bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Study of a Periodical Flapping Flag
Authors: Bai Ye  , Jia Yong-Xia  , Li Cun-Biao  , Zhu Yi-Ding  ,   State Key Laboratory for Turbulence and Complex Systems, College of Engineering, Peking University, Beijing 100871, China
Published: December 2020

A flag flapping in the wind is a classical fluid-structure interaction problem that concerns the interaction of elastic bodies with ambient fluid. The fluid-flag interaction can give rise to three self-sustained oscillation states under certain conditions, i.e., stretched-straight state, periodic state, and chaotic state. This paper reports an experimental study of a cantilevered polydimethylsiloxane (PDMS) flag flapping in a uniform flow at a periodic state. A heavy flag is well designed, with metal strips imbedded in the PDMS sheet. Immersing the elastic but self-sustaining flag into the water flow, we use the time-resolved particle image velocimetry (PIV) and visualization techniques to obtain the whole flow field around the midspan of the flapping flag. A unique PIV image processing method is used to measure the near-wall flow velocities around the flap-ping elastic flag at the periodic state. The image processing technique adopts a radon transform technology to determine the moving interface in the particle images. The interface velocity distribution is subsequently calculated. Artificial particles of uniform size with the interface velocity are added into the flag region. Therefore, the whole velocity field over a flapping period is accurately obtained, giving the basic data to analyze the flag flapping. It is found that there exists an inflection point in the envelope curve of the flag flapping. Based on the analyses of the flapping states and velocity fields, a unified flapping Strouhal number (St = 2Af/U) is proposed by choosing the amplitude of the inflection point as the characteristic length, which is similar to the Strouhal number of the flow around a circular cylinder over the same range of Reynolds number.
First-principles study of the phonon spectrum and heat capacity of TATB crystal
Authors: Jiang Wen-Can  , Chen Hua  , Zhang Wei-Bin  ,   Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China;,   Graduate School, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

The widely used energetic material 1, 3, 5-triamino-2, 4, 6-trinitrobenzene (TATB) is an extremely powerful explosive and known for its extraordinary insensitivity to external stimuli (i.e., shock, friction, impact). TATB crystal exhibits graphitic-like sheets with significant inter- and intra-molecular hydrogen bondings within each layer and weak van der Waals (vdW) interactions between layers. Although TATB has been extensively studied both theoretically and experimentally, a fully understanding of its unique detonation phenomenon at a microscopic level is still lacking. Before establishing the exact pathway through which the initial energy is transferred, a fundamental knowledge of both the lattice vibrations (phonons) and molecule internal vibrations must be gained at the first step. Recently, it has been demonstrated that density functional theory (DFT) is inadequate in treating conventional energetic materials, within which dispersion interactions appear to be major contributors to the binding forces. In the present work, phonon spectrum and specific heat of TATB crystal are calculated in the framework of DFT with vdW-DF2 correction, which has been validated in our previous studies of the equation of state, structure and vibration property of TATB crystal under pressures in a range of 0-8.5 GPa. Structure optimization is preformed at zero-pressure, followed by calculating the equation of state, crystal density and lattice energy. The computed results are found to fit well with the experimental and other theoretical values. Frozen phonon method is used to calculate the phonon spectrum and phonon density of states. We find that the phonon density of states reaches its maximum at a vibration frequency of 2.3 THz, which is in good agreement with the strong absorption peak at 2.22 THz observed by THz spectroscopy. The assignment of several Raman active vibrations of TATB above 7.5 THz is given, and a comparison with other published results is also made in this study. Furthermore, the contributions of different phonon vibration modes to the specific heat are derived from the phonon density of states. The number of doorway modes (i.e., the low frequency molecular vibrations that is critical to detonation initiation) of TATB in a range of 6.0-21.0 THz is estimated based on the phonon density of states. It is shown that the phonon modes in a range of 0-27.5 THz would contribute 93.7% of the total specific heat at room temperature. By combining a Mulliken population analysis of TATB with the relative contribution of phonon vibration modes to the specific heat at 300-600 K, we conclude that C-NO2 bond might be the trigger bond of TATB during thermolysis.
Effect of Ga vacancy on the magnetism in GaN:Gd: First-principles calculation
Authors: Hou Zhen-Tao  , Li Yan-Ru  , Liu He-Yan  , Dai Xue-Fang  , Liu Guo-Dong  , Liu Cai-Chi  , Li Ying  ,   School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China
Published: December 2020

