bluuebunny/crossref_metadata_2025 huggingface dataset:
First-principles calculation of doped GaN/AlN superlattices
Authors: Rao Xue  , Wang Ru-Zhi  , Cao Jue-Xian  , Yan Hui  ,   College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China;,   Department of Physics and Institute for Nanophysics and Rare-earth Luminescence, Xiangtan University, Xiangtan 411105, China
Published: December 2020

First-principles calculation is a quite powerful tool for explaining experimental phenomena and predicting the properties of new materials. Based on the first-principles calculation within the density functional theory, the energetic stabilities and electronic properties of Mg and Si doped GaN/AlN superlattices with wurtzite and zinc-blende structures are investigated. The results show that there is no variation in formation energy if the doping position is changed when the impurities are doped in the well (GaN) region, and the same situation also happens in the barriers (AlN) region. Thus it is equivalent for dopants to replace Ga atoms in the cation site of wells or Al atoms in the cation site of barrers. However, the formation energies of these dopants in the well region and the barrier region are different. Compared with the formation energy in the barrier region, it is much lower in the well region. That is to say, the impurities in the cation site (MgGa, MgAl, SiGa and SiAl) present lower formation energies in the wells of GaN/AlN SLs with wurtzite and zinc-blende structures. In addition, the impurities in zinc-blende GaN/AlN superlattices present lower formation energy than in the wurtzite structure. The negative formation energy illustrates that the defects are spontaneously formed if Mg-atom is mixed into the wells of the zinc-blende structure. Therefore, in experiment, for the zinc-blende superlattice structure, preparing p-type semiconductor needs less energy than preparing n-type semiconductor. And for the wurtzite superlattice structure, preparing p-type semiconductor needs the same energy as preparing n-type semiconductor. Furthermore, the relationships between the distribution of the electronic states and their structures are analyzed. It is found that the different kinds of dopants lead to different band bendings, owing to the modified polarization fields. The spatial distributions of electrons and holes, plotted by the partial charge densities, reveal that electrons and holes experience redistributions by Si or Mg dopants in different phases. The band gap of doped GaN/AlN superlattice decreases and the projected density of states also accounts for the change of defect formation energy. The calculated results provide a new reference for the fabrication of modulation-doping GaN/AlN SL under desired control, which could be considered to control phase.
The study of energy model and multi-period of discrete phase shift control technique for full-bridge DC-DC converter
Authors: Sha Jin  , Xu Jian-Ping  , Chen Yi-Ming  ,   Key Laboratory of Magnetic Suspension Technology and Maglev Vehicle, Ministry of Education, School of Electrical Engineering, Southwest Jiaotong University, Chengdu 610031, China
Published: December 2020

Phase-shift full-bridge (PSFB) DC-DC converter benefits from high efficiency by zero-voltage switching turn-on of all switches without any additional auxiliary circuit, and PSFB DC-DC converter has been widely used in high power applications. In this paper, the operating mode of PSFB DC-DC converter is studied, and the energy iteration model of PSFB DC-DC converter is established. The discrete phase shift (DPS) control technique for PSFB DC-DC converter is proposed and discussed. Unlike the conventional PWM PSFB control technique, the DPS control technique uses two preset phase shift times tpsH and tpsL as control variables where 0tpsHtpsL ≤Tw with Tw being the switching period. When output voltage is lower than the reference voltage, phase shift time tpsH is selected, and a large duty cycle DH is obtained on the secondary side, which makes output voltage increase. Similarly, when output voltage is higher than the reference voltage, phase shift time tpsL is selected, and a small duty cycle DL is obtained on the secondary side, which makes output voltage decrease. With the energy iteration model, the energy iteration process is clearly revealed, steady-state and transient performances are studied. From the analysis results it can be known that the DPS controlled PSFB DC-DC converter always operates in a multi-periodic state. The simulation reasults show that the proposed control technique has an advantage over the conventional PWM PSFB control technique in simple design, great robust and excellent transient performance.
Low-voltage pentacene photodetector based on a vertical transistor configuration
Authors: Yang Dan  , Zhang Li  , Yang Sheng-Yi  , Zou Bing-Suo  ,   Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Physics, Beijing Institute of Technology, Beijing 100081, China;,   Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
Published: December 2020

