Vol. 67, No. 18 (2018)
2018, 67 (18): 184203. doi: 10.7498/aps.67.20180877
Compared with coherent laser beams, partially coherent beams have advantages of effectively reducing turbulence-induced extra beam spreading, beam wander and intensity scintillation on propagation through turbulent atmosphere, and have promising applications in free-space optical communications, laser radar and remote sensing. Recently, more and more attention was paid to the propagation of partially coherent beams through turbulent atmosphere. In this article, we first review historically the research progress of the propagation of partially coherent beams in atmospheric turbulence. And we describe in detail the basic theory for the calculation of average intensity, second-order moment and scintillation index of partially coherent beams in turbulence based on the extended Huygens-Fresnel principle and Rytov method. We also present a phase screen method of numerically simulating the propagation of coherent beams through turbulent atmosphere, and then extend such a method to treating the propagation of partially coherent beams.
2018, 67 (18): 187302. doi: 10.7498/aps.67.20172372
Majorana zero-energy modes are their own antiparticles, which are potential building blocks of topological quantum computing. Recently, there has been growing the interest in searching for Majorana zero modes in condensed matter physics. Semiconductor-superconductor hybrid systems have received particular attention because of easy realization and high-degree experimental control. The Majorana zero-energy modes are predicted to appear at two ends of a semiconductor nanowire, in the proximity of an s-wave superconductor and under a proper external magnetic field. Experimental signatures of Majorana zero modes in semiconductor-superconductor systems typically consist of zero-bias conductance peaks in tunneling spectra. So far it is universally received that an ideal semiconductor-superconductor hybrid structure should possess Majorana zero-energy modes. However, an unambiguous verification remains elusive because zero-bias conductance peaks can also have non-topological origins, such as Kondo effect, Andreev bound states or disorder effect. Therefore, it is important to investigate additional evidences to conclusively confirm the presence of Majorana zero modes in the hybrid solid state devices. It has been suggested that the Majorana-quantum dot hybrid system might be one of the solutions to the problem. Up to now, various Majorana-dot hybrid devices have been proposed to detect the existence of Majorana zero modes. Most of these studies mainly focused on the limits of transport at zero temperature, large bias voltage or zero frequency shot noise. Then a natural question is how the current correlations between the electrons transport through the topological nanowire, especially still in the zero-bias regime. In this paper, a specific spinless model consisting of a quantum dot tunnel-coupled to topological nanowire is considered. We present a systematic investigation of the electron transport by using a particle-number resolved master equation. We pay particular attention to the effects of Majorana's dynamics on the current fluctuations (shot noise) at nonzero temperature and finite bias voltage as well as at finite frequencies, especially in the low-bias regime. It is shown that the difference between the electrode currents combined with the low-bias oscillations of finite-frequency shot noise can identify Majorana zero modes from the usual resonant-tunneling levels. When there exist Majorana zero modes, on the one hand, the current difference depends on the asymmetry of electron tunneling rate. The asymmetric behaviors can expose the essential features of the Majorana zero modes since the symmetric current difference is zero. And the zero-bias conductance peak appears for the asymmetric coupling. Moreover, as the Majorana splitting energy increases, the current difference is suppressed while it is increased with the dot-wire coupling increasing. On the other hand, the dynamics of Majorana coherent oscillations between the dot and the wire is revealed in the finite-frequency shot noise. Due to the existence of Majorana zero modes the finite-frequency shot noise shows oscillations with a pronounced zero-frequency noise enhancement. Especially in the low-bias regime, the noise spectrum still exhibits an oscillation behavior which is absent from the large-bias voltage limit. Furthermore, with the Majorana splitting energy increasing, the oscillations of shot noise become more obvious, but the zero-frequency peak is lowered. When the dot is asymmetrically coupled to the electrode, the shot noise gradually changes into the super-Poissonian statistics from the sub-Poissonian statistics. This indicates the crossover from antibunched to bunched electron transport. As a result, the combination of the current difference and the low-bias oscillations of finite-frequency shot noise allows one to probe the presence of Majorana zero modes. It is therefore expected that the findings of this work can offer additional guides for experiments to identify signatures of Majorana zero modes in solid state sy
Electron correlation effects in even Rydberg series converging to 4f13(2F7/2o)6s(7/2, 1/2)4o and 4f13(2F7/2o)6s(7/2, 1/2)3o of thulium atom
2018, 67 (18): 183102. doi: 10.7498/aps.67.20180797
In the frame work of multi-channel quantum defect theory (MQDT), the energy levels of three even Rydberg series 4f13(2F7/2o)6s(7/2, 1/2)4onp3/2, 4f13(2F7/2o)6s(7/2, 1/2)3onp3/2 and 4f13(2F7/2o)6s(7/2, 1/2)3onp1/2 converging to 4f13(2F7/2o)6s(7/2, 1/2)4o or 4f13(2F7/2o)6s(7/2, 1/2)3o of thulium atom are calculated by relativistic multi-channel theory. Compared with the experimental data from National Institute of Standards and Technology (NIST), the theoretical result shows two different types of electron-correlation effects: 1)the interaction between two Rydberg series results in energy shifts for these Rydberg series; 2)an isolated perturbed state is embedded in the energy range of a Rydberg series and interacts with the whole series, and breaks the regularity of the Rydberg series, and quantum defects show a large jump around the perturbed state. More specifically, by comparing the present calculated quantum defects with the experimental data, we reassign two Rydberg series: 1)4f13(2F7/2o)6s(7/2, 1/2)4onp3/2 Rydberg series from NIST is reassigned as 4f13(2F7/2o)6s(7/2, 1/2)4onf5/2, J=(5/2)+, 4f13(2F7/2o)6s(7/2, 1/2)4onf5/2, J=(7/2)+ and/or 4f13(2F7/2o)6s(7/2, 1/2)4onp1/2, J=(9/2)+ Rydberg series, and the difference between experimental and calculated quantum defects is generally better than 0.1; 2)4f13(2F7/2o)6s(7/2, 1/2)3onp3/2 Rydberg series from NIST is reassigned as 4f13(2F7/2o)6s(7/2, 1/2)3onf7/2, J=(5/2)+, 4f13(2F7/2o)6s(7/2, 1/2)3onf7/2, J=(7/2)+ and/or 4f13(2F7/2o)6s(7/2, 1/2)3onf5/2, 7/2, J=(9/2)+ Rydberg series, and the difference between experimental and calculated quantum defects is generally better than 0.05. As for the 4f13(2F7/2o)6s(7/2, 1/2)3onp1/2 Rydberg series from NIST, we find there is a perturbed state at about 49900 cm-1, and assign the perturbed state as 4f13(3F4)6 d5/26s2, J=7/2 and the total angular momentum for the Rydberg series is J=7/2.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2018, 67 (18): 185201. doi: 10.7498/aps.67.20180595
A series of experiments is designed in order to investigate the expansion and movement characteristics of atoms and ions of the plasma in the presence of ambient gas. To obtain two-dimensional spectral images of different components in the plasma, a nanosecond laser with a wavelength of 532 nm is used to ablate an aluminum sample, forming the plasma. A C-T type of tri-grating monochromator with an emICCD detector is used for diagnosing the plasma chronologically. At the same time, a 2400 gmm-1 grating is used to replace the narrowband filter for imaging diagnosis of different components in vacuum. The spectrally resolved images of Al I (396.1 nm), Al Ⅱ (466.3 nm), and Al Ⅲ (447.9 nm) in aluminum plasma are obtained. Besides, the spectral images of plasma components under different ambient pressures are collected to explore the influence of background gas on plasma evolution. The results show that in the plasma formation process, the ion component is distributed in the anterior segment of the plume relative to the atom component, and its angular distribution is smaller. The vacuum expansion rates of atoms and ions are all on the order of 104 ms-1. The movement speed of the ion component in the plasma is higher than that of atom component, and its movement speed increases with the valence of the ion increasing. In the energy density range used in this experiment, the velocity varies slightly with the laser energy. For the neutral atom, the velocity increases obviously as the energy increases. With the expansion process progressing, each component of the plume advances along the direction normal to the sample surface, and the emission intensity gradually decreases, the corresponding plume density and its temperature also decrease. With the ambient pressure increasing, the movement characteristics of each component are obviously different from those under high vacuum. At a pressure higher than 1 Pa, the plasma and the ambient gas are infiltrated with each other, vignetting appears in the front of the plume, disturbance occurs, causing the expansion speed to decrease. In addition, the plasma plume shrinks due to the increase of pressure, and the probability of collision with the background gas increases, so that the plume emission intensity is strengthened and the plasma lifetime is prolonged. The results of the new diagnosis method and the experimental results demonstrated in this study can provide a reference for the study of plasma component dynamic process.
2018, 67 (18): 180501. doi: 10.7498/aps.67.20180596
Fretting phenomena exist widely in structural engineering. In recent years, it has attracted more attention from scientists and technicians. In order to study the fretting wear in depth, we establish a new method of calculating the wear rate of material in vibratory environment. Firstly, according to the characteristics of friction pair and fretting wear process in fretting friction system, the asymmetric double potential well model is proposed and the potential energy function of the model is given. The transfer of particles between the two kinds of materials during the fretting is regarded as the motion of the particles in the two potential wells which are asymmetrical, and the particle motion equation in the potential well is established. Furthermore, considering the characteristics of the randomness, time-varying and irreversibility of particle motion in fretting friction system, a theoretical model is established by using the non-equilibrium statistical theory, which is based on the particle equation motion, combined with the Langevin equation in random theory and the Foker-Planck equation in the non-equilibrium statistical theory. The probability density distribution function of particles moving from the interior of the material to the material surface at any time is obtained. A method of calculating the wear rate is proposed by integrating the probability density distribution function. Secondly, by calculating the wear rate of the friction pair which consists of metal materials Mg and Fe, we obtain the potential energy function of the asymmetric double potential well model as the different surface energies of both materials. Furthermore, the probability density distribution function of particles moving in this friction pair is calculated. Then, the change of wear rate with wear time and width of potential well is derived, and the effect of normal force on wear rate is further analyzed. The results of calculation and analysis show that the wear rate of material decreases with the decrease of the width of the potential well in the friction pair system, decreases with the increase of wear time and increases with the increase of the normal force of the load, and the surface of the relatively soft material in the friction pair system is more likely to wear off. Finally, the conclusions of the theoretical model accord with the experimental results, illustrating the applicability of the theoretical model.