In recent years, GaN doped with Gd (GaN:Gd) has attracted much attention due to its potential applications in spintronic devices since the high temperature ferromagnetism and the colossal magnetic moment were observed in GaN:Gd. However, the microscopic nature of ferromagnetism in GaN:Gd still is controversial. We investigate the crystal parameters, magnetic moment, formation energies, and electronic structures of the defect complexes formed by Gd and native Ga (or N) vacancies in GaN by using the first-principles method based on the density functional theory. The calculated results show that the energy band gap of GaN:Gd becomes indirect and its width becomes small compared with that of GaN. The lattice constants of GaN:Gd expand due to the larger ionic radius of Gd than that of Ga atom, while they shrink when the Gd atom and Ga vacancies coexist. In the case of the isolated Gd dopant, the Gd-4f electrons lead to a magnetic moment of about 7 B in GaN:Gd. For the defect complex, one Ga vacancy can introduce a magnetic moment of about 2 B, while N vacancy has little effect on the total magnetic moment. In addition, when we focus on the defect complex composed of Gd and five neighboring Ga vacancies, we find that the magnetic moment of per Gd atom and the total magnetic moment depend strongly on the concentration and position of Ga vacancies. When the Ga vacancies are distributed loosely near the Gd atom, the magnetic moment of Gd atom increases slightly, while for the closely-distributed Ga vacancies the Gd magnetic moment can be increased by 2 B. We infer that the interactions among Ga vacancies result in the large magnetic moment of Gd atom. It is also found that the formation energy is very small when the Ga vacancies are distributed thickly around the Gd atom in GaN:Gd. Our results are in qualitative agreement with the results from other studies (Thiess A et al. 2012 Phys. Rev. B 86 180401; Thiess A et al. 2015 Phys. Rev. B 92 104418), where Ga vacancies were proposed to tend to cluster in GaN:Gd and induce the large magnetic moment of Gd. Moreover, the effect of distance between the Gd atom and Ga vacancies on the Gd magnetic moment is also discussed. It is found that the Gd magnetic moment is relatively large when Ga vacancies are close to the Gd atoms.
Spectroscopic ellipsometry analysis of vanadium oxide film in Vis-NIR and NIR-MIR
Authors: Wang Pan-Pan  , Zhang Yu-Zhi  , Peng Ming-Dong  , Zhang Yun-Long  , Wu Ling-Nan  , Cao Yun-Zhen  , Song Li-Xin  ,   Key Laboratory of Inorganic Coating Materials CAS, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
Published: December 2020

The monoclinic phase (M phase) VO2 film is prepared on quartz glass substrate by a model MSP-3200 three-target co-sputter coater with RF magnetron reactive sputtering. The optical properties in incident energy ranges of 0.5-3.5 eV (350-2500 nm) and 0.083-0.87 eV (1400-15000 nm) of VO2 film are investigated by spectroscopic ellipsometry with variable temperature attachment. The good results are determined point by point with the three Lorentz harmonic oscillator dispersion models in the range of 0.5-3.5 eV and four Gaussion harmonic oscillator dispersion models in the range of 0.083-0.87 eV in the state of semiconductor below the transition temperature, while adding seven Lorentz harmonic oscillator dispersion models in the high temperature metallic state film results in the characteristic absorption peaks. The results show that the refractive index of the semiconductor state of VO2 film is maintained at maximum 3.27 and extinction coefficient k is close to zero in the near infrared-mid infrared, which is due to the fact that the absorption of semiconductor thin film in the VIS-NIR range is derived from the free carrier absorption and d// orbital of the semiconductor film has less electron density. The refractive index n of high temperature metallic state VO2 film has an obviously increasing trend in the near infrared-the mid infrared which is larger than the refractive index of the semiconductor state when the incident light energy is 0.45 eV. Extinction coefficient k increases rapidly in the near infrared, which is because the density of free carrier increases in the range of 0.5-1.62 eV and electron transition absorption augments within the V3d band. When the incident energy less than 0.5 eV, k value changes gently in the film because free carrier concentration and flow rates are stable.
Magnetization distribution in exchange spring bilayers with mutually orthogonal anisotropies
Authors: Chen Chuan-Wen  , Xiang Yang  ,   College of Information Science and Engineering, Huaqiao University (Xiamen), Xiamen 361021, China
Published: December 2020

A soft/hard bilayer system with mutually orthogonal anisotropies is considered in this paper. The easy axis of the hard layer is perpendicular to the film plane, and the easy axis of the soft layer is parallel to the film plane. Pt84Co16 is chosen as the soft layer material, and TbFeCo is chosen as the hard layer material. The one-dimensional continuum micromagnetic model is used. The characteristics of nucleation fields, angular distribution and hysteresis loops are studied. The calculation results show that the nucleation field decreases rapidly and even turns negative with increasing soft layer thickness. This negative nucleation field is caused by the demagnetizing field and the easy axis orientation of the soft layer which is parallel to the film plane. Both of these two factors can induce an effective in-plane uniaxial anisotropy, which will tend to align the magnetization of the soft layer parallel to the film plane. As the magnetocrystalline anisotropy constant K of the soft layer is very small, the negative nucleation field mainly comes from the demagnetizing field of the soft layer. The angular distribution calculation shows that the change rate of magnetization deviation angle (degree per nanometer) along z axis in the soft layer is faster than that in the hard layer. The angular change rate could be adjusted by varying the anisotropy constant ratio, exchange energy constant ratio, or external field. When the anisotropy constant ratio Ks/Kh (soft/hard) or exchange energy constant ratio As/Ah (soft/hard) increases, the angular change rate ratio (soft/hard) decreases. Especially when both Ks/Kh and As/Ah increase at the same time, the angular change rate in the hard layer could become faster than that in the soft layer. If the anisotropy constant Ks becomes larger, it is more difficult for the magnetization in the soft layer to deviate from its easy axis than before. This will also enhance the pinning effect of the magnetization in the soft layer, and reduce the difference in deviation angle between the two boundaries of the soft layer. When the exchange energy constant As increases, the magnetization tends to become parallel to the neighboring magnetization, which also reduces the angular change of magnetization in the soft layer. As the anisotropy constant is roughly proportional to the square of spontaneous magnetization, the effect of spontaneous magnetization on the angular change rate comes from the anisotropy constant change. The simulation for the hysteresis loops shows that the saturation field strength increases while the remanence decreases with increasing both the values of Ks and As.
Demagnetization dynamics of C-doped FePt film
Authors: Pan Qun-Feng  , Zhang Ze-Yu  , Wang Hui-Zhen  , Lin Xian  , Jin Zuan-Ming  , Cheng Zhen-Xiang  , Ma Guo-Hong  ,   Department of Physics, Shanghai University, Shanghai 200444, China;,   Institute for superconductor and Electronic Materials, University of Wollongong, North Wollongong NSW 2500, Australia
Published: December 2020

