## Vol. 64, No. 11 (2015)

##### 2015-06-05

###### GENERAL

2015, 64 (11): 110201.
doi: 10.7498/aps.64.110201

Abstract +

Reflection is a natural phenomenon that occurs when light passes the interface between materials with different refractive index. In many applications, such as solar cells, introduction of a substrate will result in an increase in reflection. There are many ways to reduce the reflection from a substrate, which have been investigated so far, including dielectric interference coatings, surface texturing, adiabatic index matching, and scattering from plasmonic nanoparticles etc. Here we present an entirely new concept to eliminate reflection from a silicon wafer, which makes use of much simpler method than the ones reported before, and can be applied to any high-index material. Finite-difference-time-domain (FDTD) method and auxiliary differential equations are used in this paper to simulate a new structure that can suppress the reflection of light from a silicon surface over a broad spectral range. A two-dimensional periodic array of subwavelength silicon nanocylinders is designed, which possesses a phenomenon strongly substrate-coupled to the Mie resonances, and which can produce an extraordinary transmission phenomenon similar to the metal surface plasmon that yields almost zero total reflectance over the entire spectral range from ultraviolet to near-infrared. This new antireflection concept relies on the strong forward scattering that occurs when a scattering structure is placed in close proximity to a high-index substrate with a high optical density of states. For a detailed description of the problem, we have carried out some simulations. From the results, one can see that although nano-pillar covers only 30% of the substrate surface area, it can reduce the reflection from the surface from 30% to under 10% at the Mie resonance. For the purpose of reducing reflection from the substrate, this new structure designed may provide a reference for the actual solar cells and optical antenna design.

2015, 64 (11): 110501.
doi: 10.7498/aps.64.110501

Abstract +

Response characteristics of FitzHugh-Nagumo neurons to low frequency signal have been investigated by numerical simulation. Neurons are arranged on a square-lattice and are subjected to two frequency signals. Results show that, vibrational resonance of the membrane potential can be induced by varying the amplitude of the high-frequency signal, when the control parameter is selected in the excitable region. In addition, the responses of neurons to higher harmonics of low-frequency signal have been studied, and nonlinear vibrational resonances are also found. With the increase of frequency in the low-frequency signal, the response of the system to low-frequency signal can resonate. Thus, the double resonance can occur by changing the frequency in low-frequency signal and the amplitude in high-frequency signal. Moreover, effects of electrical synapses and chemical synapses on vibrational resonance and nonlinear vibrational resonance of the neurons have also been studied. Effect of the number of neurons, which are subjected to two frequency signals in the square-lattice, on the response characteristic of the system is also studied. It is found that the response characteristic of the electrical coupling neurons is quite different from that of chemical coupling neurons.

2015, 64 (11): 110502.
doi: 10.7498/aps.64.110502

Abstract +

Because the detection of small target in the background of sea clutter is strongly influenced by the sea condition, this paper studies the fractal property of sea clutter in fractional Fourier transform (FRFT) domain and proposes the fractal detection method in single and high dimensions. The FRFT deduced from mathematical definition is not consistent with the self-similar properties in orders and scales. Multifractal detrended fluctuation analysis (MF-DFA) method is used to determine the fractal parameter H(q) and analyze the fractal property of the sea clutter in different situations, distances, and polarizations. In single dimension, the small target detection method is proposed based on an adaptive order. By comparing different factors of the multifractal parameters, the results show that the transform order method in sea clutter FRFT domain can detect small signals under complicated sea conditions. The detection threshold mostly increases above 200%, which is 26.3% higher than the method of time domain signal. H(q) has an obvious multifractal difference on high negative scale, the H(q)-q curve satisfies the arctangent distribution. The fitting amplitude ratios of pure sea clutter and target data are greater than 1.8 (HH) and 1.4 (VV), which provide the basis for the small target detection in sea clutter background.

2015, 64 (11): 110503.
doi: 10.7498/aps.64.110503

Abstract +

Information of internet public opinion is influenced by many netizens and net medias; characteristics of this information are non regular, stochastic, and may be expressed by a nonlinear complex evolution system. Corresponding model is difficult to establish and effectively predicted using the traditional methods based on statistical and machine learning. Characteristics of internet public opinion are chaotic, so the chaos theory can be introduced to research first, then the information of internet public opinion having chaotic characteristic is proved by the Lyapunov index. The model to predict the development trend of internet public opinion is next established by the phase space reconstruction theory. Finally, the hybrid algorithm EMPSO-RBF which is based on EM algorithm and the RBF neural network optimized by the improved PSO algorithm is proposed to solve the model. The hybrid algorithm fully takes the advantage of the EM clustering algorithm and the improved PSO, so the RBF neural network is improved by initializing the network structure in the early stage and optimizing the network parameters later. First, the EM clustering algorithm is used to obtain the center value and variance, and the radial basis function is improved with the combination of traditional Gauss model. Then the relevant network parameters are obtained by the improved PSO algorithm which is based on error optimizing the network parameters constantly. The model algorithm can be accurately simulated in the time series of chaotic information by experiments which are validated by different chaotic time series information; and it can better describe the development trend of different information of internet public opinion. The predicted results are made for government to monitor and guide the information of internet public opinion and benefit the social harmony and stability.

2015, 64 (11): 110504.
doi: 10.7498/aps.64.110504

Abstract +

Synthesis of desired radiation patterns without an optimization algorithm is usually time consuming and inefficient. To achieve a desired radiation pattern such as cosecant squared beam and contoured beam, different evolutionary algorithms such as genetic algorithm (GA), particle swarm optimization algorithm, and invasive weed optimization algorithm have been used to find the excitation of radiation elements. Adaptive genetic algorithm (AGA) optimizer is a robust, stochastic search method, modeled on the principles and concepts of natural selection and evolution. As an optimizer, the powerful heuristic of the AGA is effective for solving complex and related problems. An improved AGA is proposed, in allusion to the characteristics of optimizing designs of antenna arrays which have many parameters and complicated structures. This algorithm constructs an adjustble formula to produce the crossover rate and mutation rate based on a logistic curve equation. In the way of combining roulette wheel selection and elitist strategy, this algorithm searches for the optimal solution in the global space, and is compared with the classical GA; the improved AGA has a better performance in seeking the solution. Taking the mutual coupling between the elements into account, we design the X band extended cosecant squared beam micro-strip antenna arrays based on the improved AGA. Specifications of the antenna are as follows:a -3 dB beam width in height is from 0° to 12°, a -10 dB beam width in height is from 12° to 65°, and a total height coverage is 65°; a frequency band ranges from 8.5 to 9.8 GHz and its center frequency is 9.05 GHz. Simulation results show that the fitness increases from 0.07 to 0.09, and the quality of the synthesized radiation pattern has a great improvement, which verifies the superiority of the improved AGA proposed in this paper. In addition, the prospect of the designed antenna which has an extended cosecant squared beam is promising in air-surveillance radar systems, where the radiation pattern of the antenna will compensate for the free-space loss.

2015, 64 (11): 110701.
doi: 10.7498/aps.64.110701

Abstract +

By combining the iterative phase retrieval algorithm in the Fresnel domain with the shift rotation permutation operations of row vectors and column vectors, a new kind of asymmetric multiple-image authentication based on complex amplitude information multiplexing and RSA algorithm is proposed, where multiple complex amplitude information in the input plane is retrieved and generated by the phase retrieval algorithm in the Fresnel domain. In original binary amplitude mask, the row vector and column vectors random numbers are randomly generated in advance, such that each sampling mask for each authenticator is obtained by the shift rotation permutation operations of corresponding row vector and column vectors random numbers for original binary amplitude mask. Thus, one synthesized complex amplitude is generated by the operations of sampling, overlap and multiplexing, and then sent to the certification center for authentication use. At the same time, the row vector and column vectors random numbers are encoded to ciphers by the public keys of RSA algorithm, and then delivered to the corresponding authenticators. During the authentication process, the row vector and column vectors random numbers are first decoded by the private keys possessed by the authenticator; second, the authenticator’s sampling mask is reconstructed by the shift rotation permutation operations of the above decoded random numbers for original binary amplitude mask. Finally, the authenticator with other additional authentication keys is prompted to place the synthesized complex amplitude information and its sampling mask at the corresponding positions, when the system is illuminated by a plane wave with the correct wavelength. A recovered image is then recorded in the output plane, by calculating and displaying the nonlinear correlation coefficient between the recovered image and the certification image, if there exists a remarkable peak in its nonlinear correlation coefficient distributions, indicating that the authentication is successful. On the contrary, if there is no remarkable peak but uniformly distributed white noise in the map, the authentication process is a failure attempt. Any intruder with randomly generated forged authentication keys will end up with a failure which enhances the security of the system to some extent.

2015, 64 (11): 110202.
doi: 10.7498/aps.64.110202

Abstract +

A fuel air cloud is formed under the driving force of the explosive detonation and then it’s ignited to explosion to attack the target. The existing numerical simulations are mainly limited to the fuel dispersal processes which are all based on mesh methods. The fuel particles in the air cloud are difficult to traced. Otherwise, the computing process is complex and could not be solved by the exiting methods for the chemical reaction and the forming and propagation of shock waves are both involved in the fuel combustion and explosion. Smoothed discrete particle hydrodynamics (SDPH), as a new method to solve the gas-particle two-phase flow, has been successfully used to simulate the aeolian sand transport, heat transfer and evaporation. Based on the previous work, the Jones-Wilkins-Lee (JWL) function is imported to describe the explosive detonation to expansion and it is solved by finite volume method. The fuel drops dispersed by explosion are traced by the improved smoothed particle hydrodynamics. The drop evaporation model and the EBU-Arrhenius combustion model for gas high-speed combustion are introduced to describe the combustion and detonation of fuel drops. Then we build a new SDPH method to simulate the warhead initiation, fuel dispersal, and the fuel second explosion. Firstly, we design a test that is the dispersal of circular fuel drops drove by explosive detonation to validate our new method. The changing of the explosive detonation pressure and the velocity fields of explosive and particles are analyzed and they are consistent with the theory. And then, the forming and developing of FAE cloud are simulated. Through comparing with the experiments, the shapes of the cloud by the two methods coincide with each other. The effects of different initiations on the cloud forming are also analyzed. Finally, based on the cloud group forming, the evaporation and combustion models are introduced to study the combustion and explosion of FAE. We obtain the velocity field and the distribution of combustion product. The result indicates that the fuel dispersal into cloud and its explosion can be simulated better with the mathematical model and computational method built in this paper. This finding supplies a more effective numerical method for the design and research on this type of weapon equipments.

