## Vol. 66, No. 11 (2017)

##### 2017-06-05

###### GENERAL

2017, 66 (11): 110301.
doi: 10.7498/aps.66.110301

Abstract +

Quantum coherence is an essential ingredient in quantum information processing and plays an important role in quantum computation. Therefore, it is a hot issue about how to quantify the coherence of quantum states in theoretical framework. The coherence effect of a state is usually described by the off-diagonal elements of its density matrix with respect to a particular reference basis. Recently, based on the established notions from quantitative theory of entanglement, a resource theory of coherence quantification has been proposed[1,2]. In the theory framework, a proper measure of coherence should satisfy three criteria: the coherence should be zero for all incoherent state; the coherence should not increase under mixing quantum states; the coherence should not increase under incoherent operations. Then, a number of coherence measures have been suggested, such as l1 norm of coherence and the relative entropy of coherence[2]. Wigner function is known as an important tool to study the non-classical property of quantum states for continuous-variable quantum systems. It has been generalized to finite-dimensional Hilbert spaces, and named as discrete Wigner function[9-16]. The magic property of quantum states, which promotes stabilizer computation to universal quantum computation, can be generally measured by the absolute sum of the negative items (negativity sum) in the discrete Wigner function of the observed quantum states. In this paper we investigate quantum coherence from the view of discrete Wigner function. From the definition of the discrete Wigner function of the quantum systems with odd prime dimensions, for a given density matrix we analyze in phase space the performance of its diagonal and off-diagonal items. We find that, the discrete Wigner function of a quantum state contains two aspects: the true quantum coherence and the classical mixture, where the part of classical mixture can be excluded by only considering the discrete Wigner function of the diagonal items of the density matrix. Thus, we propose a possible measure method for quantum coherence from the discrete Wigner function of the off-diagonal items of the density matrix. We show that the proposed measure method satisfies the criteria (C1) and (C2) of coherence measure perfectly. For the criteria (C3), we give a numerical proof in three-dimensional quantum system. Meanwhile, we compare the proposed coherence measure with l1 norm coherence, and get an inequality relationship between them. Finally, an inequality is obtained to discuss the relation between quantum coherence and the negativity sum of discrete Wigner function, which shows that the quantum coherence is only necessary but not sufficient for quantum computation speed-up.

2017, 66 (11): 110201.
doi: 10.7498/aps.66.110201

Abstract +

The sub-diffraction-limit spatially structured light patterns have attracted more and more attention for their important applications in many frontier scientific fields. The present paper aims at developing sub-diffraction-limit spatially structured beam patterns which might have great potential to improve the light performance in fields such as super resolution imagery, optical tweezer, micro/nano lithography, etc. Here, a variety of spatially structured beam patterns are obtained by the phase modulation of polarized beams and studied in detail experimentally and numerically. Firstly, a new kind of phase plate, which combines the merits of circular and vortex 2 phase plates, is proposed based on the wave front design; it is composed of two spiral-shaped phase plates with their phases changing from 0 to 2 and - to , respectively. Later, the phase plate is applied to the circularly polarized Gaussian beam modulation in a high NA system. By combining a self-made circular with a commercial vortex 2 phase plate, the designed new phase plate is implemented in the experiment. The morphology of the spatially structured light pattern, which is generated on the focal plane, is observed by a CCD camera in the experiment. The beam pattern presents a donut shape on the focal plane, while the dimension of the donut-shaped pattern becomes smaller as the imaging plane axially deviates from the focal plane. It is found that the beam patterns captured in experiment highly consist with the numerical simulation results carried out by the vectorial diffraction integral theory. It can be deduced that the spatially structured beam is capillary-shaped. Meanwhile, at the two ends of the capillary-shaped beam, the inner diameter is smaller than the diffraction limitation. Furthermore, the structured beam pattern presents a spatial voxel distribution with center and axis symmetry. Finally, the characteristics of the spatially structured beam patterns, which are generated by modulating circular, linear, radial and azimuthal polarized beams with the new designed phase plate, are analyzed and discussed in detail. It is found that for circular, linear, radial and azimuthal polarization, the full widths at half maximum (FWHMs) of the minimum dark spots in the horizontal direction are 0.31, 0.32, 0.24 and 0.36, respectively. On the optical axis, the FWHMs of the dark spots created by linearly, radially and azimuthally polarized light, are 0.8, 0.78 and 0.76 , respectively, and no axial intensity is found with circularly polarized beam incidence.

2017, 66 (11): 110501.
doi: 10.7498/aps.66.110501

Abstract +

The main purpose of this study is to investigate the characteristics as well as the bifurcation mechanisms of the bursting oscillations in the asymmetrical dynamical system with two scales in the frequency domain. Since the slow-fast Hodgkin-Huxley model was established to successfully reproduce the activities of neuron, the complicated dynamics of the system with multiple time scales has become a hot research topic due to the wide engineering background. The dynamical system with multiple scales often presents periodic oscillations coupled by large-amplitude oscillations at spiking states and small-amplitude oscillations at quiescent states, which are connected by bifurcations. Up to now, most of the reports concentrate on bursting oscillations in the symmetric systems, in which there exists only one form of spiking oscillations and quiescence, respectively. Here we explore some typical forms of bursting behavior in an asymmetrical dynamical system with periodic excitation, in which there exists an order gap between the exciting frequency and the natural frequency. As an example, based on the typical Chua's oscillator, by introducing an asymmetrical controller and a periodically changed current source, and choosing suitable parameter values, we establish an asymmetrical dynamical system with two scales in the frequency domain. Since the exciting frequency is much smaller than the natural frequency, the whole periodic exciting term can be regarded as a slowly-varying parameter, leading to the fast subsystem in autonomous form. Since all the equilibrium curves and relevant bifurcations are presented in the form related to the slowly-varying parameter, the transformed phase portraits describing the evolution relationship between the state variables and the slowly-varying parameter are employed to account for the mechanism of the bursting oscillations. With the variation of the slowly-varying parameter, different equilibrium states and relevant bifurcations in the fast subsystem are presented. It is found that for different parameter values, multiple balance curves of the fast subsystem may coexist, which affect the structure of the bursting attractor. For the other parameters fixed to certain values, the balance curve with the variation of the slowly-varying parameter is presented. Three typical cases with different exciting amplitudes are considered, corresponding to different situations of coexistence of equilibrium states in the fast subsystem. In the first case, there exist at most three stable equilibrium points in the fast subsystem. Bursting attractor that oscillates around the three points can be observed, in which fold and Hopf bifurcations lead to the alternations between spiking states and quiescent states, while in the second case, saddle on the limit cycle bifurcation may cause the repetitive spiking oscillations to jump to the equilibrium curve. In the third case with relatively large exciting amplitude, only two equilibrium curves may involve the bursting oscillations, in which fold bifurcations lead to the alternation between the quiescent states and spiking states. Unlike the structures of bursting oscillations in the symmetric system, different forms of asymmetrical bursting oscillations with different periodic exciting amplitudes can be observed, the mechanisms of which are presented. It is pointed out that the change of the external exciting amplitude, does not only cause the variation of the attracting basins corresponding to different stable equilibrium branches, but also leads to the change of the temporal intervals when the trajectory passes different bifurcation points, respectively, which results in different patterns of bursting oscillations. Furthermore, since the slowly-varying parameter determined by the whole exciting term changes between two extreme values determined by the amplitude, the trajectory of the bursting oscillations of the transformed phase portrait returns at the two extreme values. The properties of equilibrium branches between the two extreme values determine the forms of the moving attractors.

2017, 66 (11): 110502.
doi: 10.7498/aps.66.110502

Abstract +

Now, many different approaches have been presented to study the different semi-classical models derived from the Dicke Hamiltonian, which reflect a fact that the quantum-mechanical spin possesses no direct classical analog. The Hartree-Fock-type approximation is one of the widely used approaches, with which we derive the Heisenberg equations of motion for the system and replace the operators in these equations with the corresponding expectation values. In the present paper, we investigate the role of quantum phase transition in determining the chaotic property of the time-dependent driven Dicke model. The semi-classical Hamiltonian is derived by evaluating the expectation value of the Dicke Hamiltonian in a state, which is a product state of photonic and atomic coherent states. Based on the inverse of the relations between the position-momentum representation and the Bosonic creation-annihilation operators, the Hamiltonian is rewritten in the position-momentum representation and it undergoes a spontaneous symmetry-breaking phase transition, which is directly analogous to the quantum phase transition of the quantum system. In order to depict the Poincaré sections, which are used to analyze the trajectories through the four-dimensional phase space, we give the equations of motion of system from the derivatives of the semi-classical Hamiltonian for a variety of different parameters and initial conditions. According to the Dicke quantum phase transition observed from the experimental setup , we study the effect of a monochromatic non-adiabatic modulation of the atom-field coupling in Dicke model (i.e., the driven Dicke model) on the system chaos by adjusting the pump laser intensity. The change from periodic track to chaotic figure reflects the quantum properties of the system, especially the quantum phase transition point, which is a key position for people to analyse the shift from periodic orbit to chaos. In an undriven case, the system reduces to the standard Dicke model. We discover from the Poincaré sections that the system undergoes a change from the classical periodic orbit to a number of chaotic trajectories and in the superradiant phase area, the whole phase space is completely chaotic. When the static and driving coupling both exist, the system shows rich chaotic motion. The ground state properties are mainly determined by the static coupling, while the orbit of the system is adjusted by the driving coupling. If the static coupling is greater than the critical coupling, the system displays completely chaotic images in the Poincaré sections, and the periodic orbits in the chaos can also be adjusted by the strong driving coupling. While the static coupling is less than the critical coupling, the system can also show the chaotic images by adjusting the driving coupling strength and oscillation frequency. Moreover, by tuning the oscillation frequency, the Poincaré sections may change from the classical orbits to the chaos, and back to the classical orbits in the normal phase of the system.

