancement algorithms show that the proposed algorithm can not only well suppress noise but also obviously improve local details and global contrast. Experimental results show that the proposed method proves to be better in performance.

Theoretical and experimental investigation of light guidance in hollow-core anti-resonant fiber

Abstract: The inherent material imperfections of solid core optical fiber, for example, Kerr nonlinearity, chromatic dispersion, Rayleigh scattering and photodarkening, set fundamental limitations for further improving the performances of fiber-based systems. Hollow-core fiber (HCF) allows the light to be guided in an air core with many unprecedented characteristics, overcoming almost all the shortcomings arising from bulk material. The exploitation of HCF could revolutionize the research fields ranging from ultra-intense pulse delivery, single-cycle pulse generation, nonlinear optics, low latency optical communication, UV light sources, mid-IR gas lasers to biochemical sensing, quantum optics and mid-IR to Terahertz waveguides. Therefore, the investigations into the guidance mechanism and the ultimate limit of HCF have become a hot research topic. In the past two decades, scientists and engineers have fabricated two types of high-performance HCFs with loss figures of 1.7 dB/km and 7.7 dB/km for hollow-core photonic bandgap fiber (HC-PBGF) and hollow-core anti-resonant fiber (HC-ARF) respectively. In comparison with the twenty-years-old HC-PBGF technology, the HC-ARF that recently appeared outperforms the former in terms of broadband transmission and high laser damage threshold together with a quickly-improved loss figure, providing an ideal platform for many more challenging applications. While the guidance mechanism and fabrication technique in HC-PBGF have been well recognized, the HC-ARF still has a lot of room for improvement. At the birth of the first generation of broadband HC-ARF, the guidance mechanism was unclear, the fiber design was far from perfect, the fabrication was immature, and the optical properties were not optimized. In the past five years, we have developed an intuitive and semi-analytical model for the confinement loss of HC-ARF and managed to fabricate high-performance nodeless HC-ARF. We further employ our theoretical model and fabrication technique to well control and design other interesting properties, such as polarization maintenance and bending loss in HC-ARF. For a long time, the anti-resonant theory of light guidance has been regarded as being qualitative, and the leaky-mode-based HC-ARF have been considered to have worse performances than the guided-mode-based HC-PBGF. Our investigations in theory and experiment negative these prejudices, thus paving the way for the booming development of HC-ARF technologies in the near future.

Nanoscale magnetic field sensing and imaging based on nitrogen-vacancy center in diamond

Abstract: Magnetic field measurement and imaging with nanometer resolution is a key tool in the study of magnetism. There have been several powerful techniques such as superconducting quantum interference device, hall sensor, electron microscopy, magnetic force microscopy and spin polarized scanning tunneling microscopy. However, they either have poor sensitivity or resolution, or need severe environment of cryogenic temperature or vacuum. The nitrogen-vacancy color center (NV center) in diamond, serving as a quantum magnetic sensor, has great advantages such as long decoherence time, atomic size, and ambient working conditions. The NV center consists of a substitutional nitrogen atom and an adjacent vacancy in diamond. Its electronic structure of ground state is a spin triplet. The spin state can be initialized to mS=0 state and read out by laser pulse, and coherently manipulated by microwave pulse. It is sensitive to the magnetic field by measuring the magnetic Zeeman splitting or quantum phase in quantum interferometer strategies. By using dynamical decoupling sequence to prolong the decoherence time, the sensitivities approach to nano tesla for a single NV center and pico tesla for the NV center ensemble, respectively. As a sensor with an atomic size, it reaches single-nuclear-spin sensitivity and sub-nanometer spatial resolution. Combining with scanning microscopy technology, it can accomplish high-sensitivity and high-resolution magnetic field imaging so that the stray field can be reconstructed quantitatively. The magnetic field is calculated from the two resonant frequencies by solving the Hamiltonian of NV center in order to obtain the value of stray field. Recently, this novel magnetic imaging technique has revealed the magnetization structures of many important objects in magnetism research. The polarity and chirality of magnetic vortex core are determined by imaging its stray field; laser induced domain wall hopping is observed quantitatively with a nanoscale resolution; non-linear antimagnetic order is imaged in real space by NV center. It was recently reported that magnetization of the magnetic skyrmion is imaged by NV center. The magnetization distribution is reconstructed from stray field imaging. With the topological number limited to one, the Nel type magnetization is uniquely determined. These results show that the magnetic imaging method has great advantages to resolve the emerging magnetic structure materials. The magnetic imaging technology based on the NV center will potentially become an important method to study magnetic materials under continuous development.