2015, 64 (11): 110301.
doi: 10.7498/aps.64.110301

Abstract +

A measurement-device-independent quantum key distribution (MDI-QKD) protocol is immune to all detection side-channel attacks and guarantees the information-theoretical security even with uncharacterized single photon detectors. A weak coherent source is used in the current MDI-QKD experiments, it inevitably contains a certain percentage of vacuum and multi-photon pulses. The security issues introduced by these source imperfections can be avoided by applying the decoy state method. Here, through modeling experimental devices, and taking into account the weak coherent source and the threshold detectors, we have evaluated the gain, the probability to get successful Bell measurement and incorrect Bell measurement, and the quantum bit error rate (QBER), given a practical setup. In our simulation, we show how QBER varies with different transmission distances in the cases when the average photon numbers per pulse from Alice and Bob are symmetric and asymmetric. Result shows that the multi-photon pulses do not cause error in the Z basis of polarization encoding scheme, but produce a large QBER in phase encoding scheme and in the X basis of polarization encoding scheme. QBER is affected by the dark count rate and the system optical error associated with the multi-photon pulses. For different encoding schemes, QBER caused by each kind of average photon numbers from Alice and Bob increases to different degrees with the transmission distance, and finally is close to 50%. With the increase of the transmission distance, the average photon number per pulse decreases and the fraction of the dark count rate causing QBER gradually increases. Under the same effect of the dark count rate, the smaller the average photon number per pulse, the bigger the QBER. After a certain transmission and at the same transmission distance, the QBER is largest when average photon numbers used by Alice and Bob are both smallest. For the short distance transmission of phase encoding scheme and the X basis, we find that QBER is larger when average photon numbers from the two arms are asymmetric, as compared to the symmetric case. For the Z basis, the QBER caused by the system optical error and the dark count rate is very small.

2015, 64 (11): 110702.
doi: 10.7498/aps.64.110702

Abstract +

The determination of the optical constants of absorbing films, particularly on opaque substrates, is a difficult problem when solely using spectroscopic ellipsometry. First, unwanted backside reflections are incoherent with the desired reflection from the front side, which makes the fitting of optical constants difficult. Second, the optical constants of substrate must be carefully characterized in advance, as any small absorption in the substrate would be mixed into the film’s overall optical constants. Third, thickness and optical constants are strongly correlated with each other, which may prevent a unique solution for absorbing films. For the above reasons, quartz, glass slide, cover glass and float glass substrates are studied. Backside reflections of the substrates are suppressed by index matching technique. The results show that the simple technique works well for substrate materials with refractive index in a range from 1.43 to 1.64, including materials such as fused silica, float glass, etc. in a spectral range from 190 nm to 1700 nm. The refractive index and extinction coefficient of the substrate are fitted by ellipsometricψdata and the normal spectral transmittance T0. The results are consistent with the literature reported. Finally, a Combined ellipsometry and transmission approach is used to determine the thickness values and optical constants of the diamond-like carbon (DLC) film coated on the quartz and the amorphous silicon (a-Si) film coated on the glass slide and cover glass accurately.

###### ATOMIC AND MOLECULAR PHYSICS

2015, 64 (11): 113201.
doi: 10.7498/aps.64.113201

Abstract +

In the present paper, we study the average fluorescence lifetimes, detected by using the time-correlated single-photon-counting (TCSPC) technique, of three thioglycolic acid-capped CdTe quantum dots (TGA-CdTe QDs), which are ～6 ns, ～9 ns and ～11 ns; and the fluorescence kinetic process includes two parts:the slow process and the fast process. With the increase of the particle size, the slow process becomes longer, but the fast process becomes shorter. Afterwards, by using both femtosecond transient absorption and fluorescence up-conversion time-resolved spectrum techniques, we have investigated the interband relaxation process of three TGA-CdTe QD samples, with the nanoparticle diameters of 2.3, 2.8 and 3.5 nm. Investigation indicates that for the three QD samples, exciton filling rate becomes slower in the highest excited state and the lowest excited state, among them, the time of exciton filling increases from 0.33 to 0.79 ps for the highest excited state, while the time of exciton filling increases from 0.53 ps to 1 ps for the lowest excited state. Moreover, the two kinds of experiment provide complementary information and obtain the full image of interband relaxation process. Result shows that the bleach recovery of the 1 S transition shows an initial rise, but the fluorescence up-conversion signal for the 1 S transition is slower in rise time, which can provide help in the application of optoelectronic devices.

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

2015, 64 (11): 114101.
doi: 10.7498/aps.64.114101

Abstract +

High impedance surface, due to its unique property of in-phase reflection at some frequency, could be used in designing multiband Salisbury screen by replacing the metallic ground plane in a traditional structure, which is proposed, in this paper, to enhance the microwave absorbing performance of the conventional Salisbury screen. First, electromagnetic wave field intensity of different frequency in space after being reflected by a high impedance surface is analyzed, which implies that new absorption bands can be introduced at about the frequencies of in-phase reflection by sharing Salisbury screen’s resistive sheet, without adding extra lossy materials such as lumped elements or others. Then, by taking a single band high impedance surface at 6.25 GHz and a dual-band high impedance surface at 6.27 and 8.17 GHz, which are both composed of patches array with varying periodic size and a thickness of 0.6 mm, the multiband Salisbury screens can be constructed utilizing a conventional one with an absorbing peak at about 10.5 GHz. The reflectivity of these multiband absorbers are simulated by employing the commercial CST microwave studio and later measured using a reflectivity measurement system comprising two polarized horns and a vector network analyzer. Experimental results agree well with the simulations, and all results verify that the method presented at the beginning is effective. Results also show that new additional absorptions appear at the frequencies where microwaves are nearly reflected in phase from the high impedance surface, with the same number of the in-phase reflection bands. Meanwhile, the original microwave absorbing capability of the traditional Salisbury screen is reserved mostly. Compared to the single band high impedance surface, the dual-band high impedance surface performs better in the design as the absorbing bandwidth is wider and the absorbing frequency is lower. With an additional thickness of the high impedance surface (no more than 1 mm), the total absorption bandwidth of the multiband Salisbury screen with a reflection below -10 dB increases from 8.5 to 10.1 GHz, and the lowest frequency with 10 dB absorption falls from 7.5 to 5.98 GHz. So it could be concluded that the design of multiband Salisbury screen is helpful to widen the absorption, especially towards the lower frequency direction.

## EDITOR'S SUGGESTION

2015, 64 (11): 114102.
doi: 10.7498/aps.64.114102

Abstract +

High aspect ratio gratings can be made by perpendicularly cutting in the growth direction of multilayers. X-ray exposure technique using a sectioned multilayer grating based on Talbot effect is a new type of nano patterning method. Although 300 nanometer gratings through the experiment are completed, some phenomena in the experiments cannot be satisfactorily explained and the factors influencing the nano pattern quality have not been fully understood yet. Here we use a rigorous coupled-wave theory to discuss several important factors, including grating thickness, the fraction of material thickness and multilayer period, which is the first time as far as we know for Talbot self-imaging in X-ray range. Simulation results show that the grating thickness affects both X-ray transmission efficiency and fringe contrast, while the fraction of material thickness determines the quality of fringes. And the position deviation of the best image plane in near field is related to both the thickness of the grating and the multilayer period. Moreover, the multilayer gratings with smaller periods can achieve higher resolution, indicating that the Talbot effect can be used to fabricate a more detailed structure.

2015, 64 (11): 114201.
doi: 10.7498/aps.64.114201

Abstract +

The first-order quantity of the optical structure can be obtained using the ybar-y diagram and this diagram has a control point quantity. Based on the concept and quantity of the ybar-y diagram, this paper establishes a mathematical model to calculate an optical system’s first-order structure. The aberration is induced by the deflection of the transmission rays. If a first-order structure has a minimal total deflection angle of the first paraxial ray and the second paraxial ray, the higher order aberration will have a small value and the calculated first-order optical structure will be the best. According to the mathematical model, the issue of optical structure calculation is converted to a numerical optimization problem. And the objective function, i.e. the sum of the first and second paraxial ray’s deflection angles, is constructed. After comparison among etween many kinds of numerical optimization algorithms, the particle swarm optimization algorithm is used to solve the problem. Then a calculation program containing graphical user interface (GUI) is developed to calculate the first-order structure quickly and efficiently. The basic design parameters of the optical system are imported into the GUI after some treatments, then the resulting first-order structure is obtained after some clicks of the mouse. The resulting structure is thereafter converted to a practical lens system by the use of commercially available optical design software such as ZEMAX. After a lens optimization process, an actual optical system is accomplished. According to the method proposed in this paper and by the use of the calculation program, a 13 megapixel mobile phone camera lens is designed first. The F# of this lens system is 2.3 and the full field of viewing angle is 70 degree. The system has a total length of 4.5 mm and a distortion of 1.2%. Only four aspheric lenses are used and the other optical performance meets the design requirements as well. In addition, an eyepiece of helmet-mounted display system is designed, in which only two lenses are used and a visual field angle of 90 degree is achieved. The entrance pupil is 5 mm width and the image diagonal length is 65 mm. This system has a total length less than 45 mm and eye relief greater than 12 mm. Other performance of the eyepiece can also meet the requirements. These designs of the two optical systems demonstrate that the proposed method is reliable in calculating the optical structure of the optical system.