###### NUCLEAR PHYSICS

2017, 66 (11): 112501.
doi: 10.7498/aps.66.112501

Abstract +

In order to study the irradiation responses of reduced activation ferritic/martensitic (RAFM) steels which are candidates for fusion reactors, a reduced activation steel is irradiated at a terminal of HIRFL (heavy ion research facility in Lanzhou) with 63 MeV 14N ions and 336 MeV 56Fe ions at -50 ℃. The energies of the incident N/Fe ions are varied from 0.22 MeV/u to 6.17 MeV/u by using an energy degrader at the terminal, so that a plateau region of an atomic displacement damage (0.05-0.2 dpa) is obtained from the near surface to a depth of 24 μm in the specimens. Nanoindentation technique is used to investigate the nano-hardness changes of the samples before and after irradiation. The constant stiffness measurement is used to obtain the depth profile of hardness. The Nix-Gao model taking account of the indentation size effect (ISE) is used to fit the measured hardness and thus a hardness value excluding ISE is obtained. Consequently, the soft substrate effect for lower energy ion irradiation is effectively avoided. It is observed that there seems to be a power function relationship between the hardness and damage for the RAFM steel. The hardness initially increases significantly with the increase of irradiation damage, then increases slowly when the damage reaches to about 0.2 dpa. Positron annihilation is performed to investigate the defect evolution in the material. The positron annihilation lifetime spectra show that the long-lifetime proportion of the RAFM steel increases significantly after being irradiated. This means vacancy clusters are produced by the irradiation, resulting in the change of mechanics property. Even at low irradiation dose, point defects with high density are generated in the steel specimens, and subsequently aggregate into defect clusters, thereby suppressing the dislocation slip.The defect concentration in the material increases continuously with the increase of irradiation damage, which leads to the obvious irradiation hardening phenomenon. When the damage is higher than 0.1 dpa, the increment of mean lifetime gradually decreases due to the existence of a large number of vacancies and dislocations, and it eventually tends to be saturated, which explains why the irradiation hardening increment rate decreases with the increase of irradiation damage in the material.

2017, 66 (11): 112901.
doi: 10.7498/aps.66.112901

Abstract +

Low energy X-ray telescope, working over 0.7-15 keV energy band, is one of the main payloads in the hard X-ray modulation telescope satellite. The primary scientific objectives are to survey large sky area to investigate galactic X-ray transient sources as well as the cosmic X-ray background, and to observe X-ray binaries or black holes for studying the dynamics and emission mechanism in strong gravitational or magnetic field. The detector of low energy X-ray telescope is CCD236, a new generation of swept charge device, which has good time and energy resolution. Quantum efficiency (QE) of the detector has a crucial influence on X-ray spectrum fitting and absolute luminosity calculation. To provide valuable scientific data, QE should be calibrated in detail. In this paper, QE calibration is accomplished with respect to a silicon drift detector (SDD), using an Fe-55 radioactive source, at energy points Mn-Kα (5.899 keV) and Mn-Kβ (6.497 keV). The energies of Mn-Kα and Mn-Kβ are near that of iron-K, which is an important line in X-ray observation. Additionally, Mn-Kα and Mn-Kβ X-ray will partially pass through the depletion region of CCD236, and these energy points can be used to measure the depletion thickness. This experiment is set up in a vacuum cooling chamber. The X-ray source perpendicularly illuminates SDD and CCD236 through a small hole, whose area is far less than those of two detectors; therefore, QE measurements are irrelevant to neither the distance nor the azimuth angle between the X-ray source and the detector. For CCD236, split events should be corrected. Energy spectra of SDD and CCD236 are fitted with two Gaussian distributions, respectively, to obtain peak positions and standard variations of Mn-Kα and Mn-Kβ. With known structure of SDD, the QE of CCD236 can be calculated. QE values at Mn-Kα and Mn-Kβ are 71% and 62%, respectively. QE and temperature are uncorrelated with each other in a temperature range from -95 ℃ to -30 ℃. According to the specific structure of CCD236 and the measured QE, without considering the effect of channel stop, the best-fit thickness of depletion region is obtained to be 38 μm. When CCD236 is applied with different driving or substrate voltages, no obvious variation of QE is observed. It indicates that the thickness values of depletion region with high and low level voltages are equal. Furthermore, it shows that working CCD236 is deep depleted, and the thickness of depletion region will not change because it reaches its maximum, the edge of epitaxial layer and substrate layer.

###### ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

2017, 66 (11): 114202.
doi: 10.7498/aps.66.114202

Abstract +

Light transport in complex disordered medium, such as white paint, milk, is a fundamental physical phenomenon, and it plays an important role in numerous applications including imaging through turbid layers, and quantum information processes. However, all spatial coherence is lost due to the distorted incident wavefront caused by repeated scattering and interference. Incident coherent light diffuses through the medium and cannot form a geometric focus but a volume speckle field on the imaging plane. In this paper, we propose a four-element division algorithm and experimentally demonstrate that using this algorithm to modulate the incident light, the shaped wavefront can focus through disordered material. At the beginning, we start with four segments on spatial light modulator (SLM), changing the phase of each segment from 0-2πup to search for the optimal phase in terms of the maximal output intensity at a certain field. After the optimal phase of these four segments is found, each of all segments is divided further into four subsegments, so 16 subsegments are formed on the SLM. Just like the first step, the optimal phase is found by cycling the phases of these 16 subsegments. Sequentially, this procedure is repeated several times, so more and more subsegments are obtained. As a result, the modulated input light from SLM can be focused after it has passed through the turbid scattering medium. By employing this approach in the forward scattered experiment, the total pixels of spatial light modulator are divided into 4-4096 segments to shape the incident light. After separately searching for all the optimal phase distributions, we can see that a sharp focusing is gradually achieved. Likewise, in backscattered experiment, 4-1024 segments are used to focus the incident light after passing through the diffuse material. In comparison with stepwise sequential algorithm, the main advantage of our method is that the interference effect of all segments on SLM is taken into consideration, which means that the modulated and the modulating segments are connected with each other. In this way, the signal-to-noise ratio is higher and no iteration is needed. All this experiment shows that the four-element division algorithm can be employed to focus the incident light passing through a disorder material efficiently, which maybe provide a new idea and method in the field of biomedical imaging through scattering medium.

2017, 66 (11): 114204.
doi: 10.7498/aps.66.114204

Abstract +

Swept-source optical coherence tomography (SS-OCT) has high sensitivity and signalnoise ratio compare with time-domain optical coherence tomography and spectral-domain optical coherence tomography. Therefore, SS-OCT is the form of Fourier domain optical coherence tomography predominantly used in experimental research and biomedical image. However, polygon tunable laser-based SS-OCT suffers sweep range fluctuation and spectral misplacement. Under certain circumstances, in the current resampling methods cross-correlation is widely used to align spectrum misplacement, and truncate A-lines in order to ensure the consistency of frequency-scanning range, which, however, degrades the image SNR and resolution. We use the Mach-Zehnder interference (MZI) signal to quantify and analyze this problem in two typical polygon tunable lasers. The periodical change of sweep range and spectrum misplacement show the instability derived from polygon mirror. The parallelism among unwrapped phase curves indicates that polygon tunable laser output spectra have consistent wavelength distributions, and thus it is suited to implement cross-correlation between MZI signals in time domain, and an unwrapped phase curve can represent the wavelength distribution of all A-lines.According to the above conclusions, we demonstrate a resampling method in which the zero-padding interpolation and cross-correlation are used to align A-lines in time domain and eliminate the residual phase noise caused by integer shift. Then the unwrapped phase curve that has a largest sweep range is used to resample all the aligned A-lines, and the interference signals can be fully utilized. The experiments for signal truncation and Pomelo fruit flesh indicate that the proposed method can improve image SNR but does not make the intensity image dislocated. The phase noise (3.9 mrad for a 49 dB SNR) from static mirror is close to theory limit after resampling, thus showing good phase stability and resampling precision. The proposed resampling method also needs less computational work than one-to-one resampling method because it only fits unwrapped phase curve and calculates interpolation coefficient once.

2017, 66 (11): 114205.
doi: 10.7498/aps.66.114205

Abstract +

Measurement of audio signal plays a significant role in many applications, such as gravitational wave detection, bio-particle imaging and magnetometer. In this paper, low-frequency squeezed light is generated by a non-degenerate optical parametric amplifier. In order to avoid the effect of injected light on low-frequency squeezing, an auxiliary laser is used to lock the length of non-degenerate optical parametric amplifier and a method of locking quantum noise is employed to lock the phase between the local light and the squeezed light. By isolating the vibration noises at low-frequency and reducing back action of parasitic interference, the squeezing of (7.1±0.1) dB takes place at 19 kHz. Then the squeezed light is injected into the Mach-Zehnder interferometer to measure an audio signal which drives a piezoelectric transducer to generate a small phase variation between two arms of Mach-Zehnder interferometer. According to the low-frequency squeezing, we realize experimentally the measurement of phase signal at audio frequency which exceeds the shot-noise limit of (3.0±0.4) dB. The experiment provides technical supports for the generation of low-frequency squeezed light and the measurement of audio signal. Furthermore it can be extended to other quantum measurements, such as high-precision magnetometer and measurement of small-displacement.

2017, 66 (11): 114206.
doi: 10.7498/aps.66.114206

Abstract +

Coherent anti-Stokes Raman scattering (CARS) microscopy is a valuable tool for label-free imaging of biological samples, since it enables providing contrast via vibrational resonances of a specific chemical bond. However, in a conventional CARS image the Raman resonant anti-Stokes radiation is often superimposed by a nonresonant contribution arising from the electronic part of the polarization. The situation becomes worse if a sample is composed of a significant amount of water, where a strong nonresonant background over the whole image is obtained.To date, various approaches including Epi, polarization sensitive, time-resolved, and CARS phase imaging have been implemented to suppress the undesirable nonresonant background in CARS microscopy. Notably, optical heterodyne based phase imaging schemes are of particular interest due to their intrinsic ability to retrieve Im(χ(3)), which is proportional to the Raman resonant signal. Nevertheless, all the reported phase imaging methods that require an independent reference wave lead to an increase in the setup complexity, thus making the measurement sensitive to external perturbations. In order to simplify the setup, single-beam scheme has also been utilized for vibrational CARS imaging by using wave-front sensors to acquire the phase of the complex anti-Stokes amplitude. However, this method demands highly accurate wave-front sensors.In this paper we present a reference-less CARS phase imaging technique to suppress nonresonant CARS background based on transport of intensity equation (TIE). Resonant CARS radiation ECARSR can be obtained when the frequency difference between the pump and Stokes beams is tuned to match a molecular vibration frequency (Raman resonant mode). In contrast, the nonresonant background ECARSNR can be obtained when the frequency difference between the pump and Stokes beams does not match a molecular vibration frequency (Raman resonant mode). Considering the fact that there is a phase shift of π/2 between the resonant and non-resonant CARS field, the phase imaging of both resonant and nonresonant CARS field can provide a background-free image. In implementation, three intensity images of the CARS field under resonant mode are recorded at three neighboring planes by moving the CCD camera along the axial direction. In the meantime, three images of the CARS field under non-resonant mode are also recorded. Considering the fact that the TIE links the intensity distributions in three neighboring planes (through which a beam transverses) with the phase distribution of the field, the phase images of the CARS field under both resonant and nonresonant modes are reconstructed from the recorded intensity images. The phase difference φχ between the resonant CARS field and the non-resonant CARS field is calculated. Eventually, the CARS background is efficiently suppressed by using the relation ICARSbf≅ICARSR·sin2φχ.Compared with conventional CARS background suppression techniques, the proposed method is robust against environmental disturbance, since it does not require an additional reference beam. Furthermore, the proposed method is easy to incorporate in a conventional CARS configuration. Therefore, the proposed method has the potential to become a versatile technique to image deep tissue with low background signal.