Due to the excellent characteristics of field-effect transistor and its high absorption coefficient in the visible region, pentacene has been widely used in phototransistors. The channel length of the vertical transistor could be designed to be very short (on the order of nanometers). In this way, the device performances and its working frequency can be effectively improved, and the energy consumption can be reduced simultaneously. In this paper, we fabricate a kind of low-voltage pentacene photodetector ITO(S)/Pentacene/Al(G)/Pentacene/Au(D), based on the vertical transistor configuration. The threshold voltage and “on/off” current ratio are -0.9 V and 104 at a low working-voltage of -3 V, respectively. The pentacene photodetector ITO/Pentacene(80 nm)/Al(15 nm)/Pentacene (80 nm)/Au exhibits a good p-type transistor behavior and low-voltage-controlling performance. The photosensitivity and responsivity vary with incident monochromatic light from 350 nm to 750 nm, and the photosensitivity peak of 308 is obtained at 350 nm with a responsivity of 219 mA·W-1, which is even higher than that of the standard Si-based photodetector under 350 nm incident light. Therefore, this work provides an easy way to fabricate a high sensitivity all-organic photodetector working at low voltages.
Asymetric multiple-image authentication based on complex amplitude information multiplexing and RSA algorithm
Authors: Pan Xue-Mei  , Meng Xiang-Feng  , Yang Xiu-Lun  , Wang Yu-Rong  , Peng Xiang  , He Wen-Qi  , Dong Guo-Yan  , Chen Hong-Yi  ,   Department of Optics, School of Information Science and Engineering and Shandong Provincial Key Laboratory of Laser Technology and Application, Shandong University, Jinan 250100, China;,   College of Optoelectronics Engineering, Shenzhen University, Shenzhen 518060, China;,   College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China;,   College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, China
Published: December 2020

By combining the iterative phase retrieval algorithm in the Fresnel domain with the shift rotation permutation operations of row vectors and column vectors, a new kind of asymmetric multiple-image authentication based on complex amplitude information multiplexing and RSA algorithm is proposed, where multiple complex amplitude information in the input plane is retrieved and generated by the phase retrieval algorithm in the Fresnel domain. In original binary amplitude mask, the row vector and column vectors random numbers are randomly generated in advance, such that each sampling mask for each authenticator is obtained by the shift rotation permutation operations of corresponding row vector and column vectors random numbers for original binary amplitude mask. Thus, one synthesized complex amplitude is generated by the operations of sampling, overlap and multiplexing, and then sent to the certification center for authentication use. At the same time, the row vector and column vectors random numbers are encoded to ciphers by the public keys of RSA algorithm, and then delivered to the corresponding authenticators. During the authentication process, the row vector and column vectors random numbers are first decoded by the private keys possessed by the authenticator; second, the authenticator’s sampling mask is reconstructed by the shift rotation permutation operations of the above decoded random numbers for original binary amplitude mask. Finally, the authenticator with other additional authentication keys is prompted to place the synthesized complex amplitude information and its sampling mask at the corresponding positions, when the system is illuminated by a plane wave with the correct wavelength. A recovered image is then recorded in the output plane, by calculating and displaying the nonlinear correlation coefficient between the recovered image and the certification image, if there exists a remarkable peak in its nonlinear correlation coefficient distributions, indicating that the authentication is successful. On the contrary, if there is no remarkable peak but uniformly distributed white noise in the map, the authentication process is a failure attempt. Any intruder with randomly generated forged authentication keys will end up with a failure which enhances the security of the system to some extent.
Simulation of nano-grating patterning based on X-ray Talbot effect
Authors: Wen Ming-Wu  , Yang Xiao-Wei  , Wang Zhan-Shan  ,   MOE Key Laboratory of Advanced Micro-Structured Materials, School of Physics Science and Engineering, Department of Physics, Tongji University, Shanghai 200092, China
Published: December 2020

