bluuebunny/crossref_metadata_2025 huggingface dataset:
Utra-thin single-layered high-efficiency focusing metasurface lens
Authors: Guo Wen-Long  , Wang Guang-Ming  , Li Hai-Peng  , Hou Hai-Sheng  ,   Air and Missile Defense College, Air Force Engineering University, Xi'an 710051, China
Published: December 2020

For potential applications of metasurfaces in lens technologies, we propose a cross circularly polarized focusing metasurface which is capable of transforming a circularly polarized wave into cross-polarized wave and simultaneously focusing electromagnetic wave. A helicity-dependent phase change is introduced into the transmission metasurface cell, which is a single layer with a thickness of 1.5 mm and can be engineered by assembling along the spatial orientation of each Pancharatnam-Berry phase element. The phase change of the Pancharatnam-Berry phase element is analyzed theoretically, and the efficiency of the designed element is simulated under the irradiation of differently polarized waves. A phase gradient metasurface with a phase difference of 60 between neighbouring cells is designed. When simulated in CST Microwave Studio, the gradient metasurface is observed to have a ability to refract right-hand circularly polarized waves in +x direction and left-hand circularly polarized waves in -x direction but with an identical refraction angle of 33.8, which is in good accordance with the angle calculated from the general refraction law. Then we design a focusing metasurface with a size of 90 mm90 mm and 1515 cells. When the focusing metasurface lens is irradiated by left-hand circularly polarized wave, the refracted right-hand circularly polarized wave is focused at a point 40 mm away from the lens center. However, when the metasurface lens is impinged by the right-hand circularly polarized wave, the refracted left-hand circularly polarized wave is diffracted. This ultimately accords with different phase responses under different polarized waves when the metasurface cell is rotated. Furthermore, the metasurface lens diffracts the incident wave when impinged by right-hand circularly polarized wave, which validates the design principle. The beam-width at the focal spot and the focal depth are also calculated. The simulation results indicate that the beam-width at the focal spot is approximately equal to three quarters of the operating wavelength. Therefore, the circularly polarized wave refraction focusing metasurface has a good performance for focusing the refracted waves. In addition, the proposed focusing metasurface is simulated separately at f=14 GHz and f=16 GHz, and the results show a good focusing effect, which demonstrates the bandwidth characteristic of the focusing metasurface lens. This designed metasurface lens is thin, single-layered, and highly effective, and it is also convenient to fabricate. Moreover, the metasurface lens has an advantage over the conventional lens, which has potential applications in manipulating electromagnetic waves and improves the performance of lens.
Mechanism of eliminating the aerosol haze particles by using laser gradient force
Authors: Zhang Yong-Yan  , Wu Jiu-Hui  , Zeng Tao  , Zhong Hong-Min  ,   School of Mechanical Engineering, Xi'an Jiaotong University, Xi'an 710049, China;,   School of Mechanical Engineering, Sichuan University of Science and Engineering, Zigong 643000, China
Published: December 2020

The mechanism of eliminating the aerosol haze particles by destroying the force balanced system with laser gradient force is proposed in terms of the force balanced system exerted on the haze particles in the atmosphere, namely, rotary lift force balances gravity, and haze particles are balanced by repulsion between particles and particles. First of all, nonlinear equations about forces exerted on haze particles are obtained according to the Newton's second law, and these main forces (air drag force, van der Waals repulsion, rotary lift force) on the haze particles are solved by integrating these nonlinear equations by the Runge-Kutta method. The facts that the concentrations of particles with diameters in a range from 0.5 m to 0.835 $m increased obviously in the haze particles test on December 17-25, 2013 and February 20-26, 2014 in Xi'an are verified. Secondly, the laser gradient force is analyzed and calculated in the homogeneous medium of haze particles. It is found that the system balanced by the force exerted on haze particles is destroyed completely by laser gradient force, because the magnitude of laser gradient force is always larger than any other forces exerted on haze particles. Therefore, it is feasible to eliminate these haze particles by laser gradient force. It is undoubted that the proposed way of eliminating haze particles is of great significance to establish harmonious surroundings for human.
Theoretical and experimental study of atmospheric turbulence measurement using two-aperture differential scintillation method
Authors: Cheng Zhi  , Tan Feng-Fu  , Jing Xu  , He Feng  , Hou Zai-Hong  ,   Key Laboratory of Atmospheric Composition and Optical Radiation, Chinese Academy of Science, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Science, Hefei 230031, China;,   University of Science and Technology of China, Hefei 230026, China
Published: December 2020