2017, 66 (11): 114208.
doi: 10.7498/aps.66.114208

Abstract +

The photonic crystal power splitter based on the energy coupling effect between waveguides has the advantages of compact structure, wide bandwidth, low bending loss, large angle of separation, and no external electromagnetic interference. In this paper, the power splitting characteristics of two-dimensional triangular-lattice photonic crystal coupled waveguide are theoretically studied by using the finite-difference time-domain method, and a functional device is designed in order to achieve different output power ratios within different frequency ranges.In the two-dimensional photonic crystal structure with triangular lattice, we set two adjacent straight waveguides and the light beam is introduced from one of them. Because of the energy coupling effect between the two line defects, the light energy propagates alternately in them. Based on this principle, structures of different coupling lengths are simulated and the interference effect of each surface is considered. The device with the best coupling length is achieved for three different output energy propagating characteristics at different frequencies, which include three-division, two-division and single output cases. That is to say, the incident light beam within a frequency band travels through a particular waveguide; light in another frequency band only flows through the other two output waveguides; light in the third frequency band is assigned to all the three output waveguides equally. However, the frequency band width for the high-quality light beam splitting area as well as the transmittance contrast of the other two functional band areas are not very ideal.Based on the above numerical results, two transmission modes in the coupling waveguides are achieved by changing the cross section shape of the dielectric column in the coupling region and also by changing the connecting position between the output branch waveguide and the energy-coupling waveguide. Through the above change, the splitting performance is further optimized.By detecting and analyzing the relative intensity of the three output waveguides, we can determine the range of the incident light beam. Furthermore, the frequency ranges of the three different light output characteristics can be adjusted flexibly by changing the cross section shape of the dielectric column in the coupling region or by changing the connecting position of output waveguides. The functional device proposed in this paper has a high transmittance contrast ratio and a compact structure, which will promote the practical application of the all-optical functional devices in the fields of large-scale all-optical complex integration.

2017, 66 (11): 114209.
doi: 10.7498/aps.66.114209

Abstract +

The Tm-doped mode-locked pulsed fiber lasers, which are known for their wide applications in optical communication, laser medical system and special material processing, have attracted considerable interest as novel laser sources. Up to now, many reported Tm-doped mode-locked fiber lasers focused on emitting picosecond or femtosecond pulses at a few megahertz (MHz) repetition rate. Actually, due to the strong chirp, large pulse width, low peak power and little nonlinear phase accumulation characteristics in the process of power amplifier, nanosecond mode-locked fiber laser is a representative of ideal seed source in the chirped pulse amplification (CPA) system. However, nanosecond mode-locked fiber lasers are generally implemented with the kilometerlong cavity length, corresponding to the fundamental repetition rate of hundreds of kilohertz. Usually, fiber lasers with such a low repetition rate are not desirable in applications of laser material processing, nor medical treatment nor scientific researches. In this paper, we report a nanosecond mode-locked Tm-doped fiber laser with MHz repetition rate based on graphene saturable absorber (SA). As the SA, graphene has excellent optical properties, such as optical visualization, high transparency, ultra-fast relaxation time and nonlinear absorption. It is not limited by the band gap either because of its zero-band-gap structure. Therefore, graphene can be used as fast SA, with wide spectral range operated. Generally, graphene suitable for mode-locked fiber lasers can be produced by using chemical vapor deposition (CVD), liquid phase exfoliation and mechanical exfoliation. Since the CVD technique can obtain high-quality graphene with precisely controlled number of layers, it is always the first choice for the manufacture of graphene. In our work, monolayer graphene layers are grown on copper foils by CVD, and then transferred onto the end face of the fiber connector three times. Meanwhile, a narrow-band fiber Bragg grating is used to constrain longitudinal modes of the laser intra-cavity. By simply adjusting the pump power and the polarization angle of polarization controller, stable 2 μm nanosecond mode-locked pulses are obtained in a wide range from 3.8 ns to 94.3 ns at 3.8 MHz repetition rate. We believe that the results obtained will be helpful for investigating the CPA system at 2 μm.

2017, 66 (11): 114401.
doi: 10.7498/aps.66.114401

Abstract +

Recently, thermal metamaterials have attracted more and more attention, and they have been used to manipulate the flow of heat flux. As a typical case, the thermal cloak can conceal the heat signature of an object. To the best of our knowledge, most of researches on cloak have focused on the case in which the background is a single homogeneous medium. However, cloaking in the layered and gradually changing backgrounds is very common in our real life such as hiding the buried mines in several soil backgrounds. In this paper, on the basis of transformation thermodynamics, a general expression of the thermal conductivity for two-dimensional thermal cloak with arbitrary shape in the layered and gradually changing backgrounds is derived by the coordinate transformation method. According to the expression, we design the thermal cloak in different inhomogeneous backgrounds. Results of full wave simulation show that heat flux can travel around the protection area and eventually return to their original path. The temperature profile inside the thermal cloak keeps unchanged, and the temperature field outside the thermal cloak is not distorted, which proves that the cloak has a thermal protection and thermal stealth function. In the end, we propose a useful method of utilizing homogeneous isotropic materials to construct a thermal device according to the equivalent medium theory. The method is closer to the practical application of the project because of considering the complex backgrounds. At the same time, this technology provides a feasible method to control heat transfer in the future and has great significance for thermal stealth and thermal protection.

2017, 66 (11): 114701.
doi: 10.7498/aps.66.114701

Abstract +

A new curved boundary treatment in lattice Boltzmann method is developed for micro gas flow in the slip regime. The proposed treatment is a combination of the nonequilibrium extrapolation scheme for curved boundary with no-slip velocity condition and the counter-extrapolation method for the velocity and its normal gradient on the curved boundary. Taking into consideration the effect of the offset between the physical boundary and the closest grid line, the new treatment is proved to be more accurate than the traditional half-way diffusive bounce-back (DBB) scheme. The present treatment is also more applicable than the modified DBB scheme because the specific gas-wall interaction parameters need to be determined to ensure the validation of the modified DBB scheme.The proposed boundary treatment is implemented to simulate the benchmark problems, which include a Poiseuille flow in the aligned/inclined micro-channel, a flow past a microcylinder and a microcylindrical Couette flow. The results and conclusions are summarized as follows.1) The force-driven Poiseuille flow in an aligned microchannel is simulated separately with different values of wall-grid offset qδx (q=0.25, 0.5, 0.75, 1.0). With the consideration of the wall-grid offset, the numerical results with the new boundary treatment show good agreement with the analytical solutions. However, the results obtained by using the half-way DBB scheme only accord well with the analytical solutions under the condition of a fixed wall-grid offset (q=0.5).2) To demonstrate the capability of the present treatment in dealing with gas flow in a more complex geometry, the force-driven Poiseuille flow in a micro-channel is investigated separately with different inclined angles. The present numerical results fit well with the analytical solutions. However, the discrepancy between the results obtained with the half-way DBB scheme and the analytical solutions can be clearly observed near the inclined boundaries.3) The gas flow past a microcylinder is simulated to prove that the present treatment can deal with the curved boundary. The slip velocity profile along the micro cylinder periphery obtained with the present treatment accords well with the available data in the published literature. However, the results obtained with the half-way DBB scheme show lower values than the results from the published work.4) In the simulations of the microcylindrical Couette flow between two coaxial rotating cylinders for different Knudsen numbers the results obtained by using the present treatment show excellent agreement with the analytical solutions. However, the results obtained with the half-way DBB scheme and the modified DBB scheme deviate obviously from the analytical solutions near the inner and outer cylindrical walls, respectively.In summary, the new boundary treatment proposed in this work is capable of dealing with the complex gas-solid boundary in the slip regime. The new treatment has a higher accuracy than the half-way DBB scheme and shows a better applicability than the modified DBB scheme.

2017, 66 (11): 114702.
doi: 10.7498/aps.66.114702

Abstract +

In view of the current status that different literature applies different coupling methods to the calculation of shale gas flow, and in order to clarify the relation between slippage and several diffusions, in this paper the slippage effect and various diffusions are analyzed first by theoretical analysis and mathematical models according to the definitions and the mechanisms of microscopic motions. Afterwards, allowing for the spatial effect of the adsorbed molecules on gas flow, the concept “wall-associated diffusion” is proposed for the first time to represent the gross effects of Knudsen diffusion and surface diffusion, and it is pointed out that wall-associated diffusion is equivalent to slippage effect. Therefore a new coupling way where wall-associated diffusion and slippage effect are replaceable and no superposition of them is needed in flow calculation, is proposed. The case study shows that when the capillary radius ranges from 5 nm to 2000 nm, the relative error between wall-associated diffusion and slippage effect mass flux is fairly small, namely less than 10% in the vast majority of the range. The difference between mean values of wall-associated diffusion and slippage effect mass flux in the whole aperture range is 1.4×10- 6 kg·m-2·s-1. That is, the relative error between the mean values is only 5.8%. Therefore, the new method satisfies the requirements for engineering calculations. Taking parameter selection, unfinished improvements in mathematical models of relevant mechanisms and other factors into account, there is some room for further promoting the verification of the proposed method. The development of wall-associated diffusion has practical significance and multiple research significance. And the new coupling way reveals the relation between slippage and diffusions, which prevents reduplicated superposition of shale gas flow mechanisms in nano-scale pores and can well change the status where the current coupling methods for shale gas flow are not consistent, thus specifying a new direction in the quantitative calculations for shale gas development.

2017, 66 (11): 114207.
doi: 10.7498/aps.66.114207

Abstract +

Using few-layer tungsten disulfide (WS2) doped polyvinyl alcohol as a saturable absorber for the initiation of the pulse generation, we experimentally demonstrate stable passively Q-switched mode-locked operations of Tm, Ho:LiLuF4 laser at 1895 nm for the first time. The laser is designed with an X-type four-mirror cavity and pumped by a Ti:sapphire laser operated at 785 nm, and its continuous operation is initiated when the absorbed pump power is 143 mW. When the absorbed pump power reaches 2.645 W, we obtain a maximum output power of 985 mW and a crystal slope efficiency of 39.8% by linear fitting. When the saturable absorber WS2 is inserted in the cavity, the threshold of the absorbed pump power is increased to 234 mW. With the increase of the pump power, Q-switch pulse sequence is first observed. When the absorbed pump power reaches 1.39 W, the stable operation of the Q-switched mode locked pulse is realized. A maximum average output power of 156 mW is achieved at an absorbed pump power of 2.6 W, which corresponds to a 25 kHz Q-switched repetition rate and a 300 μs-long pulse envelope. In this case, the modulation depth in Q-switching envelopes is close to 100%. After the passively Q-switched mode-locked is obtained stably, the mode-locked pulses inside the Q-switched pulse envelope have a repetition rate of 131.6 MHz, corresponding to a mode locked pulse energy of 1.19 nJ and a cavity length of 1.14 m. According to the definition of the rise time and considering the symmetric shape of the mode locked pulse, we can assume that the duration of the pulse is approximately 1.25 times more than the rise time of the pulse. Then the width of the mode locked pulse is estimated to be about 878 ps. These experimental results show that WS2 is a promising broadband saturable absorption material for generating a 2 μm-wavelength mid-infrared solid-state laser pulse. By increasing the pump power and reducing the loss of WS2 material, it is possible to realize a continuous mode locking operation which has a narrower pulse duration. The mode-locked mid-infrared pulses are very stable and have a lot of potential applications such as ultrafast molecule spectroscopy, mid-IR pulse generation, laser radar, atmospheric environment monitoring, etc.