Magneto-optical information storage has been a hot research subject for several years. FePt exhibits abundant physical properties and has received much attention as a candidate material. Its alloy film with perpendicular anisotropy and small grain size has important applications in magnetic recordings due to the large intrinsic magnetic anisotropy which ensures long-time thermal stability of nanometer sized bits. However, the large coercive field of FePt is a significant factor that hinders its application. As is well known, the magnetic anisotropy in FePt alloy can be precisely modulated by carbon-doping, and as a result, the coercive field of FePt film can be modified effectively with the carbon dopant. On the other hand, the microscopic mechanism of magnetic storage relies on the motion of spin system. Ultrashort femtosecond laser has been demonstrated to be a very effective tool to investigate the dynamical coupling among different degrees of freedom, such as electron, spin and lattice in a ferromagnetic film. The research on spin dynamics has become a new frontier of condensed matter physics, which is crucial for ultrafast magnetic recording materials. In this work, by using the time-resolved magneto-optical Kerr effect spectroscopy, we study the ultrafast spin dynamics of two FePt alloy films with different carbon dopants under the applied magnetic field along the film surface. The FePt alloy films with different carbon dopants are fabricated on silicon substrates by the sputtering method. The main experimental findings in this work are as follows. (i) The transient Kerr signal is linearly proportional to the magnetization with the magnetic field up to 0.8 T, while the transient reflectivity of the film is independent of the applied magnetic field. (ii) For FePt alloy films with different coercive fields, it is found that the demagnetization time of the film with smaller coercive field is significantly faster than that of the larger counterpart: the former shows 0.8 ps demagnetization time, and the latter has a magnitude of 1.2 ps. The demagnetization times for both soft and hard magnetic films are independent of the applied magnetic field. (iii) With ultrafast laser pulse radiation, we observe the propagation of acoustic phonon with a resonance frequency of ～ 49 GHz, and the frequency of the acoustic phonon is independent of the applied magnetic field. From the above, the spin dynamics of the samples shows strong correlation with carbon-doping. Our experimental findings are desired for basic research as well as for the design and development of novel magneto-optical devices.
Synthesis of large size monolayer MoS2 with a simple chemical vapor deposition
Authors: Dong Yan-Fang  , He Da-Wei  , Wang Yong-Sheng  , Xu Hai-Teng  , Gong Zhe  ,   Key Laboratory of Luminescence and Optical Information Technology of Ministry of Education, Beijing Jiaotong University, Beijing 100044, China;,   Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China
Published: December 2020