We report the basic theory and first horizontal results of a method called two-aperture differential scintillation method which is aimed at monitoring the vertical profile of atmospheric optical turbulence strength. The method is based on irradiance fluctuation of active light source, but can extract the optical turbulence information in the single-passage path. In this paper, the theoretical principle of two-aperture differential scintillation method is derived in detail. A concise expression is proposed for irradiance fluctuation structure function with differential aperture in the Rytov approximation under a weak fluctuation regime based on the cross-path theory. The mathematic relationship between irradiance fluctuation structure function and atmospheric optical turbulence strength is then developed. The effects of beacon aperture and beacon altitude on path weighting function of this method are analyzed for Kolmogorov turbulence. In order to test the validity of the new method, the experiments are conducted to compare the two-aperture differential scintillation method and single-aperture scintillation method in atmospheric boundary layer over 2 km horizontal single-passage path. In this arrangement, we employ a differential image motion monitor system to measure differential scintillation. Simultaneously, a large aperture scintillation instrument is placed 5 m away at the same altitude to measure the single-aperture scintillation. It is shown that the results of atmospheric refractive index structure constant deduced from the two methods are in good agreement. The measurements of atmospheric coherence length for spherical wave corresponding to the two methods indicate a linear correction factor (R2) of 0.96, in a slope of 0.98 with an offset of -0.09 cm. Feasibility and effectiveness of two-aperture differential scintillation method are thus verified experimentally. The novel method can separate single-passage scintillation information of active beacon double-passage propagation, thereby providing an accurate technique for measuring the atmospheric turbulence of active beacon.
Influence of laser linewidth on the conversion efficiency of single-pass frequency doubling with a PPMgO: LN crystal
Authors: Zhang Kong  , Bai Jian-Dong  , He Jun  , Wang Jun-Min  ,   State Key Laboratory of Quantum Optics and Quantum Optics Devices (Shanxi University), and Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China
Published: December 2020

We report on the generation of 2.06 W of tunable cw light at 780 nm by a single-pass frequency doubling in a PPMgO:LN crystal with a seeded high-power fiber amplifier. A 1560 nm distributed feedback (DFB) diode laser, a Littman-type grating external-cavity diode laser (ECDL) and a DFB-type erbium-doped fiber laser (DFB-EDFL) were separately used as a seeding laser source of an erbium-doped fiber amplifier (EDFA). Here we use the EDFA which has a narrow linewidth option, the fundamental frequency light will not be obviously broadened. The influence of laser linewidth on the conversion efficiency of frequency doubling is investigated. The injection power and output power of EDFA must be consistent. So when temperature of the PPMgO:LN crystal is fixed, the conversion efficiency for different seeding resources can be obtained as follows. When the fundamental power is 12.42 W, using the DFB as seeding resource yields 1.36 W of 780 nm doubling output, and the corresponding conversion efficiency is 11.0%. Using the ECDL as seeding source yields 1.78 W of 780 nm doubling output, and the corresponding conversion efficiency is 14.3%. While using the DFB-EDFL as seeding source yields 2.06 W of 780 nm doubling output, and the conversion efficiency is 16.6%. The measured laser linewidths of the three seeding sources are 1.2 MHz, 200 kHz, and 600 Hz for the DFB, ECDL, and DFB-EDFL, respectively. The experimental results show that the narrower laser linewidth, the higher doubling efficiency, and the experimental results agree with our theoretical analysis.
Simulation of cherenkov radiation oscillation in a plasma-filled metallic photonic crystal
Authors: Fu Tao  , Ouyang Zheng-Biao  ,   Shenzhen Key Laboratory of Micro-Nano Photon Information Technology, THz Technical Research Center of Shenzhen University, College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, China;,   Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
Published: December 2020

Plasma filling can significantly improve the efficiency and power of a vacuum device. In this paper, we first analyze the dispersion properties of a plasma-filled metal-photonic-crystal slow-wave structure (SWS), and then investigate the interaction procedure between a relativistic electron beam and the Cherenkov radiation in the plasma-filled metallic-photonic-crystal by the particle in cell method. We pay our attention to the influences of plasma density, cathode voltage, and guiding magnetic field on output frequency and power. The results show that the electric field strength in the SWS increases obviously at a fixed plasma density of 50 mTorr (1m mTorr=0.133 Pa). The device works at a stable single TM01 mode due to the good mode properties of the metal photonic crystal even if plasma is filled in it. The maximum value of Ez field along the z axis of the device increases from 46.34 MV/m without plasma to 79 MV/m with plasma. The value along the x axis increases from 136 MV/m without plasma to 185 MV/m with plasma. The working frequency (35.5 GHz) of the device, obtained from simulation, is consistent with the theoretical estimation (35.4 GHz). The power increases with the cathode voltage between 500 kV and 600 kV while the frequency increases only a little. When the magnetic field B increases, the output power first increases and then decreases. But the frequency is not affected due to the dispersion property. The output power of the device increases 20% when the air pressure increases from 0 to 100 mTorr. However, there is a pretty distribution of the field Ez along the angular direction only in an appropriate plasma density around 50 mTorr. According to the theory and simulation, the output power and efficiency can be improved in an appropriate range of plasma density. These results provide a basis for developing the plasma-filled vacuum devices.
Abnormal behaviors in lock-in transition of the vortices in melt textured growth of YBa2Cu3O7- crystals
Authors: Wu Dong-Jie  , Xu Ke-Xi  , Tang Tian-Wei  ,   Department of Physics, Shanghai University, Shanghai 200444, China
Published: December 2020