High aspect ratio gratings can be made by perpendicularly cutting in the growth direction of multilayers. X-ray exposure technique using a sectioned multilayer grating based on Talbot effect is a new type of nano patterning method. Although 300 nanometer gratings through the experiment are completed, some phenomena in the experiments cannot be satisfactorily explained and the factors influencing the nano pattern quality have not been fully understood yet. Here we use a rigorous coupled-wave theory to discuss several important factors, including grating thickness, the fraction of material thickness and multilayer period, which is the first time as far as we know for Talbot self-imaging in X-ray range. Simulation results show that the grating thickness affects both X-ray transmission efficiency and fringe contrast, while the fraction of material thickness determines the quality of fringes. And the position deviation of the best image plane in near field is related to both the thickness of the grating and the multilayer period. Moreover, the multilayer gratings with smaller periods can achieve higher resolution, indicating that the Talbot effect can be used to fabricate a more detailed structure.
Particle transport simulation and effect analysis of CCD irradiated by protons
Authors: Zeng Jun-Zhe  , He Cheng-Fa  , Li Yu-Dong  , Guo Qi  , Wen Lin  , Wang Bo  , Maria  , Wang Hai-Jiao  ,   Key Laboratory of Functional Materials and Devices for Special Environments of CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of CAS, Urumqi 830011, China;,   Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Monte Carlo method is used to calculate the energy deposition of proton-irradiated scientific CCD (charge coupled device) structure, and the radiation damage mechanism of the device is analyzed by combining the proton irradiation with the annealing experiments. The ionizing dose in gate oxide layer and the displacement damage dose in silicon deposition are simulated. During irradiation and annealing experiments two main parameters, dark signal and charge transfer efficiency, are investigated. Results show that variations of dark signal and charge transfer efficiency are the same as those with ionizing dose and displacement damage dose. During irradiation, dark signal rises obviously as the fluence of 10 MeV proton increases. Defects and their annealing temperature:the divacancy levels show little annealing effect below 300℃, while the oxygen-vacancy complex is stable up to 350℃, and the phosphorous-vacancy has a characteristic annealing temperature of 150℃. Interface states are annealed totally at 175℃. So the annealing only affects oxide-trapped-charges. Dark signal is greatly reduced after annealing, this phenomenon means that the dark signal is mainly affected by ionization. The surface dark signal proportion of the total dark signal can be calculated by the reduction of dark signal during annealing and this is at least 80% or more. As the fluence of 10 MeV proton increases, the charge transfer efficiency reduces obviously. After annealing, the recovery of charge transfer efficiency changes very little, so the charge transfer efficiency is unaffected by oxide-trapped-charges, since it is reduced due mainly to bulk defects. The final device damage will always be proportional to the amount of initial damage and also to the electrical effect on the device. Hence NIEL scaling implies a universal relation:device damage=kdamage×displacement damage dose, where kdamage is a damage constant depending on the device and the parameter affected, and the displacement damage dose (DD) is the product of the NIEL and the particle fluence. MULASSIS is used to calculate the displacement damage dose in depletion area of P-area and deduce kdamage by combining with the experimental value of charge transfer efficiency; kdamage is calculated to be about 3.50×10-14. The formula for degradation degree of charge transfer efficiency is CTEafter irradiated = 1-Dd×kdamage, this formula is used to estimated CTE and the result is compared with the value from experiment. It is shown that the simulated data is in agreement with the experimental data.
Theoretical study on the model of metalic melt shearing flow near the surface and its effect on solidification microstructure
Authors: Wang Xiang  , Chao Run-Ze  , Guan Ren-Guo  , Li Yuan-Dong  , Liu Chun-Ming  ,   School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China;,   Luoyang Ship Material Research Institute, Luoyang 471000, China;,   School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, China
Published: December 2020

In this paper, the model of metalic melt shearing flow near the surface is established, and the effect of shearing flow on solidification microstructure of the metal is also analyzed. Calculated results based on A356 alloy melt show that in the laminar flowing melt, the shear stress decreases with increasing length along the vertical direction of the surface of the slope, and the shear stress first decreases rapidly and then stabilizes with increasing length along the flowing direction of the surface of the slope; while in the turbulent flowing melt, the shear stress firstly decreases rapidly and then stabilizes with increasing length along the vertical direction of the surface of the slope, and increases with increasing length along the flowing direction of the surface of the slope. The shear stress at the same position in the melt on the surface of the slope increases with increasing angle of the slope; the shear stress acting on the columnar crystal in the melt on the surface of the slope increases with decreasing length along the vertical direction of the surface of the slope. The shear stress acting on the columnar crystal at the same position in the melt on the surface of the slope increases with increasing angle of the slope; with the increase of the length along the flowing direction, the shear stress acting on the columnar crystal rapidly decreases first and then stabilizes in the laminar flowing melt on the surface of the slope, while the shear stress increases in the turbulently flowing melt on the surface of the slope. Based on the theoretical calculation, the maximum shear stress acting on the columnar crystal in the melt during the shearing flow near the surface of the metalic melt is lower than the yield strength of α-Al grain, so the shear stress induced by shearing flow cannot break the columnar crystal, and only by sweeping the grain into the melt to induce the multiplication of grain, which agrees with the experimental results. So, the proposed model can explain the constitutive relations of the metalic melt shearing flow near the surface and the effect of shear stress on the solidification microstructure.
Junction temperature measurement of light-emitting diodes using temperature-dependent capacitance
Authors: Zhao Yu  , Wei Ai-Xiang  , Liu Jun  ,   School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China;,   Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, Guangzhou 510006, China
Published: December 2020