2017, 66 (11): 114301.
doi: 10.7498/aps.66.114301

Abstract +

There are two kinds of definitions for waveguide invariant β. One is defined according to the striation slope of acoustic interference patterns, and the other is defined on the basis of dispersion characteristics of acoustic modes. The first definition is appropriate for engineering applications, while the second is suitable for theoretical analysis. However, the two definitions are not consistent with each other for a waveguide with thermoclines, because modal dispersion in such a waveguide can be very different for different modes and different frequencies. In such cases, the waveguide invariant defined according to modal dispersion can take many different values, which are referred to as the spectrum of waveguide invariant (β spectrum for short) in the paper. Each β spectrum can be related to some interference striation patterns with corresponding striation slopes. The sound field is composed of many modes, so the interference pattern is the summation of many components of different striation slopes and may be very complicated as a result of the diversity of β spectrum. In such a case one single β is not able to describe the complicated interference pattern adequately; instead multiple values of β spectrum are required. From the point of view of engineering application, however, the present β-extracting methods can only give one optimal value, and thus a lot of information is lost. In this paper an algorithm for doing so, called β spectrum separation technique, is proposed. By adopting the concept of integral projection used in digital image processing, the image of acoustic intensity is projected at different angles to separate out the striations of different slopes; and then fast Fourier transform (FFT) is applied to the projected curve in order to isolate striations of different spacing from each other. The values for β spectrum can be computed according to striation slopes, which are also mapped into the positions of their corresponding acoustic modal horizontal wavenumber differences in the wavenumber domain. The applicability of this algorithm for the extraction of β spectrum is tested and verified by simulation results and experiment data. It is shown that the algorithm can separate out each β spectrum of different intensity components from acoustic interference structure. The algorithm maps β spectrum into a two-dimensional plane thereby being able to suppress noise more effectively and work in the condition of low signal-to-noise ratio compared with the already-existing β-extracting algorithms.

2017, 66 (11): 114501.
doi: 10.7498/aps.66.114501

Abstract +

In recent years, research on space debris removal technique has received wide attention in aerospace field. Many novel concepts on active flexible debris remover have been proposed, such as flexible flying net, tethered cable manipulator. In view with the high flexibility and large deformation of this kind of structure, the implementation of attitude control is challenging. An accurate dynamic model of highly flexible structure is important and needed. The beam element is the most common element adopted in flexible remover models. So, in this investigation, a rotation field-based curvature shear deformable beam using absolute nodal coordinate formulation (ANCF) (named RB-curvature ANCF beam) is proposed and its geometrically nonlinear characteristic under large deformation motion is studied. Curvature is first derived through planar rotation transformation matrix between the reference frame and current tangent frame of beam centerline, and written as an arc-length derivative of the orientation angle of the tangent vector. Using the geometrically exact beam theory, the strain energy is expressed as an uncoupled form, and the new curvature is adopted to formulate bending energy. Based on the ANCF, the dynamic equation of beam is established, where mass and external force matrices are constant. In order to validate the performance of proposed beam element, other two types of beams are introduced as the comparative models. One is the classical ANCF fully parameterized shear deformable beam derived by continuum mechanics theory, and the other is position field-based curvature ANCF shear deformable beam (named PBcurvature ANCF beam). The PB-curvature model is evaluated by differentiating unit tangent vector of beam centerline with respect to its arc length quoted from differential geometry theory. A series of static analysis, eigenfrequency tests and dynamic analysis are implemented to examine the performance of the proposed element. In static analysis, both small and non-small deformation cases show that the proposed RB-curvature ANCF beam achieves the faster speed, higher precision and good agreement with analytical solution in the case of cantilever beam subjected to a concentrated tip force, which is compared with other two beam models. The eigenfrequency analysis validates RB-curvature ANCF beam in a simply supported beam case that converges to its analytical solution. Meanwhile, the mode shapes of the proposed ANCF beam could be correctly corresponded to vibration state of element with respect to each different eigenfrequency. In the dynamics test, a flexible pendulum case is used and simulation results show that the proposed RB-curvature ANCF beam accords well with ANSYS BEAM3, classical ANCF shear beam and PB-curvature ANCF beam in vertical displacements of tip point and middle point. Since deformation modes are uncoupled in the cross section of proposed beam element, its shear strain is achieved with much better convergence in the case of lower elastic modulus, and shear locking is significantly alleviated, compared with classical ANCF beam. Therefore, RB-curvature ANCF shear deformable beam element proposed in this paper is able to describe accurately geometric nonlinearity in large deformation problem, and can be a potential candidate in the modeling of flexible/rigid-flexible mechanisms.

2017, 66 (11): 114201.
doi: 10.7498/aps.66.114201

Abstract +

Sheared-beam imaging, which is a nonconventional coherent laser imaging technique, can be used to better solve the problem of taking pictures with high resolution for remote targets through turbulent medium than conventional optical methods. In the previous research on this technique, a target was illuminated by three coherent laser beams that were laterally arranged at the transmitter plane into an L pattern. In order to obtain a high quality image, a series of time-varying scattered signals is collected to reconstruct speckled images of the same object. To overcome atmospheric turbulence, multiple sets of three-beam laser should be emitted, which increases data acquisition time. In this paper, aiming at the quasi real-time problem of conventional sheared beam imaging technique, we use four-beam laser with rectangular distribution instead of the traditional L type sheared three-beam laser to illuminate the target. According to this, we propose a target reconstruction algorithm for four-beam sheared coherent imaging to reconstruct four target images simultaneously in one measurement, which can acquire high quality images by reducing the amount of measurement and the speckle noise. Meanwhile, it can greatly reduce the amount of beam switching in multi-group emission and improve the imaging efficiency. Firstly, the principle of the four-beam sheared coherent imaging technique is deduced. Secondly, in the algorithm, the speckle amplitude and phase difference frames can be extracted accurately by searching for the accurate positions of the beat frequency components. Based on the speckle phase difference frames, four sets of wavefront phases can be demodulated by the least squares method, and wavefront amplitude can be obtained by algebraic operation of speckle amplitude. The reconstructed wavefront is used for inverse Fourier transform to yield a two-dimensional image. A series of speckled images is averaged to form an incoherent image. Finally, the validity of the proposed technique is verified by simulations. From the simulation results, the image quality of the proposed method is better than that of the traditional method in the same amount of measurement. Furthermore, on the premise of the same image quality, the data acquisition amount of the proposed method is 2-3 times as large as that of the traditional method. In other words, compared with that of the traditional method, the data acquisition time of the proposed method is reduced at least by half and the algorithm processing time is less. It can be concluded that the proposed imaging technique can not only improve the efficiency of target reconstruction, but also present a better way of imaging the remote moving targets.

2017, 66 (11): 114203.
doi: 10.7498/aps.66.114203

Abstract +

The research on simulation of pollutant-gas-cloud infrared spectra is very important for studying the spectral identification algorithms by using simulated spectra. Some good results of the simulation of pollutant-gas-cloud infrared spectra under single-detector detecting are achieved, and have been used for studying the spectral identification. With the development of infrared detection technology, the infrared imaging spectrometer is used to detect pollutant gas cloud. The gas identification algorithms in the way of plane-array detecion based on the imaging spectrometer also need a number of measured gas-cloud infrared spectrum data cubes. Due to the lack of measured data in studying the spectral identification algorithm that is based on imaging spectrometer, the multiple-layer model of the cloud infrared spectrum is well studied by using the high-precision physics-based gas-cloud explosion model and its gridding simulation results, and the way of simulating pollutant-gas-cloud infrared spectra under plane-array-detector detecting is proposed to generate the infrared spectrum data cube with both spectral and spatial information, which obtains a new research method for the research field. Validations are made by comparing the measured data with the simulated data, and the comparison contains three parts: i) the comparison of measured gas-cloud explosion with simulated gas-cloud explosion, ii) the comparison of spectral identification imaging results between the measured and the simulated gas-cloud infrared spectrum data cubes, and iii) the comparison between the measured and the simulated gas-cloud infrared spectra. The comparison results have two aspects: the first aspect is that the simulated gas-cloud explosion is consistent with the measured explosion and has little difference in separate parts, and the second aspect is that the simulated gas-cloud spectra have relative errors of less than 10% compared with the measured gas-cloud spectra. The conclusion is that the simulation model of pollutant-gas-cloud infrared spectra under plane-array detecting is correct, which is obtained from the validation results that simulated gas-cloud infrared spectrum data cubes are highly precise, whether in the comparison with spectral identification imaging results or in the comparison with gas-cloud spectra. The simulation of pollutant-gas-cloud infrared spectra under plane-array detecting which directly reflects the explosion of pollutant gas cloud and provides complete and realistic infrared spectrum data cube of pollutant gas cloud, is significant for improving and perfecting the spectral identification algorithms.