2018, 67 (18): 180201. doi: 10.7498/aps.67.20180794
Indium tin oxide (ITO) thin films,as a heavy doping n-type semiconductor material with a high carrier concentration,can realize the surface plasma effect and regulation of surface plasmon resonance wavelength in the near infrared region:the surface plasma has broad application prospect in surface plasmon devices.The ITO thin films are deposited on float glass substrates (20 mm20 mm) via the direct current (DC) magnetron sputtering by regulating substrate temperature from 100 ℃ to 500 ℃.The deposited ITO thin films present a cubic polycrystalline iron manganese structure,in which the ITO film shows the strong crystallinity at 400 ℃,so that it is conducive to reducing the defects of bound electrons and the damping force of thin film.The surface roughness of ITO thin film first decreases and then increases with the temperature increasing,correspondingly the root-mean-square roughness (Rq) of these films decreases from 4.11~nm to 2.19 nm,then increases to 2.56 nm.The Rqvalue of 2.19 nm corresponds to a preferable surface smoothness of ITO thin film,indicating that it can effectively increase carrier concentration of ITO thin film at 400 ℃.The effects of the different substrate temperature on the photoelectric and surface plasma properties of ITO thin films are analyzed by UV-Vis absorption spectra,Hall measurement,refractive index and dielectric constant.As the temperature increases from 100 ℃ to 500 ℃,the carrier concentration of ITO thin film is enhanced from 4.11020 cm-3 to 2.481021 cm-3,and thus increasing the probability of the Fermi level to the conduction band of ITO thin film.And the enhancement of carrier concentration induces the Moss-Burstein effect,which makes the edges of absorption spectrum of the ITO thin film gradually blue-shift from 340 nm to 312 nm,correspondingly broadening the optical band gap from 3.64 eV to 3.97 eV.These results cause the difficulties of electrons interband transition to be enhanced,and thus suppressing the phenomenon of absorbing photons for the electron transition from low level to high level,which ultimately reduces the optical loss of ITO thin film.In addition,the surface plasma effect is realized in a range from 1100 nm to 1700 nm for ITO thin film by regulating the substrate temperature.Meanwhile,the electronic mobility in the ITO thin film is also improved from 24.6 cm2V-1s-1 to 32.2 cm2V-1s-1,which reduces the electronic scattering,and is beneficial to the increase of propagation length of surface plasma waves.The above results imply that we have attained the goal of the reducing the electrical loss of ITO thin film.
ATOMIC AND MOLECULAR PHYSICS
Stimulated radiation characteristics and quantum phase transition for two-component Bose-Einstein condensate in optical cavity
2018, 67 (18): 183701. doi: 10.7498/aps.67.20180971
Dicke model describes a collective interaction between the two-level atoms and the light cavity and has been predicted to show a peculiar quantum phase transition, which is a second-order phase transition from a normal phase (in a weak-coupling strength) to a superradiant phase (in a strong-coupling strength). The model plays an important role in illustrating the quantum ground-state properties of many-body macroscopic quantum states. In the experiment, Dicke quantum phase transition in an open system could be formed by a Bose-Einstein condensate coupled to a high-finesse optical cavity. This experiment on the Bose-Einstein condensate trapped in the optical cavity have opened new frontiers, which could combine the cold atoms with quantum optics and makes it possible to enter into the strongly coupled regime of cavity quantum electrodynamics. In strong coupled regime, the atoms exchange the photons many times before spontaneous emission and cavity losses set in. It has become a hot research topic in recent years and plays an important role in many fields of modern physics, such as condensed matter physics, nuclear physics, etc. It can be applied to the manipulation of the geometric phase and entanglement in quantum information and computing. Quantum phase transition has been widely studied for the Dicke model as a typical example. Many different research methods about the mean-field approximation have been used to analyze the ground state properties of the Dicke model. In this paper, we study the ground state properties of two-component Bose-Einstein condensate in a single-mode cavity. Meanwhile, the associated quantum phase transition is described by the spin-coherent-state variational method, whose advantage is that the ground state energy and wave function can be obtained without the thermodynamic limit. By taking the average in the boson coherent state, we obtain an equivalent effective pesudospin Hamiltonian, which will be diagonalized by using the spin coherent state. Finally, we can obtain the energy functional, which is the basics of the variation to obtain the numerical solution of photon number and the expression of the atomic number and the ground state energy. This paper presents a rich phase diagram, which can be manipulated by changing the atom-field coupling imbalance between two components and the atom-field frequency detuning. While in the single-mode Dicke model there exist only the normal phase and the superradiation phase. When the frequency of one component atom is zero or the frequency of the two component atoms are equal in optical cavity, the system returns to the standard Dicke model, in which there occurs the second-order phase transition from the normal phase to the superradiant phase by adjusting the atom-field coupling. In conclusion, we discover that the stimulated radiation comes from the collective state of atomic population inversion, which does not exist in the single-mode Dicke model. Meanwhile, the new stimulated-radiation state S and S, which can only be produced by one component of the atom, are observed in the two-component Bose-Einstein condensates in the single-mode optical cavity. By adjusting the atom-field coupling imbalance and the atom-field frequency detuning (the blue or red detuning), the order of the superradiation state and the stimulated-radiation states can be exchanged between the two components of the atom.
Molecular structure and properties of salt cross-linked polyethylene under external electric field based on density functional theory
2018, 67 (18): 183101. doi: 10.7498/aps.67.20180808
Cross-linked polyethylene is the main power cable insulation material and is widely used in high voltage cables. In order to study the effect of external electric field on the molecular structure of salt cross-linked polyethylene, in this paper we use the basis set of def2-TZVP for Zn atom, uses the basis set of 6-31(d) for C, H, O atoms, and uses the Minnesota density functional (M06-2X) to optimize the molecular structure of salt cross-linked polyethylene, then we obtain the stable structure of its ground state. On this basis, the molecular structure, total energy, kinetic energy, potential energy, dipole moment and polarizability changes of salt cross-linked polyethylene under the action of different external electric fields (from 0 to 0.020 a.u.) are studied by the same method. The influence of external electric field on energy level, energy gap, orbital distribution and composition of frontier orbit are studied. And the effect of external electric field on bond level, breaking bond and infrared spectrum of atoms are also discussed. The research results show that as the external electric field intensity increases, the cross-linked polyethylene molecule is gradually transformed from the spatial network structure into a linear structure, and the total energy and kinetic energy of the molecule are reduced, but its potential energy, dipole moment and polarizability are gradually increased. The highest occupied molecular orbital energy level increases with the increase of external electric field intensity. The lowest unoccupied molecular orbital energy level starts to decrease continuously from the electric field intensity of 0.011 a.u. (1 a.u. = 5:1421011 V/m), the energy gap decreases continuously, and the critical breakdown field intensity is 11.16 GV/m. With the external electric field increasing dramatically, the highest occupied molecular orbital is obviously converged at chain end in the direction of inverse electric field. Its orbital composition is more than 60%, contributed by the C atom of methyl group in the polyethylene terminal. The molecular polyethylene chain end of the inverse electric field direction exhibits an electrophilic reactivity, and C atoms are more likely to lose electrons. The Mayer bond order value of the CC bond decreases gradually, which leads the CC bonds to break more easily, and thus forming the methyl carbon negative ions. The lowest unoccupied molecular orbital moves along the electric field direction and is converged at the other end of polyethylene chain, nearly 80% of its orbital composition is contributed by the methyl of polyethylene chain end. The molecule shows a nucleophilic reactivity at the polyethylene end along the electric field direction, methyl is easier to obtain the electrons. The Mayer bond order value of the CH bond decreases gradually, and it brings about the CH bond more likely to break into H positive ions. The infrared absorption peaks of polyethylene chains are mainly concentrated in the high frequency region. With the increase of electric field intensity, the red shift occurs and the bond energy of polyethylene chain decreases. The infrared absorption peak of the cross-linked salt bridge is mainly concentrated in the low frequency area. Although there are both red shift and blue shift, the effect of red shift is more obvious, and the energy of the whole salt bridge decreases. From the variation of molecular potential energy, energy gap and Mayer bond order value, it is found that the stability of salt cross-linked polyethylene molecular system decreases with the increase of external electric field intensity.
Spectroscopic exploration of upconversion luminescence behavior of rare earth-doped single-particle micro/nanocrystals
2018, 67 (18): 183301. doi: 10.7498/aps.67.20172191
In recent years, rare earth-doped upconversion (UC) micro/nanocrystals are useful for many applications, especially in biology because of their unique luminescent properties and specific geometry. The luminescence efficiency of lanthanide-doped UC nanoparticles is of particular importance for their applications. However, the unsatisfactory UC efficiency is still one of the main hurdles. In the present article, a series of Yb3+/Er3+ doped NaYF4 micro/nanoparticles with different ratios of length to diameter are successfully synthesized by a facile hydrothermal route. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX) analyses, photoluminescence spectra, and the dynamic process of the luminescence are used to characterize the samples. The intrinsic structural feature of fluoride, the solution pH value, and organic additive Cit3- account for the ultimate shape evolution of the final products. The ratio of length to diameter of NaYF4 microrod can be tuned only by varying the value of pH or the amount of an organic additive (Cit3-). The UC characteristics of a single NaYF4:Yb3+/Er3+ microrod obtained by tuning the value of pH or the amount of Cit3- are investigated by laser confocal microscopy with a 980 nm laser. The two series of codoped fluoride crystals both exhibit the characteristic UC luminescence from Er3+ ions and display the rich luminescence patterns in space. The UC luminescence from a single NaYF4:Yb3+/Er3+ microrod obtained by tuning the value of pH is brighter than that from a single NaYF4:Yb3+/Er3+ microrod with the same size obtained by tuning the amount of Cit3-. The EDX analysis indicates that the number of Na+ defects depends on the specific synthesis conditions of the sample. The Na+ defects of samples obtained by tuning the values of pH are lower than those of samples with the same size obtained by tuning the amount of Cit3-. It conduces to reducing Na+ defects at lower pH value. The parameters of the luminescence kinetics are found to be unambiguously dependent on the size of sample, which relates to higher energy phonon of surface and Na+ defects. The mechanism of luminescence enhancement by pH controlling is explored, and a mechanism based on the reduced intrinsic defects of Na+ is proposed. The investigation not only enriches the controllable synthesis approach of fluoride micro/nanomaterials, but also indicates the potential applications of rare earth materials with a rich luminescence pattern in the photonic devices and anti-counterfeiting devices.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2018, 67 (18): 184101. doi: 10.7498/aps.67.20180592
Metasurfaces, the two-dimensional counterparts of metamaterials composed of subwavelength building blocks, can be used to control the amplitude, phase, and polarization of the scattered wave in a simple but effective way and thus have a wide range of applications such as lenses, holograms, and beam steering. Among these applications, metasurfacebased beam steering is of great importance for antenna engineering in communication systems, because of its low loss and easy manufacture. The capability of beam steering is mainly controlled by the phase profile which is determined by the phase shift applied to the wave scattered by each of unit cells that constitute the metasurface. It should be noted that the required phase profile achieved by distributing the unit cells with different phase responses can operate well only at a certain frequency. The guidance in determining the required phase profile to steer the beam into a certain direction is the generalized Snell's law. According to this law, the reflection angle of the wave reflected by the metasurface interface depends on the linear phase gradient along the metasurface. Therefore, by forming different linear phase gradients covering the whole phase shift 2 periodically, one can steer the reflected waves to different angles. However, the obtained reflection angles are limited because the phase gradient of a metasurface is limited by the unit cell size, which cannot be infinitely small. Recently, a new pattern shift theory based on the convolution theorem has been proposed to realize wide angle range steering, enabling flexible and continuous manipulation of reflection angle. Because the electric field distribution and the scattering pattern in the far-field region are a Fourier transform pair, we can pattern the electric field of the metasurface to control the scattered waves of far field. Specifically, the multiplication of an electric distribution by a gradient phase sequence leads to a deviation of the scattering pattern from its original direction to a certain extent in the angular coordinate. However, we have not considered the tunability of metasurfaces so far, which is required in applications. The ways to reach tunability in metasurface include diode switches, micro-electro-mechanical system, and the use of tunable materials such as graphene. Graphene, an atomically thin layer of carbon atoms arranged in a honeycomb lattice, has aroused the enormous interest due to its outstanding mechanical, thermal, and electrical properties. With the capability of being electrically tunable, graphene has manifested itself as a promising candidate for designing the tunable metasurfaces. Although the reflection angle can be changed by electrically reconfiguring the graphene Fermi level distribution of the metasurface, the steering angle is still limited. In this paper, we propose and design a tunable graphene metasurface with the capability of dynamically steering the reflection angle in a wide range, which is achieved based on the new pattern shift theory. The theoretical results and the numerically simulated results both show that the reflection angle can be steered from 5 to 70 with an interval less than 10, implying the promising potential in the design of tunable antenna.