2015, 64 (11): 114202.
doi: 10.7498/aps.64.114202

Abstract +

Optical Tamm state (OTS) refers to a kind of interface state between the metal layer and the photonic crystal (PC) reflectors. Given the matching conditions being satisfied, the electromagnetic waves tend to tunnel through the metal-PC hetero-structure efficiently. Quite different from the conventional surface plasmon polaritons (SPPs) on metal surface, OTSs can be excited directly by normally incident propagating waves for both TE and TM polarizations to occur. In the meantime, strong electromagnetic (EM) localization around the interface can be achieved, leading to potential applications such as polariton lasers, enhancement of Faraday rotation, various nonlinear effects, and so on.#br#To further enhance the EM localization around the interface, some well designed artificial structures are patterned on the thin metal layer. For instance, confined Tamm plasmon modes with the aid of metallic microdisks are proposed by Gazzano et al. to control the spontaneous optical emission. Moreover, in 2013 it was also demonstrated that planar plasmonic metamaterials (PPM) with electromagnetically-induced-reflection-like (EIR-like) dispersion can boost the Q-factor of OTS tunneling mode, as well as the EM localization around the interface between planar plasmonic metamaterials and PC. Both these methods can be understood in the same scheme:the structure-induced dispersion provides exotic power of modulating the propagation of OTS.#br#In this paper, the enhancement of optical Tamm state and related lasing effect is investigated by introducing planar plasmonic metamaterials with EIR-like dispersion. The planar plasmonic metamaterials are achieved by periodic patterning some plasmonic units on the planar metal layer. Through fine tuning each unit cell, EIR-like dispersion can be achieved, making the properties of hetero-structure more tunable. One-dimensional photonic crystals composed of TiO2/SiO2 are also designed properly to support the optical Tamm state in PPM-PC hetero-structure. First, to analyze the possibility of enhancing local electromagnetic field density of optical Tamm state, a transfer matrix method is performed when EIR-like dispersion of PPM structure is hired. Next, full wave simulations based on FDTD method are also carried out to verify a hetero-structure composed of PPM and one-dimensional photonic crystal embbed with gain media. By introducing gain medium into (or near) the PPM structure, where the maximum local electromagnetic field density exists, the lasing effect is found obviously enhanced. Better emitting efficiency and monochromic response can be observed compared to the common metal-PC hetero-structure. These features make our structure promising to reduce the lasing threshold, enhance the fluorescence, and so on.

2015, 64 (11): 114203.
doi: 10.7498/aps.64.114203

Abstract +

For the polarization switching (PS) and the nonlinear dynamic behaviors (NDBs) of the optically injected laser system composed of master vertical-cavity surface-emitting laser (M-VCSEL) and slave vertical-cavity surface-emitting laser (S-VCSEL), we put forward a novel manipulation scheme and explore their control law by means of electro-optic (EO) modulation with quasi-phase matched technology in periodically poled LiNbO3. It is found that the PS of the S-VCSEL subjected to parallel or orthogonal optical injection undergoes a change of periodic oscillation with the applied transverse electric field. The envelope trajectory of the oscillation peak appears to be a cosine curve, and that of the oscillation wave trough becomes a sine curve. Besides, the PS of the S-VCSEL only depends on the applied transverse electric field and the bias current of the M-VCSEL, and is independent of the bias current of the S-VCSEL. When the bias current of the M-VCSEL takes a different value, the PS of the S-VCSEL shows a different evolution law in one period of the applied electric field. For a certain fixed bias current of the M-VCSEL, the optically injected S-VCSEL can emit an arbitrary polarization mode and its NDBs experience different evolutions when the light from the M-VCSEL goes through EO intensity modulation. If the output light of the M-VCSEL is subjected to EO intensity modulation and EO phase modulation simultaneously, while the bias current of the S-VCSEL is fixed at 1.06, that of the M-VCSEL is fixed at 1.18, and the optical injection strength is set at 5 ns-1, then the output polarization of the S-VCSEL is in turn switched from the y-LP to the left-handed elliptic polarization (EP), then the right-handed EP circular polarization, and lastly the left-handed EP. And its NDB shows in turn a single period, four doubled periods, chaos, four doubled periods, and chaos with the increase of the applied electric field.

2015, 64 (11): 114204.
doi: 10.7498/aps.64.114204

Abstract +

We propose a vector beam generation method based on the spatial light modulator composed of twisted nematic liquid crystal. According to the relation between the rotation angle and the applied voltage on the spatial light modulator, a common optical system for generating a variety of vector beams is designed in experiments. By using this common optical system, a variety of vector beams in axisymmetric phases as well as the complex vector beams are generated, and their polarization characteristics are observed and measured experimentally, where a tight focusing field is obtained, and this may be applied in optical tweezers and lithography. In addition, the device structure is simple and easy to operate, its efficiency of producing vector beam is very high and the laser spectral characteristics are not changed in the proposed generation of vector beams. Therefore, the method we proposed can find important potential applications in strong laser of vector beams interacting with matter, and laser acceleration, etc.

2015, 64 (11): 114205.
doi: 10.7498/aps.64.114205

Abstract +

This paper studies the equivalent refractive index method and other methods to measure the liquid diffusion coefficient. Based on this, a quick method to measure the liquid diffusion coefficient is proposed, i.e. using a specially designed asymmetric liquid-core cylindrical lens as both diffusive pool and imaging element. By means of this system with the liquid-core cylindrical lens to measure the diffusion coefficient, we can eliminate the spherical aberration and improve the accuracy in refractive index measurement. Based on the spatially resolving ability of the cylindrical lens in measuring the refractive index, only one instantaneous diffusive picture is required. Depending on the correspondence between the image width and the refractive index, we thus can quickly calculate the diffusion coefficient D by the Ficks second law. Then the diffusive process of ethylene glycol in water at 25℃ is investigated by this method. We calculate the diffusion coefficient between 660-3000 s with the method to analyse an instantaneous diffusion picture. At the beginning, injection will cause the liquid turbulent, and thus create a larger diffusion coefficient. In the course of diffusion, the influence of turbulence on the diffusion coefficient gradually decreases, but the image narrowing can make inaccurate results. Therefore, this method is required to be used at an appropriate time and an appropriate position to reduce experimental errors. After repeated experiments we can conclude that, between 1500-2700 s we may select the appropriate measurement of location for measuring liquid diffusion coefficient by the method to analyze an instantaneous diffusive picture. This not only can avoid the effect of turbulence but also avoid the effect of fewer sampling points. Compared with other methods reported in the literature, the results show that this method is characterized by short time (~20 ms) in data acquisition, faster measurement ( 1 s), high-accuracy (relative error 3%), and easy operation, thus providing a new method for measuring the diffusion coefficient of liquids rapidly.

2015, 64 (11): 114214.
doi: 10.7498/aps.64.114214

Abstract +

Monte Carlo method is used to calculate the energy deposition of proton-irradiated scientific CCD (charge coupled device) structure, and the radiation damage mechanism of the device is analyzed by combining the proton irradiation with the annealing experiments. The ionizing dose in gate oxide layer and the displacement damage dose in silicon deposition are simulated. During irradiation and annealing experiments two main parameters, dark signal and charge transfer efficiency, are investigated. Results show that variations of dark signal and charge transfer efficiency are the same as those with ionizing dose and displacement damage dose. During irradiation, dark signal rises obviously as the fluence of 10 MeV proton increases. Defects and their annealing temperature:the divacancy levels show little annealing effect below 300℃, while the oxygen-vacancy complex is stable up to 350℃, and the phosphorous-vacancy has a characteristic annealing temperature of 150℃. Interface states are annealed totally at 175℃. So the annealing only affects oxide-trapped-charges. Dark signal is greatly reduced after annealing, this phenomenon means that the dark signal is mainly affected by ionization. The surface dark signal proportion of the total dark signal can be calculated by the reduction of dark signal during annealing and this is at least 80% or more. As the fluence of 10 MeV proton increases, the charge transfer efficiency reduces obviously. After annealing, the recovery of charge transfer efficiency changes very little, so the charge transfer efficiency is unaffected by oxide-trapped-charges, since it is reduced due mainly to bulk defects. The final device damage will always be proportional to the amount of initial damage and also to the electrical effect on the device. Hence NIEL scaling implies a universal relation:device damage=kdamage×displacement damage dose, where kdamage is a damage constant depending on the device and the parameter affected, and the displacement damage dose (DD) is the product of the NIEL and the particle fluence. MULASSIS is used to calculate the displacement damage dose in depletion area of P-area and deduce kdamage by combining with the experimental value of charge transfer efficiency; kdamage is calculated to be about 3.50×10-14. The formula for degradation degree of charge transfer efficiency is CTEafter irradiated = 1-Dd×kdamage, this formula is used to estimated CTE and the result is compared with the value from experiment. It is shown that the simulated data is in agreement with the experimental data.

2015, 64 (11): 114301.
doi: 10.7498/aps.64.114301

Abstract +

Ultrasound contrast agent (UCA) microbubbles have been commonly used in clinic to enhance the acoustic backscattering signals in ultrasound imaging diagnosis. With increasing demand for the continuous improvement of imaging resolution and sensitivity, new type UCAs (e.g., targeted microbubbles and multifunctional microbubbles) have attracted growing interest in both medical and scientific communities. Many efforts have been made to modify microbubble shell properties, which can strongly affect microbubble dynamic behaviors, so as to enable to create some new functionalities of UCAs. However, accurate characterization of the shell mechanical properties of UCAs has been recognized to be rather challenging. In previous work, microbubble’s mechanical properties are normally estimated by fitting measured dynamic response signals with coated-microbubble models. Inevitable uncertainty will be introduced in fitting results because there are more than one unknown shell parameters are adopted in these dynamic models. In the present paper, a comprehensive approach is developed to quantitatively characterize the visco-elasticity of the encapsulated microbubbles. By combining the techniques of atomic force microscopy (AFM), single particle optical sensing (SPOS), acoustic attenuation measurement, and the coated-bubble dynamics simulation, the size distribution, shell thickness, shell elasticity and viscosity of UCA microbubbles are determined one by one in sequence. To examine the validity of this approach, a kind of albumin-shelled microbubbles with diameters ranging from 1 to 5 μm are fabricated in our lab. Based on AFM technology, the microbubble effective shell stiffness and bulk elasticity modulus are measured to be 0.149±0.012 N/m and 8.31±0.667 MPa, respectively. It is noteworthy that the shell elastic property is shown to be independent of the initial size of microbubbles. Furthermore, the size distribution and acoustic attenuation measurements are also performed of these bubbles. Then, combined with microbubble dynamic model simulations, the UCA shell viscosity is calculated to be 0.374±0.003 Pa·s. Compared with previous estimation method, the current technology can be used as an effective tool to assess UCA shell visco-elasticity with improved accuracy and certainty. It is also shown that the feasibility to optimize the design and fabrication of UCAs can satisfy different requirements in ultrasound diagnostic and therapeutic applications.