Monolayer molybdenum disulfide (MoS2) has recently aroused the great interest of researchers due to its direct-gap property and potential applications in electronics, catalysis, photovoltaics, and optoelectronics. Chemical vapor deposition (CVD) has been one of the most practical methods of synthesizing large-area and high-quality monolayer MoS2. However, The process of preparation is complex and cumbersome. Here we report that high-quality monolayer MoS2 can be obtained through using sulfurization of MoO3 by a simple and convenient CVD on sapphire substrates.The substrate cleaning is simplified. Substrates are cleaned in detergent solution, deionized water and acetone without sopropanol or piranha solution (H2SO4/H2O2=3:1) in sequence, avoiding their potential dangers. The MoO3 powder (Alfa Aesar, 99.995%, 0.02 g) is placed in an alumina boat, and a sapphire substrate is faced down and is placed 6 cm away from MoO3 powder in the same boat. The sapphire substrate is placed in the center of the heating zone of the furnace. Another alumina boat containing sulfur powder (Alfa Aesar, 99.999%, 0.2 g) is placed upstream with respect to the gas flow direction in the low temperature area. We adopt an atmospheric pressure chemical vapor deposition method, so it does not require a vacuum process. After 30 min of Ar purging, the furnace temperature is directly increased from room temperature to 800 ℃ in 30 min, reducing the heating steps. After 60 min, the furnace is cooled down naturally to room temperature. Optical microscopy (OM) images, Raman spectra and photoluminescence (PL) are all obtained by confocal Raman microscopic system (LabRAM HR Evolution). From the OM images, we can see that isolated islands (triangles) have edge lengths up to 50 m, which is far larger than that grown by micromechanical exfoliation. The color of the triangles is uniform, which has a strong contrast with the substrate. We can obtain a preliminary result that the sample is a uniform monolayer MoS2. Raman spectra are collected for MoS2 samples on sapphire substrates. Two typical Raman active modes can be found: E2g1 at 386.4 cm-1 and A1g at 406 cm-1 ( =19.6 cm-1), which correspond to single-layered MoS2 sample. Raman mapping shows that the sample is a uniform monolayer MoS2. The PL spectrum of MoS2 shows a pronounced emission peak at 669 nm, which is consistent with other reported results for MoS2 thin sheets obtained from exfoliation methods. When the layer number of MoS2 decreases, with its bandgap transforming from indirect to direct one, the fluorescence efficiency will be significantly enhanced. So the results further prove that the sample is high-quality monolayer MoS2.
Research and data processing of double locked cavity ringdown absorption spectroscopy
Authors: Jia Meng  , Zhao Gang  , Hou Jia-Jia  , Tan Wei  , Qiu Xiao-Dong  , Ma Wei-Guang  , Zhang Lei  , Dong Lei  , Yin Wang-Bao  , Xiao Lian-Tuan  , Jia Suo-Tang  ,   Institute of Laser Spectroscopy, State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, Taiyuan 030006, China;,   Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: December 2020

A continuous wave cavity ringdown spectroscopy based on a double-locking loop is proposed to improve the shortcoming of low acquisition rate of concentration in traditional scheme. A small portion of laser is separated to pass through a C2H2 reference cell, used to lock the laser frequency to the 1+3 band P(9)e absorption line of C2H2 at 6534.3634 cm-1 by the 1st harmonic demodulation of the frequency modulation spectroscopy. The remaining portion is incident on a high finesses cavity to observe the ringdown events. Meanwhile, the reflected light of cavity is used to extract the error signal to lock the laser based on the PDH frequency locking technique. As a consequence, the frequency drift of the laser and the jitter of the cavity length are improved, therefore a more relatively accuracy result is expected. The laser light is dual frequency modulated by a fiber coupled electro optic modulator (FEOM)in the above system. In order to optimize, to some extent, the asymmetry of the error signal caused by the residual amplitude modulation due to the inconsistency of the laser polarization direction with the extraordinary axis of the FEOM, the demodulation phase is adjusted carefully until the error signal is smoothed up and close to symmetry. Then, the effect of locking loop is examined. The frequency of laser, based on the measurement by a wavelength meter, is more stable and the relative frequency discrimination between the laser and the longitudinal mode of cavity is about 9.8 kHz. In addition, the PDH locking, ensuring the efficient coupling of the laser with the cavity, can gain a high acquisition rate of the concentration information. In order to obtain a complete ringdown event, the frequency of square wave to the fiber coupled acoustic optical modulator (FAOM) is limited to 30 kHz with the duty cycle of 85%, which is determined by the ringdown time and re-lock time. However, there exists a relatively large random noise in a series of ringdown time measurements of empty cavity, which is mainly caused by the errors of fitting and measurement. For the further improvement of the accuracy of experiment, an efficient digital filter, Kalman filter which can suppress the noise considerably at no expense of real-time capability, is used. The standard deviation of the ringdown time is reduced from 0.00333 to 0.00153. According to Allan variance analysis, the detection limit can reach 410-9 cm-1 for a 2 s integration time. Finally, the C2H2 gases with different concentrations from 100 ppb to 5 ppm are measured to demonstrate the linear response of this system.
Optimization of the loading rate of magneto-optical trap for neutral mercury atom
Authors: Gou Wei  , Liu Kang-Kang  , Fu Xiao-Hu  , Zhao Ru-Chen  , Sun Jian-Fang  , Xu Zhen  ,   Key Laboratory for Quantum Optics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China;,   University of Chinese Academy of Sciences, Beijing 100049, China;,   Center of Cold Atom Physics, Chinese Academy of Sciences, Shanghai 201800, China
Published: December 2020