###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

2017, 66 (11): 115201.
doi: 10.7498/aps.66.115201

Abstract +

Laser-induced breakdown spectroscopy (LIBS), which is also known as laser-induced plasma spectroscopy (LIPS), is a very promising spectral analysis technique for detecting elemental composition. The possibility of remote operation of LIBS is one of the properties, which expands the application scope of this technique. The remote LIBS technique is based on a long-range lens. With the increase of focusing distance, it is difficult to tightly focus laser pulse due to the diffraction limits. The size of focusing spot increases with focusing distance increasing. This will require extremely high laser energy. Femtosecond laser filamentation due to optical Kerr effect can be applied to the remote LIBS. During the filament propagation, the waist of laser beam is close to a constant value. The laser intensity inside the filament is about 1013 W/cm2 (intensity clamping). The intensity is sufficient to ablate sample and produce the plasma. It can overcome the influence of the diffraction limit in nanosecond LIBS. Although many researchers have studied the femtosecond geometrical focusing and femtosecond filamentation LIBSs, the spectral characteristics have not been completely understood. In this paper, we study the femtosecond laser filament-induced Cu plasma spectroscopy. Femtosecond laser system is an ultrafast Ti:sapphire amplifier (Coherent Libra). The full-width at the half maximum is 50 fs at a wavelength of 800 nm with a repetition rate of 1 kHz and its output energy is 3.5 mJ. A quartz lens with a focal length of 1 m is used to focus the laser to generate a filament channel. The spectral intensity of produced Cu plasma along the filament channel is measured by using the optical emission spectroscopy, and the distribution of Cu(I) intensity versus the distance between sample and focused lens is obtained. The results indicate that in a longer distance range along the filament, plasma spectroscopy has stronger emission due to the intensity clamping effect in femtosecond laser filamentation. In addition, we also calculate the plasma temperature and electron density by using the Boltzmann plot and the Stark broadening. The plasma temperature and electron density along the filament channel can be divided into three main regions: region 1) from 950 mm to 970 mm, in which the plasma temperature and electron density increase with the increase of distance; region 2) from 970 mm to 1030 mm, in which the change of plasma excitation temperature is opposite to the change of electron density; region 3) from 1030 mm to 1050 mm, in which the plasma temperature and electron density decrease with the increase of distance.

2017, 66 (11): 115202.
doi: 10.7498/aps.66.115202

Abstract +

Spectrally smooth X-ray sources can be used in point projection radiography and absorption spectrometry diagnostics of dense plasmas. But conventionally they are end at about 3.5 keV, which can only be used to diagnose materials up to Z=18. Spectrally smooth X-ray sources above 3.5 keV are needed to study higher-Z materials. Bremsstrahlung radiation from a laser driven implosion target can produce a small size, short duration and spectrally smooth X-ray source in the range of 1-100 keV. They have been successfully applied in the investigations of middle-Z materials in the 3-7 keV X-ray range. Despite much interest for backlit X-ray studies of middle- and high-Z dense materials, research on implosion X-ray sources are scarce. Characterization of the implosion X-ray source is needed to understand and improve its performance.To provide a physical basis for optimization, the properties of the deuterium-tritium (DT) implosion target X-ray source driven by 30-180 kJ laser pulses were explored using a radiation hydrodynamics code.We focus on laser pulse energies of 30-180 kJ at 351 nm wavelength to match the range of the OMEGA laser on the low end and the SG-Ⅲ laser on the high end. The laser pulse parameters are scaled with the target size in identical fashion to that of the OMEGA laser and the ignition designs of the National Ignition Facility to maintain the same irradiance on the surface of the capsule.The temporal and spatial evolution of the implosion targets was calculated using Multi-1D, a one-dimensional radiation hydrodynamics code. The emergent X-ray spectrum is calculated by post-processing from the time histories of the temperature and density profiles output by the Multi-1D code. We adjusted the laser absorption fraction to ensure neutron yield in accordance with OMEGA's 1D simulation results.It shows that the rapid increase of density and temperature at stagnation time develops a 150 ps point X-ray flash with approximately 100 μm size. The dominant X-ray emission comes from the inner layer of the dense compressed shell, which should be the focus of future efforts to improve the X-ray emission. Softer X-rays below 30 keV carry most of the energy due to the exponentially decaying spectral profile of implosion X-ray source. Opacity of the dense compressed shell plasma can markedly reduce the very softer X-ray emission of 1-3 keV. DT fusion reactions can enhance the share of harder X-rays above 30 keV greatly, while show negligible effect on the brightness of the implosion X-ray source. Thus higher-Z plastic target or glass target may be a better choice in generating the implosion X-ray source.

2017, 66 (11): 115203.
doi: 10.7498/aps.66.115203

Abstract +

Direct-drive inertial confinement fusion (ICF) requires a symmetric compression of the fuel target to achieve physical conditions for the ignition. The fast ignition scheme reduces the symmetry requirements for the target compression and the necessary driving energy, but symmetrically compressed target will certainly help improve the efficiency of the nuclear fuel burning. In this paper, with the particle-in-cell (PIC) simulation method, characteristics of the anisotropic pressure tensor of hot electrons are reported for the ultra intense laser pulse interaction with over dense plasmas, which mimics the scenario of the last stage when hot electrons are utilized to ignite the compressed fuel core in the ICF fast ignition scheme. A large number of hot electrons can stimulate pressure oscillations in the high density plasma. As the component parallel to the electron velocity dominates the pressure tensor, the electron density distribution perturbation propagates rapidly in this direction. In order to keep those hot electrons in the high density fuel plasma core for a period long enough for them to deposit energy and momentum, a magnetic field perpendicular to the electron velocity is used. The PIC simulation results indicate that the hot electrons can be trapped by the magnetic field, and the components of the anisotropic pressure tensor related to the parallel direction are significantly affected, thereby producing a high peak near the incidence surface. Since it is a relatively long process for the energy transfer from electrons to fuel ions and the nuclear interaction to be completed, the fluid effects take their roles in the fuel target evolution. The anisotropic electron pressure will deteriorate the fuel core symmetry, reduce the density, and achieve a lower efficiency of nuclear fuel burning and a lower gain of nuclear reaction than expected. The effects of the hot electron anisotropic pressure tensor in the fast ignition scheme should be considered as a factor in experiments where the nuclear reaction gain is measured to be much lower than the theoretical prediction.

###### CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

2017, 66 (11): 117101.
doi: 10.7498/aps.66.117101

Abstract +

The photocatalytic properties of TiO2 improved by modifying its surface have attracted more and more attention, because they play an important role in the photocatalytic degradation of greenhouse gases. Based on the fact that the photocatalytic reactions main occur on the catalyst surface, the surface modification becomes an effective method to improve the photocatalyst properties while the reaction mechanism research can give us a clear picture about it. Using the first principle calculations, the formation energies of TiO2 are calculated with doped and codoped by Cu and Ag atoms at different positions of the (001) and (101) surfaces. Comparing the formation energies, the most stable crystal structures are obtained while the electronic structures are calculated. Based on the analysis of the band structures and the density of states of atoms, it is proved that the oxidation activity of the active group formed on the (001) surface is stronger than that on (101) surface, which is more conducive to the improvement of photocatalytic oxidation properties. Meanwhile, the TiO2 compounds codoped by bimetal on the two surfaces have better light response than doped by one species of ions, which is in good agreement with the former experimental results.

2017, 66 (11): 117102.
doi: 10.7498/aps.66.117102

Abstract +

All-optical diode is the most basic photonic device in integrated optical circuits. It is of great significance to develop a modulated optical diode for preparing complex optical circuits in the near future. However, there are few studies on constructing all-optical diodes in subwavelength metal micro-nano structured devices based on the hybrid model of surface plasmon polaritons (SPPs) and composite diffracted evanescent wave (CDEW). In fact, most of the researches have been focusing on how to effectively enhance the unidirectional nonreciprocal transmission of the optical diode and improve the extinction ratio. According to SPPs-CDEW hybrid states, in this paper we put forward a novel method of designing an optical diode and its structure. The structure consists of a subwavelength single micro-nano slit surrounded by symmetric multi-pair grooves on a silver film. First of all, on the basis of the single slit structure of the silver film, the pairs of the groove structures are etched on both sides of the silver film: the positions and quantities of the grooves on the top and bottom surfaces are asymmetric. Then combining with an effect similar to Fabry-Perot resonance effect inside the micro-nano slit, the function of beam unidirectional transmission is achieved by controlling SPPs through changing the geometric parameters of the structure. Furthermore, in order to realize unidirectional nonreciprocal transmission, by means of theoretical derivation and the finite element method (FEM), in this paper we analyze the transmission enhancement phenomenon of single slit-symmetric pair of groove micro-nano structure, discuss the physical mechanisms of transmission enhancement and weakening, and also give the far field transmission spectrum of the normalized transmission changing with the distance between slit and pair grooves. The results obtained from the rigorous theoretical formula are in excellent agreement with the numerical results obtained by using FEM. Finally, as the position and number of the pair grooves are precisely determined by this transmission spectrum, the optimized all-optical diode structure, of which the unidirectional transmission is effectively enhanced and the extinction ratio of the optical diode is improved, is achieved with five pairs of enhanced transmission grooves formed on the top surface of the Ag film and six pairs of weakened transmission grooves formed on the bottom surface. The maximum extinction ratio reaches 38.3 dB, which means that the forward transmittance is 6761 times the reverse transmittance, i.e., it increases 14.6 dB over the result from previous theoretical work. And there appears a 70 nm wavelength band width (20 dB) in the operating wavelength 850 nm. The proposed optical diode has the advantages of simple structure, wide working bandwidth, easy integration, and high coupling efficiency. The research of the optical diode is valuable for the potential applications in optical signal transmission, optical integrated optical circuit, super-resolution lithography and other related fields.

## EDITOR'S SUGGESTION

2017, 66 (11): 117301.
doi: 10.7498/aps.66.117301

Abstract +

In this work, we report that NiO thin film can be used as a back contact buffer layer in CdTe thin film solar cells. The NiO layer is prepared by electron beam evaporation. To optimize the thickness of the NiO thin film, we fabricate some CdTe solar cells with different NiO thickness values. A NiO/Au back contact CdTe solar cell with an efficiency of 12.17% and an open-circuit voltage Voc of 789 mV is obtained, which are comparable to those of a standard Cu/Au back contact solar cell. The X-ray photoelectron spectroscopy (XPS) is used to quantitatively characterize the band alignment at the CdTe/NiO interface. It can be seen from the band alignment that the valence band offset (EVBO) is 0.52 eV and the conduction band offset (ECBO) is 2.68 eV. The EVBO presents no energy barrier for hole to transport from CdTe to NiO. The value of ECBO indicates that NiO can act as a back surface field layer (BSF) to dramatically reduce carrier recombination in the contact region of a CdTe cell, leading to an improved Voc. The band alignment obtained from XPS measurement shows that the band alignments of NiO and CdTe are perfectly matched. However, the conductivity of NiO film is poor. The insertion of a NiO buffer layer in the back contact increases the series resistance and reduces the fill factor (FF). We propose to use Cu/NiO composite structure as a bi-layer contact to improve the conductivity of the NiO buffer layer, which at the same time can be used to dope the CdTe film surface by Cu to obtain a low resistive contact. We fabricate a cell with a contact structure of 3-nm-Cu/20-nm-NiO/Au and the cell has a Voc of 796 mV, a Jsc (short-circuit currrent) of 24.2 mA/cm2, an FF of 70.2% and an efficiency of 13.5%. In order to study the stability of the solar cell with a Cu/NiO/Au back contact, a thermal stressing test is carried out at a temperature of 80 ℃ in the air atmosphere. For the Cu/NiO/Au back contact structure solar cell, the efficiency decreases from 13.1% to 12.9% after the cell is stressed for 80 h, showing that the stability of the Cu/NiO/Au back contact cell is significantly improved compared with that of the standard Cu/Au contact cell. In summary, the experimental results obtained in this study demonstrate that NiO thin film is a promising buffer layer for manufacturing stable and high efficiency CdTe thin film solar cells.