2015, 64 (11): 114302.
doi: 10.7498/aps.64.114302

Abstract +

The existing blind beamforming methods are effective only under the condition that the source signals have some special statistical or structural characteristics. Additionally, the structure of cascade model is complicated and the stability of parallel model is poor when dealing with multi-target signals. To address these problems, a novel blind beamforming algorithm for multi-target signals based on time-frequency (TF) analysis is proposed in this paper. The received array signals are first transformed into time-frequency domain by using quadratic time-frequency distributions (TFDs). Then, the single-source auto-term TF points which show energy concentration at a single signal are extracted through three operations:(i) removing noise points by setting a reasonable threshold, (ii) separating auto-term TF points from cross-term points, and (iii) selecting the single-source auto-term TF points from the auto-term ones. Moreover, these single-source auto-term TF points are classified by the principal eigenvector of their spatial time-frequency distribution matrixes. For each class of TF points, the uncertain set of signal steering vector is given, whose radius is defined as the ultimate range between the center and the elements in the class. Within the uncertain set, an optimization algorithm is provided to get the optimal estimation of the signal steering vector. Finally, the blind beamforming for multi-target signals is achieved based on the Capon method, which can enhance the desired signals and suppress the noise and interference signals. In addition, the influence of parameters selection, the clustering method of unknown source number, and the computational complexity of the proposed algorithm are analyzed. The proposed algorithm can achieve parallel output of multi-target signals under the condition that the array manifold and the direction of arrival (DOA) are unknown. Also, the complex iterative solving processing may be avoided and special limitations on signal characteristics are unnecessary. As a result, it has wide applicability and superior stability compared with the existing blind beamforming methods. Simulations illustrate that the proposed algorithm can well separate multi-target signals which are TF-nondisjoint to a certain extent. It can achieve a higher output signal to interference plus noise ratio (SINR) compared with the constant modulus algorithm (CMA), the independent component analysis (ICA) algorithm, and the joint approximate diagolization of eigenmald (JADE) algorithm. Furthermore, the output performance of the proposed algorithm is close to the optimal Capon beamformer.

2015, 64 (11): 114401.
doi: 10.7498/aps.64.114401

Abstract +

In this paper, a mathematical relationship between particle melting rate and its surface heat flux is established to solve the problem of melting of elliptical particle sedimentation based on the direct numerical simulations of particle sedimentation when taking account of thermal convection within the framework of the arbitrary Lagrangian-Eulerian technique. The elliptical particle with different initial angles is released in a mesoscale channel under gravity. Compared with the isothermal elliptical particle sedimentation, the melting elliptical particle shows large differences in moving trajectories, the forces exerting on the particle and velocities, which come from the consideration of fluid convection, mass loss, and shape change. More specifically, 1) in the case of isothermal elliptical particle sedimentation, the velocity, the horizontal trajectory, and the force vary periodically. However, the amplitude recedes gradually, and finally becomes zero in the case of the melting elliptical particle, which is caused by the mass lost and shape change. 2) The equilibrium position of the major axis will finally be perpendicular to the direction of sedimentation. So, the initial angle of slope (θ) usually affects the sedimentation in the beginning, and vanishes after a period of time. 3) The downward convection induced by the cold fluid accelerates the velocity of the melting particle. The angular velocity, force and horizontal amplitude of the melting particle become smaller than those of the isothermal particle, and finally recedes to zero. In our study, the investigation of coupled heat transfer, fluid-solid system and shape change is carried out, and some new features are found out.

2015, 64 (11): 114501.
doi: 10.7498/aps.64.114501

Abstract +

To investigate the motion characteristics and the law of identical property for particles obtained under segregation to uniform distribution conditions in forced agitation mixing, the mixing process of the same sized ellipsoidal particles at different rotating speeds in a U-tank is simulated using three-dimensional discrete element method. Macroscopic mixing law and partial mixing characteristics in particle mixing process are analysed in the view of single particle random motion trajectory and motion vector diagram of macroscopic particle flow. And the mathematical relation between mixability and revolutions of agitating blades is described quantitatively. Results show that convective mixing and four partial mixing characteristics control the mixing homogeneity process of identical property of segregation particles in forced agitation mixing. Mixability of segregation particles is independent of rotating speed of the agitating shaft, but has a direct correlation with revolutions. The relation between mixability and revolutions agrees with the exponential growth model. Research results can provide the basis and reference for equipment improvement and operating control of bulk material in the industry of the augmenting of mix.

2015, 64 (11): 114502.
doi: 10.7498/aps.64.114502

Abstract +

There often occurs traffic accident or road construction in real traffic, which leads to partial road closure. In this paper, we set up a traffic model for the partial road closure. According to the Nagel-Schreckenberg (NS) cellular automata update rules, the road can be separated into cells with the same length of 7.5 m. L = 4000 (corresponding to 30 km) is set to the road length in the simulations. For a larger system size, our simulations show that the results are the same with those presented in the following. In our model, vmax rtial road is closed (for convenience, we define the road length as L1), vmax 2= 2 (corresponding to 54 km/h) in the section of normal road (we define the road length as L2). In our simulations, let L1= L2 = 2000. We would like to mention that changing these parameter values does not have a qualitative influence on the simulation results. The simulation results demonstrate that three stationary phases exist, that is, low density (LD), high density (HD) and shock wave (SW). Two critical average densities are found:the critical point ρcr 1= 3/8 separates the LD phase from the SW phase, and ρcr 2= 1/2 separates the SW phase from the HD phase. We also analyze the relationship between the average flux J and average density ρ. In the LD phase J = 4/3ρ, in the HD phase J= 1 -ρ and J is 0.5 in the SW phase. We investigate the dependence of J on ρ. It is shown that with the increase of ρ, J first increases, at this stage J corresponds to the LD phase. Then J remains to be a constant 0.5 when the critical average density ρcr 1 is reached, and J corresponds to the SW phase (this time,J reaches the maximum value 0.5). One goal of traffic-management strategies is to maximize the flow. We find that the optimal choice of the average density is 3/8 ρρcr 2 is reached, J decreases with the increase of average density, which corresponds to the HD phase. We also obtain the relationship between the shock wave position and the average density by theoretical calculations, i.e. Si = i+4-8ρ, which is in agreement with simulations.

2015, 64 (11): 114701.
doi: 10.7498/aps.64.114701

Abstract +

Research on the droplet impact on a hydrophobic surface is of important theoretical significance and engineering value, both in mesoscopic fluid mechanics and interactions between microfluid and special materials. The van der Waals (vdW) equation of state relates the pressure to the temperature and the density of the fluid, and gives the long-range attractive force and short-range repulsive force between particles. The van der Waals equation of state can be used to describe the surface tension between liquid and vapor. As a pure meshless particle method, the smoothed particle hydrodynamic (SPH) method can use the vdW equation of state written in SPH form of N-S equations to describe the surface tension. The vdW surface tension mode is validated by simulating the coalescence of two equally sized static droplets in vacuum. Repellant of the hydrophobic surface is derived from a core potential. By combining the vdW surface tension and the repulsive force of the surface, the phenomenon of a liquid droplet impact with a certain initial velocity on the hydrophobic surface is studied. The SPH model is not only capable to describe the spreading of the droplet after it contacts the surface, but also clearly reproduces the springback, bouncing and secondary impact of the droplet. During the deformation of the droplet, the inertia force impels the spreading process of the droplet whilst the springback and bouncing behavior is dominated by the surface tension. The simulated results are in good agreement with the published experimental observations and VOF simulated results, indicating that the way we treat the surface tension and the repulsive force of the hydrophobic surface is effective and applicable in droplet impact surface problems. The impact velocity and liquid viscosity are considered to be two important factors that affect the deformation of the droplet after it contacts the surface. By varying the impact velocity within a certain range it is concluded that the maximum liquid-solid contact area increases as the impact velocity grows, and the bounced droplet will leave the surface when the velocity is big enough. Another comparison between different liquid viscosities shows that the maximum contact area decreases as the liquid viscosity increases because of the viscous dissipation, and the droplet barely rebound when the liquid viscosity is big enough.

2015, 64 (11): 114206.
doi: 10.7498/aps.64.114206

Abstract +

Based on terahertz time domain spectroscopy, a false-color imaging system is demonstrated by experiments. Three frequency ranges are defined as color ranges for three primary colors (red, green and blue). The mixture of the spectral integral values in each color range presents the final color of each pixel on the false-color THz image. Since the absorption frequencies of different materials are different, the spectral integral values in defined ranges are different, leading to different color on the false-color THz image. The false-color THz images of two kinds of white powder which are lactose and 4-aminobenzonic acid are obtained from the imaging system with two different definitions of color ranges. From the first color range definition, the absorption frequency of lactose lies in the green range, so only the green light is absorbed, and the color of lactose is magenta. In the meanwhile, there are two absorption frequencies for 4-aminobenzonic acid lying in the green and blue ranges, so both green and blue light are absorbed, and the color of 4-aminobenzonic acid is red. They can be told easily by different colors on the false-color THz image. From the second color range definition, the colors of two kinds of powder are more different. Both false-color THz images can present the cuvette and two kinds of powder clearly. By comparing the THz imaging with grayscale images, false-color THz imaging can display different materials by different colors in one image, instead of the requirement of many grayscale images. It is no need to generate grayscale images at each frequency, making false-color THz imaging consume less time. The false-color imaging is clearer and more efficient, which is more suitable for recognition in a rapid security check. In the situation of complex materials, more false-color THz images can be generated by different color range definitions to assist the detection. The spatial resolution of the imaging system is also investigated. The resolution of imaging system is investigated by imaging home-made standard sample plate. For the frequency range that is higher than 0.3 THz, the resolution can reach 0.4 mm, which is larger than enough for most practical applications.

2015, 64 (11): 114207.
doi: 10.7498/aps.64.114207

Abstract +

Miniature Fourier transform spectrometer (FTS) has attracted considerable interest because of its important application in spaceborne spectroscopy and as a portable analytical tool for rapid on-site chemical/biochemical detection. In a previous paper, a stationary miniature FTS constructed with an electro-optic (EO) modulator of a LiNbO3 (LN) waveguide Mach-Zehnder interferometer (MZI) containing push-pull electrodes was demonstrated. This stationary miniature FTS is operated in the near-infrared region with either nonlinear or linear scanning of the modulating voltage. The simple and mirrorless structure renders the device compact, vibration resistant, and cost-effective. However, the spectral resolution of the proposed prototype FTS was not satisfactory due to the limited optical pathlength difference (OPD), thereby limiting the device application. To improve its spectral resolution, the factors affecting the spectral resolution of the LN waveguide-based FTS is investigated in this paper. Findings show that the spectral resolution is inversely proportional to the maximum OPD, which is proportional to the length of the EO modulating region. A simple method for two-fold enhancement of the spectral resolution of the FTS is proposed based on the end-face reflection in LN waveguide interferometer. With the end-face reflection geometry the guided mode can propagate back and forth in the LN waveguide, making the mode passing through the EO modulating region twice and consequently leading to two times enhancement of the OPD. Therefore, the end-face reflection geometry enables to double the maximum OPD of the interferometer without increasing the device size and thus to offer the device a two-fold enhanced spectral resolution according to the equation for FTS resolution. Two prototypes of FTS with and without the end-face reflection structure are prepared using the same commercial LN waveguide EO modulator. The spectral resolutions in terms of the full-width at half maximum (FWHM) at different wavelengths for the two prototypes of FTS are measured using a series of distributed feedback lasers. The FWHM measured at a specific wavelength with the end-face reflection structure is half as large as that obtained without the end-face reflection structure. Experimental results are in excellent agreement with the theoretical data, demonstrating the applicability of the end-face reflection method to the spectral resolution enhancement.