Optics lattice clock is a hot topic in the researches of frequency standard and metrology. Neutral mercury atom is one of the most promising candidates for optical lattice clock. Due to its large atomic number, mercury atom is insensitive to black body radiation, which is the severe limitation for developing the optical lattice clocks. To realize the optical lattice clock of neutral mercury atoms, the first step is to implement laser-cooling and trapping of neutral mercury atoms. The cooling transition of mercury atom is 1S0-3P1 transition. The wavelength is 253.7 nm, the line width is 1.27 MHz, and the saturation intensity is 10.2 mW/cm2. Quantum projection noise (QPN) is an important parameter that affects optical lattice clock. Increasing the loading rate of magneto-optical trap (MOT) can help lower the QPN, thereby improving the performance of optical lattice clock. In this work, we calculate the scattering force of deep UV cooling laser, which is exerted on mercury atom in our single chamber MOT, and numerically simulate the one-dimensional motion of the atom in the MOT. It gives us the capture velocity under optimized parameters of the MOT. Then we calculate the loading rate of three-dimensional MOT by a high efficient random sampling method. According to the rate equation of MOT, the loading rate is proportional to the atom number of the steady state, which is the accessible parameter in the experiment. An experimental setup of MOT is established with a high vacuum system and a frequency quadrupled semiconductor laser system. The fluorescence imaging on an EMCCD gives the atom number in the MOT. We also calibrate the vapor density of background mercury gas in the vacuum, and measure the atom number in a steady MOT. We numerically simulate and experimentally study the influences on the atom number on the parameters of MOT, such as laser intensity, laser detuning and magnetic field gradient. The calculated results are in consistent with the experimental results. We also find the optimized parameters to maximize the loading rate. The numerical simulation also gives some results beyond current experimental condition, especially for laser intensity. From the simulations we obtain the optimized MOT parameter and relation for laser cooling of neutral mercury atom. Especially, current deep UV cooling laser with 20 mW of power does not have enough power to enhance the loading rate. These results are rather valuable for designing the next generation of optical lattice clocks for neutral mercury atoms.
Density functional theory study of structure stability and electronic structures of graphyne derivatives
Authors: Chi Bao-Qian  , Liu Yi  , Xu Jing-Cheng  , Qin Xu-Ming  , Sun Chen  , Bai Cheng-Hao  , Liu Yi-Fan  , Zhao Xin-Luo  , Li Xiao-Wu  ,   Institute of Materials Physics and Chemistry, School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China;,   Department of Physics, Materials Genome Institute, and International Centre for Quantum and Molecular Structures, Shanghai University, Shanghai 200444, China;,   College of Sciences, Shenyang Ligong University, Shenyang 110159, China;,   School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China
Published: December 2020

Due to the diversified atomic structures and electronic properties, two-dimensional monolayer nanocarbon materials (graphyne or graphdiyne) composed of sp and sp2 hybridization C atoms have received the widespread attention in recent years. The fundamental questions include how the sp orbital hybridization affects the electronic structure of graphyne. In order to investigate the structure dependent electronic structures of graphyne, the energetic stabilities and electronic structures of -graphyne and its derivatives (-N) with N carbon atoms on each edge of the hexagons are investigated by density functional theory (DFT) calculations in this work. In our DFT calculations we adopt generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) using the CASTEP module implemented in Materials Studio. The studied -Ns consist of hexagon carbon rings connected by vertexes whose edges have various numbers of carbon atoms N= 1-10. The structure and energy analyses show that -Ns with even-numbered carbon chains have alternating single and triple C-C bonds, energetically more stable than those with odd-numbered carbon chains possessing continuous C-C double bonds. The calculated electronic structures indicate that -Ns can be either metallic (odd N) or semiconductive (even N), depending on the parity of number of hexagon edge atoms regardless of the edge length due to Jahn-Teller distortion effect. Some semiconducting -graphyne derivatives (-N, N= 2, 6, 10) are found to possess Dirac cones (DC) with small direct band gaps 10 meV and large electron velocities 0.255106-0.414106 m/s, ～30%-50% of that of graphene. We find that Dirac cones also appear in -3 and -4 when we shorten the double bonds and elongate the triple bonds in -3 and -4 respectively. These results show that the bond length change will affect the characteristics of band structure and suggests that the band structure characteristics may be influenced by Peierls distortion in a two-dimensional system. Our DFT studies indicate that introducing sp carbon atoms into the hexagon edges of graphene opens the way to switching between metallic and semiconductor/DC electronic structures via tuning the parity of the number of hexagon edge atoms without doping and defects in nanocarbon materials and nanoelectronic devices.
Atmospheric window characteristic and channel capacity of THz wave propagation
Authors: Wang Yu-Wen  , Dong Zhi-Wei  , Li Han-Yu  , Zhou Xun  , Luo Zhen-Fei  ,   Graduate School of Chinese Academy of Engineering Physics, Beijing 100088, China;,   Institute of Applied Physics and Computational Mathematics, Beijing 100094, China;,   Terahertz Research Center, Chinese Academy of Engineering Physics, Mianyang 621900, China;,   Research Center of Laser Fusion, Chinese Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