Junction temperature, as one of the most important properties of light-emitting diodes (LEDs), has great impact on LEDs’ power efficiency, luminosity, reliability, life-time, and so on. Precise measurement of junction temperature for LED device is quite important in the research of chip’s fabrication, device packaging and related applications. In this paper, we propose a new approach to measure the junction temperature of LEDs by using temperature-dependent capacitance. The capacitance of white LEDs at room temperature is measured and found to be decreased first and then increased with an increasing reverse bias. Equivalent model using vertical and horizontal capacitances connected in parallel is proposed to qualitatively explain the variation of capacitance under different reverse bias. Result obtained from the model fitting agrees well with the experimental result. The capacitance-temperature (C-T) curve of white LEDs under different reverse bias is measured and analysed. Results show that the capacitance of LEDs is sensitive to temperature at all biases. Under a reverse voltage of 0.5 V, the capacitance has the maximal response of 1.971 pF/℃ and a good linear temperature-dependent property. The C-T curve is used as the calibration for the measurement of junction temperature. By monitoring the change of capacitance of the working LEDs and comparing it with the C-T curve, the junction temperature of the LED device is successfully measured. The junction temperature of a white LED obtained by the proposed C-T method is compared with that by tranditional forward voltage method, and they are in good agreement. The C-T method is also used to measure the real-time junction temperatures of white LEDs under a constant current of 350 mA and a constant voltage of 3.2 V, respectively. In both conditions, the junction temperature of an LED needs approximately 110 sec to rise from room temperature to a steady value, and subsequently needs approximately 500 sec to fall back to room temperature after the LED is turned off. Compared with traditional methods, C-T method only needs to measure one calibration and this calibration can be applied to LEDs working at any current and voltage. Therefore, C-T method is a simple and flexible alternative to the existing technique of temperature measurement in electronic device.
Simulation and sesign of single event effect radiation hardening for SiGe heterojunction bipolar transistor
Authors: Li Pei  , Guo Hong-Xia  , Guo Qi  , Wen Lin  , Cui Jiang-Wei  , Wang Xin  , Zhang Jin-Xin  ,   Key Laboratory of Functional Materials and Devices for Special Environments of CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices; Xinjiang Technical Institute of Physics & Chemistry of CAS, 40-1 South Beijing Road, Urumqi 830011, China;,   University of Chinese Academy of Sciences, Beijing 100049, China;,   Northwest Institution of Nuclear Technology, Xi’an 710024, China;,   Xi’an Jiao Tong University, Xi’an 710049, China
Published: December 2020

With the rapid development of satellite, manned space flight and deep space exploration technology, semiconductor devices are used in extreme environments, especially in radiation and low temperature environment. SiGe HBT is a potential candidate for space applications because of its inherent robustness to total ionizing dose (TID) radiation. However, due primarily to charge collection through the collector-substrate (CS) junction and the relatively low substrate doping., SiGe HBTs are vulnerable to single event effects (SEEs) because of new features of process and structure. Thus, the SEE becomes a key factor in restricting space applications of SiGe HBTs. This paper presents an SEE hardening approach that uses a dummy collector to reduce charge collection in the SiGe HBT. The dummy collector is obtained by using the silicon space between adjacent HBTs. It is obtained without any process modification or area penalty. At first, we build simulation models for both normal and hardened SiGe HBTs, and then carry out SEE simulations respectively. The charge collection mechanism is obtained by analyzing the transient current and charge collection changes at different ion incident positions. Unlike the normal HBT, we can see that charge is continuously collected by the dummy CS junction. This causes more charges diffuse outward and the charges available for collector terminal to be reduced. For all ion incident positions, in the case of hardening, the drift components of charge collection are approximately the same, while the diffusion charge collection components are nearly completely compressed. During SEE, the CS junction either directly collects the deposited charges through drift within the potential funnel or indirectly collects charges after they have arrived at the junction after diffusion. The diffusion length of the carriers is on the order of tens of microns or more. Hence a dummy CS junction should be able to reduce the quantity of diffusive charges collected by the HBT collector. The actual charges collected by the collector are effectively reduced. The emitter and base charge collection also decrease by the dummy collector to different extents. Dummy-collector effectively mitigates the SEE of SiGe HBT. The SEE sensitive area of SiGe HBT is also effectively reduced by half. This work is carried out for the SiGe HBT circuit level radiation hardening design of single event effects
Non-uniformity study on readout circuit for uncooled IR detector
Authors: Yuan Hong-Hui  , Chen Yong-Ping  ,   Key Laboratory of Infrared Imaging Materials and Detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
Published: December 2020