2017, 66 (11): 117802.
doi: 10.7498/aps.66.117802

Abstract +

Photoacoustic temperature measurement is a novel technique in which photoacoustic effect is used to measure temperature. It has the advantages of non-invasiveness, high sensitivity and deep penetration depth, which is suitable for monitoring the temperature distribution for the safe deposition of heat energy and efficient destruction of tumor cells during thermotherapy or cryotherapy. However, the present reported methods usually use one single wavelength for photoacoustic temperature measuring and are vulnerable to systematic and environmental influence, including the instability of system caused by fluctuation of laser energy, position displacement of transducer, and tissue complexity, which could reduce the measuring accuracy and stability. To solve this problem, a new photoacoustic temperature measuring method by employing two laser wavelengths is proposed in this paper. Firstly a brief theoretical analysis of dual-wavelengths photoacoustic temperature method is performed based on the linear relationship between photoacoustic signal and tissue temperature under two different wavelengths. Then two different samples including phantom of graphite and ex vivo pig blood are experimented respectively. The experimental temperature is set to be in a range of 26 ℃-48 ℃, which is controlled by a precise hot plate. And for improving the detection accuracy, the dual-wavelengths are selected as 760 and 900 nm for graphite phantom, 820 nm and 860 nm for ex vivo pig blood according to their absorption spectrum repetitively. The obtained results reveal that the temperature measuring correlation coefficients by dual-wavelength method can reach to 0.98 in graphite phantom and 0.99 in ex vivo tissue, respectively. And the average measurement deviation decreases to 0.88 ℃ in dual-wavelength method from 1.31 ℃ for the traditional single wavelength method for graphite phantom. While in ex vivo tissue, the measurement deviation decreases to 0.90 ℃ in dual-wavelength method from the average value 1.45 ℃ for the single wavelength method. Furthermore, the standard deviations of error are respectively reduced by an average of 38% in graphite phantom and an average of 30% in ex vivo tissue, respectively. These results indicate that the dual-wavelength method of photoacoustic temperature measurement can improve both the measuring accuracy and stability, and has a potential to be applied to medical therapy and other biomedical fields.

2017, 66 (11): 117401.
doi: 10.7498/aps.66.117401

Abstract +

Nowadays, the experimental results of absorption spectrum distribution of Ni doped ZnO suffer controversy when the mole fraction of impurity is in a range from 2.78% to 6.25%. However, there is still lack of a reasonable theoretical explanation. To solve this problem, the geometry optimizations and energies of different Ni-doped ZnO systems are calculated at a state of electron spin polarization by adopting plane-wave ultra-soft pseudo potential technique based on the density function theory. Calculation results show that the volume parameter and lattice parameter of the doping system are smaller than those of the pure ZnO, and they decrease with the increase of the concentration of Ni. The formation energy in the O-rich condition is lower than that in the Zn-rich condition for the same doping system, and the system is more stable in the O-rich condition. With the same doping concentration of Ni, the formation energies of the systems with interstitial Ni and Ni replacing Zn cannot be very different. The formation energy of the system with Ni replacing Zn increases with the increase of the concentration of Ni, the doping becomes difficult, the stability of the doping system decreases, the band gap becomes narrow and the absorption spectrum is obviously red shifted. The Mulliken atomic population method is used to calculate the orbital average charges of doping systems. The results show that the sum of the charge transitions between the s state orbital and d state orbital of Ni2+ ions in the doping systems Zn0.9722Ni0.0278O, Zn0.9583Ni0.0417O and Zn0.9375Ni0.0625O supercells are all closed to +2. Thus, it is considered that the valence of Ni doped in ZnO is +2, and the Ni is present as a Ni2+ ion in the doping system. The ionized impurity concentrations of all the doping systems exceed the critical doping concentration for the Mott phase change of semiconductor ZnO, which extremely matches the condition of degeneration, and the doping systems are degenerate semiconductors. Ni-doped ZnO has a conductive hole polarization rate of up to nearly 100%. Then the band gaps are corrected via the LDA (local density approximation)+U method. The calculation results show that the doping system possesses high Curie temperature and can achieve room temperature ferromagnetism. The magnetic moment is derived from the hybrid coupling effect of p-d exchange action. Meanwhile, the magnetic moment of the doping system becomes weak with the increase of the concentration of Ni. In addition, the absorption spectrum of Ni-interstitial ZnO is blue-shifted in the ultraviolet and visible light bands.

2017, 66 (11): 117701.
doi: 10.7498/aps.66.117701

Abstract +

Ferroelectric ceramics have been widely used in lots of fields, such as mechanical-electric transducer, ferroelectric memory, and energy storage devices. The dielectric breakdown process of ferroelectric ceramic has received much attention for years, due to the fact that this issue is critical in many electrical applications. Though great efforts have been made, the mechanism of dielectric breakdown is still under debate. The reason is that the electrical breakdown is a complex process related to electrical, thermal, and light effects. In the present work, we investigate the breakdown process of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3(PZT95/5) ceramic, which is a kind of typical ferroelectric ceramic working in the high voltage environments. The high voltage pulse generator is used in the breakdown experiments to apply a square pulsed voltage with an amplitude of 10 kV and a width of 7 s. The resistivity change in the breakdown process is recorded by the high-frequency oscillograph in nano-second. The results show that there are two different breakdown types for our sample, i.e. body-breakdown and flashover. To better understand the breakdown mechanism of the PZT95/5 ceramic, the formation of the conductive channel in ceramic in the process is investigated by comparing the resistivity development in body-breakdown and flashover processes. The development of the conductive channel formation can be divided into three steps in body-breakdown. In the first step that lasts for the first 40 ns of breakdown, the conductive channel starts forming, with the equivalent resistance sharply decreasing to about 105 in the mean time. Then, i.e. in the second step, conductive path grows into a stable one with the equivalent resistance decreasing to the magneitude of about 102 . The resistance decreases slowly to about 130 in the third step, which means that the conductive channel is completely formed. The channel formation of flashover can also be divided into three steps. The first step is similar to that of body-breakdown, with the equivalent resistance decreasing to about 105 in about 40 ns. In the second step of flashover, the conductive path keeps growing into a stable one with the equivalent resistance decreasing to 102 , but with a different resistance changing rate from that in body-breakdown, and the resistance decreases slowly to about 20 in the end. Different behavior between the body-breakdown and the surface flashover can be explained by different carrier densities on the conductive paths in the two breakdown processes. In the body-breakdown, the carrier density in the conductive channel is higher than that in the surface flashover, which improves the electron transfer and reduces the resistance. This may explain the reason why the channel formation in body-breakdown is faster than in flashover. This study is helpful for further materials design and applications.

2017, 66 (11): 117801.
doi: 10.7498/aps.66.117801

Abstract +

Generally, the Eu3+-activated red phosphors suffer narrow 4f-4f excitation lines ranging from near-UV to blue part of the spectrum, resulting in poor spectral overlapping with the emission spectrum of the pumping LED and low energy conversion efficiency. In this paper, the strategy of Te6+/Mo6+ mixing is adopted to enhance the excitation bandwidth of Eu3+ via the energy transfer from Mo6+-O2- charge transfer state to Eu3+, which is crucial for LED applications. A series of (Gd1-xEux)6(Te1-yMoy)O12 red phosphors are synthesized by the solid state method at 1200 ℃. The crystal structure, morphology and luminescent properties are investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescent spectrum. The XRD patterns of (Gd1-xEux)6(Te1-yMoy) O12 (x = 0.2, y = 0, 0.4) match well with that of Gd6TeO12 (JCPDS No. 50-0269), but differ from that of Gd6MoO12 (JCPDS No. 24-1085). The phosphor consists of irregular particles with an average size of 10 m. Upon excitation at 393 nm, the (Gd1-xEux)6TeO12 phosphors emit red light corresponding to the intraconfigurational 4f-4f transitions of Eu3+, and the color coordinates are calculated to be (0.647, 0.353). The 5D07F2 electron-dipole transition dominates the emission spectrum, which reveals that Eu3+ occupies a crystallographic site without an inversion center. Moreover, this transition gives rise to three distinguishable emission lines situated at 605, 618, and 632 nm, respectively. This unusual spectral splitting is supposed to originate from the strong interaction exerted by the crystal field of host on the 4f electrons. The optimum doping content of Eu3+ in (Gd1-xEux)6TeO12 phosphor is 20% (mole fraction), the critical distance for energy transfer is 0.75 nm, and the concentration quenching is confirmed to be induced by the dipole-dipole interaction from the linear relationship between lg(I/x) and lg x (I represents the luminescence intensity, and x represents the doping concentration of Eu3+). As the temperature increases, the emission intensity decreases gradually due to thermal quenching. The integrated emission intensity at 423 K is 70% of the initial value at ambient temperature. The thermal activation energy is determined to be 0.1796 eV from the temperature dependence of luminescence intensities. The partial substitution of Te6+ by Mo6+ does not change the emission position nor intensity significantly, but promotes the excitation bandwidth and conversion efficiency remarkably. Compared with (Gd0.8Eu0.2)6TeO12, the compositionoptimized (Gd0.8Eu0.2)6(Te0.6Mo0.4)O12 presents a relatively flat excitation spectrum in the near-UV region. It also provides more intense emission since (Gd0.8Eu0.2)6MoO12 undergoes the strong concentration quenching arising from the high density of [MoO6] groups. In conclusion, the results indicate that (Gd0.8Eu0.2)6(Te0.6Mo0.4)O12 can serve as a broadband-excited red phosphor for near-UV-based white LEDs.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