The magnetization behavior of the layered anisotropic high-Tc superconductor in the mixed state Hc1 H Hc2 has a feature that when the angle between the applied magnetic field H and the CuO plane (a-b plane) is less than a critical value ( L), the vortex lattice is converted from three-dimensional structure into two-dimensional structure, forming a phenomenon so called the lock-in transition, where the flux lines are completely parallel to the a-b plane, and the vertical component of the magnetic induction B丄 (perpendicular to the a-b plane) is consequently zero. So far, there have still existed the differences in the physical explanation of the lock-in phenomenon. For the lock-in phenomenon occurring in the region between the CuO planes, it can be considered to be caused by the transverse Meissner effect. However, for the one occurring in other extended correlated defect areas, such as twin boundaries in YBa2Cu3O7- (YBCO) crystal, this phenomenon is believed to be the results of the energy linearization of the vortices trapped in the defect channels. Many theoretical and experimental studies have revealed the existence of the lock-in behaviors related to the microstructure properties of the superconductor crystals. Therefore, the research of the lock-in transition behavior will be helpful to understand the intrinsic pinning properties of the layered anisotropic superconductors, and the phase transition process in the vortex system.
In this paper, we systematically measure the magnetic torque signal in melt texture growth YBCO (MTG-YBCO) bulk and observe an abnormal lock-in transition behavior in the vortex system. The critical angle of the lock-in transition is found to be directly proportional to the strength of the magnetic field, which is contrary to the observations in the common cases. According to the framework of the Ginzburg-Landau theory and the kink structure model of the vortex line, we discuss the abnormal phenomenon, and propose that there is a type of extend-correlated defect structure, which is parallel to the a-b plane, in the MTG-YBCO crystal. The relationship between the critical angle of the lock-in transition to the temperature and the magnetic field is established theoretically, and the theoretical results coincide well with the torque measurements.
Numerical extraction of electric field distribution from thermal pulse method based on Monte Carlo simulation
Authors: Liang Ming-Hui  , Zheng Fei-Hu  , An Zhen-Lian  , Zhang Ye-Wen  ,   Department of Electrical Engineering, Tongji University, Shanghai 201804, China
Published: December 2020

Thermal-pulse method is a powerful tool for measuring space charge distributions in polymer films. The data analysis for thermal-pulse method involves the Fredholm integral equation of the first kind, which requires an appropriate numerical procedure to obtain a solution. Various numerical techniques, including scale transformation and regulation method, are proposed. Of those numerical methods, the scale transformation (ST) is the simplest and the most widely used method. However, it presents a high spatial resolution only near the sample surface. Monte Carlo (MC) method is one of the recently proposed ways to solve the equation numerically and has been successfully applied to the analysis of laser intensity modulation method data, which also involves the Fredholm integral equation of the first kind. In this paper we attempt to analyze thermal-pulse data in frequency domain with the MC method and discuss its effectiveness based on some numerical simulations. The simulation results indicate that the electric field profiles can be effectively extracted by the MC method. The computed profiles by the MC method consist well with the supposed distributions in the entire thickness of the sample, while the profiles reconstructed by the ST method fit very well to the supposed one at the vicinity of the target surface and distort sharply along the direction of the thermal pulse propagation in the sample bulk. On the other hand, the oscillations in the computed results by the MC method could deteriorate its accuracy in this study.
The influence of noise level on the analysis based on the MC method is also tested by the use of the simulated data. The results show that the computed profiles would become more fluctuant as the noise level increases. This problem can be solved by selecting a larger value of tolerance during the singular value decomposition procedure. Thus, the value of tolerance is considered to be one of the key parameters in this algorithm, which is actually hard to determine. Additionally, the experimental data obtained from a polypropylene film under applied electric field are analyzed to illustrate the feasibility of MC method to be applied to the thermal-pulse experimental data. The results also show that the spatial accuracy by the MC method in the entire sample thickness is higher than by the ST method, which verifies that the MC method is more suitable for detecting the electric field distribution in the deep bulk of the sample. Owing to noise and error, the accuracy of MC calculation depends on the chosen tolerance value, which is now considered to be an obstacle in applying this method to the practical thermal-pulse measurement.
Influences of InGaN/GaN superlattice thickness on the electronic and optical properties of GaN based blue light-emitting diodes grown on Si substrates
Authors: Qi Wei-Jing  , Zhang Meng  , Pan Shuan  , Wang Xiao-Lan  , Zhang Jian-Li  , Jiang Feng-Yi  ,   National Engineering Technology Research Center for LED on Si Substrate, Nanchang University, Nanchang 330047, China;,   School of Materials Science and Engineering, Nanchang University, Nanchang 330031, China
Published: December 2020