The increasing demand of unoccupied and unregulated bandwidth for wireless communication systems will inevitably lead to the extension of operation frequencies toward the lower THz frequency range. Since atmospheric transmission windows exist in the lower THz frequency range, it can be realized that carrier frequencies of 300 GHz and beyond will be used for communications once the technology for high bitrate data transmission is available. However, the free-space path-loss and the attenuation due to molecules in the atmosphere can significantly reduce the transmittable data rate in the lower THz frequency range.The main factor affecting the behavior of terahertz band is the absorption by water vapor, which not only attenuates the transmitted signal, but also disperses the signal. A new model of the terahertz wave atmospheric propagation of attenuation and dispersion is developed by using the radiation transmission theory and the empirical continuum absorption based on the HITRAN database. Theoretical aspects of absorption are presented, emphasizing those that deserve special attention as frequency increases. The THz wave atmospheric attenuation experimental results and self- and foreign-continuum coefficients obtained with the improved THz-time domain spectroscopy (THz-TDS) technique are analyzed by this model. The intensities and locations of the observed absorption lines are in good agreement with spectral databases. This model accounts for the group velocity dispersion and the total path loss that a wave in the THz band suffers when propagating 1 km distance. The channel capacity of the THz band is investigated by this model under different conditions including antenna gains, channel bandwidth and transmitter power. In order to keep the considerations as general as possible, the derivations are based on simple assumptions and equations. The special requirement for antenna is also discussed.Three communication channels (340 GHz, 410 GHz and 667 GHz) are obtained in terms of the spectrum. The four parameters of the three channels, i.e., available bandwidth, center frequency, dispersion and transmittable data rate, are summarized and quantized. The signals through the atmosphere for the three communication channels within the corresponding atmospheric windows are not easy to broaden due to the low group velocity dispersion; high data rates of up to 10 Gbps or beyond per 1 GHz bandwidth can be transmitted via these channels if the antennas with high gains are used.
Sensitivity analysis of terahertz wave passive remote sensing of cirrus microphysical parameters
Authors: Li Shu-Lei  , Liu Lei  , Gao Tai-Chang  , Huang Wei  , Hu Shuai  ,   College of Meteorology and Oceanography, PLA University of Science and Technology, Nanjing 211101, China
Published: December 2020

Cirrus clouds play an important role in the energy budget and the hydrological cycle of the atmosphere. It is still one of the largest uncertainties in the global climate change studies. This is mainly attributable to the measurement discrepancies of cirrus parameters, especially the microphysical parameters, which are constrained by the existing methods. With THz wavelengths on the order of the size of typical cirrus cloud particles and therefore being sensitive to cirrus clouds, THz region is expected to have a promising prospect concerning measuring cirrus microphysical parameters (ice water path and effective particle size). In order to evaluate the effects of cirrus microphysical parameters on THz transmission characteristics and the sensitivity of cirrus in THz region, the THz radiation spectra at the top of atmosphere in the clear sky and the cloudy situations are simulated and calculated based on the atmospheric radiative transfer simulator. The effects of cirrus particle shape, particle size and ice water path on THz transmission characteristics are obtained by analyzing the brightness temperature difference between the two situations, and the sensitivity parameters that quantitatively describ the effects. The results indicate that cirrus particle shape, particle size and ice water path have different effects on the THz wave propagation. The cirrus effect varies also with channel frequency. Overall, in the low frequency channels, cirrus effects are enhanced with the increases of particle size and ice water path; in the high frequency channels, cirrus effects are more complicated and vary with particle size and ice water path. The effects are first enhanced and then turned into saturation. The THz wave is sensitive to cirrus cloud ice water path and effective particle size, and THz wave may be the best waveband for remote sensing of cirrus microphysical parameters in theory. For thin clouds, the sensitivity parameters are approximately constant, indicating that the spectral brightness temperature at the top of the atmosphere almost shows linear relationship with ice water path, and the sensitivity parameters increase with frequency increasing. For thick clouds, the sensitivity of cirrus to ice water path decreases and gradually becomes saturated, and the higher the frequency, the more quickly it tends to saturation level. Compared with the microwave and infrared, THz wave can provide many detailed information about cirrus. The two-channel look-up table indicates that THz wave passive remote sensing of cirrus may be a stable and effective method. The results will be conducible to developing the technology of THz wave remote sensing of cirrus microphysical parameters. Moreover, it is also beneficial to improving the cirrus detection precision.
Effect of half wave plate angle mismatch on channel modulating imaging result and its compensation
Authors: Li Hao  , Zhu Jing-Ping  , Zhang Ning  , Zhang Yun-Yao  , Qiang Fan  , Zong Kang  ,   Key Laboratory for Physical Electronics and Devices of the Ministry of Education, Shaanxi Key Laboratory of Information Photonic Technique, Xi’an 710049, China
Published: December 2020