2017, 66 (11): 118201.
doi: 10.7498/aps.66.118201

Abstract +

Two-dimensional (2D) materials have shown great potential for electronic and optoelectronic applications. Among the 2D materials, molybdenum disulfide (MoS2) has received great attention in the transition metal dichalcogenides family. Unlike graphene, 2D MoS2 can exhibit semiconducting properties and its band gap is tunable with thickness. A demonstration of a single-layer MoS2 based field-effect transistor (FET) with a high on/off current ratio (about 108) has aroused the considerable interest. Although 2D MoS2 exhibits fascinating intrinsic properties for electronics, the contact may limit the device performance severely. In a real device such as FET, semiconducting 2D MoS2 needs contact with a metal electrode, and a Schottky barrier is always formed at the semiconductor-metal interface. The formation of low-resistance contact is a challenge, which is important for achieving high on current, large photoresponse and high-frequency operation. Therefore, understanding and tuning the interfaces formed between metals and 2D MoS2 is critical to controlling the contact resistance. In this work, some efforts have been made to investigate the 2D MoS2-metal interface in order to reduce the Schottky barrier height. By using the first-principles calculations based on density function theory, we investigate the effects of halogen doping-on metal-MoS2 interface, including the formation energy of defect, electronic structure, charge difference, and population. All calculations are performed using the ultrasoft pseudopotential plane wave method implemented in the CASTEP code. We use the generalized gradient approximation for the exchange and correlation potential as proposed by Perdew-Burke-Ernzerhof. Firstly, we calculate the formation energy to find the thermodynamically stable positions for the halogen elements located in 2D MoS2. It is shown that the halogen elements tend to occupy the S site of a MoS2 monolayer. Meanwhile, for the MoS2 monolayer, the halogen doping may introduce the defect level into the forbidden gap and make the Fermi level shift. For the metal-MoS2 interface, halogen doping can modulate its Schottky barrier height effectively in terms of Schottky-Mott model. This is because the Schottky barrier height at the metal-semiconductor interface depends on the difference between the Fermi level and the band edge position of the semiconductor. At the metal-MoS2 interface, the Fermi level is partially pinned as a result of the interface dipole formation and the production of the gap states. Therefore, using different metals with different work functions cannot modify the Schottky barrier height effectively. Here we demonstrate that F and Cl doping can reduce the Schottky barrier height, while Br and I doping can increase it. According to the results of the differential charge density analysis, we can ascribe the tuning of Schottky barrier height to the influence of the dipole caused by the charge transfer among the interfaces. This study can explain the relevant experimental results very well and provide a potential route to achieving low-resistance contact in the future applications of 2D materials.

2017, 66 (11): 118202.
doi: 10.7498/aps.66.118202

Abstract +

It is one of the important issues for electric vehicle to utilize power batteries which have long lifetime and excellent performance. For optimizing electrochemical performance and lifetime of the lithium ion battery, an electrochemical-thermal model based on dynamic response is developed by COMSOL MULTIPHYSICS. The modeling theory is the reaction mechanism of lithium iron phosphate battery which also includes a parasitic reaction occurring in the constant current and constant voltage charging process. The model consists of three parts: electro-chemical model, thermal model and capacity fade model. A series of temperature-dependent parameters and lithium ion concentration-dependent parameters relevant to the reaction rate and Li+ transport are employed in this model. Comparing with the results of the experimental test, the model shows high accuracy and reliability. The capacity losses and electrochemical behaviors of the battery in cyclic processes with different rates are investigated. The results show that when the battery is cycled at a rate of 1C, the capacity fading rate is about 6.35%, meanwhile the solid electrolyte interface membrane impedance of the battery is increased by 15.6 mm-2 after 800 time cycle. In the charge process, the side reaction rate within the anode shows a decreasing trend along the direction from the cooper current collector to separator, which is consistent with the lithium concentration in the anode. Besides, the effects of charge/discharge rate, negative active material particle radius and negative solid volume fraction on the battery cycle life are also discussed respectively. Compared with the fading rate of 3.31% after 400 time cycle with 1C rate, the capacity fading rates for 2C, 3C, 4C reach to 3.93%, 4.69% and 5.04% respectively. When the average particle radii of the anode are 2 m and 10 m, corresponding capacity fading rates are 2.89% and 3.87%, showing a difference of nearly 1%. The study for solid volume fraction demonstrates that the battery with a solid volume fraction varying in a range of [0.5, 0.6] will keep a longest battery life. These results show that the model has great potential to optimize the design of the battery.

## EDITOR'S SUGGESTION

2017, 66 (11): 118701.
doi: 10.7498/aps.66.118701

Abstract +

The single-molecule fluorescence resonance energy transfer (smFRET) technique plays an important role in the development of biophysics. Measuring the changes of the fluorescence intensities of donor and acceptor and of the FRET efficiency can reveal the changes of distance between the labeling positions. The smFRET may be used to study conformational changes of DNA, proteins and other biomolecules. Traditional algorithm for smFRET data processing is highly dependent on manual operation, leading to high noise, low efficiency and low reliability of the outputs. In the present work, we propose an automatic and more accurate algorithm for smFRET data processing. It consists of three parts: algorithm for automatic pairing of donor and acceptor fluorescence spots based on negative correlation between their intensities; algorithm for data screening by eliminating invalid fluorescence spots sections; algorithm for global data fitting based on Baum-Welch algorithm of hidden Markov model (HMM).
Based on the law of energy conservation, the light intensity of one pair of donor and acceptor shows a negative correlation. We can use this feature to find the active smFRET pairs automatically. The algorithm will first find out three active smFRET pairs with correlation coefficient lower than the threshold we set. This three active smFRET pairs will provide enough coordinate data for the algorithm to calculate the pairing matrix in the rest of automatic pairing work. After obtaining all the smFRET pairs, the algorithm for data screening will check the correlation coefficient for each pair. The invalid pairs with correlation coefficient higher than the threshold value will be eliminated. The rest of smFRET pairs will be analyzed by the data fitting algorithm. The Baum-Welch algorithm can be used for learning the global parameters. The global parameters we obtained will then be used to fit each FRET-time curve with Viterbi algorithm. The global parameter learning part will help us find the specific FRET efficiency for each state and the curve fitting part will provide more kinetic parameters.
The optimization algorithm significantly simplifies the procedures of manual operation in the traditional algorithm and eliminate several types of noises from the experimental data automatically. We apply the new optimization algorithm to the analyses of folding kinetics data for human telomere repeat sequence, the G-quadruplex DNA. It is demonstrated that the optimization algorithm is more efficient to produce data with higher S/N ratio than the traditional algorithm. The final results reveal clearly the folding of G-quadruplex DNA in multiple states that are influenced by the K+ concentration.

2017, 66 (11): 118801.
doi: 10.7498/aps.66.118801

Abstract +

The organic-inorganic metal halide perovskite materials have excellent optical and electrical properties such as high absorption coefficient, high carrier mobility, long carrier lifetime, tunable bandgap, facile fabrication process, etc. Owing to the above excellent properties, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased significantly from 3.8% to 22.1% in the last few years. The PSCs have attracted intensive interest in recent years and show great commercial potential. Previous approaches to increasing the PCE of PSCs have focused on the optimization of the morphology of perovskite film. However, there are relatively few studies on the electron transport layer (ETL) in the typical p-i-n sandwiched structure. In this work, the PCE of PSCs with device structure of ITO/PEDTO: PSS/CH3NH3PbI3/PCBM/Al is improved from 10.8% to 12.5% by using polystyrene (PS) and 1,8-diiodooctane (DIO) as binary additives during the deposition of phenyl-C61-butyric acid methyl ester (PCBM) layer. With the addition of PS, a highly smooth and uniform PCBM ETL is formed due to the increase of viscosity. The morphologies of the PCBM films prepared with and without PS are analyzed using an atomic force microscope in the tapping mode. The root-mean-square surface roughness decreases from 1.270 to 0.975 nm with the addition of PS increasing, which is more effective in preventing electron and hole from recombining at the interface between the perovskite layer and the top electrode. Addition of DIO improves the morphology of PCBM, which plays an important role in charge dissociation, charge transportation, and charge collection. From the time-resolved photoluminescence decay curves of ITO/CH3NH3PbI3/PCBM (with different additives), it is clear to conclude that the exciton dissociation between the perovskite layer and PCBM layer is faster and faster. Electrons and holes can be quickly separated, indicating that charge transport performances of electron transport layer with the addition DIO turn better. The addition of two additives is a simple and low-cost approach to improving the morphology of the electron transport layer, which provides a path-to the further improvement of the performance of p-i-n PSCs.

###### REVIEW

2017, 66 (11): 110701.
doi: 10.7498/aps.66.110701

Abstract +

refrigeration technology. It has been considered as one of promising alternatives to traditional vapor compression refrigeration technology. Magnetic refrigeration, in which solid magnetic materials instead of gaseous refrigerants are used, is based on the magnetocaloric effect. When magnetocaloric material moves in or out of magnetic field, it releases heat due to magnetization or absorbs heat due to demagnetization, respectively. In this paper, magnetocaloric effects (MCEs) and basic thermodynamic cycles are briefly described at first. Some typical magnetic refrigeration cycles are introduced from the viewpoint of thermodynamics, which include hybrid cycle, cycle based on the active magnetic regenerator and cycle based on the active magnetic regenerator coupled with gas regenerative refrigeration. Specifically, magnetic refrigeration cycle based on the active magnetic regenerator (AMR) coupled with gas regenerative refrigeration is a novel idea that combines the magnetocaloric effect with the regenerative gas expansion refrigeration. And it has been under the way to try to achieve greater refrigeration performance of the coupled refrigerator in the research institutions. Thereafter, the paper reviews the existing different numerical models of AMR refrigerator. Analyzing and optimizing an AMR magnetic refrigerator are typical complicated multi-physics problems, which include heat transfer, fluid dynamics and magnetics. The majority of models published are based on one-dimensional simplification, which requires shorter computation time and lower computation resources. Because a one-dimensional model idealizes many factors important for the system performance, two- or three- dimensional numerical models have been setup. Besides, some key items for the model are described in detail, such as magnetocaloric effect, thermal conduction, thermal losses, demagnetizing effect and magnetic hysteresis. Considering the accuracy, convergence and computation time, it is quite vital for numerical models to choose some influential factors reasonably. Then, the recent typical room magnetic refrigeration systems are listed and grouped into four types, i.e., reciprocating-magnet type, reciprocating-regenerator type, rotary-magnet type, and rotaryregenerators type. Different characteristics of these four types are compared. Reciprocating magnetic refrigerators have the advantages of simple construction and max magnetic field intensity difference. Rotary magnetic refrigerator due to compact construction, higher operational frequency and better performance is deemed as a more promising type, in the progress of magnetic refrigeration technology. Meanwhile there are still some key challenges in the practical implementation of magnetic refrigeration technology, such as the development and preparation technologies of high-performance MCE materials, powerful magnetic circuit system and flowing condition. Finally, possible applications are discussed and the tendency of future development is given.