GaN based light-emitting diodes (LEDs) are subjected to a large polarization-related built-in electric field in c-plane InGaN multiple quantum well (MQW) during growth, which causes the reduction of emission efficiency. To mitigate the electric field, a superlattice layer with a numerous good characteristics, such as a small thickness, a high crystalline quality, is embedded in the epitaxial structure of LED. However, the effect of the superlattice thickness on the properties of LED is not fully understood. In this paper, two blue-LED MQW thin film structures with different thickness values of InGaN/GaN superlattice inserted between n-GaN and MQW, are grown on Si (111) substrates by metal-organic chemical vapor deposition. Electronic and optical properties of the two kinds of samples are investigated. The obtained results are as follows. 1) Comparing two samples, it is observed that more serious reverse-bias leakage current exists in the one with thicker superlattice; 2) Room temperature electroluminescence (EL) measurement shows that the emission spectrum peak between two samples is blue-shifted to different extents as the injection current increases. With superlattice thickness increasing, the extent to which the peak is blue-shifted decreases. Nevertheless, there is no obvious discrepancy in the EL intensity between two samples with different thickness values at 300 K. In addition, the V-shaped pit characteristics including density and size, and the dislocation densities of two samples are studied by high-resolution X-ray diffraction, scanning electron microscope, and transmission electron microscope. The experimental data reveal that the reason for a tremendously different in reverse-bias leakage current between two samples is that there are larger and more V-pits in the superlattice sample with a large thickness. Whereas, V-pits also act as preferential paths for carriers, resulting in the fact that the thicker superlattice suffers more serious reverse-bias leakage current. According to reciprocal space X-ray diffraction intensity around the asymmetrical (105) for GaN measurement, the relaxed degree of InGaN quantum well on GaN is proportional to the superlattice thickness. On the other hand, it is useful for increasing superlattice thickness to reduce a huge stress in c-plane InGaN. Owing to joint effects of above factors, the EL intensities of the superlattice sample with different thickness values are almost identical. Our results show the functions of superlattice thickness in electronic and optical characteristics. What is more, the conclusions obtained in the present research indicate the practical significance for improving the performances of LED.
Barrier and well thickness designing of InGaN/GaN multiple quantum well for better performances of GaN based laser diode
Authors: Zhou Mei  , Zhao De-Gang  ,   Department of Physics, China Agriculture University, Beijing 100083, China;,   State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
Published: December 2020

The effects of barrier and well thickness in InGaN/GaN (with in content of 15%) multiple quantum well (MQW) on the performances of GaN based laser diode (LD) are investigated by using LASTIP software, and the relevant physical mechanisms are discussed. It is found that when the barrier-thickness in InGaN/GaN MQW is fixed to be 7 nm, for the well thickness values of 3.0, 3.5, 4.0, 4.5, and 5.0 nm, the threshold currents of LD are 76.31, 67.96, 57.60, 64.62, and 74.59 mA, and the output light powers of LD are 12.05, 15.64, 24.70, 18.21, and 11.35 mW under an injection current of 100 mA, respectively. It indicates that too thick or too thin well may lead to a higher threshold current and a lower output power of GaN based LD. A high performance device can be obtained by using an optimized well thickness of around 4.0 nm. It is found that the LD performance is degraded by using too thin well in the device structure mainly due to the high leakage current, while strong polarization will lead to the decrease of overlap integral and luminescence intensity if the well layer is too thick, and thus a poor performance is obtained. It is found that the LD performance can be improved obviously by appropriately increasing barrier thickness from 7 nm to 15 nm. When the barrier thickness in InGaN/GaN MQW is fixed at 15 nm and the well thickness values are 3.0, 3.5, 4.0, 4.5 and 5.0 nm, the threshold currents of LD are 59.54, 52.42, 52.17, 51.38, and 58.99 mA, and the output light powers of LD are 36.12, 39.69, 40.79, 40.27, and 33.19 mW under an injection current of 100 mA, respectively, i.e., LD device parameters are improved. It suggests that the higher performances of GaN based laser diode can be realized by appropriately increasing the thickness of barrier when the thickness of well is optimized to be around 4 nm.
Two kinds of generalized gradient representations for generalized Birkhoff system
Authors: Li Yan-Min  , Chen Xiang-Wei  , Wu Hui-Bin  , Mei Feng-Xiang  ,   Department of Physics and Information Engineering, Shangqiu Normal University, Shangqiu 476000, China;,   School of Mathematics, Beijing Institute of Technology, Beijing 100081, China;,   School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, China
Published: December 2020