2015, 64 (11): 114208.
doi: 10.7498/aps.64.114208

Abstract +

Multi-frame super resolution reconstruction is a technology for obtaining a high resolution image from a set of blurred and aliased low resolution images. The most popular and widely used super resolution methods are motion based. However, the estimation of motion information (registration) is very difficult, computationally expensive and inaccurate, especially for aerial image. The sub-pixel registration error restricts the performance of the subsequent super resolution. Instead of trying to parameterize the motion estimation model, this paper proposes an image super resolution framework based on the polyphase components reconstruction algorithm and an improved steering kernel regression algorithm. Given an image observation model, a reversible 2D polyphase decomposition, which breaks down a high resolution image into polyphase components, is obtained. Though the assumption of diversity sampling, this paper adopts a fundamentally different approach, in which the low-resolution frames is used as the basis and the reference frame as the reference sub-polyphase component of the high resolution image for recovering the polyphase components of the high resolution image. The polyphase components, which fuse the low resolution frames with the complementary details, can be obtained by computing their expansion coefficients in terms of this basis using the available sub-polyphase components and then inversely transforming them into a high resolution image. This paper accomplishes this by formulating the problem as the maximum likelihood estimation, which guarantees a close-to-perfect solution. Furthermore, this paper proposes an improved steering kernel regression algorithm, to help restore the fusion image with mild blur and random noise. This paper adaptively refines the steering kernel regression function according to the local region context and structures. Thus, this new algorithm not only effectively combines denoising and deblurring together, but also preserves the edge information. Our framework develops an efficient and stable algorithm to tackle the huge size and ill-posedness of the super resolution problem, and improves the computational efficiency via avoiding registration and iterative computation. Several experimental results on synthetic data illustrate that our method outperforms the state-of-the-art methods in quantitative and qualitative comparisons. The proposed super resolution algorithm can indeed reconstruct high-frequency information which is otherwise unavailable in the single LR image. It can effectively suppress blur and noise, and produce visually pleasing resolution enhancement in aerial images.

## EDITOR'S SUGGESTION

2015, 64 (11): 114209.
doi: 10.7498/aps.64.114209

Abstract +

Digital micro holography offers an in-situ, non-contact and three-dimensional way to explore the microscopic world. However, as it is difficult to focalize the whole object in one single reconstructed image, the application of digital micro holography to cases with a large longitudinal object volume is limited by the microscopes depth of field. By extending the depth of field in reconstructed micro holograms in the wavelet domain, this paper fully takes advantage of numerical reconstruction algorithms to solve this problem. First, a recorded hologram is rebuilt using the wavelet transform approach by setting up an appropriate longitudinal interval to obtain a series of reconstructed hologram planes. Then each plane is decomposed with wavelet into its sub-images of both high and low frequencies. Furthermore, the local variance of the maximum intensity gradients of the high- and low-frequency coefficients is calculated and utilized as the focus criterion. Finally, the image planes are fused into a single one with the depth of field extended to a large extent. The feasibility and robustness of this reconstruction procedure for both continuum and particle fields are investigated. One of the demonstrations is made in an experiment of a tilted continuum:carbon fiber. It is different from most of the previous applications where the interrogated is the particles and where the area involved is parallel to the CCD. The carbon fiber gets successfully reconstructed in three dimensions, and the measurement errors of its diameter are presented together with the reconstruction distances. Another is an experiment of a dispersed particle field:micro transparent particles are generated by an ultrasonic atomizer, for which the reconstruction procedure achieves an extended depth of field. In addition, a numerical model based on generalized Lorenz-Mie theory is used to simulate the holograms of both opaque and transparent particles of 1-15 m in diameter. Variations of the longitudinal location errors with the Fraunhofer number are analyzed, and comparisons are made between the results of opaque and transparent particles. Both the experimental and simulation outcomes show that this reconstruction procedure is a reliable one to acquire an extended-depth-of-field hologram for both the continuum and the dispersed particle fields, and then to accurately measure the objects.

2015, 64 (11): 114210.
doi: 10.7498/aps.64.114210

Abstract +

High-repetition-rate fiber laser has widely applications in the field of femtosecond frequency comb, ultra-fast optical sampling, and so on. In this paper, an Er-doped femtosecond fiber laser with a repetition rate of 303 MHz is demonstrated based on the mechanism of nonlinear polarization rotation. By means of optimization of cavity dispersion, the net dispersion in fiber cavity is a little negative nearby zero point. After mode locking, the laser is operating in the stretched-pulse regime. At a pump power of 817 mW, the output power of the laser is 125 mW in continue-wave state, and 69 mW in mode-locking state. The laser directly generates 90 fs before dispersion compensation. Mode locking can self-start at the pump power of 700-817 mW. Repetition rate drift of the mode-locked laser is 30 Hz in five hours.

2015, 64 (11): 114211.
doi: 10.7498/aps.64.114211

Abstract +

The distribution of tropospheric NO2 vertical column desity shows a characteristic of inhomogeneity. Such information is important for the study of pollution formation. A horizontal distribution of tropospheric NO2 vertical column desity based on mobile MAX-DOAS is studied in this paper, especially for a retrieval method of tropospheric NO2 with mobile MAX-DOAS. Using a low-order polynomial fitting can remove the conbtibuiton of the Frauenhofer and stratosphere, and then the tropospheric NO2 vertical column desity can be detected on the mobile platform. The total tropospheric NO2 error is lower than 25% with the model simulation by setting the different aerosol optical densities, aerosol layer heights, NO2 layer heights and azimuths. The mobile MAX-DOAS system is designed by ourself and the pattern of scanning sequentially is selected for this system. On the other hand, using electronic compass sensors, inclinometer, and software control method, the system can determine the elevation, the azimuth angle drift due to unstability of mobile platform during measurement, as well as the elevation and azimuth angle acquisition exactly, and automatically refer to the north and reduce measurement errors. In addition, the observation of tropospheric NO2 is carried out in Hefei city based on the mobile MAX-DOAS. The horizontal distribution of tropospheric NO2 across Hefei ring expressway and the 2nd ring in Hefei city is obtained during the measurement period. Furthermore, the tropospheric NO2 vertical column density from the mobile DOAS is compared with those from ozone monitoring instrument (OMI). Three pixels are covered by OMI in Hefei city during the measurement period of mobile MAX-DOAS, reprsenting “clean area”, “more mobile MAX-DOAS data area” and “polluted area” respectivley. A good agreement is found for “clean area” and the pixel including more data of mobile MAX-DOAS with 3.34×1015 molec/cm2 from mobile MAX-DOAS and 3.00×1015 molec/cm2 from OMI for “clean area” as well as 5.10×1015 molec/cm2 from mobile MAX-DOAS and 5.60×1015 molec/cm2 from OMI for “more mobile MAX-DOAS data area”. While there is a small difference between the two results for polluted area with 9.16×1015 molec/cm2 from mobile MAX-DOAS and 4.50×1015 molec/cm2 from OMI. The unsensitivity of OMI to sources near surface may be accounted for by this difference. These results indicate that the mobile MAX-DOAS can well detect the regional distribution of tropospheric trace gas rapidly. This is important for validation of the model and satellite and study of transport process.

Spatial correlation experimental analysis of atmospheric optical turbulence in the near ground layer

2015, 64 (11): 114212.
doi: 10.7498/aps.64.114212

Abstract +

Atmospheric optical turbulence means refractive index random fluctuation of atmosphere. In this article, according to the concept of correlation function, the measurement principle, measurement schemes, and data processing method of spatial correlation function are given based on a high-quality fiber optical turbulence sensing array. Determining the statistical time and the calculation principle of the spatial correlation is the main point of current research. Emphasis is put on demonstrating the kinds of structural forms and analyzing the impact elements of spatial correlation function in turbulence as clear as possible. Using the sensing array, experimental measurement is promoted in the near ground layer and many forms of correlation functions are revealed. Results show that there are two main structural forms of the spatial correlation function:the first one shows an isotropy-model form, which tends to decrease with the increase of spatial displacement, and then tends to zero after outer scale, the coincidence rate is about 58.7%. The other one tends to oscillate around zero, and the coincidence rate is about 37.9%. By analyzing the probability and impact elements, it is not difficult to know that the spatial correlation of an optical turbulence mainly depends on the intensity and development degree of the optical turbulence; and the coherent structure is an important factor of oscillation in the correlation functions. On the one hand, the value of correlation coefficient is mainly determined by the intensity of the optical turbulence; and on a certain scale, the stronger the turbulence, the bigger the value of correlation coefficient becomes. On the other hand, the variation tendency of correlation function is not only determined by the intensity of turbulence, but also by the development degree of the optical turbulence. When the atmosphere is in advection or anisotropy, its spatial correlation coefficient will oscillate around zero and be unrelated to the spatial displacement. The spatial correlation function obtained by the sensor array set is not only the foundation of analyzing the spatial structure, but also the beginning of giving non-Kolmogorov model of turbulence.

2015, 64 (11): 114213.
doi: 10.7498/aps.64.114213

Abstract +

In this paper, the properties of electronic structure and band-gap change of Zn2GeO4 under high pressures are investigated using the first principles method based on the density functional theory (DFT). We demonstrate that the density functional theory calculations performed with the local density approximation (LDA) allows for a significantly better reproduction of lattice constants, the unit cell volume and the band gap of Zn2GeO4 than those performed with the generalized gradient approximation (GGA), so the electronic structure and the band-gap changes of Zn2GeO4 under high pressures can be systematically investigated by LDA. Result of the state density without application of pressures shows that Zn2GeO4 is a wide direct-band-gap semiconductor, and the top of the valence band is mainly composed of Zn 3d and O 2p states, while the conduction band is dominated by the Zn 4 s and Ge 4p. Calculated results about the energy band structure of Zn2GeO4 show that the band gaps of Zn2GeO4 first increase and have a peak at around 9.7 GPa, and then gradually decrease with increasing pressure. The Mulliken charge populations and the value of net charges of Zn2GeO4 at different pressures reveal that the charge distribution of O atoms does not change obviously, while the s and p orbital charges of Zn and Ge atom distributions have obviously charge transfer above 9.7 GPa, and result in an increase of Zn and Ge atom net charges. Analysis of the state density, the Mulliken charge populations, and the electronic density difference of Zn2GeO4 in (210) plane at different pressures indicate:in the low-pressure region (0PP>9.7 GPa), the delocalization phenomenon becomes dominant due to the fact that the delocalization action exceeds the force between the bonding state and anti-bonding state, which induces the decrease of the band gap. These results will not only help to understand the germanate crystal structures in Zn2GeO4 materials under high pressures and the unique characteristics and laws, and may provide a reference for the design of electronic devices of Zn2GeO4 crystals.