For long line uncooled infrared detectors, the non-uniformity of different detecting elements is the key parameter in measuring the circuit performance. So far there have been few research reports in this area. Most uncooled infrared detector circuits require corresponding blind detector for readout circuit design, which increases the complexity of uncooled infrared detector. In addition, the performances of these circuits need to be further improved in practical applications. In order to achieve high performance readout of the long line uncooled infrared detectors, a kind of 160 element readout circuit based on current mirror is designed in this paper. The readout circuit is composed of current mirror input part, capacitor feedback transimpedance amplifier (CTIA), and correlated double sampling (CDS) output circuit. The circuit is fabricated by using the 0.5 micron technology. The non-uniformity of circuit is obviously improved by reasonable parameter setting and current mirror circuit layout. Transconductance amplifier CTIA with capacitance negative feedback is used in the circuit. The integral capacitor consists of three capacitors whose capacitances are 10 pF, 20 pF and 20 pF respectively, thus the circuit can realize different integration capacitances, which forms different magnifications. The circuit can meet different response rates of uncooled detectors. Folded-cascode structure is adopted as the CMOS differential amplifier. The open loop gain is over 80 dB. This single-state folded-cascode construct can overcome the two-stage amplifier’s disadvantages, which easily leads to oscillations. The CDS N SF (source follow) and P SF are adopted as the circuit output, the output swing can easily be greater than 2 V. On average, the CDS N SF and P SF power consumptions are very low. So the total power consumption of 160 line circuit is lower than 100 mW. In the test, the non-uniformity of the readout circuit decreases from 10% to 1%. This result is in accordance with simulation result on non-uniformity. The other test results of total power consumption and the output amplitude also agree with simulation results. The readout circuit has good noise characteristics and the output noise is lower than 1 mV. When the readout circuit and uncooled infrared detector are connected, the infrared signal can be well read out. When the integration time is 20 μups, the device response is 0.294 mV/Ω. The overall system performance is very good. This circuit design based on current mirror has laid the technical foundation for developing readout circuit of the very large scale uncooled infrared detector in the future.
Ranging characteristic and uncertainty of airborne multi-pulse laser
Authors: Kou Tian  , Wang Hai-Yan  , Wang Fang  , Wu Xue-Ming  , Wang Ling  , Xu Qiang  ,   Aeronautics and Astronautics Engineering College, Air Force Engineering University, Xi’an 710038, China;,   Unit No. 95949 of PLA, Cangzhou 061000, China
Published: December 2020

Under the special environment of airborne platform, the theoretical model is built based on probability characteristic of laser ranging, and the probability density distribution functions of pulse-echo signal envelope under different signal-to-noise ratios (SNRs) are obtained. The theoretical results show that the decrease of SNR causes different distributions of ranging data through echo wave characteristic. The experimental data present Gauss distribution with larger SNR, or Rayleigh distribution with poor SNR, or Rice distribution with general condition. According to the constant-ratio timing method, experimental results verify the rationality of theoretical model under a number of ranging data. When the ranging accuracy defined by variance is not applicable, the concept of uncertainty of measurement is introduced. Combined with the theory of uncertainty, a new method of evaluating airborne laser ranging performance is put forward. This method could overcome the unity and irrationality of traditional evaluating method, and meanwhile, it could provide an important reference for evaluating and testing airborne optoelectronic system performance.
Circularly polarized wave reflection focusing metasurfaces
Authors: Li Yong-Feng  , Zhang Jie-Qiu  , Qu Shao-Bo  , Wang Jia-Fu  , Wu Xiang  , Xu Zhuo  , Zhang An-Xue  ,   College of Science, Air Force Engineering University, Xi’an 710051, China;,   Key Laboratory of Electronic Materials Research of Ministry of Education, Xi’an Jiaotong University, Xi’an 710049, China;,   School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China
Published: December 2020

The phase profiles of the reflected circularly polarized waves can be freely manipulated by virtue of a co-polarization reflective metasurface. Based on the co-polarization reflective metasurface, a circularly polarized wave reflection focusing metasurface can be achieved, it can make the reflected waves focus at a focal spot under the normal incidence of circularly polarized plane waves. In this paper, a reflection focusing metasurface is designed. It is found that around the central frequency f=16 GHz, the reflected waves focus on a focal spot above the metasurface with a focal distance L=200 mm under the normal incidence of right-handed circularly polarized waves. However, in the case of normal incidence of left-handed circularly waves, the reflected waves focus on an imaginary focal spot below the metasurface with the focal distance L=-200 mm. The beam-width at the focal spot and focal depth are also calculated by using CST Microwave Studio. The simulation results indicate that the beam-width at the focal spot is approximately equal to the operating wavelength. Therefore, the circularly polarized wave reflection focusing metasurface has a good performance for focusing the reflected waves. In addition, the proposed focusing metasurface displays the advantages of the long focal depth and the broad operating bandwidth.
Performance of chaos synchronization and security in dual-chaotic optical multiplexing system
Authors: Mu Peng-Hua  , Pan Wei  , Li Nian-Qiang  , Yan Lian-Shan  , Luo Bin  , Zou Xi-Hua  , Xu Ming-Feng  ,   Center for Information Photonics and Communications, Southwest Jiaotong University, Chengdu 610031, China
Published: December 2020