Focusing white laser into samples, many colorful rings (coherent rainbows) come out. Such phenomena have been observed in many materials like water, acetone, absolute ethyl alcohol, soft drink and other liquids, and ice, colored glass, plastics, wax and other solids. From the center of the coherent rainbows to the outer side, the distance between neighboring rings becomes larger and larger. The coherent rainbow is an interference effect, whose optical path difference is induced by locally heating the material with the laser beam. Especially, the coherent rainbows from colored glass in reflection mode can be described with a simple formula, with which simulated results fit the observed interference pattern very well. Several possible mechanisms like nonlinear optical effect, thermal lens effect and self-phase modulation effect are excluded.
2018, 67 (18): 184207. doi: 10.7498/aps.67.20180548
Time precision switching is crucial to a high-precision synchronization control system with several synchronized sources. Compared with the other high-power switches, a GaAs photoconductive semiconductor switch (PCSS) with a litter time jitter has been widely used in a precision synchronization control system. There is little work on the time jitter of a GaAs PCSS. In this paper, a formula of GaAs PCSS time jitter is derived by the qualitative theoretical derivation through using the probability distribution of the output electrical pulse and the corresponding relation between the time and electrical waveform of GaAs PCSS, and combining the carrier transport process. In experiment, a neodymium-doped yttrium aluminum garnet nanosecond laser beam is split by a semipermeable half mirror into two optical beams, and then these two beams simultaneously trigger two identical GaAs PCSSs in two parallel circuits. As the energy of a triggering laser pulse is fixed at 0.35 mJ, four different laser pulse widths, namely 30 ns, 22 ns, 16 ns and 11 ns, respectively, are used to trigger the GaAs PCSSs. The bias voltage changes from 0.1 kV to 1 kV in steps of 0.1 kV, and it is used in the above-mentioned experiment. The PCSSs are triggered 20 times at each of the bias voltage values. The time jitter of the GaAs PCSS with a 3-mm gap can be measured. By analyzing the experimental data, we conclude that the time jitter of the GaAs PCSS decreases with the triggering laser pulse width decreasing under the condition of different bias voltage. In the linear mode, the GaAs PCSS illuminated by a photon with a proper wavelength creates an electron-hole pair. The characteristic of the triggering laser pulse determines that of the output electrical pulse. With the energy of triggering laser pulse fixed, the fluctuation of electrical pulse increases fast with its pulse width decreasing. Moreover, according to the derived formula for a time jitter, the GaAs PCSS time jitter decreases with triggering laser pulse width narrowing, under the different externally applied bias voltages. It is demonstrated that the theoretical and experimental results of the relationship between the triggering laser pulse width and the GaAs PCSS time jitter are consistent. The obtained results provide a basis for further reducing the GaAs PCSS time jitter, which is important for a next-generation fusion research facility and laser trigger antenna array of generating short pulse sequence.
Multi-wavelength light-emitting diode light source radar system and near-ground atmospheric aerosol detection
2018, 67 (18): 184208. doi: 10.7498/aps.67.20180721
Near-ground atmospheric aerosol has a direct effect on the living and production of human, and the research on its detection attracts substantial attention from engineers and scholars in the community of environment. Traditional monitoring instruments can accurately and continuously detect the aerosols above the ground, but it is difficult for them to obtain the information about vertical distribution of near-ground aerosols. As is well known, lidar can act as an efficient method to detect the aerosol's temporal and spatial distribution. However, this technique is restricted in the potential applications of the detection of aerosol at a certain wavelengths or near range. That is because it usually presents fixed wavelengths and a large blind area. In this work, a new multi-wavelength light-emitting diode (LED) light source radar system is designed and established for detecting the characteristics of atmospheric aerosol near ground. The paper is outlined as follows. Firstly, the composition and working principle of the LED light source radar system are introduced. Based on the structure of radar's transmitter and receiver, the geometric overlap factor is analyzed and calculated. The minimum detection height of the LED light source radar system is then determined to be 60 m. Secondly, the inversion method for the echo signal of LED light source radar is studied. In consideration of the suitability of near-range detection of LED light source radar, the Fernald forward integration method is used for inversing the aerosol extinction coefficient. The calibration value of aerosol extinction coefficient is further determined with the ground visibility data. Finally, by using the designed multi-wavelength LED light source radar system (475 nm, 530 nm, and 625 nm), near-ground aerosol observation at night in Xi'an city is carried out and three atmospheric conditions including light, moderate and severe pollution days are considered. The height distribution curves of atmospheric aerosol extinction coefficient of three wavelengths within a height of nearly 300 m are obtained. The characteristics of the distribution and change of near ground aerosols are accordingly discussed. The experimental results show that the multi-wavelength LED light source radar provides an efficient implementation for detecting the vertical distribution of atmospheric aerosol near the ground, and can make up for the inadequacy of lidar in near range detection.
2018, 67 (18): 184703. doi: 10.7498/aps.67.20180593
Boundary-layer receptivity is the initial stage of the laminar-turbulent transition, which is the key step to implement the prediction and control of laminar-turbulent transition in the boundary layer. Current studies show that under the action of acoustic wave or vortical disturbance, the variation of leading-edge curvature significantly affects the boundary-layer receptivity. Additionally, the free-stream turbulence is universal in nature. Therefore, direct numerical simulation is performed in this paper to study the receptivity to free-stream turbulence in the flat-plate boundary layer with an elliptic leading edge. To discretize the Navier-Stokes equation, a modified fourth-order Runge-Kutta scheme is introduced for the temporal discretization; the high-order compact finite difference scheme is utilized for the x- and y-direction spatial discretization; the Fourier transform is conducted in the z-direction. The pressure Helmholtz equation is solved by iterating a fourth-order finite difference scheme. In addition, the Jaccobi transform is introduced to convert the curvilinear coordinate system into Cartesian coordinate system. And elliptic equation technique is adopted to generate the body-fitted mesh. Then the effect of elliptic leading-edge curvature on the receptivity mechanism and the propagation speed of the excited Tollmien-Schlichting (T-S) wave packet in the flat-plate boundary layer are revealed. Subsequently, a group of multi-frequency T-S waves is extracted from the T-S wave packets by temporal fast Fourier transform. The influences of different leading-edge curvatures on the amplitudes, dispersion relations, growth rates, phases and shape functions of the excited T-S waves are analyzed in detail. Finally, the position occupied by leading-edge curvature in the boundary-layer receptivity process for the excitation of T-S wave is also confirmed. The numerical results show that the more intensive receptivity is triggered in the smaller leading-edge curvature; on the contrary, the less intensive receptivity is triggered in the greater leading-edge curvature. But in different leading-edge curvatures, the structures of the excited T-S wave packets are almost identical, and the group velocity is close to constant, which is approximate to one-third of the free-stream velocity. Similarly, the greater amplitude of the excited T-S wave can be induced with the smaller leading-edge curvature; whereas the smaller amplitude of the excited T-S wave can be induced with the greater leading-edge curvature. Moreover, the dispersion relations, growth rates, phases and shape function of the excited T-S waves in the boundary layer are found to be nearly invariable in different leading-edge curvatures. Through the above study, a further step can be made to understand the boundary-layer leading-edge receptivity and also improve the theory of the hydrodynamic stability.
2018, 67 (18): 184704. doi: 10.7498/aps.67.20180660
The thermocapillary migration characteristics of a self-wetting drop on the non-uniformly heated, horizontal, solid substrate are investigagted by numerical simulation. Based on the lubrication theory, an evolution equation for the height of the two-dimensional drop is established. The substrate underlying the drop is subjected to a temperature gradient which induces surface tension gradient-driven drop deformation and migration. The self-rewetting fluid has non-monotonic dependence of the surface tension on temperature with a well-defined minimum, and the position of the minimum corresponding to the temperature on the substrate is called the critical point. The effect of the relationship between the critical point and the drop position on drop dynamics is analyzed. With the temperature sensitivity coefficient of three interfaces under the same condition, the substrate is illustrated with constant wettability. The direction of drop migration will alter as the initial drop location moves to the left relative to the critical point position, resulting from the variation of the interplay among thermocapillary, gravity, and capillarity forces within the drop. But the drop always migrates toward the high interfacial tension region due to the thermocapillary force. In the presence of substrate wettability variations, the drop migrates toward the low temperature region no matter where the drop is placed relative to the critical point. This is due to the fact that the deterioration of substrate wettability on the right side of the drop prevents the drop from migrating toward the hot region. Under the critical point being on the left or within the drop, as the initial drop location moves to the left relative to the critical point position, the enhancement of the thermocapillary force toward the left leads to increased moving speed of the left contact line and increased spreading area. When the critical point is positioned on the outer right side of the drop, the speed of the left contact line sharply decreases at t=6103, caused by the suddenly deteriorating substrate wettability. Hence, it is effective to manipulate the self-wetting drop movement by regulating the relationship between the critical point and the initial drop location. To inhibit the migration of the drop toward the cold region, the drop should be placed on the right side of the critical point.