2015, 64 (11): 114702.
doi: 10.7498/aps.64.114702

Abstract +

This paper analyzes theoretically and numerically the refraction phenomenon of detonation wave at the explosive-metal interface, motivated by the problem that there exist large discrepancies between the experimental results and the classical shock polar theory. After pointing out the major defects of the classical shock polar theory based on CJ model of detonation, an improved shock polar theory based on ZND model of detonation is presented to give the styles of the refraction of detonation wave and the pressure values at the interaction point between the refracted shock wave and the incident shock wave, to show the pressure values at free-surface of copper remarkably lower than the ones at the shock interaction point due to the attenuation effects from the chemical reaction expansion and the following Taylor rarefaction. A second-order cell-centered Lagrangian hydrodynamics method with high resolution based on the subcharacteristics theory is develped to solve the reactive flow equations of detonation in condensed explosive, and then to numerically simulate a representative refraction experiment about T2 explosive interacting with copper. The simulated pressure values at the interaction point agree well with the ones from the improved shock polar theory, and the simulated pressure values at free-surface of copper agree well with the experimental values, meanwhile, the refraction styles predicted by the improved shock polar theory are confirmed by the numerically simulated flowfield images. From the theoretical and numerical results, there exist three kinds of refraction styles of detonation waves at explosive-metal interface:i) the regular refraction with reflecting shock wave, ii) the irregular refraction with Mach reflection, and iii) the regular refraction without any reflecting wave; in particular, the regular refraction with no reflecting wave is a kind of refraction style unable to be predicted by the classical shock polar theory, meanwhile, the pressure values at the free-surface and the interaction point inside the shocked metal both monotonically decrease with the increase of the incident angle.

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

2015, 64 (11): 116101.
doi: 10.7498/aps.64.116101

Abstract +

A water pumping system model has been designed based on the double-walled carbon nanotube. In this system, the inner tube is fixed as the water channel, while the exterior one can move, similar to the piston motion along the axial direction, to create a pumping force. Molecular dynamics simulations confirm that both the water flux and the water dipole orientation are sensitive to the velocity of motions of the outer tube so that a controllable unidirectional water flow can be achieved in this system by varying the velocity. Its pumping ability comes mainly from the carbon-water van der Waals driving forces of the exterior tube. The piston motion of the outer tube changes the position of the vdW balance point, which not only leads to the increase of vdW force on the water molecules already residing in the inner tube, but also enlarges their accelerated distance. Meanwhile, the orientation of water molecules inside the inner tube is strongly coupled to the water flux, the probability of +dipole states attains unity at v = 0.05 Å/ps, where the water flux reaches its maximum value (2.02 ns-1). Compared to the pump which is controlled by uniform electric field, the transmission efficiency of our mechanical pump is higher. This design may open a new way for water pumping in the field of nanodevices.

2015, 64 (11): 116102.
doi: 10.7498/aps.64.116102

Abstract +

In this paper, we study the electrical properties of ion-beam-etched Hg1-xCdxTe (x=0.236) crystal with the help of mobility spectrum analysis technique. In step-by-step chemical etching, it is shown that the p-HgCdTe is completely converted to the n-type one which includes a damaged surface electron layer with a low mobility and a bulk electron layer with a higher mobility after ion etching. The mobility spectra at different temperatures show that the mobility of the surface electrons is independent of temperature in the measurement temperature range while the bulk electrons exhibit a classical behavior of n-HgCdTe with characteristics that are strongly dependent on temperature. Hall data for different thicknesses show that the electrical properties of the bulk layer are uniform. Otherwise, the surface electron layer may be found to consist of a concentration about 2-3 order of magnitude higher than the bulk electron layer.

2015, 64 (11): 116301.
doi: 10.7498/aps.64.116301

Abstract +

Hexagonal α-Fe2O3 is one of the most common functional material used as magnetic semiconductor, and plays an important part in various applications, such as electronic devices etc. Based on the density functional theory, the lattice parameters, density of states and Bader charge analysis of α-Fe2O3 have been calculated using the first-principles calculation with GGA+U method. As Fe is a transition metal element, the value of U can be more accurate by considering the influence of the strong on-site Coulomb interaction between 3d electrons. First, the crystal equilibrium volume, the magnetic moment of Fe atom, and the band gap value of α-Fe2O3 are synthetically researched and compared with those with different U. Results indicate that the calculation model of α-Fe2O3 are in good agreement with the experimental model when the value of U is 6 eV. These parameters can also be adapted to the following doping calculaton. The α-Fe2O3 unit cell has both tetrahedral and octahedral interstitial sites. The calculation of doping formation energy shows that the α-Fe2O3 system is most stable when the doped hydrogen atom is in the tetrahedral interstitial site. The density of states show that the valence band and conduction band compositions are similar for the bulk and hydrogen-doped α-Fe2O3. That is, the valence bands are dominated mainly by both O 2p and Fe 3d orbitals with the O 2p orbitals playing a leading role, while the conduction band is dominated by Fe 3d orbitals. The band gap of α-Fe2O3 decreases from 2.2 to 1.63 eV after hydrogen doping. Also, a strong hybrid peak occurs near the Fermi level after hydrogen doping, which is chiefly composed of Fe 3d orbital, and the O 2p orbital also has a small contribution. The H 1s orbital is mainly in the lower level below the top valence band. Results of the Bader charge analysis and the density of states calculation for partial correlated atoms suggest that the new hybrid peak is chiefly caused by Fe atom which is closest to the hydrogen atom in the crystal cell. In this process, H atom loses electrons, and the nearest neighbors of H atom, i.e. O and Fe atoms, almost obtain all the electrons H atom loses, so H and O atoms are bonded together strongly, causing the hybrid peak, to expand the width of the top valence band and shift down the bottom of the conduction band, so that the band gap decreases and the electrical conductivity increases. Hydrogen doping is suggested to be an effective method to modify the band.

2015, 64 (11): 116302.
doi: 10.7498/aps.64.116302

Abstract +

We experimentally investigated the correlation between local structures and phonon modes in quasi-2D colloidal glasses. The glass samples consist of thermo-sensitive poly-N-isopropylacrylamide microgel (PNIPAM) particles, whose diameter can be tuned by small changes of sample temperature. A binary mixture of these particles is confined between two coverslips and forms a monolayer of quasi-2D glass. By changing the number ratio between large and small particles, the structure or the overall degree of disorder of the samples can be systematically tuned. We employ a video microscopy to record the motion of the colloidal particles in the sample for 11 min at a rate of 60 fps. The trajectories of individual particles are obtained by particle tracking software. Dynamical matrix is constructed using covariance matrix analysis, from which the eigenfrequency and eigenvector of vibrations are extracted. In this study, we focus on the evolution of the low-frequency quasi-localized phonon modes in glasses, as the system becomes more and more disordered from the increased dopants. To compare the results from different samples, we choose those with packing fraction of 86%, and rescale the eigenfrequencies by the median frequency of each sample. For the four doping levels investigated (2%, 9%, 29%, 61%), the density of states at low frequencies increases with the doping level, suggesting that the fraction of low-frequency modes increases with disorder, which is corroborated by the higher boson peaks at higher dopant fractions. We have measured the participation ratio of the obtained phonon modes, and find that the boson peak corresponds to quasi-localized vibration modes, or soft modes. We also examine the correlation between the soft modes and local structural parameter. Specifically, we have calculated the local orientational order parameter in our samples, and computed the correlation coefficients between the relative amplitude and the local orientational order parameter for each mode. The soft modes are found to have a significantly negative correlation with the local orientational order parameter, which implies that the soft modes are concentrated in regions with poor local order. We therefore conclude that the local disorder is probably the structural origin of soft modes in glasses.

2015, 64 (11): 116801.
doi: 10.7498/aps.64.116801

Abstract +

A capillary model is developed for calculating the wetting angle of molten silicon on different walls by using the microfluidic two-phase flow level set method and studying the characteristics of the rising process. A mathematical model formulation rigorously accounts for the mass and momentum conservation by using the improved Navier-Stokes equation and considering the Marangoni effect. Compared with the experimental data, the change of the wetting angle on the chemical vapor deposition (CVD) diamond wall indicates the grids independence and the validity of the numerical algorithm. We also discuss the influence of surface tension, and Marangoni stress induced by the gradient of surface tension coefficient, and wall adhesion to the change of wetting angle for three different walls, which include SiC wall, graphite wall, and CVD diamond wall, at different temperatures (1683-1873 K). Result shows that at the same temperature, the thermal-capillary effects that induce the molten silicon to undulation are raised. The wetting angle is reduced after first being increased and finally stabilized. At the initial stage, the fluctuation of the liquid-air interface is volatile due to the large changes of the liquid-air and the wall-air surface tensions, and subsequently, the fluctuation tends to be stable while the wetting angle is close to a fixed value. It is also found that with the graphite wall, these changes are more likely to be stable. This research provides a theoretical guide to obtain a stable growth environment for silicon belt fabricated from the molten silicon.

2015, 64 (11): 116201.
doi: 10.7498/aps.64.116201

Abstract +

In this paper molecular dynamics (MD) method and the modified analytical embedded atom model (MAEAM) are used to investigate the size effect on the elastic properties of Ni, Al and V nanowires and the role the free surface plays. For convenience of comparison, the elastic properties of these corresponding perfect bulk materials are also studied. Results obtained indicate that the calculated values of the elastic properties of these perfect materials are in good agreement with those previously given theoretical and experimental ones. But the calculated bulk moduli of the nanowires, which are lower than those of the prefect materials, increase exponentially with increasing size of the nanowire and are nearly close to a constant (180.20 GPa for the Ni nanowire, 83.98 GPa for the Al nanowire and 162.48 GPa for the V nanowire). Meanwhile, the surface energy of the nanowire decreases exponentially with the increase of its size and reaches a minimal value (1.84 J·m-2 for the Ni nanowire, 0.77 J·m-2 for the Al nanowire, and 1.71 J·m-2 for the V nanowire), which is consistent with the corresponding bulk material. And the critical value of the size, which has a distinct effect on the elastic properties and the surface energy, is about 5.0 nm for all nanowires. On this basis, the free surface dependence of the elastic properties of these metallic nanowires and the inherent mechanisms are further discussed by exploring the size effect on the surface energies of Ni, Al and V nanowires and their distribution characteristics, showing that the free surface plays a more and more important role in the diminution of the elastic properties of nanowires as the size decreases. The mode of the surface impacting on the elastic properties of nanowire is described as follows:The surface first reduces the compressional stress of the internal core region of nanowires and then the reduced compressional stress results further in the decrease in the elastic properties of nanowires.