Based on a novel multiplexing system of two distinct chaotic signals, the corresponding modified Lang-Kobayashi rate equations are established. The numerical investigations into the performance of chaos synchronization are carried out. In more detail, the influences of single-parameter mismatch, multi-parameter mismatch, feedback-strength discrepancy, and frequency detuning between the two master semiconductor lasers (MLs) on synchronization performance are investigated, respectively. Moreover, the security and spectrum characteristics are addressed briefly in this work. The numerical simulations show that by adopting parameter mismatch, i.e., choosing appropriate system parameters of the two MLs, the correlation between the two MLs becomes extremely low, while the matched master-slave laser pairs can achieve high-quality chaos synchronization, indicating that the condition of optical chaos multiplexing is satisfied; the parameter mismatch between the MLs has a significant influence on their synchronization quality, but no obvious influence on their synchronization quality of the matched master-slave laser pairs, which further demonstrates the validity and feasibility of the chaos multiplexing scheme. More importantly, in this paper, the multiplexed chaotic signals in the time and frequency domains in terms of autocorrelation function and power spectrum are analyzed, demonstrating that the present system could provide higher security than the single external-cavity semiconductor laser.
Rotational viscosity comparison of liquid crystals based on the molecular dynamics of mixtures
Authors: Wang Qi-Dong  , Peng Zeng-Hui  , Liu Yong-Gang  , Yao Li-Shuang  , Ren Gan  , Xuan Li  ,   State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130022, China;,   University of Chinese Academy of Sciences, Beijing 100039, China;,   State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

It is critical to improve the response speed of a liquid crystal wavefront corrector in order to increase the bandwidth of a liquid crystal adaptive optics system. The design of liquid crystal molecules with small rotational viscosity becomes a basic method of increasing the response speed of a liquid crystal wavefront corrector. Various phases of liquid crystal from molecular dynamics simulation are given in this paper, and the detailed computational methods of order parameter and rotational viscosity are also presented. Rotational viscosities of liquid crystals are compared based on the molecular dynamics of mixtures. The data fluctuation is reduced effectively through several simulations and the multiple analysis of original data. A detailed process of molecular dynamics of mixtures is given in this paper and the result is greatly satisfactory. We believe that one can perform a better molecular design using this process and obtain a better understanding of molecular interactions of LCs.
Reflective Ni/Ag/Ti/Au electrode with low specific contact resistivity
Authors: Wei Zheng-Hong  , Yun Feng  , Ding Wen  , Huang Ya-Ping  , Wang Hong  , Li Qiang  , Zhang Ye  , Guo Mao-Feng  , Liu Shuo  , Wu Hong-Bin  ,   Key Laboratory of Physical Electronics and Devices of Ministry of Education, Shaanxi Provincial Key Laboratory of Photonics and Information Technology, Xi’an Jiaotong University, Xi’an 710049, China;,   Solid-State Lighting Engineering Research Center, Xi’an Jiaotong University, Xi’an 710049, China;,   Shaanxi Supernova Lighting Technology Co. Ltd., Xi’an 710077, China
Published: December 2020

The specific contact resistivity and reflectivity of Ni/Ag/Ti/Au contact with p-GaN are studied. It is found that the thickness of Ag, anneal time and deposition temperature have a great effect on the performance of Ni/Ag/Ti/Au electrode. Its optical reflectivity is measured by reflectivity spectrophotometer, and its specific contact resistivity is calculated by circular transmission line model. It is observed that the contact reflectivity values of Ni (1 nm)/Ag/Ti (100 nm)/Au (100 nm), when the thickness values of Ag are 25 nm and 50 nm, are low: their values are 68.5% and 82.1% at 450 nm, respectively, and they start to increase with increasing the Ag thickness, then reach their saturated values when Ag thickness is beyond 200 nm. When the anneal time changes from 1 min to 10 min in oxygen atmosphere, the specific contact resistivity decreases at 300 ℃, decreases further and then increases at 400-600 ℃. After annealing at temperatures at 300 ℃ and 400 ℃ in oxygen atmosphere, the contact reflectivity value of Ni/Ag/Ti/Au remains almost unchanged, even when anneal time increases from 1 min to 10 min. However, The contact reflectivity of Ni/Ag/Ti/Au decreases significantly after annealing at a temperature higher than 400 ℃ and it becomes smaller with longer annealing time. After 400 ℃ annealing in oxygen atmosphere for 3 min, the specific contact resistivity reaches 3.6×10-3 Ω·cm2. Additionally, the deposition temperature of Ni/Ag is investigated. It is noticed that the specific contact resistivity decreases and the reflectivity increases with increasing the deposition temperature from room temperature to 120 ℃. The reflectivity rises to 90.1% at 450 nm and the specific contact resistivity reaches 6.4×10-3Ω·cm2 for the Ni/Ag/Ti/Au electrode at a deposition temperature of 120 ℃. However, the effects of improving the electrical and optical characteristics weaken when deposition temperature changes from 120 ℃ to 140 ℃. With a overall consideration, Ni (1 nm)/Ag (200 nm)/Ti (100 nm)/Au (100 nm) is made at a deposition temperature of 120 ℃, and then anneals at 400 ℃ for 3 min in oxygen atmosphere to achieve the optimized electrode. The vertical light emitting diode with this Ni/Ag/Ti/Au electrode is fabricated. Its working voltage is 2.95 V and the light output power is 387.1 mW at 350 mA. The electro-optical conversion efficiency reaches 37.5%.
Electronic structures and second hyperpolarizabilities of annulenes derivatives
Authors: Chen Ying-Ying  , Han Kui  , Li Hai-Peng  , Li Ming-Xue  , Tang Gang  , Shen Xiao-Peng  ,   Department of Physics, College of Science, China University of Mining and Technology, Xuzhou 221116, China
Published: December 2020