2018, 67 (18): 184201. doi: 10.7498/aps.67.20172473
In order to improve the performance of laser diode (LD) array pumping field in high-power solid state laser, an LD array pumping coupling system based on microlens array is used to achieve a high-uniformity pumping source with a longer transmission distance. The homogenizer has two structures based on microlens array, which are called diffracting homogenizer and imaging homogenizer. In this paper, we mainly study imaging microlens array due to its advantages of simple structure, better output homogeneity, flexibility of changing pumping field size, and insensitive to change in the input beam. First, the mathematical expression of the intensity distribution of target surface is derived based on the theory of geometrical optical. According to the geometrical optical formula, we obtain the relationship between the intensity distribution of target surface and system parameters, i.e., maximum incident angle of LD array, the distance between two microlens arrays, and the aperture and focal length of microlens. The boundary condition of microlens Fresnel number is derived based on the LD array beam parameters. Second, the influence of the number of microlens array elements on the output field homogeneity is studied theoreti-cally by the mathematical statistics method. As the input beam is considered to be divided randomly, the central limit theorem is employed to derive the mathematical expression of calculating the integrated output field non-homogeneity. The formula shows that the non-homogeneity is in inverse proportion to the root of the number of microlens array elements and the related maximum and minimum value of input field intensity distribution. And the spatial period of microlens array is designed to be unrelated to the spatial period of LD array to reduce the coherence of LD beam. According to the luminescence field parameters of an LD array consisting of 25 bars, an LD coupling imaging microlens array homogenizer test system is designed and constructed based on the theoretical analysis above. Another contrast system with a different microlens array which is not optimized is constructed at the same time. The coupling characteristics of two coupling systems with different microlens arrays are compared. The simulation and experimental test are carried out. The experimental result accords well with the simulated result, and thus proving the correctness of the theoretical studies. The coupling system with optimized microlens array shows better homogeneous effect with an output field non-homogeneity of 7.9%, and a coupling efficiency of 90.7%, proving the feasibility of the system for LD array pumping field homogenization.
2018, 67 (18): 184202. doi: 10.7498/aps.67.20180692
Underwater imaging is widely applied to mariculture, archaeology, and hydrocarbon exploration, because it can provide the information about visualized target. Among various underwater imaging techniques, polarization imaging is of particular interest to us, due to its simple system structure and low cost. It images the waterbody through using the polarization characteristics of light, specifically, the background light and target light. Active polarization imaging method illuminates a target scene with an artificial polarized light source to provide polarization information for imaging. But in neritic area, active imaging leads to complex light scattering conditions when artificial light and natural light are superimposed together, which further leads to poor image quality. Passive underwater polarization imaging attempts to recover a clear image by utilizing the polarization characteristics of background light and target light. However, serious color cast always appears in the final image, resulting from light absorbed by water, which may further result in target distortion. In this manuscript, we present a passive underwater polarization imaging method for detecting a target in neritic area. A depth-information-based underwater Lambertian reflection model is established by incorporating the depth information into the traditional Lambertian reflection model. First, we attribute the light changes in color and brightness of a Lambertian surface to the spatial variation of the light. According to Lambertian reflection model, the appearance of a target on a detector depends on the light source, the surface reflectance, and the camera sensitivity function. But in underwater imaging, light attenuation at different wavelengths also varies with depth. By analyzing the transmission characteristics of background light in water, we build a physical relationship between the depth information of the scene and the background light. After that, we take the depth information as the weight of light intensity distribution. Then we calculate the product of the light intensity and the camera sensitivity function in the underwater scene according to gray world algorithm, and the real color information of the target can be obtained. Finally, the clear image of an underwater target scene can be obtained, where color cast is calibrated and background light is removed. Underwater experiments are conducted to demonstrate the validity of the proposed method. Besides, the quantitative analyses also verify the improvement of the quality in final target image. Compared with conventional passive underwater polarization imaging methods, the proposed method is capable of detecting targets in various conditions, with the color cast problem solved. It can provide underwater images with better quality and valid detailed information. Furthermore, the proposed method is easy to conduct with no need to change the conventional polarization imaging system and is promising in various practical applications.
Generation of multi-wavelength femtosecond laser pulse based on nonlinear propagation of high peak power ultrashort laser pulse in single-mode fiber and spectral selectivity technology
2018, 67 (18): 184205. doi: 10.7498/aps.67.20181026
Highly-integrated high-reliability widely-tunable femtosecond laser sources have important application values in various research and application fields, such as ultrafast spectroscopy, quantum optics, remote sensing and bio-imaging. In multi-photon excited fluorescence microscopy, femtosecond laser sources with moderate pulse energy and wide wavelength tunable range can not only meet the application requirements of the different tissue structures for the peak power and excitation wavelength, but also improve the nonlinear fluorescence efficiency and imaging resolution of the sample, and thus enhancing the penetration depth. Considering the extensive application prospect and important scientific research significance of the widely tunable femtosecond laser, in this paper we conduct an experimental research of the high repetition rate multi-wavelength femtosecond laser generation in compact sized and low-cost configuration based on the nonlinear propagation scheme of the high peak power femtosecond laser pulses in single-mode fiber. In experiment, we first construct a highly-integrated reliable all-polarization-maintaining fiber femtosecond laser amplifier, which mainly consists of an environmentally stable all-polarization-maintaining fiber mode-locked laser oscillator, single-mode fiber stretcher, a single-mode power pre-amplifier, a dual-cladding Yb-fiber amplifier, and transmission grating-pair compressor. Self-starting mode-locked operation is assured with a semiconductor saturable absorber mirror, and intra-cavity dispersion compensation is realized by a chirped fiber Bragg grating in the mode-locked oscillator. The mode-locked oscillator, which delivers laser pulses with center wavelength peaked at 1035 nm, is robust operation as temperature changes from 10℃ to 40℃ and the measured power fluctuation is less than 1% RMS over 168 hours at 23℃. The amplified high repetition rate laser pulses are compressed in a double-pass 1000 lines/mm transmission grating-pair compressor. After compression, laser pulses with 5.83 W average power and 264 fs pulse duration at 34 MHz repetition rate can be obtained. Simultaneously, we also study the dependence of the compressed pulse duration on the amplified output power. Employing a home-made high reliable compact sized all-polarization-maintaining fiber femtosecond laser as a pump source and low-cost single-mode fiber as a nonlinear medium, the generation technology of the widely tunable femtosecond laser in only fiber format is also studied based on the self-phase modulation nonlinear spectral broadening mechanism. Simultaneously, in order to reduce the effect of the dispersion on the spectral broadening as much as possible, an 80-mm-long fiber is used in experiment. The used single-mode spectral broadening fiber has a 6-m-diameter core and 20 fs2/mm dispersion coefficient. By coupling the femtosecond pump laser pulses into the 6-m-diameter fiber core, the output spectrum presents a significant nonlinear broadening. The coupled pump power can be continuously adjusted by a combination of a half-wave plate and a Glan laser polarizer. After bandpass filtering the leftmost and rightmost spectral lobes in self-phase modulation and self-steeping induced broadened spectrum with bandpass filters centered at 980, 1000, 1050, 1070 and 1100 nm, the laser pulses with 203, 195, 196, 187, and 194 fs pulse duration can be obtained at the corresponding center wavelengths. The experimental scheme presented in this paper, which is based on the nonlinear spectral broadening of the highreliability femtosecond laser pulse in single-mode fiber and the spectral selectivity technology, provides a new research approach to the realization of the highly-compacted reliable widely-tunable femtosecond laser sources and has important research significance.
Influence of temperature on supercontinuum generation induced by femtosecond laser filamentation in NaCl solution
2018, 67 (18): 184206. doi: 10.7498/aps.67.20180686
Supercontinuum generation is an important nonlinear phenomenon that occurs during the femtosecond laser filamentation in transparent medium, and its potential and promising applications like remote sensing, biomedical imaging and generation of few-cycle femtosecond pulses, etc. have aroused a great deal of interest. With the extensive and thorough theoretical simulation and experimental research of the supercontinuum generation in air, the mechanism of the supercontinuum induced by femtosecond laser filament in gaseous medium has become clear. However, the femtosecond laser filament-induced supercontinuum in liquid is still an open question. In this work, by taking NaCl solution for example, we investigate the influence of solution temperature on the supercontinuum induced by the femtosecond laser filamentation in solution. It is found that when the laser pulse energy is relatively low (e.g. 20 and 50 J), the influence of solution temperature on supercontinuum generation can be neglected. In contrast, when the laser pulse energy is relatively high (e.g. 200 J), with the increase of solution temperature, the supercontinuum generation shows a suppression tendency. The water molecules in NaCl solution are photo-ionized due to the high intensity of femtosecond laser filament, generating a great deal of oxygen (O2), hydrogen (H2) and water vapor (H2O), and thus forming bubbles that float upwards. In the case of lower pulse energy, the multi-photon ionization rate is low, therefore, only a few bubbles are generated, and they are small in size, which hardly affects the supercontinuum generation. In the case of higher pulse energy, a large number of bubbles can be observed in the NaCl solution, and their sizes become increasingly large when the temperature of NaCl solution increases. The generation of bubbles leads to the reflection and refraction of light, which inevitably influences the spectral intensity. Furthermore, the components (e.g. O2, H2 and H2O) in the bubbles also absorb the supercontinuum, which further lowers the spectral intensity. This work reveals that the main factors leading to the supercontinuum suppression in solution can be attributed to the generation of bubbles during femtosecond laser filamentation and the scattering and absorption of light caused by water vapor in bubbles. When we detect the components in solution via the femtosecond laser filament-induced supercontiunum, the influence of tempera-ture can be effectively eliminated by adjusting the incident pulse energy. Moreover, in the case of high pulse energy, the supercontinuum generation can be controlled by adjusting the solution temperature. This study is conducible to the application of supercontinuum as well as its generation.
First-principle calculation of electronic structures and absorption spectra of lithium niobate crystals doped with Co and Zn ions
2018, 67 (18): 184209. doi: 10.7498/aps.67.20180735
In this paper, the electronic structures and absorption spectra of Co doped and Co, Zn co-doped LiNbO3 crystals are studied by the first-principle using the density functional theory, to explore the characteristics of charge transfer in Co, Zn co-doped LiNbO3 crystals, and to build the relationship between these characteristics and the holographic storage quality. The basic model is built as a supercell structure of 211 of near-stoichiometric pure LiNbO3 crystal with 60 atoms, including 12 Li atoms, 12 Nb atoms and 36 O atoms. Four models are established as the near-stoichiometric pure LiNbO3 crystal (LiNbO3), the cobalt doped LiNbO3 crystal (Co:LiNbO3), the zinc and cobalt co-doped LiNbO3 crystal [Co:Zn(L):LiNbO3] with doping ions at Li sites, and the other zinc and cobalt co-doped LiNbO3 crystal [Co:Zn (E):LiNbO3)] with zinc ions at Li sites and Nb sites. The last two models would represent the concentration of Zn ions below the threshold (6 mol%) and near the threshold, respectively. The charge compensation forms are taken as CoLi+-VLi-, CoLi+-ZnLi+-2VLi- and CoLi+-ZnNb3--2ZnLi+ respectively in doped models. The results show that the conduction band and valence band of pure LiNbO3 crystal are mainly composed of O 2p orbit and Nb 4d orbit respectively, and energy gap is 3.48 eV. The band gap of the doped LiNbO3 crystal is narrower than that of pure LiNbO3 crystal, due to the Co 3d and Zn 3d orbit energy levels superposed with that of O 2p orbit energy levels, and thus forming the upside of covalent bond. The band gap of Co:LiNbO3 crystal is 3.32 eV, and that of Co:Zn:LiNbO3 crystals are 2.87 eV and 2.75 eV respectively for Co:Zn(L):LiNbO3 and Co:Zn(E):LiNbO3 model. The Co 3d orbit is split into eg orbit and t2g orbit with different energies. The absorption peak at 2.40 eV appears in the band gap of Co:LiNbO3 crystal, which is attributed to the transfer of the Co 3d splitting orbital t2g electrons to conduction band. The absorption peaks of 1.58 eV and 1.10 eV could be taken as the result of eg electron transfers of both Co2+ and Co3+ in crystal, especially the latter ion. These two absorption peaks are obviously enhanced in Co:Zn (E):LiNbO3 crystal compared with in other samples in this paper. Based on that, it could be proposed that a charge transfer between Zn2+ and Co2+ as Co2++Zn2+Co3++Zn+ exist in the crystal, which results in the decrease of eg orbital electron number, but hardly affect the t2g orbital electron. The Co ion in crystal could act as the deep-level center (2.40 eV) or the shallow-level center (1.58 eV) with the different accompanying doped photorefractive ions in the two-light holographic storage applications. In both cases, the choice of Zn ion concentration near threshold could be helpful for the photo damage resistance and recording light absorption in storage applications.