Brikhoff system is a kind of basic dynamical system. The theory and method of Brikhoff system dynamics have been applied to the hadron physics, quantum physics, relativity and rotational relativistic system. The properties of gradient system not only play an important role in revealing the internal structure of dynamical system, but also help to explore the dynamical behavior of the system. In this paper, two kinds of generalized gradient representations for generalized Birkhoff system are studied. First, two kinds of generalized gradient systems, i. e., the generalized skew gradient system and the generalized gradient system with symmetric negative definite matrix, are proposed and the characteristics of the systems are studied. Second, the relations of stability between these two kinds of gradient system and the dynamical system are discussed. Third, the condition under which a generalized Birkhoff system can be considered as one of the two generalized gradient systems is obtained. Fourth, the gradient discrimination method of stability of the generalized Brikhoff system is given, and the characteristics of the generalized gradient systems can be used to study the stability of the generalized Birkhoff system. Finally, some examples are given to illustrate the application of the result. Therefore, once the mechanical system is expressed as the generalized gradient system, the stability and the asymptotic stability can be conveniently studied by using the properties of generalized gradient system. The difficulty in constructing Lyapunov functions is avoided, and a convenient method of analyzing the stability of mechanical system is provided.
Search algorithm of structure anomalies in complete graph based on scattering quantum walk
Authors: Xue Xi-Ling  , Chen Han-Wu  , Liu Zhi-Hao  , Zhang Bin-Bin  ,   School of Computer Science and Engineering, Southeast University, Nanjing 210096, China
Published: December 2020

Quantum walks have been proven to be a useful framework in designing new quantum algorithms, of which the search algorithm is the most notable. Besides a general search for a special vertex, recent researches have shown that quantum walks can also be used to find structural anomalies. Suppose a vertex of complete graph KN is attached to a second graph G, then the kind of structure anomaly will break the symmetry of the complete graph. The search algorithm based on scattering quantum walk model is presented to speed up locating this kind of structure anomaly. The concepts of scattering quantum walk model and collapsed graphs are presented. The definition of the evolutionary operator, which is different from that of a general search, is given. Based on the specific definition of evolutionary operator and the obvious symmetry of complete graph, it is shown that the search space is confined to a low-dimensional collapsed space, and the initial state is chosen to lie in this subspace. To illustrate the evolutionary process of the search algorithm, an example is given in the case that G is a single vertex. Taking advantage of our earlier study on the evolutionary operator of coined quantum walks with Grover coin, calculations of the unitary operator in the collapsed space are greatly simplified. To quantify the evolutionary process of the algorithm, we use the matrix perturbation theory involving a perturbative approach to find the eigenvalues and eigenstates. It is the degenerate zeroth-order eigenvalue 0 = 1 that leads to the interesting parts of the Hilbert space. Most of the recent researches of searching the structure anomalies focus on star graph SN with an unspecified graph G attached to one of its external vertices, where the overall graph is divided into two parts by the central vertex. It is shown that quantum speedup will occur if and only if the eigenvalues associated with these two parts in the N limit are the same. In this paper, we find that the collapsed graph of complete graphs can also be divided into two parts by a single collapsed vertex. As these two parts roughly correspond to the initial state and the desired state respectively, the techniques and results in star graphs can be generalized to the collapse graph on complete graph. What is more, under our definition of unitary evolution operator these two parts in the N limit will always share the same eigenvalue, i.e. 0 = 1, no matter what the structure of graph G is. Based on this, we prove that the search algorithm can find the target vertex in O(N) time steps with a success probability of 1-O(1N). That is to say, the quantum search algorithm gains a quadratic speedup over classical counterpart.
Nano surface interaction and model of vibrating probe
Authors: Chen Li-Juan  , Chen Xiao-Huai  , Liu Fang-Fang  , Wang Jing-Fan  ,   School of Instrument Science and Opto-electronic Engineering, Hefei University of Technology, Hefei 230009, China
Published: December 2020