Organic nonlinear optical materials have attracted considerable attention in recent years because of their potential applications in photonic devices and optical information processing. Recent studies have shown that annulene derivatives exhibit good second-order nonlinear optical properties, but their third-order nonlinear optical properties are studied little. In this paper, the values of molecular static linear polarizability α and second hyperpolarizability γ of substituted annulenes have been investigated with different levels of HF, B3LYP, BHandHLYP and CAM-B3LYP at different basis sets, respectively. Their ultraviolet spectra have also been calculated by using the TD-B3LYP method. It is found that the quality of the basis set is important for the hyperpolarizability calculations, and diffuse functions are important to obtain accurate results for the second hyperpolarizability. We also study the structure-optical property relationship for annulene. It is found that annulene molecular structure has a significant influence on third-order nonlinear optical response. Increasing the conjugation length and introducing push-pull electronic groups can enhance the second hyperpolarizability. But the introduction of push-pull electronic groups can enhance the hyperpolarizability more remarkably than increasing the conjugation length dose, which may be due to the fact that the introduction of push-pull electronic groups can provide a large number of polarizable electrons whereas increasing the conjugation length can only enhance the electron delocalization. Meanwhile the push-pull electronic group substituted annulenes can also exhibit high transparency in visible region. Thus, this work has a good reference for designing nonlinear optical material with high, nonlinear optical coefficient and good transparency. In addition, for the same push-pull electronic groups, the higher conjugation degree and the longer πup -conjugated bridge result in the decrease of HOMO-LUMO energy gap and transition energy which benefits the enhancement of nonlinear optical response. Our results demonstrate that annulene derivative shows both high transparency and large second hyperpolarizability, and thus becomes a promising candidate for third-order nonlinear optical material. In addition, the dynamic (hyper) polarizabilities of considered annulene molecules are calculated by using CAM-B3LYP method. It is found that in near-infrared region, with the increase of frequency of incident light, α (ω; ω), γ (-ω; ω, 0, 0) and γ (-2ω; ω, ω, 0) are all increased, and the near-resonance enhancement effect occurs. Under the condition of far resonance, α (ω; ω), γ (-ω; ω, 0, 0) and γ (-2ω; ω, ω, 0) change little. This dispersion effect may be helpful for the experimental study and applications as well.
Finite-temperature properties of N two-level atoms in a single-mode optic cavity and phase transition
Authors: Jia Shu-Fang  , Liang Jiu-Qing  ,   Institute of Theoretical Physics, Shanxi University, Taiyuan 030006, China
Published: December 2020

In this paper, we investigate the finite-temperature properties and phase transition of the Dicke model. Converting the atomic pseudo-spin operator to the two-mode Fermi operators, we obtain the partition function in terms of the imaginary-time path integral. The atomic population and average photon number as analytic functions of the atom-photon coupling strength are found from the thermodynamic equilibrium equation, which leads to the stationary state at a finite temperature and is determined by the variation in an extremum-condition of the Euclidean action with respect to the bosonic field. In particular we study the phase transition from normal to superradiation phase at a fixed low-temperature, in which the phase transition is dominated by quantum fluctuations. The phase transition induced by the variation of the atom-photon coupling strength indeed obeys the Landau continuous phase-transition theory, in which the average photon-number can serve as an order parameter with non-zero value that characterizes the superradiation phase. In the zero temperature limit our results recover exactly all those obtained from the quantum phase transition theory at zero temperature. In addition, we discuss the thermodynamic properties and compare the difference between finite-temperature phase transition and zero-temperature quantum phase transition. It is discovered that the average photon-number and mean energy in the low-temperature stationary state coincide with the corresponding values of zero-temperature in the strong coupling region. The entropy of the superradiation phase decays rapidly to zero with the increase of coupling strength.
Realisation of orbital angular momentum sorter of photons based on sagnac interferometer
Authors: Fu Dong-Zhi  , Jia Jun-Liang  , Zhou Ying-Nan  , Chen Dong-Xu  , Gao Hong  , Li Fu-Li  , Zhang Pei  ,   MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Department of Applied Physics, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