2015, 64 (11): 116601.
doi: 10.7498/aps.64.116601

Abstract +

In this paper, the model of metalic melt shearing flow near the surface is established, and the effect of shearing flow on solidification microstructure of the metal is also analyzed. Calculated results based on A356 alloy melt show that in the laminar flowing melt, the shear stress decreases with increasing length along the vertical direction of the surface of the slope, and the shear stress first decreases rapidly and then stabilizes with increasing length along the flowing direction of the surface of the slope; while in the turbulent flowing melt, the shear stress firstly decreases rapidly and then stabilizes with increasing length along the vertical direction of the surface of the slope, and increases with increasing length along the flowing direction of the surface of the slope. The shear stress at the same position in the melt on the surface of the slope increases with increasing angle of the slope; the shear stress acting on the columnar crystal in the melt on the surface of the slope increases with decreasing length along the vertical direction of the surface of the slope. The shear stress acting on the columnar crystal at the same position in the melt on the surface of the slope increases with increasing angle of the slope; with the increase of the length along the flowing direction, the shear stress acting on the columnar crystal rapidly decreases first and then stabilizes in the laminar flowing melt on the surface of the slope, while the shear stress increases in the turbulently flowing melt on the surface of the slope. Based on the theoretical calculation, the maximum shear stress acting on the columnar crystal in the melt during the shearing flow near the surface of the metalic melt is lower than the yield strength of α-Al grain, so the shear stress induced by shearing flow cannot break the columnar crystal, and only by sweeping the grain into the melt to induce the multiplication of grain, which agrees with the experimental results. So, the proposed model can explain the constitutive relations of the metalic melt shearing flow near the surface and the effect of shear stress on the solidification microstructure.

2015, 64 (11): 116802.
doi: 10.7498/aps.64.116802

Abstract +

Theoretical analyses are given to the known approaches of nano-contact angle and arrive at the conclusions:1) All the approaches based on the assumptions of Qusi-uniform liquid film, or uniform liquid molecular density, or uniform liquid molecular densities respectively inside and outside the interface layer cannot give the correct nano-contact angle, and it is difficult to improve them. Among these approaches, both the conclusions of nano-contact angle sure being 0° and sure being 180° are false. 2) Density functional theory (DFT)approach and Molecular Dynamics (MD) approach are capable to treat of nano-contact angle, however, the work is very heavy for using the DFT approach. 3) In 1995, Ruzeng Zhu (College Physic [Vol. 14 (2), p1-4 (in Chinese)], corrected the concept of contact angle in a earlier false theory for macro contact angle and obtained the most simple and convenient approximate formula of nano-contact angle α = (1-2EPS/EPL)π,where EPL is the potential of a liquid molecule in the internal liquid and EPS is the interact potential between a liquid molecule and the solid on which it locats. Both EPS and EPL can be obtained by MD, therefore this theory as a approximate simplified form belongs to Molecular Dynamics approach of nano-contact angle. The results of 0° and 180° for complete wetting and complete non-wetting given by this formula are correct under the assumption of incompressible fluid, therefore, this theory is worthy of further development. For this end, based on the physical analysis, we assume that the potential energy of a liquid molecule on the Gibss surface of tension outside the three-phase contact area is EPL/2x and that of a liquid molecule on the three-phase contact line is (1+kEPS/EPL)α EPL/2xπ, where x and k are optimal parameters. According to the condition that the potential energy is the same everywhere on the Gibss surface of tension, an improved approximate formula for nano-contact angle α = π(1-2xEPS/EPL)/(1+kEPS/EPL) is obtained．To obtain the value of x and k, MD simulations are carried on argon liquid cylinders placed on the solid surface under the temperature 90 K, by using the lennard-Jones (LJ) potentials for the interaction between liquid molecules and for that between a liquid molecule and a solid molecule with the variable coefficient of strength a. Eight values of a between 0.650 and 0.825 are used. The Gibss surfaces of tension are obtained by simulations and their bottom angles are treated as the approximate nano-contact angles. Combining these data with the physical conditions (when EPS/EPL=0, α = π), the optimized parameter values x=0.7141, k=1.6051 with the correlation coefficient 0.9997 are obtained by least square method. This correlation coefficient close enough to 1 indicates that for nano liquid solid contact system with different interaction strength, the parameter of optimization x and k really can be viewed as constants, so that our using MD simulation to determine of the optimized parameters is feasible and our approximate formula is of general applicability.

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

2015, 64 (11): 117101.
doi: 10.7498/aps.64.117101

Abstract +

Fracture behavior and mechanical properties of SiC nanofiber (SiCNF) reinforced SiC nanocomposites as influenced by the thickness of amorphous carbon (a-C) coatings are studied via molecular dynamics simulations using Tersoff potential. To simulate the condition that a matrix crack arrives at the interface between matrix and coating, a pre-setting matrix crack is created. Results show that the tensile stress-strain curve of nanocomposites without and/or with thin a-C coatings (e.g., t≤ 0.3 nm) demonstrates an abrupt drop after achieving a maximum value, while nonlinear tails appear in the curves of nanocomposites with thick a-C coatings (e.g., t >2.0 nm). It is demonstrated that the SiCNF is penetrated by the matrix crack when it is uncoated and/or coated by a thin a-C layer (t ≤ 0.3 nm) and the nanocomposite fails in a typical brittle mode; whereas the crack deflection path changes and the SiCNF is pulled out from the matrix when the a-C coatings are thick enough (e.g., 4 nm), showing a different fracture mode in nanocomposites. Compared to nanocomposites without an a-C coating, the tensile strength of nanocomposites with a-C coating of 4.0 nm thickness is about four times higher, and the fracture energy increases around an order of magnitude. Furthermore, the average stress concentration factor for SiCNF in nanocomposites, defined as the ratio of tensile strength of single SiCNF to the average stress of the nanofiber in the composite when it is broken, is extracted and shows a decreasing trend with increasing coating thickness, indicating that a-C coating can therefore be expected to simultaneously enhance the tensile strength and fracture energy of the SiCNF/SiC nanocomposites. This work sheds light on the toughening mechanism in SiCNF/C/SiC nanocomposites where a-C coating plays a significant role, indicating that the toughening mechanism in conventional ceramic matrix composites on a microscale is still valid on a nanoscale. Simulation results suggest that coating thickness in material design is efficient for engineering SiCNF/SiC nanocomposites with high strength and toughness.

2015, 64 (11): 117301.
doi: 10.7498/aps.64.117301

Abstract +

Due to its smoothest surface, fewer defects, and better crystal quality, [100] textured diamond film is well suited for the application of optoelectronic and microelectronic devices. Carrier concentration and mobility are very important parameters of semiconductor materials. In order to further broadening the application of diamond films in optoelectronics and microelectronics, it is necessary to made a research on Hall effect characteristics of [100] textured and [111] textured films. In this paper, different textured polycrystalline diamond films are deposited on silicon substrates by hot filament chemical vapor deposition (HFCVD) method under different conditions. Microstructures of diamond films are characterized by X-ray diffraction (XRD). High quality [100] textured and [111] textured diamond films are obtained. Dark current-voltage (I-V) characteristics of different-oriented films after annealing are investigated at room temperature. The carrier concentration and mobility of diamond films are measured by Hall effect test system as the temperature changing from 100 to 500 K. Results indicate that the textures of diamond films affect the value of carrier mobility:carrier concentration increases and mobility decreases with the decrease of temperature; and the deposited films are of p-type materials. The carrier concentration and mobility of polycrystalline [100]-textured diamond films at room temperature are 4.3×104 cm-3 and 76.5 cm2/V·s, respectively.

2015, 64 (11): 117701.
doi: 10.7498/aps.64.117701

Abstract +

Researches on electrocaloric effects of ferroelectric materials and their applications in solid-state refrigeration have attracted great interest in recent years. EuTiO3 is a new multiferroic material with many special physical properties, such as high dielectric constant, low dielectric-loss, as well as their responses to tunable external electric field and temperature. With EuTiO3 ferroelectric thin films, their polarization size and phase transition process not only can be changed by regulating external electric field and temperature applied, but also can be controlled by adjusting the external stress applied and the lattice mismatch with the substrate in a large scale. Accordingly, in this paper a phenomenological Landau-Devonshire thermodynamic theory is used to investigate the ferroelectric properties and electrocaloric effects of EuTiO3 ferroelectric films under different external tensile stresses (σ3 > 0) perpendicular to the film surface and different in-plane compressive strains. We have calculated the electric polarizations, electrocaloric coefficients and adiabatic temperature differences as a function of temperature for EuTiO3 ferroelectric films with a biaxial in-plane misfit strain um =-0.005 under different applied stresses. Results demonstrate that the changes of the electric polarization, the electrocaloric coefficient and the adiabatic temperature differences conform with the regulation of externally applied stresses. With the enhancement of applied tensile stress perpendicular to the film surface, the phase transition temperature and adiabatic temperature change of EuTiO3 thin film increase, and the operating temperature corresponding to the maximum adiabatic temperature difference moves toward high temperature region. For the thin films with a biaxial in-plane misfit compressive strain um =-0.005 and the external tensile stress σ3 = 5 GPa, when the change of electric field strength is 200 MV/m, the adiabatic temperature differences at room temperature can be over 14 K, and the maximum electrocaloric coefficient may approach 1.75×10-3 C/m2·K. In the meantime, the working temperature range, when the adiabatic temperature differences go beyond 13 K, is over 120 K. Then we investigate the effect of in-plane compressive strains on the changes of adiabatic temperature, showing that with the increase of compressive strain um, the adiabatic temperature change will also increase and the peak of the curve of adiabatic temperature change versus temperature will shift toward high temperature zone far away from room temperature. Therefore, the above results show that we can not only have relatively bigger adiabatic temperature differences in epitaxially grown EuTiO3 thin films through the regulation of external stresses and in-plane lattice misfit strain, but also a sound application prospect of ferroelectric EuTiO3 thin film in solid-state refrigeration at room temperature.