###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

2017, 66 (11): 116401.
doi: 10.7498/aps.66.116401

Abstract +

Verification, validation and uncertainty quantification (V&V&UQ) is a method of assessing the credibility of physical model and quantifying the confidence level of numerical simulation result in complex engineering. Verification is used to answer the question whether the physical model is well solved or the program is implemented correctly, and it will give the ranges of error and uncertainty. Validation is used to answer the question whether the physical model reflects the real world or the confidence level of the physical model. This article deals with the detonation computational fluid dynamics model, and analyses the uncertainty factor in modeling, then presents the key factor which affects the accuracy of the simulation result. Due to the complexity of the explosive detonation phenomenon, there are a huge number of uncertainty factors in the detonation modeling. The sensitivity analyses of these uncertainty factors are utilized to distinguish the main factors which influence the output of the system. Then uncertainty quantification is conducted in these uncertain factors. After comparing the simulation result with the experiment data, the adaptation of the model is validated. This procedure is applied to the cylindrical test with TNT explosive. From the result, we can see that the parameters in the JWL EOS are calibrated and the accuracy of the model is validated. By the way, through conducting the uncertainty quantification of this system, we obtain that the expectation and standard deviation of detonation pressure for TNT are 1.6 and 2.2 GPa respectively. Detonation velocity and position of the cylindrical wall accord well with the experiment data. That means that the model is suited in this case. This technique is also extended to the detonation diffraction phenomenon. We can conclude that simulation result is greatly affected by the scale of the cell. From these examples, we can infer that this method also has a wide application scope.

2017, 66 (11): 116601.
doi: 10.7498/aps.66.116601

Abstract +

How impurity atoms move through a crystal is a fundamental and renewed issue in condensed matter physics and materials science. Diffusion of oxygen (O) in titanium (Ti) affects the formation of titanium-oxides and the design of Tibased alloys. Moreover, the kinetics of initial growth of titania-nanotubes via anodization of a titanium metal substrate also involves the diffusion of oxygen. Therefore, the understanding of the migration mechanism of oxygen atoms in -Ti is extremely important for controlling oxygen diffusion in Ti alloys.
In this work, we show how the diffusion coefficient can be predicted directly from first-principles studies without any empirical fitting parameters. By performing the first-principles calculations based on the density functional theory (DFT) through using the Vienna ab initio Simulation Package (VASP), we obtain three locally stable interstitial oxygen sites in the hexagonal closed-packed (hcp) lattice of titanium. These sites are octahedral center (OC) site, hexahedral center (HE) site, and TiTi bond center crowdion (CR) site with interstitial energies of -2.83, -1.61, and -1.48 eV, respectively. From the interstitial energies it follows that oxygen atom prefers to occupy the octahedral site. From electronic structure analysis, it is found that the TiO bonds possess some covalent characteristics and are strong and stable. Using the three stable O sites from our calculations, we propose seven migration pathways for oxygen diffusion in hcp Ti and quantitatively determine the transition state and diffusion barrier with the saddle point along the minimum energy diffusion path by the climbing image nudged elastic band (CI-NEB) method. The microscopic diffusion barriers (E) from the first-principles calculations are important for quantitatively describing the temperature dependent diffusion coefficients D from Arrhenius formula D = L2v* exp(-((E)/(kBT)), where v* is the jumping frequency and L is the atomic displacement of each jump. The jumping frequency v* is determined from
where vi and vj are the vibration frequency of oxygen atom at the initial state and the transition state respectively. This analysis leads to the formula for calculating the temperature dependent diffusion coefficient by using the microscopic parameters (vi and E) from first-principles calculations
without any fitting parameters.
Using the above formula and the vibration frequencies and diffusion barriers from first-principles calculations, we calculate the diffusion coefficients among different interstitial sites. It is found that the diffusion coefficient from the octahedral center site to the available site nearby is in good agreement with the experimental result, i.e., the diffusion rate D is 1.046510-6 m2s-1 with E of 0.5310 eV. The jump from the crowdion site to the octahedral interstitial site prevails over all the other jumps, as a result of its low energy barrier and thus leading to markedly higher diffusivity values. The diffusion of oxygen atoms is mainly controlled by the jump occurring between OC and CR sites, resulting in high diffusion anisotropy. This finding of oxygen diffusion behavior in Ti provides a useful insight into the kinetics at initial stage of oxidation in Ti which is very relevant to many technological applications of Ti-based materials.

2017, 66 (11): 116501.
doi: 10.7498/aps.66.116501

Abstract +

both the theoretical and the experimental aspects. Bulk polyethylene is regarded as a thermal insulator because its thermal conductivity is typically on the order of 0.35 W·m-1·K-1. However, recent studies demonstrate that a polyethylene chain has an extremely high thermal conductivity and the reported thermal conductivity of ultra-drawn polyethylene nanofibers is as high as 104 W·m-1·K-1, about 300 times higher than that of bulk polyethylene. In order to cast off this dilemma, several simulation methods are used to detect the unusually high thermal conductivity of a polyethylene chain. Molecular dynamics (MD) simulation results are highly sensitive to the choice of empirical potential or simulation method. Even using the same potential (AIREBO potential), the obtained thermal conductivity of a polyethylene chain is different. By combining the Green-Kubo method with a modal decomposition approach, equilibrium molecular dynamics (EMD) indicates that the thermal conductivity is able to exceed 100 W·m-1·K-1 while the polyethylene chain is longer than 40 nm at room temperature. Compared with the simulation result obtained by equilibrium molecular dynamics, the simulation result provided by using the non-equilibrium molecular dynamics (NEMD) method is only 57 W m·m-1·K-1 for a 160-nm-long polyethylene chain at room temperature. We use the first-principles method to calculate the force constant tensor, and the characteristics of quantum thermal transport in a polyethylene chain can be revealed. In our algorithm, several shortcomings of molecular dynamics, i.e., different potential functions or simulation methods may lead to obviously different results for the same quantum thermal transport system, are overcome. Based on the density functional theory (DFT), the central insertion scheme (CIS) combined with nonequilibrium Green's function (NEGF) is used to evaluate the isotope effect on quantum thermal transport in a polyethylene chain, which includes 432 atoms in scattering region and has a length of 18.533 nm. It is found that the upper limit of thermal conductivity of a 100-nm-long pure 12C polyethylene chain reaches a high value of 314.1 W·m-1·K-1 at room temperature. Moreover, for the case of a pure polyethylene chain of 12C, with other conditions unchanged, the reduction of average thermal conductance caused by 14C impurity is more remarkable than that by 13C. The most outstanding isotope effect on quantum thermal transport can be detected in the polyethylene chain. When the doping concentration of 14C in 12C is 50% at room temperature, the average thermal conductance will be reduced by 51%. It is of great significance for studying the mechanism of isotope effect on thermal transport in polyethylene.

###### GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

2017, 66 (11): 119401.
doi: 10.7498/aps.66.119401

Abstract +

The sporadic-E (Es) layer is a thin layer of several kilometers existing at an altitude around 100 km and features extremely dense ionized irregularities, which can reflect or scatter high frequency (HF) and very high frequency (VHF) radio waves. The most popular theoretical explanation for mid-latitude Es formation is the wind shear theory. Measurements by rocket souding have shown that Es has high electron density and relatively sharp density gradient in the vertical direction. The one-hop propagation of VHF signal in Es can even reach as far as 2000 km. In this paper, we consider incident radio waves influenced by Es via both reflecting and scattering processes at low and middle latitudes, the coefficients of which are related to and vary with the critical frequency of Es (foEs). Firstly, with a supposed parabolic density distribution and the autocorrelation function of the electron density given by Booker, HF and VHF radio wave propagations in Es are analyzed according to the reflection and scattering theory. Secondly, a numerical model for the combined reflecting and scattering processes is developed in the form of piecewise function, the contribution of which can be distinguished by the portion factor of reflection (kr). According to the model, there are two threshold ratios of the critical frequency to the wave frequencies fr and fs respectively. The incident radio waves are totally reflected by Es when foEs/f is higher than fr and mostly scattered when foEs/f is lower than fs. A transition zone exists between two critical points, with the combined processes working together. Thirdly, HF/VHF radio wave propagations in low and middle latitudes of Es are are in the north-southern direction and east-western direction separately. The experiment link in the north-southern direction is from Kunming to Xi'an at distance of 1065 km, and the ionosonde used for Es observation is located at Chongqing. Two east-west links are arranged, one of which is from Dehong to Huaihua and the other is from Dehong to Chenzhou, with the ionosonde located at Kunming and the ground distance as far as 1240 km and 1590 km respectively. The measurement data are treated and parameters of the above mentioned model for wave propagation in Es are experimentally determined. Finally, our model is verified by comparing with ITU-R model. Our results are consistent with the results from the ITU-R model when the foEs/f is high (i.e., the reflecting process plays a main role). When the scattering process dominates, the attenuation value of VHF signal is far less than that predicted by the ITU-R model, which is closer to actual measurements. It is concluded that our model is more preferable for HF and VHF radio wave propagations in Es at low and middle latitudes.

###### ATOMIC AND MOLECULAR PHYSICS

2017, 66 (11): 113101.
doi: 10.7498/aps.66.113101

Abstract +

The isotope shift parameters for the atomic transitions 1S0-1P1 and 1S0-3P1 of Mg are calculated by the relativistic multiconfiguration Dirac-Hartree-Fock (MCDHF) method, including the normal mass shift (NMS) coefficients, the specific mass shift (SMS) coefficients and the field shift (FS) factors. The detailed calculations of the isotope shifts for the three stable isotopes 24Mg, 25Mg and 26Mg are also carried out, in which the GRASP2K package is used together with another modified relativistic isotope shift computation code package RIS3. The two-parameter Fermi model is used here to describe the nuclear charge distribution in order to calculate the field shift by the first-order perturbation. A restricted double excitation mode is used in our calculations, one electron is excited from the two electrons in the 3s shell (3s2), another electron is excited from the eight electrons in the 2s or 2p shells (2s22p6), and the two electrons in the 1s shell (1s2) are not excited. The active configurations are expanded from the occupied orbitals to some active sets layer by layer, each correlation layer is numbered by the principal quantum numbers n (n= 3, 4, 5, …) and contains the corresponding orbitals s, p, d, …. The active configurations with the mixing coefficients in the added layer can be optimized by the MCDHF calculations. In this work, the atomic state functions are optimized simultaneously by the self-consistent field method and the relativistic configuration interaction approach in which the Breit interaction is taken into account perturbatively as well. The maximum principal quantum number n equals 10 and the largest orbital quantum number lmax is g. In our calculations, the NMS coefficients are -576.8 and -359.9 GHz·u, the SMS coefficients are 133.9 and -479.6 GHz·u, and the FS factors are -62.7 and -78.0 MHz·fm-2 for the 1S0-1P1 and 1S0-3P1 transitions of Mg, respectively. The difference between our isotope shift calculations and the previous experimental measurements is in a range from 6 MHz to 20 MHz with the relative error range from 0.6% to 1.3%, which shows that our results are in good agreement with experimental values. Our calculations are also coincident with other theoretical results. The isotope shift parameters provided here can be applied to the quick calculations of isotope shifts for the short-lived Mg isotopes, including 20-23Mg and 27-40Mg, and can be referred to for the corresponding isotope shift experiments. The methods used here canbe applied to calculating the isotope shifts and the atomic spectroscopic structures for other Mg-like ions with twelve extranuclear electrons.