2018, 67 (18): 184211. doi: 10.7498/aps.67.20180684
Surface-enhanced Raman scattering (SERS) technology can effectively enhance the Raman signal of sample molecules. It has a higher sensitivity to detect biomolecule and thus has many potential applications in biochemistry. The combination of hollow-core microstructured fiber and SERS technology not only enables remote real-time and distributed detection, but also can increase the effective action area between the light field and the object to be measured, and further reduce silica glass background signal that is unavoidable in traditional fiber probes. In this paper, the hollow-core microstructure fiber Raman probes with excellent performance are investigated from the aspects of fiber preparation and SERS experi-mental testing. First, we design and manufacture a kind of hollow-core microstructured fiber with multi-bands in the visible and near-infrared wavelength. The fibers show good light guide performance and thus can fully meet the requirements for surface-enhanced Raman excitation and signal transmission. At the same time, the large core size facilitates the coupling of excitation light, and provides enough room for the test object and the light field. Then, this hollow-core microstructured fiber is used in surface-enhanced Raman experiment. A layer of nano-Ag film is modified on the inner surface of the hollow-core microstructure fiber to prepare the SERS probe by the vacuum physical sputtering method, and Rhodamine 6G (R6G) alcohol solutions with different concentrations are prepared by the dilution method. The hollow-core microstructured fiber deposited with the Ag nano-film is immersed in R6G alcohol solution for 2 min. The alcohol solution of R6G is sucked into the air hole of the hollow-core microstructured fiber by the capillary effect. Then this fiber with R6G alcohol solution is placed in a drying oven at 40 ℃ for 3 h until the alcohol solvent in the air hole is completely volatilized. After that, this fiber is taken out and tested under a detection environment full with air. The fiber SERS probes are tested by microscopic confocal Raman spectroscopy, then the Raman spectra of R6G alcohol solvents with different concentrations are obtained. An R6G Raman signal with a concentration as low as 10-9 mol/L is successfully detected on the front side of the probe. In the far-end back-side detection mode, the detected concentration of SERS probe can be less than 10-6 mol/L. The designed hollow-core microstructured fiber probe has a simple structure and is easy to prepare and test. Compared with the traditional optical fiber, it has advantages of large effective area for the test object and the light field, small interference from the silica glass background signal. This hollow-core microstructured fiber probe has wide application prospects in biochemical detection and other fields.
Simultaneous measurement of magnetic field and temperature based on photonic crystal fiber with eliminating cross-sensitivity
2018, 67 (18): 184212. doi: 10.7498/aps.67.20180680
Measurement of magnetic field is very important in many fields, such as industrial manufacture, marine environmental monitoring, medical testing, etc. However, there is a cross sensitivity between the measurement of magnetic field and the fluctuation temperature in the environment. So how to accurately measure the magnetic field and the temperature simultaneously by eliminating the cross-sensitivity has been an urgent problem. In recent years, photonic crystal fiber (PCF) sensor has been widely used due to its particular advantages, such as high sensitivity, small size and its flexibility of filling various sensitive media into the air hole. So the PCF provides a new idea for designing the high-sensitivity magnetic sensor. In this paper, a new PCF sensing structure based on the mixed effects of directional resonance coupling and surface plasmon resonance (SPR) is proposed. In the cladding of the PCF, one air hole infiltrated with the magnetic fluid (MF) forms a defect core and is used as a directional coupling channel. When the wave vector matching condition is satisfied in the directional coupling channel, the power is transferred from the fiber core region to the clad defect core at a particular wavelength, and a loss peak is generated in the transmission spectrum. The MF has its unique magneto-optical effect. This is because its refractive index changes with external magnetic field. So the loss peak can be shifted with the magnetic field at a fixed temperature. Another air hole coated with a gold nano film and infiltrated with the methylbenzene is used as the SPR channel. So plasmon modes are excited, and the resonance peak occurs when the real part of the effective index of the core mode is equal to that of the SPR mode at a particular wavelength. The resonance peak can also be shifted with the index of the methylbenzene at changed temperature. The simulation and numerical analysis of the magnetic field and temperature sensing characteristics of the structure are carried out, and the structure parameters of PCF are optimized by COMSOL Multiphysics through using the finite element method under the boundary condition of perfectly matched layer. In a magnetic field range of 90-270 Oe and in a temperature range of 25-60 ℃, the highest magnetic field sensitivity and temperature sensitivity are respectively 1.16 nm/Oe and -9.07 nm/℃, each with a good linearity in the sensing structure. To eliminate the cross sensitivity between the temperature and magnetic field, a sensitivity coefficient matrix is established. As a result, the highly sensitive double-parameter detection of magnetic field and temperature is realized. Moreover, this sensing structure can be used in an extensive range, which has a certain potential value and practical significance.
2018, 67 (18): 184701. doi: 10.7498/aps.67.20180364
Droplet impact on a solid surface is ubiquitous in daily life and various engineering fields such as ink-jet printing and surface coating. Most of existing studies focused on the droplet impact on flat or convex surface whereas the droplet impact on a concave surface has been less investigated. The purpose of this paper is to investigate the dynamic process of droplet impact on the inner surface of a cylinder numerically by using the phase-field-based lattice Boltzmann method. This method combines the finite-difference solution of the Cahn-Hilliard equation to capture the interface dynamics and the lattice Boltzmann method for the hydrodynamics of the flow. Besides, a recently proposed method is employed to deal with the wetting boundary condition on the curved wall. The method is first verified through the study of the equilibrium contact angle of a droplet on the inner surface of a cylinder and the droplet impact on a thin film, for which good agreement is obtained with theoretical results or other numerical solutions in the literature. Then, different droplet impact velocity, initial height of the droplet, surface wettability and radius of the cylinder are considered for the main problem and their effects on the evolution of the droplet shape are investigated. The physical properties of the droplet including the density and viscosity are also varied to assess their effects on the impact outcome. It is found that the impact Weber number, the liquid/gas density and dynamic viscosity ratios, the wettability of the inner surface of the cylinder, and the radius of the cylinder may have significant effects on the deformation and spreading of the droplet. At low Weber numbers, when the density and dynamic viscosity ratios are sufficiently high, their variations have little effect on the droplet impact process. At high Weber numbers, changes of these two ratios have more noticeable effects. When the Weber number is high enough, droplet splashing appears. When the density and dynamic viscosity ratios are high, the initial height of the droplet only has a minor effect on the impact results. The increment of the cylinder radius not only increases the maximum spreading radius but also enlarges the oscillation period of the droplet after its impact. Rebound of the droplet may be observed when the contact angle of the inner surface of the cylinder is large enough. Besides, the gravity force is found to suppress the oscillation of the droplet on the cylinder's inner surface. This work may broaden our understanding of the droplet impact on curved surfaces.
2018, 67 (18): 184702. doi: 10.7498/aps.67.20180879
In shock bubble interaction (SBI), the baroclinic vorticity generated by misalignment of pressure and density gradient will lead to flow instability which promotes the mixing between the bubbles and surrounding gas. A numerical study on the flow and mixing of shock-accelerated elliptic helium cylinder with the surrounding air is presented in this study. To well simulate the SBI, compressible multi-component two-dimensional Navier-Stokes equations are solved by combining with double-flux model and five-order weighted essentially non-oscillatory scheme. Both the wave system evolution and the interface deformation are clearly illustrated by using the present numerical method. Quantitatively, the length scales of distorted interface, compressibility of helium cylinder, circulation, and total mixing rates of helium are measured and compared to investigate the mixing mechanism and structure effect of the helium cylinder. It is found that the evolution of elliptic interface is closely related to its shape. In the case of elliptic gas cylinder shock-accelerated along major axis, the most remarkable feature is the air jet which grows constantly with time and penetrates the downstream interface boundary, forming two independent vortices. The penetration speed of the air jet is found to increase with ellipse eccentricity increasing. In addition, like the case of the circular helium cylinder, typical free-precursor irregular shock wave refraction occurs when incident shock wave passes through the interface. In the case of shock-accelerated elliptic gas cylinder along minor axis, a distinct flat structure appears due to the shock compression during the evolution of interface, and then vorticity concentrates at the two ends of the ellipses, which finally bends the interface severely. Simple regular shock wave refraction occurs in the large frontal area of the helium cylinder. These features also grow intensely with the eccentricity of the initial elliptic interface increasing. The distinct morphologies of these elliptic interfaces also lead to the different behaviors of the interface features including the length and height. The comprehensive analysis shows that for the elliptic helium cylinder, the structure effect not only affects the interface evolution in a length-scale manner but also plays a role in their mixing process. The mixing rate of helium cylinder shocked along the major axis is significantly superior to that along the minor axis.