The high precision measurement has been a focus in the field of manufacturing and microelectronics in this year. The micro/nano probe for coordinate measuring machine (CMM) acts as a key characteristic because it can measure the high-aspect-ratio components with high precision. Various micro/nano-CMM probes with different principles and different structures have been developed in the last decade. However, most of these studies focused on the sensing principle and measurement methods. There is little research on the behavior of the surface interaction between the probe tip and the workpiece. And the measurement accuracy and reliability of the current probe, especially those of the low stiffness probe, are limited by interaction forces including capillary force, van der Waals force, electrostatic force and Casimir force. Therefore, it becomes a challenge to reduce the effect of the surface interaction forces for the Micro/nano CMM probe. A new trigger probe based on the vibrating principle is analyzed and an optimal method for the appropriate vibrating parameters is presented in this paper.
The structure and principle of the probe are briefly described in the first part. In this system, a tungsten stylus with a tip-ball is fixed to the floating plate, which is supported by four L-shape high-elasticity leaf springs. The fiber Bargg grating (FBG) sensors are used in the probe for micro-CMM due to their superiority in t of small size, high sensitivity, large linear measuring range, immunity to electromagnetic interference, and low cost. One end of FBG is attached to a floating plate, and the other end to a retention plate which is connected with the piezoelectric ceramic actuator (PZT). The probe is driven by the PZT vibrating. Assuming that the driving forces can offset the surface interaction forces, then the probe can be described as a forced vibration model of the spring oscillator. Therefore, the equivalent model of the probe is set up.
In the second part, a relationship between the vibration parameters of the probe and the surface interaction can be confirmed. Through theoretical analysis and numerical simulation, the appropriate vibrating parameters including resonance amplitude, velocity and frequency of the probe are designed, which can offset the surface interaction forces.
In the third part, a probe is designed based on the above theories and an experimental system is set up to verify its rationality. The results show that the resonant micro/nano probe after optimizing its parameters can effectively reduce the influence of surface forces and improve the measurement accuracy.
Multifunctional metasurfaces for terahertz polarization controller
Authors: Yang Lei  , Fan Fei  , Chen Meng  , Zhang Xuan-Zhou  , Chang Sheng-Jiang  ,   Institute of Modern Optics, Nankai University, Tianjin 300071, China
Published: December 2020

Polarization is one of the basic properties of electromagnetic wave conveying valuable information about signal transmission and sensitive measurements. Manipulations of polarization state and amplitude have aroused a lot of research interest in many different fields, especially in the terahertz (THz) regime. Although many researches on THz polarization controller have been carried out, their transmission losses are still difficult to lower in a broad bandwidth. And there are few reports on THz polarization controller which can rotate the polarization state and split beams at the same time. Multifunctional THz devices are required to meet the needs of the progress of THz technology and its applications. In order to overcome this constraint, semiconductor silicon is integrated into the proposed structure to manipulate the polarization state and the amplitude, because of its optical properties with the external pump light. Here, according to the electromagnetic resonance between split rings and silicon rings in Fabry-Prot-like cavity, we propose a metasurfaces-based terahertz polarization controller. The unite cell structure is composed of metal grids-split ring/Si ring-metal grids spaced by silica layers. By using the finite element method in CST Microwave Studio, we simulate the transport and polarization properties under different conditions. The results show that a linear polarization state can be nearly perfectly converted into its orthogonal one from 0.39 to 1.11 THz with a transmission loss of 1 dB, which fits well to the one of multiple-beam interference theory. Then we study the properties of the proposed metasurface structure for oblique incidence. The property of rotating polarization basically keeps stable even at an incident angle of 60 from 0.52 to 1.05 THz. At the end of the paper, the polarization splitting feature of the device is discussed in the THz regime. The results demonstrate that the transmitted and reflected beam power of the device can be tuned by changing the pump light power. The modulation depths of two beams reach more than 90% at 0.5 THz. It is worth noting that the proposed structure can not only rotate the polarization state of transmitted light in a broad bandwidth of 0.72 THz, but also modulate the transmitted and reflected beam power with a wide modulation depth. It can be used as a broad-band, low-loss and tunable terahertz polarization controller which is also insensitive to the incident angle. So it meets the requirements in THz communication, spectrum detection and imaging systems.
Nonsequential double ionization of aligned molecules by few-cycle laser pulses at low intensity
Authors: Huang Cheng  , Zhong Ming-Min  , Wu Zheng-Mao  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715, China
Published: December 2020

Using the three-dimensional classical ensemble model, nonsequential double ionization (NSDI) of aligned molecules by the few-cycle laser pulse at the low intensity is investigated. Here the two electrons involved in NSDI finally are ionized through a transition doubly excited state induced by the recollision. The results show that the electron correlation behavior in NSDI is strongly dependent on the molecular alignment and the carrier-envelope phase (CEP) of the laser pulse. There are more anti-correlated emissions for the perpendicular molecules than those for the parallel molecules regardless of CEP. The dependence of the electron correlation behavior on molecular alignment can be well explained by the potential energy curves of molecules. That is because the suppressed potential barrier for perpendicular molecules is higher and the electron is more difficult to ionize than for parallel molecules. Thus for perpendicular molecules the ionization of the two electrons has longer time delay, which results in more anticorrelated emissions. Additionally, because the potential barrier for the perpendicular molecules is higher than that for the parallel molecules, the ionization yield of NSDI is about an order of magnitude smaller than that for the parallel molecules. With CEP increasing from 0 to , the anti-correlated emission first increases and then decreases. For parallel alignment, the correlated emission is always dominant at all CEPs. However, for perpendicular alignment, the dominant correlation behavior depends on the CEP of the laser pulse. When the CEP is in a range from 0.3 to 0.7, the anti-correlated emission is dominant. At other CEPs, the correlated emission is dominant. The dependence of the electron correlation behavior on the CEP of the laser pulse is well explained by the dependence of the returning energy of the electron on the CEP of the laser pulse. For different CEPs, the single ionization times resulting in NSDI and the corresponding acceleration electric field are different, which leads to at some CEPs the returning energy of the electron being large and at some other CEPs the returning energy of the electron being small. When those CEPs are available where the returning energy of the electron is larger, the doubly excited state induced by the recollision is more energetic. Thus at those CEPs the emissions of the two electrons from the doubly excited state have smaller time delays and more correlated emissions occur. On the contrary, at those CEPs where the returning energy of the electron is small, more anti-correlated emissions are produced.
Output pulse compressibility of the chirped pulse fiber amplification based on the dissipative solitons
Authors: Shi Lei  , Ma Ting  , Wu Hao-Yu  , Sun Qing  , Ma Jin-Dong  , Lu Qiao  , Mao Qing-He  ,   Anhui Provincial Key Laboratory of Photonics Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China;,   Division of Optics, National Institute of Metrology, Beijing 100029, China
Published: December 2020