Polarization imaging technology is a powerful tool in remote sensing, bioscience, or other scientific areas science, and it can extract the polarization information of target effectively. As a novel polarization imaging technology, the channel modulating polarization imaging has been widely investigated in recent years, owing to its prominent advantages of compact, snapshot and full-Stokes acquirable. In the polarization imaging system based on Savart Plate, a half wave plate with its optical axis at an angle of 22.5 is used to rotate the vibration direction of each incident light to 45. Thus, the amplitude of the light could be equally divided by the second Savart Plate polariscope. Finally, the different components of light will interfere with each other on the focal plane and the target polarization information will be modulated in the interference pattern. Since the intensity distribution of interference pattern is sensitive to the orientation of the half wave plate, a small mismatch angle of half wave plate will lead to a wrong polarization image. In order to solve this problem, we investigate the relationship between the mismatch angle and the image intensity grabbed by focal plane array (FPA) and propose an error eliminating method to improve the accuracy of polarization imaging. We analyze how the mismatch angle affects the light intensity and deduce the expression of the image obtained by the FPA. According to the expression, the raw image we grabbed directly by the FPA is a superposition of the modulated Stokes images with different carrier frequencies. Compared with the ideal expression, the expression we obtained shows that the channels of Stokes parameter S1 and S2,3 each contain a constant factor which is related to the mismatch angle . On the basis of this expression, we propose a method to measure the mismatch angle by imaging a target twice, one is behind a polarizer that is oriented at 0 and the other at 45. Then we can calibrate the system by calculating the mismatch angle through the demodulated images. To image a target with the calibrated system, we just demodulate the raw image obtained by the FPA and then divide the reconstructed stokes images by the constant factor which is determined by . For a mismatch angle of 0.5, a computer simulation is conducted. The result shows that through the compensation method, the accuracies of S1, S2 and S3 can be increased by 0.06%, 3.49% and 3.49%, respectively.
A broadband passive cavity for analyzing and filtering the noise of a femtosecond laser
Authors: Xiang Xiao  , Wang Shao-Feng  , Hou Fei-Yan  , Quan Run-Ai  , Zhai Yi-Wei  , Wang Meng-Meng  , Zhou Cong-Hua  , Xu Guan-Jun  , Dong Rui-Fang  , Liu Tao  , Zhang Shou-Gang  ,   Key Laboratory of Time and Frequency Primary Standards, National Time Service Center, Chinese Academy of Sciences, Xi’an 710600, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

In this paper, the noise filtering effect on a femtosecond laser source via a broadband passive cavity is analyzed in detail. The results show that a passive optical cavity not only can be used as a low-pass noise filter, but also can inter-convert the phase and amplitude fluctuations of a light beam after transmission or reflection. Therefore, by measuring the intensity noise of the light field under test after transmission and reflection from a passive cavity, its phase noise properties can be explored. Based on this theoretical model, an eight-mirror ring passive cavity with a finesse of 1500 and a free spectral range of 75 MHz is designed and built. With a commercial Ti:sapphire femtosecond laser as a source, its intensity noises after transmission and reflection from the above cavity are measured with home-made self-homodyne detection setup. Furthermore, with the help of the noise conversion model of the passive cavity, the phase noise of the femtosecond laser as well as its evolution through the cavity transmission and reflection is indirectly derived. The result shows that after transmission through the passive cavity, both the amplitude and phase noise of the femtosecond laser source are evidently suppressed and reach the shot noise limit at the analyzing frequency of 2 MHz.
Effects of location and polarization of a dipole source on the excitation of a photonic crystal H1 cavity
Authors: Zhao Yan-Hui  , Qian Chen-Jiang  , Tang Jing  , Sun Yue  , Peng Kai  , Xu Xiu-Lai  ,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

The integration of photonic crystal cavity with quantum dot paves the way for photonic-based quantum information processing. Photonic crystal cavity has a high-quality factor and small mode volume, which can be utilized to enhance the interaction between light and matter. Two degenerate fundamental modes with orthogonal polarizations exist in photonic crystal H1 cavity. Entangled photon pairs can be generated with a single quantum dot coupled to degenerate H1 cavity modes. Therefore a coupling system comprised of quantum dot and photonic crystal H1 cavity is a promising platform to implement quantum information processing. The excitations of cavity modes are mostly affected by the location of the single quantum dot, namely a dipole source. For the two degenerate photonic crystal H1 cavity modes, the location of the dipole source determines which mode is excited. In this paper, the effects of location and polarization of a dipole source on the excitation of photonic crystal H1 cavity are investigated with the finite-difference time-domain method, a numerical analysis technique for computing the electrodynamics. We first design a photonic crystal slab structure patterned with hexagonal lattice of air holes. Combining the light modulation by the period lattice in the slab plane and the total internal reflection in the perpendicular direction, photonic bandgap is generated, which inhibits the propagation of photon with certain frequencies. By removing one of the air holes from the photonic crystal slab, an H1 cavity is formed with two degenerate fundamental modes. One mode is x-polarized, and the other one is y-polarized. Next, a dipole source is used to excite the H1 cavity modes. When the dipole source is located at the left to the H1 cavity center, only y-polarized mode is excited. While locating the dipole source above the H1 cavity center, only x-polarized mode is excited. Therefore each degenerate mode of H1 cavity can be selectively excited with the diploe source located at different positions in the cavity. Following that, the H1 cavity modes excited with the dipole sources with different polarizations are also studied. The x-polarized dipole source can only excite the cavity mode with x-polarization, while the y-polarized dipole source can only excite the y-polarized cavity mode accordingly. It can be seen that the dipole source with specific polarization can only excite the modes with corresponding polarization. The effects of location and polarization of a dipole source on the excitation of a photonic crystal H1 cavity are important for understanding the fundamental physics of entangled photon generation with a coupled quantum dot and photonic crystal system.
First-principles study on the structure stability and doping performance of double layer h-BN/Graphene
Authors: Chen Qing-Ling  , Dai Zhen-Hong  , Liu Zhao-Qing  , An Yu-Feng  , Liu Yue-Lin  ,   Computational Physics Laboratory, School of Opto-electronic Information Science and Technology, Yantai University, Yantai 264005, China;,   National Natural Science Foundation of China, Beijing 100085, China
Published: December 2020