2018, 67 (18): 184210. doi: 10.7498/aps.67.20180848
A structure containing substrate/narrow groove metal grating/covering layer/graphene is constructed. The operational principle of the structure is based on the surface plasmon polariton (SPP) resonance excited by the metal grating and the Fabry-Prot (FP) resonance supported by the narrow grating groove. Double-channel absorption enhancement of monolayer graphene is realized in the visible range, and a simplified model is used to estimate the locations of the double-absorption channels. At the wavelengths of 462 nm and 768 nm, the light absorption efficiencies of graphene are 35.6% and 40.1%, respectively, which are more than 15.5 times the intrinsic light absorption of the monolayer graphene. Further analysis shows that the energy of the absorption peak at the short-wavelength position mainly concentrates on the surface of the metal grating, which has an obvious characteristic of the SPP mode. The resonant wavelength of SPP=476 nm, estimated by the simplified model, is basically consistent with the location of the short-wavelength absorption peak at 1=462 nm. The absorption characteristics are less affected by the thickness of the covering layer, the depth and width of the groove. For the long-wavelength absorption peak at 2=768 nm, the energy of the light field in the structure is mainly localized in the metal groove, which has a significant cavity resonance characteristic. Because the SPP resonance generates a strong electromagnetic coupling in the metal groove, the energy of the optical field is strongly confined by the grating groove. The localized light field energy gradually leaks out and is absorbed by the graphene layer above the groove, resulting in a significant increase in the light absorption efficiency of the graphene. The resonance position estimated by the FP cavity resonance model is 658 nm, which is larger than the actual absorption peak position 2=768 nm. This is because the exact length of the FP cavity is affected by the thickness of the SiO2 covering layer, and the presence of the SiO2 covering layer will enlarge the exact length of the FP cavity. To further increase the depth of the groove, the agreement between the estimated resonance position and the actual absorption peak will continue to increase. However, the increase of the thickness of the SiO2 covering layer will weaken the magnetic field enhancement effect in the groove, resulting in the decrease of light absorption efficiency of the structure and graphene. Since the absorption enhancement at the long-wavelength peak originates from the FP resonance in the narrow groove, it exhibits a good angle-insensitive absorption characteristic. The double-channel absorption enhancement of graphene based on the narrow grooved gratings may have potential applications in the fields of photodetection and solar cells.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2018, 67 (18): 187101. doi: 10.7498/aps.67.20180450
Spintronic devices utilize the electron charge and spin degree of freedom to achieve novel quantum functionalities. Diluted magnetic semiconductors (DMS) constitute an important category of spintronic materials that have the potential to be successfully incorporated into the existing semiconductor industry. The prototypical DMS (Ga,Mn) As, discovered in the 1990s, accomplishes spin and charge doping simultaneously through the heterovalent substitution of the magnetic ion Mn2+ for Ga3+. Two challenges have presented themselves in this material. First, the heterovalent nature of this integrated spin/charge doping results in severely limited chemical solubility in (Ga,Mn) As, restricting specimen fabrication to metastable thin films by molecular beam epitaxy; second, the simultaneous spin and charge doping precludes the possibility of individually tuning the spin and charge degree of freedom. A new type of ferromagnetic DMS based on I-Ⅱ-V group can overcome both of these challenges. Li(Zn,Mn) As utilizes excess Li concentration to introduce hole carriers, while independently making the isovalent substitution of Mn2+ for Zn2+ in order to achieve local spin doping. With no heterovalent substitution to restrict chemical solubility, bulk samples of Li(Zn,Mn) As are successfully fabricated. However, one drawback of Li(Zn,Mn) As is its use of the toxic element As. The isostructural direct-gap semiconductor LiZnP also undergoes a ferromagnetic transition upon Mn doping, and its bulk magnetic properties are very similar to those of LiZnAs. In this paper, the geometric structure of pure LiZnP, Ag doped, Cr doped, and Ag-Cr co-doped LiZnP new diluted magnetic semiconductor are optimized by using the first-principles plane wave ultra-soft pseudo-potential technology based on the density function theory. Then we calculate the electronic structure, magnetism, formation energy, differential charge density, and optical properties of the doped systems. The results show that the material is a paramagnetic metal after single doping of the nonmagnetic element Ag. When magnetic element Cr is doped with LiZnP, sp-d orbital hybridization makes the peak of density of state nearly EF-split, leading the system to become metallic ferromagnetism. However, Ag-Cr co-doped LiZnP changes into half-metallic ferromagnetism, which is completely different from the single doping system. The band gap decreases slightly, and the electrical conductivity is enhanced. Meanwhile, the formation energy of the system becomes lower, the bond between atoms strengthens, and the stability of the unit cell becomes stronger. A comparison of the optical properties indicate that the imaginary part of dielectric function and the optical absorption spectrum both present new peaks in low energy region in the doped systems. Ag-Cr co-doped LiZnP has the highest dielectric peak. Meanwhile, the complex refractive index function changes obviously in a low energy region, and the absorption edge extends to the low energy direction. The system enhances the absorption of low-frequency electromagnetic waves.
2018, 67 (18): 187303. doi: 10.7498/aps.67.20180904
To date, there has not been a consensus about the resistance switching mechanism of donor-doped SrTiO3. The La doped STO (LaSTO) single crystal is a donor-doped material and has an N-type conductivity since La3+ could easily substitute Sr2+. In this study, the Pt/LaSTO/In memory device is fabricated based on (100) LaSTO single crystal with 0.5 wt% La doping. Through a series of electrical tests, it is found that the Pt/LaSTO/In memory device has a stable multi-stage resistive switching property, and the maximum switching ratio is 104. The fitting I-V curve at the high resistance state (HRS) shows that there is an interface barrier in the memory device. However, the fitting I-V curve at low resistance state (LRS) is consistent with the characteristic of the electron tunneling model. The spectrum of electron paramagnetic resonance (EPR) indicates that LaSTO single crystal has only one EPR signal of g=2.012. Considering the fact that g=gobs-ge (where gobs is the g factor obtained from the sample, ge=2.0023 is the free electron value) is positive, the signal can be regarded as being due to hole center. The hole center is positively charged and can trap electrons. Comprehensive analysis indicates that the transition between the HRS and LRS of the device can be explained by the modulation of Pt/LaSTO interface barrier, which is caused by the electron trapping and detrapping of interfacial vacancy defects. In addition, it is found that illumination could reduce the low resistance of the Pt/LaSTO/In device. This is due to the photo-generated carriers causing a tunneling current because of the narrow Schottky barrier when the Pt/LaSTO/In device is in the LRS. However, the Schottky barrier plays a leading role in HRS, so the change in carrier concentration, caused by illumination, does not lead to a significant change in current for HRS. The experimental results provide theoretical and technical guidance for the applications of LaSTO single crystals in resistive memory devices.
2018, 67 (18): 187102. doi: 10.7498/aps.67.20180867
Based on density functional theory, the extraordinary magnetic properties of finite graphene fragments (graphene nanoflake, GNF) with different shapes are studied by first-principles electronic structure calculations with all electron numerical-orbital basis set scheme as implemented in DMol3 code of Materials Studio 8.0 software package. According to the graph theory, the spin characteristics of several typical hydrogen-terminated GNF shaped into 3-fold and 6-fold highly rotational symmetries as well as two specific geometrical structures related to graphene nanoribbon are analyzed and verified by first-principles calculations. In some characteristic GNFs shaped into a singular graph, the electron energy matrix becomes singular and multiple states of zero eigenvalue appear which is called nonbonding state (NBS). In these singular graph structures, all the -bonds cannot be satisfied simultaneously and spin-aligned singly occupied molecular orbitals are generated from degeneracy at Fermi-level, which means that the topological frustration occurs. It is proved that the electronic spin magnetic order of GNF originates from topological frustration of conjugate -bonds determined by its shape. The net spin of triangular GNF with zigzag edges is not zero, like an artificial ferromagnetic atom, increasing proportionally with its linear dimension. According to the principle of topological frustration, the large net spins and specific spin distributions can be reasonably introduced into graphene nanocrystals, such as by triangulation. The NBSs of zigzag-edged triangular GNFs with nanoscale dimension create 0.4-0.7 eV energy gaps at Fermi-level to achieve stable spin-alignment at ambient temperature. Even though the linear size of zigzag-edged triangular GNF increases beyond nanoscale, the maximum energy gap is still ~0.68 eV and thus the magnetic moment measurement is feasible at room ambient temperature. The total spin of the complex fractal structure constructed by zigzag-edged triangular GNF unit increases exponentially with the fractal level, due to the increased possibility of topological frustration from aggrandizing boundary. In addition, a large net spin can also be acquired by hollowed-out zigzag triangle in graphene with a net spin value of at least 1.00 and a spin-polarization split band gap of ~0.40 eV. The basic design principle for obtaining large spin and controlling spin state distribution by etching GNF of various patterns in graphene atomic layer is presented. In contrast to traditional chemical synthesis of obtaining large spin limited by complicated reaction pathways, the GNF with large spin easily exceeding the reported highest spin in conjugated polymers can be practically produced by carving lithography. It is suggested that the GNF with designed topological structures fabricated by pattern carving technique can be efficiently used to realize the controllable spintronic nanomaterials and devices.
2018, 67 (18): 187301. doi: 10.7498/aps.67.20180769
Great attention has been paid to the terahertz (THz) technology due to its potential applications, in which THz radiation source and detector with excellent performances at the room temperature are most desired. The semi-classical Boltzmann equation is employed to study the response of electrons and holes to the electromagnetic radiation field in InAs/GaSb based type Ⅱ quantum well system (QWS). The balance equation method is used to solve the Boltzmann equation, and the influences of the structure of the QWS on the photoconductivity is studied in detail to reveal the mechanism of the photoconductivity in the QWS. The photoconductivity is influenced by the carrier density, the subband energy of the carriers and the coupling of the wavefunctions which can be modulated conveniently by the structure of the QWS. In this study, our attention focuses on the influence of the structure of the QWS on the conductivity. When the width of the InAs layer and the GaSb layer are both 8 nm, a sharp peak in photoconductivity is observed at about 0.2 THz due to the electron transition in different layers. The strength of the peak decreases slightly with the increase of the temperature, and a red shift is observed. However, the photoconductivity is not sensitive to the temperature and has good performances at relatively high temperatures up to the room temperature, which indicates that the InAs/GaSb based type-Ⅱ QWS can be used as a THz photoelectric device at room temperature.
2018, 67 (18): 187701. doi: 10.7498/aps.67.20181130
Ferroelectric (FE) materials have been extensively applied to the multifunctional electronic devices, particularly the FE memories due to their excellent physical properties. The FE memory is a kind of nonvolatile memory device, and it could overcome the shortcomings of the traditional memory. But the development of the FE memory is very slow due to the FE failure problem. However, with the continuous decrease of the thickness of FE thin film, when it reaches microns or nanometers in magnitude, the leakage current is the main cause of the FE failure of FE thin film. The leakage current of FE thin film is directly related to whether the FE memory is applicable, and it has been the hot spot of scientific researches. There are still a lot of factors influencing the FE memory leakage current except for the thickness of the film, such as interface, processing temperature, defect, domain wall, etc. Of these factors, the defect and domain wall are the most common and the most probable. In this paper, the first-principle calculation method through combining the density function theory with the nonequilibrium Green's function is used to systematically study the influence of oxygen vacancy defect on the leakage current of the FE thin film. The doping with four kinds of Cu, Al, V, and Fe cations is used to regulate and control the leakage current of the FE thin PbTiO3 film caused by the oxygen vacancy defects. We investigate the leakage current induced by oxygen vacancies in PbTiO3 films, and the doped PbTiO3 thin FE films having oxygen vacancies. It is found that Fe and Al doping will increase the leakage current of oxygen vacancy defects of FE thin films, while the Cu and V doping significantly reduce the leakage current of oxygen vacancy defects of FE thin films. This is because the Cu and V doping have obvious pinning effect on oxygen vacancy defect. In addition, we find that the oxygen vacancies are pinned by Cu and V atoms due to the fact that the formation energy of oxygen vacancies can be remarkably reduced. So Cu and V doping in PbTiO3 not only induce the leakage current but also improve the fatigue resistance of the FE thin film induced by oxygen vacancies. Moreover, since the ionic radius of V is closer to the ionic radius of Ti than the ionic radius of Cu, V is easier to implement doping to suppress the leakage current caused by the oxygen vacancy defects. These conclusions are of important theoretical significance and application value for improving the performance of FE thin films and their FE memories.