The all-normal-dispersion mode locked fiber laser can produce the dissipative soliton pulses because the laser can tolerate much more nonlinear phase shift than the other mode locked fiber lasers. Such large energy mode locked fiber lasers are excellent seed pulse sources for generating very large-energy ultrashort pulses with fiber chirped pulse amplification (CPA) systems. However, the spectral amplitude modulation carried by the dissipative soliton pulses will severely restrict the compressibility of the output pulses from the typical CPA system. Therefore, it is necessary to investigate and design a suitable CPA system for improving the compressibility of the output pulses according to the properties of dissipative solitons. In this paper, using the dissipative solitons generated by the all-normal-dispersion fiber laser with different spectral filter bandwidths as the input seed pulses, the compressible properties of the pulses for the CPA system with both the grating pair stretcher and the fiber stretcher are investigated. Our simulation results show that, for such a large-energy dissipative soliton seed pulse, when the grating pair stretcher is used in the CPA system, the spectral amplitude modulation of the seed pulse can be mapped to the temporal amplitude modulation by the stretcher, and amplified by the subsequent fiber amplifier, which introduces additional nonlinear phase, finally restricts the compressibility of the output pulses; when the normal-dispersion fiber stretcher is used, the interaction between the group velocity dispersion and the self-phase modulation can not only eliminate the influence of the modulated spectrum of the dissipative soliton on the compressible properties of the pulses, but also make it possible to evolve the pulse self-similarity in the fiber stretcher, and thus improve the compressibility of the output pulses of the CPA system. For the normal-dispersion fiber stretcher CPA system, the compressibility of the output pulses is mainly determined by the fiber stretcher length. If the fiber length is too short, the compressibility of the output pulses may be affected by the uncompleted self-similar evolution of the pulse, while the pulse compressibility is also restricted because the pulse spectral width may exceed the amplifier gain bandwidth due to the self-similar evolution process if the fiber length is too long. Moreover, for the dissipative soliton seed pulses, both the compressibility of the output pulses and the energy ratio of the main pulse to the total pulse for the CPA system with the fiber stretcher are better than those with the grating pair stretcher when the normal fiber stretcher length is suitably optimized.
Effect of laser linewidth on the performance of heterodyne detection
Authors: Li Cheng-Qiang  , Wang Ting-Feng  , Zhang He-Yong  , Xie Jing-Jiang  , Liu Li-Sheng  , Guo Jin  ,   State Key Laboratory of Laser Interaction with Matter, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China;,   University of Chinese Academy of Sciences, Beijing 100049, China;,   Key Laboratory of Optical System Advanced Manufacturing Technology, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China
Published: December 2020

According to the statistical theory, the influence of laser linewidth on the performance of optical heterodyne system is studied. Also, the effect of laser linewidth on the visibility of fringe pattern is discussed. The expressions of self-correlation function power-spectrum and definition of visibility of fringe patterns are obtained in this paper. Based on the analytical expressions, the numerical simulation is performed. The obtained results demonstrate that the laser linewidth influences the visibility of fringe patterns according to the result shown in Fig. 3, and that the intermediate frequency can be still detected by heterodyne detection technique as the laser linewidth increases. For different linewidths, the measurement of intermediate frequency is accurate without the influence of noise as the delay d between the received signal and locally generated signal is less than the coherence time c of laser source. If the delay d is greater than the coherence time c, the full width at half maximum of intermediate frequency in the frequency spectrum of the output signal of photodetector will broaden as the laser linewidth increases. However, for a wide linewidth, the measurement of intermediate frequency is inaccurate due to the influence of noise when the delay d is greater than the coherence time c. The wider the linewidth, the less accurate the measurement of intermediate frequency will be. In order to check the correctness of theoretical results, an experiment is carried out by using a laser with a linewidth of 1 MHz, which has an 8.1 km channel path. In our experimental set-up, a cooperative target is employed to modulate and reflect the transmitted beam. In this way, an echo signal is received. The mixing process of the received signal and local signal on the photodetector surface produces an electrical current known as the photomixing current. A spectrum analyzer is used to observe the output signal of detector. The obtained spectrum shows that intermediate frequency can be checked, which is in agreement with the theoretical result. In this work, the obtained conclusions can be directly used to choose a proper laser for optical heterodyne system. According to the target characteristics and measurement requirements, and by following the conclusions obtained in this paper, the laser linewidth can be evaluated.
Optical communication in turbid and turbulent atmosphere
Authors: Mao Tian-Yi  , Chen Qian  , He Wei-Ji  , Zhuang Jia-Yan  , Zou Yun-Hao  , Dai Hui-Dong  , Gu Guo-Hua  ,   School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China;,   Jiangsu Key Laboratory of Spectral Imaging and Intelligent Sense, Nanjing University of Science and Technology, Nanjing 210094, China
Published: December 2020