Orbital angular momentum (OAM) of photons has both classical and quantum applications due to its feature of optical vortex and infinite dimension. OAM discrimination is one of the basic problems, which has been paid much attention recently. Here we present an interferometer method in which a Sagnac interferometer with a Dove prism is placed on each arm to separate the different OAM of photons into different output ports, namely, OAM sorters. We demonstrate experimentally the feasibility of OAM sorter by dividing different OAM states into different output ports. Using the cascade interferometers, we also sort the superposition state successfully. Experimental results are in good agreement with the theoretical predictions. Compared with other methods, this method is more stable and can be used to separate superposition states into single photon levels. Furthermore, this method can also be used to couple OAM modes with spatial modes, a very important method for manipulating OAM states. It is a useful method and has potential applications in high-capacity optical communication, quantum entanglement, quantum cryptography, quantum computation and quantum information.
Quantum control of the XUV photoabsorption spectrum of helium atoms via the carrier-envelope-phase of an infrared laser pulse
Authors: Yang Zeng-Qiang  , Zhang Li-Da  ,   School of Physics, Beijing Institute of Technology, Beijing 100081, China;,   College of Science, Shihezi University, Shihezi 832003, China
Published: December 2020

In the present paper, we investigate the quantum control of the XUV photoabsorption spectrum of helium atoms via the carrier-envelope-phase (CEP) of an infrared (IR) laser pulse by numerically solving the time-dependent one-dimensional (1D) two-electron Schrödinger equation. The advantage of the 1D model is that the associated time-dependent Schrodinger equation (TDSE) can be solved numerically with high precision as taking full account of the interaction between the electrons and without making any assumptions about the dominant physical mechanisms. In our study, a single attosecond XUV pulse with broad bandwidth is used to create a wave packet consisting of several doubly-excited states. Helium atoms subjected to the XUV pulse can be ionized through two different pathways: either direct ionization into the continuum or indirect ionization via the autoionization of doubly excited states. The interference of these two paths gives rise to the well-known Fano line shape in the photoabsorption spectrum, which is determined by the ratio and relative phases of the two paths. In the presence of an IR laser pulse, however, we find that the Fano line profiles are strongly modified, in good agreement with recent experimental observations [C. Ott et al., Science 340, 716 (2013); C. Ott et al., Nature 516, 374 (2014)]. At certain time delays, we can observe symmetric Lorentz, inverted Fano profiles, and even negative absorption cross sections, indicating that the XUV light can be amplified during the interaction with atoms. We fit the absorption spectra with the Fano line profiles giving rise to the CEP-dependent Fano q parameters, which are modulated from extremely large positive value to extremely large negative value. Since the q parameter is proportional to the ratio between the dipole matrix of the indirect ionization path and the dipole matrix of the direct ionization path; these results indicate that the quantum interference between the two ionization paths can be efficiently controlled by the CEP of an ultrashort laser pulse, thus offering another possibility (in addition to the laser intensity and the time delay between the XUV pulse and the IR laser) of manipulating the extreme ultrafast electronic motion in atoms. Our predictions can be experimentally verified easily with the present experimental technique.
Repetition rate optimization of passively mode-locked fiber laser for high-speed linear optical sampling
Authors: Peng Han  , Liu Bin  , Fu Song-Nian  , Zhang Min-Ming  , Liu De-Ming  ,   National Engineering Laboratory for Next Generation Internet Access System, Huazhong University of Science and Technology(HUST), Wuhan 430074, China
Published: December 2020

Optical fiber communication systems are going to adopt the use of advanced modulation formats. It is thus important to develop measurement techniques and solutions capable of quantifying such signals. Linear optical sampling is an effective technique to characterize the quality of an advanced modulation format for high-speed optical signal with high fidelity, while the passively mode-locked fiber laser is an enabling module to implement the linear optical sampling. In this paper, we obtain a trade-off relationship between the repetition rate of passively mode-locked fiber laser and the linewidth of high-speed signal under test, after the introduction of operation principle for linear optical sampling. It is found that, for the quadrature phase shift keying (QPSK) signal, when the ratio of the linewidth of the signal under test to the repetition rate of passively mode-locked fiber laser is less than 1.5×10-3, the linear optical sampling-induced impairments can be ignored when there occurs phase noise. Therefore, the phase estimation can be successfully made by using the Viterbi-Viterbi algorithm applied to the block of samples corresponding to the modulation format phase states. Next, we use an optical sampling pulse with a repetition rate of 95.984 MHz, and carry out the optical linear sampling to a 28 Gbaud QPSK signal with a linewidth of 100 kHz. The error vector magnitude (EVM) has long been a commonly used parameter for quantifying the quality of advanced modulation signals. Using the standard coherent detection algorithm, we can successfully recover the constellation with the error vector magnitude (EVM) error less than 1%. Theoretical investigations agree well with the experimental characterization. Such a conclusion is helpful to optimize the design of passively mode-locked fiber laser for optical sampling application.