Using the firs-principles method based on density functional theory, we study the stability and doping performance of double h-BN/Graphene structure, here the exchange correlation potential is expressed through the local density approximation and the interactions between ions and electrons are described by the projective-augmented wave method. Because double layer h-BN/Graphene represents a kind of epitaxial semiconductor system, which can be applied to tunnel pressure sensor, the research is very meaningful. In order to improve the application of this special double layer structures, we often carry out the dopings of some atoms. Unlike previous research work, in which the dopings of the metals Au, Co, Mn and other atoms were took into account, we now mainly consider the dopings of the active metal atoms, such as the dopings of Li, Na, and K atoms. The band structure, electronic density of states, as well as the charge density and stability are studied on the double h-BN/Graphene structure after alkali metal doping. At the same time, bonding and electronic properties of double h-BN/Graphene are discussed under the different biaxial strain conditions. The results show that for the dopings of Li and K atoms, the structure deformation is very large, and the band structure of double h-BN/Graphene can show a small band gap at the K point in the first Brillouin zone, taking on a linear dispersion relation the same as that of the perfect graphene. We can tune the band gap by applying external strain and dopings of atoms, and find a new level appearing near the Fermi level after doping, which is mainly due to the contribution of N atoms. In addition, there exists charge transfer between Na atom and N and C atoms, and the material is converted into metal. We find obvious charge overlapping in the vicinity of Na atoms, these charge overlaps appearing around the Na and C atoms indicate the existence of covalent bond and this covalent bond also appears around the Na atoms and N atoms. We prove the existence of the chemical bonds by adopting the Bader charge analysis, which suggests that the C atoms in the lower graphene layer obtain 0.11 e and dopant atoms around the three N atoms obtain 0.68 e. We infer that the increasing of Na atom doping can increase the charge transfer, so the method of changing the substrate to increase the graphene layer charge density is very conducive to the application of graphene in electronic devices. Because the double h-BN/Graphene has been successfully synthesized, our calculations provide a theoretical basis for the further development and application of technology. We can expect that Na atom doped double h-BN/Graphene can be well applied to the future electronic devices.
Raman spectroscopy based on plasmon waveguide prepared with mesoporous TiO2 thin film
Authors: Wan Xiu-Mei  , Chen Chen  , Fan Zhi-Bo  , Lu Dan-Feng  , Gao Ran  , Qi Zhi-Mei  ,   State Key Laboratory of Transducer Technology, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Gold film (40-nm-thick) sputtered on the glass substrate was decorated by using the sol-gel copolymer templated mesoporous TiO2 thin film (275-nm-thick) to fabricate the plasmon waveguide (PW). The Raman spectroscopy based on the Au/TiO2 PW is studied theoretically and experimentally. The surface morphology of the mesoprous TiO2 thin film and the cross-section of the PW chip are obtained by scanning electron microscopy (SEM) and the porosity (P) of mesoporous TiO2 thin film is determined to be about 0.589 by fitting the calculated waveguide coupling dips to the measured resonance wavelengths based on Fresnel equations. The angular distributions of Raman power from the molecular dipole located in the core layer of the waveguide are theoretically investigated based on the optical reciprocity theorem. The calculated results suggest that the Raman light radiated into the substrate consists of the directional Raman signal propagating at the resonant angle and the non-directional Raman signal whose radiation angles are smaller than the critical angle of total reflection. The directional Raman signal could be detected with the aid of the prism coupler, while the non-directional Raman signal can be detected directly on the back of the sensor chip. Furthermore, the angular distribution of the backscattered Raman signal is divergent and it is unaffected by the use of the prism coupler. The highest power of the directional Raman signal is much larger than that of the non-directional Raman signal and the backscattered Raman signal. The Raman spectroscopy based on the PW is studied by experiment with CV molecules adsorbed into the mesoporous TiO2 thin film. The Raman spectrum is obtained with the 532 nm laser radiating directly onto the waveguide surface. The experimental results show that the Raman signal including the directional Raman signal, non-directional Raman signal and the backscattered Raman signal can be detected with the PW chip. Besides, the directional Raman signal can only be detected by using the prism coupler, while the non-directional Raman signal can be detected directly on the back of the chip. Then the results also show that the peak intensity of the directional Raman signal is twice higher than that of the non-directional Raman signal. The further measurements reveal that the backscattered Raman signal hardly changes under the condition with or without the prism coupler. The experimental results mentioned above are in accordance with the theoretical calculations. The Raman spectroscopy based on PW in this work has potential value in further developing the Raman sensing technique.