Free space optical-communication (FSO) has gained significant importance due to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power and low mass requirement. However, the performance of FSO is degraded in the turbid and turbulent atmosphere, dramatically. Various techniques are proposed to cope with the turbid media and turbulence in atmosphere, e. g. aperture averaging, diversity, adaptive optics, modulation and coding and orbital angular momentum. However, in the above systems with point-to-point optical communication structure, there exist obvious drawbacks or they are complex and expensive, and thus difficult to use in practice. In this article, array-to-point optical communication (APOC) with good performance in turbid and turbulent atmosphere is demonstrated. The strongly disturbed communication channel can be expressed as a circular complex Gaussian transmission matrix, and the transmitted field is described as a linear combination of the fields coming from different and independent segments of the digital micro-mirror device (DMD), so that the cross terms are averaged on the surface of bucket detector. Instead, the contributions of all segments for each light field nearly becomes equally weighted. Turbulence and other noises are reduced for the incoherence with sampling matrix based on the second-order correlation which has widely been used in ghost imaging and LIDAR. Consequently, narrow-band optical filter is not required at the receiver. The decoding algorithm is a new signal processing strategy from information technology, compressed sensing, which discards low frequency components in sampling process and recovers the signal by conducting convex optimization. Numerical simulations and experiments with binary and multi-bits level signals are demonstrated to show that the bit error rate of the proposed method APOC is approximately 10-4-10-2, which is feasible for the optical communication in such complex communication channels. The communication rate, limited by the frequency of the DMD and the sampling rate of the receiver, could reach hundreds of kbit/s, and with improved technology a rate of Mbit/s should be attainable. This proposed APOC could realize optical communication in turbid and turbulent atmosphere and thus offers a very effective approach to promoting the implementation in military and rescue.
Response property of a factional linear system under the base excitation
Authors: Lou Zheng-Kun  , Sun Tao  , He Wei  , Yang Jian-Hua  ,   School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou 221116, China;,   School of Mechanical Engineering, Shanghai JiaoTong University, Shanghai 200240, China;,   Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment, China University of Mining and Technology, Xuzhou 221116, China
Published: December 2020

We investigate the response property of a linear system that is excited by the base excitation. The linear system contains the ordinary damping or the fractional-order damping. In our studies, the base excitation is in the harmonic form or in the general periodic form. When the base excitation is in the harmonic form, we obtain the dynamic transfer coefficient by the undetermined coefficient method. When the base excitation is in the general periodic form, we first expand the excitation into the Fourier series, then, according to the linear superposition principle, we obtain the dynamic transfer coefficient that is induced by each harmonic component in the excitation. By expanding the general periodic excitation into the Fourier series, we can solve the non-differentiable problem that is induced by the periodic base excitation for the numerical calculations. Based on the Grnwald-Letnikov definition, the discretization formula for the fractional-order system is obtained explicitly. The analytical results are in good agreement with the numerical simulations, which verifies the validity of the analytical results. Both the analytical and the numerical results show that the dynamic transfer coefficient depends on the fractional-order of the damping closely. The dynamic transfer coefficient can be controlled by tuning the value of the fractional-order. For the general periodic excitation, when the frequency is fixed, the dynamic transfer coefficient that is induced by the high-order harmonic component may be stronger than that induced by the low-order harmonic component in the base excitation. Hence, the effect of the high-order harmonic component in the excitation cannot be ignored although its amplitude is small. Further, when the base excitation is in the full sine form, or the square form, or the triangular form, the response property of the system can be described by center frequency, resonance peak, cutoff frequency, and the filter bandwidth. For a fixed fractional-order, the center frequencies of each order corresponding to the response, obtained by the three kinds of the periodic base excitations mentioned above, are identical. However, the corresponding resonance peaks are different. The resonance peak and the filter bandwidth are both maximal when the base excitation is in the square form. The resonance peak and the filter bandwidth are both minimal when the base excitation is in the triangular form. We believe that our results are useful for solving the vibration problem in the engineering field such as the vibration isolation and the vibration control.