Optimization exploration of laser ablation propulsion performance of infrared dye doped glycidyl azide polymer
2018, 67 (18): 187901. doi: 10.7498/aps.67.20180479
The energetic polymer glycidyl azide polymer (GAP) is selected as the propellant of laser ablation micro thruster, and the effect of infrared dye doping on the propelling performance of laser ablative GAP is analyzed. By comparing the propulsion performance data with the plumes of infrared dyes doped GAP under different laser intensities, doping concentrations, target thickness and laser ablation modes, the optimization of the propulsion performance of infrared dye doped GAP is explored preliminarily. The experimental results show that the exponential attenuation characteristics of laser energy and the strong viscosity of GAP doped with infrared dye in the transmission mode lead to the existence of incomplete ablative GAP in the plume. The propulsion performances of GAP are influenced by the doping concentration of infrared dye and the thickness of propellant. Only when the target thickness is close to the laser absorption depth, can the mass of incomplete ablation along the direction of laser propagation be the least and can the laser energy be fully absorbed by the propellant to make the central ablation region reach the temperature threshold of the release of chemical energy. At the same time the optimum value of propulsion performance can be achieved. The GAP doped with infrared dyes in which laser ablation process follows the rule of absorbing laser energy first and spraying first is decomposed adequately under the reflection mode and the propelling performance is better than that in the transmission mode.
2018, 67 (18): 187801. doi: 10.7498/aps.67.20180778
Exploration of efficient deep red phosphor based on non-rare-earth ion activated oxide is of great practical value in the field of phosphors converted white light-emitting diode lighting. A spinel Mg1+yAl2-xO4:xMn4+, yMg2+ phosphor with deep red emission is synthesized by a solid-state reaction route. The crystal structure and morphology are characterized by powder X-ray diffraction and scanning electron microscopy. The luminescent performance is characterized by fluorescence spectrophotometer and fluorescence decay curves. The results demonstrate that the synthesized phosphor shows that two excited spectrum bands centered at 290 nm and 438 nm cover a broad spectral region from 220 nm to 500 nm due to the Mn4+-O2- charge transfer band and the 4A2-4T1 and 4T2 transitions of Mn4+ ions. Upon excitation at 300 nm, a strong, narrow red emission band is observed between 600 and 700 nm peaked at 652 nm as a result of the spin-forbidden 2Eg-4A2g electron transition of Mn4+. The corresponding chromaticity coordinate is (0.7256, 0.2854). Additionally, the concentration quenching of Mn4+ in the MgAl2O4 host is evaluated in detail, which indicates that the optimum doping concentration of Mn4+ is experimentally determined to be 0.14 mol%. The critical distance is calculated to be 52.15 according to the Blasse equation, which elucidates that the concentration quenching mechanism is consequently very likely to be induced by the multipole-multipole interaction. The crystal field strength (Dq) and the Racah parameters (B and C) are estimated to evaluate the nephelauxetic effect of Mn4+ suffered in MgAl2O4:Mn4+ host lattice. Luminous mechanism is explained by Tanabe-Sugano energy level diagram of Mn4+ ion. The ratio of Dq/B equals 1.74, indicating that Mn4+ ions experience a weak crystal field in the MgAl2O4 host and emission peak energy of 2Eg-4A2g transition is dependent on the nephelauxetic effect. The red emission intensity of Mg1+yAl2-xO4:xMn4+, yMg2+ increases on account of excess Mg2+ which would compensate for the local charge balance surrounding Mn4+ ions, furthermore, lead the Mn4+-Mn4+ pairs connected with interstitial O2- to transform into isolated Mn4+ ions, and thus eliminating energy transfer and enhancing the luminescence efficiency effectively. The decay times of two time-dependent curves of Mg1+yAl2-xO4:xMn4+,yMg2+ are 0.672 ms and 0.604 ms, and each entire decay curve could be well-fitted to single-exponential, confirming that there is only a single Mn4+ ion luminescence center. The decay time of Mn4+ luminescence is prolonged with the increase of Mg2+ content, indicating that excitation energy transfer and non-radiative relaxation between Mn4+-Mn4+ pairs decrease, the reason is that photoexcitation energy can be temporarily stored in the trapping centers induced by excess positive charges. These results imply that Mn4+ doped Mg1+yAl2 -xO4:xMn4+, yMg2+ is a promising candidate of deep-red phosphors for near-UV and blue light emitting diodes. These findings in the paper would be beneficial not only to developing a low-cost and safe strategy to produce high-efficient Mn4+ activated luminescent materials for white light emitting diodes, but also to providing a new insight into improving the photoluminescence properties of Mn4+.
2018, 67 (18): 182801. doi: 10.7498/aps.67.20180919
In an atomic vapor laser isotope separation process, the required isotope atoms are ionized selectively by a pulsed laser with a specific narrow line width, and then the produced isotope ions are extracted to the collected plates under an externally applied electromagnetic field. In the whole ion separation process, the ion extraction sub-process is one of the most important physical processes. Previous studies have shown that the key parameters of the laser-induced plasma, e.g., the initial electron number density and temperature, have a significant influence on the ion extraction features. In an actual isotope separation process, a specifically designed laser is necessary to produce the required isotope ions, which, however, leads the whole facility to have a very complicated structure, high capital cost, and especially, very narrow window of the key plasma parameters. These will, to some extent, limit a more in-depth investigation of the influences of the key plasma parameters on the ion extraction characteristics. In this paper, an ion extraction platform (ion extraction simulation experimental platform-2015, IEX-2015) is developed on the basis of a gas discharge plasma jet driven by a kilo-hertz high-voltage power supply. And an argon plasma collisional-radiative model is established to measure the electron temperature and number density in the plasma jet region. The experimental results show that the power input and driving frequency of the power supply and the argon mass flow rate can all affect the electron temperature and electron number density. The measured variation ranges of the electron number density and temperature are 109-1011 cm-3 and 1.7-2.8 eV, respectively, under a chamber pressure on the order of 10-2 Pa, which are close to the parameter levels in the actual ion extraction process. Subsequently, the preliminary ion extraction experiments are conducted under different extraction conditions including different externally applied voltages, different electrode distances and different plasma densities. The experimental results are also qualitatively consistent with those in an actual ion extraction process. The preceding preliminary experimental results show that it is feasible to conduct the ion extraction simulation study on IEX-2015. This is very helpful for systematically studying the ion extraction characteristics under different operating conditions in our future research.
2018, 67 (18): 182101. doi: 10.7498/aps.67.20181069
The Lipkin-Meshkov-Glick (LMG) model originally describes a Fermionic many-body system in nuclear physics. However, in recent years, it has been widely found in condensed matter physics, quantum information systems, and quantum optics, and it is of wider and wider interest. Previous studies on this model mainly focused on the physics under the thermal dynamical limit, such as quantum phase transitions and quantum entanglement. There are also some researches about LMG model with finite size in some special limits, but the finite-size effect on energy spectrum is not very clear yet. This is the main motivation of this work. In this paper, the exact diagonalization method and the quantum perturbation theory are used to calculate and analyze the energy-level structure of the LMG model at a finite N. To solve it, we first map this model into the angular-momentum space to obtain a reduced LMG model. By this mapping, the dimension of Hilbert space is reduced to N+1 from 2N. The exact solution of its energy levels can be obtained easily in the U(1) limit where the total spin is conserved. We find that the levels are woven into a fishing-net structure in the U(1) limit. While away from the U(1) limit, the crossings between even and odd levels will open a gap, and the system's energy levels will be grouped into pairs with an odd and an even level, forming some bound states, called doublet states, and the parity of each doublet state will oscillate as the Zeeman field increases. This work gives the values of the critical Zeeman field for the parity crossings. These critical values shift as the interacting parameters and disappear at zero in the Z2 limit. In the Z2 limit, the system energy levels form splittings near the zero Zeeman field. In this article, we analytically calculate the relationship between these energy gaps and the Zeeman field. For odd and even number N, the parity of each state has a different behavior. Specifically, the ground state and the doublet excited states of the system with odd N will suffer a parity reversion at zero Zeeman field, while the states with even N will not. By tuning the interacting parameters, we also study the crossover from the U(1) limit to the Z2 limit. The parity oscillation we find in this system is a very important physical phenomenon, which also exists in some other systems like optical cavity quantum electrodynamics and magnetic molecule system.
2018, 67 (18): 182401. doi: 10.7498/aps.67.20181095
Silicon carbide (SiC), as a representative of the third-generation semiconductor materials, is widely used in some fields which may suffer strong radiation such as in the cases of military affairs, aerospace and reactor. SiC possesses the superior radiation-resistance characteristic. However, SiC under the proton irradiation generate a lot of defects, resulting in degradation of device performance and even complete loss of its function. Therefore, the study on the irradiation damage to SiC under proton irradiation possesses important significance. A large number of studies have shown that for most of electronic devices and different types of incident particles, the degradation of device performance caused by displacement damage is linearly dependent on non-ionizing energy loss (NIEL), so the displacement damage can be evaluated by NIEL. In this work, the Monte Carlo software Geant4 is used to simulate the relationship between NIEL and proton energy, and the variation of NIEL with the depth of the material and the contribution of different types of primary recoil atoms to the total NIEL are also studied. The NIEL simulation results show that the NIEL in SiC material is less than that in Si and Ga semiconductor material under the same proton irradiation, proving that the stability and the radiation-resistance of SiC are stronger. The simulation results of NIEL at different depths show that the most serious damage regions of the material under different energy protons are diverse. Under the irradiation of low energy proton, the most serious region of the displacement damage occurs at the end of the proton range. With the increase of proton energy, the worst damage region of material will gradually move from the end of the proton range to the surface of SiC material. According to the contribution of different types of primary recoil atoms to the total NIEL, when the energy of the incident proton is low, the displacement damage of the proton in the SiC is mainly caused by 28Si and 12C, and the damage caused by 28Si is obviously higher than that by 12C. As the energy of proton increases, the 28Si and 12C are still the main causes of Bragg peak of the NIEL at the end of the proton range, but the number of ions generated by nuclear reactions increases accordingly, and the displacement damage caused by these ions increases in the shallow area of SiC, leading the surface of the material to be the worst damaged region. The combination of the two factors caused the most serious damage region moves from the end of the proton range to the surface of the material with the increase of proton energy. The results of this study are useful for the application of SiC devices to irradiation environment.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2018, 67 (18): 188101. doi: 10.7498/aps.67.20180220
The nanowires (NWs) of heterostructure with GaAs based materials have received great attention in the past decades, due to their potential applications in electronics and optoelectronics. Therefore it becomes more and more important to investigate the technology of fabricating NWs with GaAs based materials. In our study, Au-catalyzed GaAs nanowires and GaAs/InGaAs heterostructures are grown by metal-organic chemical vapor deposition following the vapor-liquid-solid mechanism. The growth process, which is vital for morphology research, is found to be strongly affected by growth temperature via scanning electron microscope testing. The GaAs NWs are grown at varying temperatures to investigate the influence of temperature on NW morphology. It is observed that the axial growth decreases with growth temperature increasing while radial growth exhibits the opposite trend, which causes the length of NWs to decrease with temperature increasing at the same time. As radial growth rate is inhibited and radial growth rate is enhanced at relatively high temperature, the geometry of GaAs nanowires turns from columnar to taper and eventually pyramid with temperature rising. The GaAs/InGaAs nanowire heterostructures with distinct heterostructure interfaces, which are columnar and vertical to substrates, are obtained and analyzed. Energy dispersive X-ray spectroscopy (EDX) is used for element monitoring while radial growth is hardly observed during axial heterostructure fabrication, indicating well controlled fabrication technology of NWs growth. The InGaAs segments of axial heterostructures are grown after GaAs segments and occur at the bottom of NWs instead on the top, the analysis of which shows that In atoms would take part in the growth of NWs via migrating at the surface of substrate preferentially, rather than being absorbed in Au-Ga alloy catalytic droplets. Radial heterostructures of GaAs/InGaAs nanowires are grown with GaAs as cores and InGaAs as shells, respectively. Because the axial growth rate would be restricted with temperature increasing, the growth temperature of radial heterostructures is higher than that of axial heterostructures. A small amount of axial growth occurs during the growth of radial heterostructures as indicated by the EDX monitoring result, which is analyzed to be caused by the diffusion of In atoms at radial growth temperature, resulting in a segment of InGaAs nanowire at the interface of nanowires and Au-Ga alloy catalytic droplets.