Adaptive stochastic resonance system in terahertz radar signal detection

Abstract: Terahertz radar research has attracted widely attention of researchers due to its advantages such as short wave length, wide bandwidth, no blind spot, low power, and low intercept rate. It is generally considered that the echo signal of terahertz radar system is a signal with noise. Therefore, it is necessary to reduce the noise in the process of the frequency spectrum analysis of different-frequency signals. The fast Fourier transform (FFT) and the filtering method are commonly used in radar signal processing. The FFT method has lower ability to estimate the frequency of signal due to the interference noise. The filtering method detects the signal from the angle of noise elimination, but at the same time, it weakens useful characteristics, blurs position information about the signal, and affects detection capability of terahertz radar system. Aiming at the problem above, a method of detecting terahertz radar signals based on adaptive stochastic resonance (SR) system is proposed in this paper due to a phenomenon that the noise can be suppressed while amplifying the weak signal by transferring the noise energy after going through the SR system. With the different-frequency signal processing method of the twice sampling, the adaptive SR system and the scale recovery, the optimal parameters can be obtained automatically and the ranging calculation can be completed. Comparing with the FFT method, the mean output signal-to-noise ratio (SNR) gain through the SR system is 9.6843 dB at different measuring distances. When the measuring distance is 1000 mm, the initial spectrum value increases from 110.1 to 7172, which is 64.1 times higher than original value. The initial SNR of the whole system is improved from -11.94 to -0.179 dB, the gain is 11.761 dB. Comparing with the filtering method, the largest SNR gain is 6.485 dB when the measuring distance is 1000 mm, which is increased by 70.56%. When the input noise intensity is between 0.5 V and 1 V, the output SNR of the adaptive SR system is higher than that of the traditional filter system, but the gain is small and the maximum SNR gain is 2.148 dB. When the noise intensity of the system is between 1 V and 5 V, the SNR of the adaptive SR system is obviously higher than that of the filter system, and the largest SNR gain is 14.018 dB when the noise intensity D=5 V. The SNR curve of the adaptive SR system tends to be smoother and the curvature is 0.507, while the SNR curvature of the filtering model is 3.765, which is reduced by 86.5%. The method proposed in this paper not only solves the problem of noise coverage in the different-frequency signal, but also uses the characteristic that the noise energy can be transferred to the signal, to improve the output SNR of terahertz radar system, which is beneficial to further signal processing. Experimental results demonstrate that the ranging capability of the THz radar system is greatly improved, which has high application value and wide prospect in practical engineering research.

Precise measurement of 6Li transition frequencies and hyperfine splitting

Abstract: In this paper, we report a precision measurement of hyperfine splitting and absolute frequency of D1 line in cold 6Li atoms. The gray molasses is realized in the experiment and the tempreature is cooled to about 50 μK, which is lower than the Doppler cooling limit, 140 μK. By use of an optical comb, the absolute frequencies and corresponding hyperfine splitting are measured. We obtain frequencies of 446789503.080(35) MHz, 446789529.198(36) MHz, 446789731.316(50) MHz and 446789757.476(29) MHz for the D1 line. The results are in reasonable agreement with the theoretical calculations and consistent with earlier measurements. They could provide an important foundation for future frequency measurement, α constant and nuclear radius.

Field effect transistor photodetector based on graphene and perovskite quantum dots

Abstract: Graphene is an attractive optoelectronic material for various optoelectronic devices, especially in the field of photoelectric detection due to its high carrier mobility and fast response time. However, the relatively low light absorption cross-section and fast electron-hole recombination rate can lead to rapid exciton annihilation and small light gain, which restrict the commercial applications of pure graphene-based photodetector. The perovskite has attracted much attention because of its high photoelectric conversion efficiency in the field of solar cells. The perovskite has the advantages of long carrier diffusion distance and high optical absorption coefficient, which can effectively make up for the shortcomings of pure graphene-based field-effect transistor. In this work, a field-effect transistor photodetector is demonstrated with the combination of graphene and halide perovskite quantum dots (CsPbI3) serving as conductive channel materials. The graphene is prepared by plasma enhanced chemical vapor deposition, and the quantum dots are CsPbI3 perovskite. The electrical properties of graphene and pure graphene-based field-effect transistor are detected and analyzed by using the Raman spectrum. The results show that the graphene has good intrinsic electrical properties. Unlike previous report in which bulk perovskite was used, the perovskite quantum dot field-effect transistor photodetector has an obvious light response to 400 nm signal light, and shows the excellent photoelectrical performance. Under the illumination of 400 nm light, the signal light could be detected steadily and repeatedly by the graphene-perovskite quantum dot photodetector and converted into photocurrent. The photocurrent of the photodetector has a rapid rise, and the maximum value can reach 64 A at a light power of 12 W. The corresponding responsivity is 6.4 AW-1, which is two orders of magnitude higher than that of the general single graphene photodetector (10-2 AW-1), and it is also higher than that of perovskite-based photodetector (0.4 AW-1). In addition, the photoconductive gain and detectivity arrive at 3.7104 and 6107 Jones (1 Jones=1 cmHz1/2W-1), respectively. The results of this study demonstrate that the graphene-perovskite quantum dot photodetector can be a promising candidate for commercial UV light detectors.