2018, 67 (18): 188501. doi: 10.7498/aps.67.20180643
The streak tube imaging lidar (STIL) community requires the streak tube with characteristics of small-size, high edge spatial resolution, high luminance gain, and large working area. In this work, with the aid of the computer simulation technology software, a streak camera with high edge spatial resolution and high luminance gain is designed, in which there are adopted 1) a spherical photocathode and screen to increase the edge spatial resolution and detection area, further enlarging the field of view for the STIL; 2) a slit accelerating electrode instead of the mesh one favorable for improving the electrical resistance and reliability for streak camera; 3) a streak tube with lower magnification combining with -15 kV working voltage to be able to achieve high luminance gain, thus further increasing the detection distance for STIL. Some static and dynamic properties of the tube are analyzed by observing different electron trajectories emitted from a number of different points on the photocathode. As for the spatial resolution, spatial modulation transfer function method is used to evaluate the spatial resolution characteristics of the streak tube. The 36.9 lp/mm at MTF=5% in static mode and 23 lp/mm at MTF=5% in dynamic mode of the high resolution across the 16 mm long slit on the photocathode can be obtained. As for the temporal resolution, three electron pulses at intervals of 54.6 ps can be well resolved by the streak tube in the dynamic mode. Thus, the temporal resolution of the streak tube is better than 54.6 ps. Furthermore, the influence of shape of the photocathode and screen on spatial resolution are analyzed. Compared with the P-streak tube (streak tube with plane photocathode and plane screen), S-streak tube (steak tube with spherical photocathode and spherical screen) can greatly improve the spatial resolution. The slit image of the spherical and plane photocathode are simulated. The spatial dispersion of the off-axis 8 mm slit image along the scanning direction is analyzed. The experimental results demonstrate that the spatial resolution of the small-size streak tube is 29.3 lp/mm at MTF=5% over the whole working area 16 mm2 mm, and the luminance gain is higher than 39.4. The static spatial resolution of the small-size streak tube is much higher than 15 lp/mm at CTF = 11.64%; the dynamic spatial resolution is higher than 9.8 lp/mm at CTF=5.51%; the temporal resolution is higher than 54.6 ps at Tscreen=4.3 ns and has good consistency on the whole photocathode, and the dynamic range is 345:1 at 54.6 ps. The streak camera contains 6 scanning levels for different depth of field and detection accuracy to achieve ultrafast signal diagnosis at different scanning speeds. The streak tube has a smaller dimension of 40 mm140 mm. It is of great significance in unmanned aerial and spaceborne laser imaging lidar detection.
2018, 67 (18): 188201. doi: 10.7498/aps.67.20180864
In this paper, the antispiral and antitarget wave patterns in two-dimensional space are investigated numerically by Brusselator model with three components. The formation mechanism and spatiotemporal characteristics of these two waves are studied by analyzing dispersion relation and spatiotemporal variation of parameters of model equation. The influences of equation parameters on antispiral and antitarget wave are also analyzed. Various kinds of multi-armed antispiral are obtained, such as the two-armed, three-armed, four-armed, five-armed, and six-armed antispirals. The results show that antispirals may exist in a reaction-diffusion system, when the system is in the Hopf instability or the vicinity of wave instability. In addition to the above two types of instabilities, there is the Turing instability when the antitarget wave emerges. They have the periodicity in space and time, and their propagation directions are from outside to inward (the phase velocity vp 0), just as the incoming waves disappear in the center. The rotation directions of the various antispiral tips are the same as those of the waves, which can be rotated clockwise or anticlockwise, and the rotation period of wave-tip increases with the number of arms. Furthermore, it is found that the collision sequence of the multi-armed antispiral tip is related to the rotation direction of the wave-tip. With the increase of the number of anti-spiral arms, not only the dynamic behavior of the wave-tip turns more complex, but also the radius of the center region increases. Due to the influence of perturbation and boundary conditions, the multi-armed antispiral pattern can lose one arm and become a new antispiral pattern in the rotating process. Under certain conditions, it can be realized that the single-armed antispiral wave transforms into an antitarget wave. It is found that the change of control parameters of a and b can induce the regular changes of the space scale of antispiral waves, and antispiral waves gradually turn sparse with the increase of a, on the contrary, they gradually become dense with the increase of b. When the parameter of D_w exceeds a critical value, the propagation direction of wave is changed, and the system can produce the transformation from antispiral wave to spiral wave and from antitarget wave to target wave.
2018, 67 (18): 188701. doi: 10.7498/aps.67.20180828
Based on the Helfrich elastic curvature energy model, the stable shapes for the two patterns of three-domain phase separation are studied in detail for the experimental parameters with direct minimization method in order to explain the interesting experimental results by Yanagisawa et al. (2010 Phys. Rev. E 82 051928). According to their experimental results, there are two transition processes. In the first process, the three-domain vesicles are formed, which are metastable. After several tens of minutes, the three-domain vesicles begin to bud, which is the second process. In the first process, the three-domain vesicles are formed with two patterns. The pattern with the liquid-ordered (Lo) phase in the middle with roughly cylindrical shape and two cap-shape liquid disordered (Ld) domains on each side of the Lo domain is termed pattern I in our paper, and the pattern with Ld domain in the middle with roughly cylindrical shape and two cap-shape Lo domains on each side is referred to as pattern Ⅱ. In the same paper of M. Yanagisawa et al., an approximate calculation is made with the vesicle shapes of the two patterns approximately represented by spheroids. Their calculation shows that the transition point of the two patterns is at o* 0.27 in the case of = 0.02 (or v = 0.942) and = 50, in contrast with the experimental result of o* 0.5. Here o is the area fraction of Lo phase, and is the excess area (which is usually represented by reduced volume v in the previous literatures), is the reduced line tension at the boundary of two adjacent domains. Thus the problem comes down to whether the transition point of the two patterns conforming with the experimental result can be obtained by the Helfrich elastic curvature energy theory if one performs a more precise calculation. Our calculation is performed with the direct minimization method, with the two boundaries of domains constrained in two parallel planes, this is an effective method to guarantee the smoothness of the boundary. To allow the vesicle to have a sufficient freedom to evolve, only constraints of fixed reduced volume and area fraction are imposed (The usual implementation method of constraints with the enclosed volume and the area of each phase fixed is not appropriate in this case. It does not allow the vesicle to have enough freedom to evolve, since the two boundaries are constrained in two preassigned planes). For the experimental parameters of = 50 and = 0.02, the transition point for the two patterns is obtained to be o* = 0.49, which is quite close to the experimental result of o* = 0.5. In order to understand the budding process in the second process, a detailed study is also made with the direct minimization method. It is found that the budding process can occur only for high enough value ( qslant 7.0) and permeable membrane (in other words, no constraint of reduced volume is exerted). One possible mechanism of the permeation is the temporary passage caused by the defect in the bilayer membrane due to large reduced line tension, which needs to be further checked experimentally. The three-domain vesicles found in the experiment have rotational symmetry in the case of small (or large v). What is more, they have a reflective symmetric plane perpendicular to the rotational symmetric axis, thus only vesicles with Dh symmetry are considered in this paper.
Design and simulation of output mode conversion structure of relativistic magnetron with all cavity output
2018, 67 (18): 188401. doi: 10.7498/aps.67.20180358
A relativistic magnetron using all cavity extraction and semi-transparent cathode has the virtues of compactness, high output power and high efficiency. The three-dimensional particle-in-cell simulations show that 1.15 GW output microwave with an efficiency about 50% can be obtained at S-band with pure TE11 mode of the fan waveguide. However, due to the fact that the output structure is composed of three detached fan waveguides, mode conversion structure in the output region is required for the convenience of practical applications. Therefore, two mode conversion structures are studied for the output mode conversion. The first structure is to widen gradually or abruptly the fan waveguide in the azimuthal direction from a given position (starting point) along the microwave transport direction. And then the three fan waveguides are connected into one coaxial waveguide. The effects of the position of the starting point on the beam-wave interaction and microwave extraction are numerically studied. For the convenience of description, we define L as the axial distance between the center of the output coupling hole and starting point. Simulation results show that for the abrupt and gradual variation case, when the length of L changes in a relatively wide region, the output power is larger than 1.0 GW in TEM mode at S-band. It is about 90% of the conventional fan waveguide with 1.15 GW. For the gradual variation case, the optimal value of L equals 10.0 cm, and the corresponding output power is beyond 1.0 GW. For the abrupt variation case, the optimal value of L equals 13.75 cm, the corresponding output power is about 1.15 GW. But in the abrupt variation case, the output power is a little more sensitive to the value of L. The second structure is to convert the fan waveguide into a rectangular waveguide. Ａcompound waveguide composed of a section of fan waveguide and a section of rectangular waveguide is designed for studying its feasibility. In the compound waveguide, the wide edges of the cross section of the rectangular waveguide are tangent to the inner and outer arc of the fan cross section respectively. And the narrow edges cross the end points of the outer arc. Simulation results show that in the compound waveguide the microwave with TE11 mode of the fan waveguide input at the inlet can be changed into the TE10 mode of the rectangular waveguide at the outlet with almost no power loss. In all, the output microwave power larger than 1.0 GW could be obtained after using the two proposed mode conversion structures. In practical applications, one could choose the relevant mode conversion structure according to the requirement.