Memristor-based multi-scroll chaotic system and its pulse synchronization control

Abstract: The memristor is a nonlinear element and intrinsically possesses memory function. When it works as nonlinear part of a chaotic system, the complexity and the randomness of signal will be enhanced. In this paper memristor is introduced into a three-dimensional chaotic system based on the augmented L system. The interesting and promising behaviors of complex single, double and four-scroll chaotic attractors generated only by varying a parameter have not been reported in memristive chaotic system and thus they deserve to be further investigated. It is also obvious that such a simple change of one parameter could be used to generate a variety of quite complex attractors. Therefore, as a nonlinear device the memristor plays an important role in this system. Firstly, some basic dynamical properties of the memristive chaotic system, including symmetry and in-variance, the existence of attractor, equilibrium, and stability are investigated in detail. By numerically simulating the power spectrum, Lyapunov exponent, Poincare map and bifurcation diagram, in this paper we verify that the proposed system has abundant dynamical behaviors. The sensitivities of system parameters to the chaotic behaviors are further explored by calculating, in detail, its Lyapunov exponent spectrum and bifurcation diagrams. The results of simulation and experiment are in good agreement, thereby proving the veracity of analysis. The memristive chaotic circuit is designed using the memristor, operational amplifier, analog multiplier and other conventional components. The circuit implementation of the memristive system is simulated using SPICE (simulation program with integrated circuit emphasis). The SPICE simulation results and the theoretical analysis are found to be in good agreement, and thus verifying that the system can produce chaos. Pulse synchronization has the following characteristics: low energy consumption, fast synchronization and easy-to-implement single-channel transmission. Therefore, it is more practical in chaotic secure communication. Subsequently the pulse chaos synchronization is realized from the perspective of the maximum Lyapunov exponent, and numerical simulations show the existence of new memristive chaotic system and the feasibility of pulse synchronization control, and also provide an experimental basis for further studying the applications of the memristive chaotic system in voice secure communication and information processing.

Analysis of orbital angular momentum spectra of Hankel-Bessel beams in channels with oceanic turbulence

Abstract: Beams with different-mode-number (l) orbital angular momenta (OAMs) are mutually orthogonal to each other, which makes it possible to enlarge the channel capacity in an OAM multiplexed underwater optical communication (UOC) system. Nevertheless, the implementation of this strategy is limited by oceanic turbulence. Hankel-Bessel (HB) vortex beams carrying OAM are relatively less affected by atmospheric turbulence due to their ability to propagate without changing the intensity profile (non-diffraction nature) and remarkable ability to be reconstructed after encountering an obstacle (self-healing mechanism). Consequently, HB vortex beams can be used as the carriers to increase the channel capacity of information transmission. In this paper, based on the Rytov approximation theory, the analytical expressions of OAM spectra for HB vortex beams under weak horizontal oceanic turbulent channels are derived. The influences of oceanic turbulence parameters on the OAM spectra of HB vortex beams are investigated via numerical calculations. The results indicate that oceanic turbulence leads to the decline of detection probability of transmitted OAM mode and the broadening of OAM spectra as well. Similarly, the spatial coherence length in oceanic turbulence decreases with increasing propagation distance and the dissipation rate of mean-squared temperature and with decreasing the dissipation rate of turbulent kinetic energy, which lead to the decline of detection probability of transmitted OAM mode for HB vortex beams. On the other hand, beams with larger OAM mode numbers each have a wider beam spreading after propagating in the turbulence, which results in the decrease of the detection probability for transmitted OAM modes of HB vortex beams. And the HB vortex beams are more affected by salinity fluctuation than by temperature fluctuations, which indicates that salinity fluctuations are much more effective than temperature fluctuations in determinating the effect of oceanic turbulence. In addition, for weak turbulence and a distance of several tens of meters, the transmission performance of HB vortex beams is worse than that of Laguerre-Gaussian vortex beams with the optimal waist setting. These results provide references for the realization of optical communication links in the marine environment.

Solid quantum sensor based on nitrogen-vacancy center in diamond

Abstract: Solid-state electronic spin system of the nitrogen-vacancy (NV) center in diamond is attractive as a nanoscale quantum sensor under room-temperature dueto its unique characteristics such as stable fluorescence, long coherent time, and near-atomic size under ambient conditions. Nowadays, the NV center plays a significant role in super-resolution microscopies. Different super-resolution microscopies have been used on NV center to archievenanoscale spatial resolution. Moreover, the spin state in NV center can be regraded as a solid-state qubit, which can be optically polarized and read out. The spin state can couple with electromagnetic fields and strain, which enables the NV center to be an excellent quantum sensor with high spatial resolution and high sensitivity. Such an NV-center based quantum sensing technique is being developed for applications in newmateriales, single protein nuclear spin dynamic field, life science, etc. This review will introduce the basic principle of such a nanoscale quantum sensor, the experimental realization, methods of enhancing the sensitivity, and some applications in high-spatial-resolution and high-sensitivity sensing.