2015, 64 (11): 117401.
doi: 10.7498/aps.64.117401

Abstract +

The discovery of superconductivity in iron-based superconductors by Professor Hosono in Japan in 2008 has triggered off an enormous group of researches the world wide. The iron-based superconductors are regarded as another kind of high-Tc superconductors, which possess lots of merits, such as very high upper critical field (Hc2), high critical current density (Jc), and small crystal anisotropy (), are promising for high field applications. Ba1-xKxFe2As2, as a typical FeAs-122 superconductor, is focused on by both theoretical physicists and material scientists since its discovery. In this paper, we first successfully fabricate Ba1-xKxFe2As2 single crystal. It has an onset transition temperature up to 38.5 K, while its zero resistivity temperature reaches 37.2 K. Both the R-T and M-T data of it show very sharp superconducting transition, and its critical current density at 5 K and self field is over 106 Acm-2 and almost field independent. The flux pinning force and the relative pinning mechanisms in Ba1-xKxFe2As2 are discussed by analyzing the data obtained from the measurements about the R-T and M-H under different conditions. Results indicate that the Ba1- xKxFe2As2 superconductors have very strong intrinsic vortex pinning force, and the vortex potentials (U0) under 9 T field are 5800 K and 8100 K for the H//c and H//ab, respectively. Furthermore, the vortex pinning mechanism is also investigated by analyzing the relationship Jc-B. According to the present results, the flux pinning mechanism should be (l) pinning because of the change of mean free path for electrons induced by nano-size crystallographic defects in Ba1-xKxFe2As2.

2015, 64 (11): 117501.
doi: 10.7498/aps.64.117501

Abstract +

In this paper, polycrystalline BaFe4-xTi2+xO11 (x=0, 0.25, 0.5, 0.75, 1) samples have been synthesized by the conventional solid-state reaction method. X-ray diffraction (XRD) patterns of all the samples show that the diffraction peaks correspond to that of an R-type hexagonal ferrite structure, and no trace of second phase is detected. Measurement of X-ray photoelectron spectroscopy (XPS) reveals that most of the Fe ions in BaFe4Ti2O11 are trivalent and the fitting of two peaks in Fe 2p spectrum corresponding to different Fe ion sites, while the amount of Fe2+ ions increases with the increase of Ti ions in BaFe4-xTi2+xO11. The spectroscopy of Ti ions confirms that the valence of Ti in BaFe4-xTi2+xO11 are tetravalent. Magnetic susceptibility of BaFe4-xTi2+xO11 (x= 0, 0.25, 0.5, 0.75, 1) reveals two magnetic transitions at T1～250 K and T2～83 K, which indicate a complex magnetic order driven by competing interactions on a frustrated lattice with a noncentrosymmetric structure. For all the samples, the magnetic susceptibility obeys Curie-Weiss law above T1, and M-H curves exhibit a linear variation with magnetic field in this temperature range, which is consistent with the paramagnetic behavior. A decrease of the effective magnetic moment is due to the increase of Fe2+ ions with the increase of Ti content in BaFe4-xTi2+xO11. Below T1, the magnetization curve as a function of temperature (M-T) and the magnetization versus magnetic field (M-H) at different temperatures imply its characteristic of a typical canted antiferromagnetic or ferrimagnetic state. Meanwhile, the transition temperature T2 drops gradually with the increase in Ti content, which might be related to the change of occupying of Fe ions in the kagome layers.

2015, 64 (11): 117801.
doi: 10.7498/aps.64.117801

Abstract +

Recently, metamaterials have attracted considerable attention because of their unique properties and capability of being used in many areas of science. Among these applications, metamaterial absorber is the one researchers show much interests. On the basis of its electromagnetic responses to other material parameters, the metamaterial absorber can be applied to sensing. In this paper, a metamaterial absorber with an I-shaped unit cell is proposed and its favorable sensing characteristics in terahertz frequency range are numerically simulated in terms of frequency-domain algorithm. Influences of the thickness of the sample to be tested and the thickness of dielectric spacer of the sensing of metamaterial absorber on the frequency sensitivity, amplitude sensitivity, and the figure of merit of the refractive index, are studied in detail. Research results indicate that as the refractive index of the sample, whose thickness being fixed, increases, the resonant frequency red-shifts and the reflected amplitude increases. And when the thickness of the sample with a particular refractive index increases, the resonant frequency red-shifts and the reflected amplitude increases correspondingly. The above researches indicate that the sensing of thickness or refractive index of the sample to be tested (abbreviated as specimen) can be realized in a metamaterial absorber. The frequency sensitivity of the refractive index can reach 153.17 GHz/RIU and the amplitude sensitivity of the refractive index can reach 41.37%/RIU when the thickness of the sample is fixed at 40 μm. The frequency sensitivity of the refractive index increases as the thickness of the sample tested increases, but the increasing range gradually decreases. In addition, the amplitude sensitivity of the refractive index increases linearly with the increase of thickness of the sample tested. The frequency sensitivity of thickness decreases linearly with the increase of the thickness of the sample to be tested which is of a particular refractive index. As the thickness of dielectric spacer increases, the frequency sensitivity of the refractive index increases until the thickness reaches 30 μm. Besides, when the refractive index takes a particular value, the frequency sensitivity of thickness decreases linearly as the thickness of dielectric spacer increases. Along with the gradual increase of the thickness of the sample tested, RFOM increases but the increasing range decreases. And TFOM gradually decreases as the thickness of sample tested increases. Both the RFOM and TFOM decrease with the increase of the thickness of dielectric spacer. In the end, the sensing mechanism of metamaterial absorber is discussed in detail. The reflectance spectra and the sensitivity can be adjusted with changing the refractive index and thickeness of the sample tested and the thickness of dielectric spacer, and this will provide important instructive means for terahertz sensing with metamaterial absorbers.

###### NUCLEAR PHYSICS

## EDITOR'S SUGGESTION

2015, 64 (11): 112101.
doi: 10.7498/aps.64.112101

Abstract +

Experimental nuclear charge radii for 885 nuclei with N8 and Z8 have been systematically investigated. Results show that the formula for single parameter Z1/3 law is superior to that for the A1/3 law in describing nuclear charge radii. For two-parameter and three-parameter formulae, the Z1/3 law is as good as the A1/3 law. Considering the importance of shell effect and deformations for nuclear charge radii, we add a term including the Casten factor P into the conventional three-parameter formula and thus obtain very good results. The corresponding root-mean-square deviation falls to =0.0273 fm, i.e. reduced by about 50% when compared with the result obtained with the old three-parameter formula. Shell effect can be well reproduced for some elements by adding the Casten factor term. It is shown that the Casten factor plays a key role for nuclear charge radii. The odd-even staggering is a common phenomenon in many nuclear fields. This phenomenon can be observed with nuclear charge radii for most elements. For this reason, we add a term into the formula (10) in this paper. The root-mean-square deviation falls to =0.0266 fm. A five-parameter formula can well reproduce the variation of the nuclear charge radii for most elements. Calculated results are well consistent with the experimental data available. The differences between the experimental nuclear charge radii and the results calculated using the conventional three-parameter formula and the present five-parameter formula for the 885 selected nuclei are presented. A comparison of the formulae mentioned in this paper is given. The present five-parameter formula including the Casten factor P and the odd-even staggering is the best formula to fit available RC data and gives the smallest root-mean-square deviation . Our calculated results may be useful for future experiments.

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

2015, 64 (11): 115201.
doi: 10.7498/aps.64.115201

Abstract +

The volt-ampere (V-I) characteristic curve obtained using single probe diagnostic system for collisionless plasma will be distorted when the system is applied to collisional plasma. The V-I characteristic curves of collisionless and collision inductively coupled plasma is obtained at the gas pressure of 20 Pascal and 400 Pascal, respectively. The distortion phenomenon was studied by measuring the V-I characteristic curves at different positions of the probe and introducing interference electrode to adjust the level of overall electrical-neutrality constraint. The inferential and experimental results show that the overall electrical neutrality in collisional plasma will compose constraint on the single probe diagnostic system for collisionless plasma. Moreover, it is concluded from experiments that the plasma will adjust the potential of itself, which makes the potential of vacuum chamber wall as reference, in order to satisfy the overall electrical neutrality, and that the V-I characteristic curve obtained by single probe is not the V-I characteristic of probe sheath predicted by theory.

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

2015, 64 (11): 118101.
doi: 10.7498/aps.64.118101

Abstract +

Ba0.6Sr0.4TiO3 (BST) nanotubes are fabricated successfully by sol-sel method with the through-hole anodic aluminum oxide (AAO) template for the first time so far as we know. This fabrication method is easy to realize at low cost because the through-hole AAO template and the BST sol can be acquired easily at low cost, so this is very valuable in the fabrication of other similar nanostructures. First, the steady BST sol is prepared and the well aligned through-hole anodic aluminum oxide template is fabricated by a two-step anodization method; second, the BST sol is introduced into the ordered nanohole arrays of the through-hole AAO template by dipping and spinning; and finally, the samples are fired in air at 650℃ for 1 h to get BST nanotubes. X ray diffraction (XRD) patterns reveal that the BST nanotubes are of cubic perovskite structures, and grow mainly along [110] crystal orientation. Scanning electron microscope (SEM) results show that the thickness and pore size of the through-hole AAO template are about 16 μm and 75 nm, respectively. The length, external and inner diameters of the BST nanotubes are about 16 μm, 75 nm and 50 nm, respectively. Measurements of BST nanotubes give results highly matched with that of the through-hole AAO template. Fourier transform infrared spectroscopy (FTIR) results shows that in the 1350-1650 cm-1 waveband, the composite structure of AAO/BST nanotubes has two obvious absorption peaks which are respectively at 1470 and 1550 cm-1, while the BST film does not have; the absorption property of the composite structure is about two times of the pure through-hole AAO membrane. Finally, the possible reasons of this phenomenon about infrared absorption are discussed.

2015, 64 (11): 118102.
doi: 10.7498/aps.64.118102

Abstract +

The gap flow field formed by two rotating cylinders and the fiber orientation in the gap flow field are studied numerically. The finite volume method on the collocated body fitted grid is used for solving the field. On the assumption that there is no relative motion between the fibers and the fluid, the motion of the fibers is determined. The velocities of fibers are calculated by bi-linear interpolation method. The orientation of fibers is obtained by solving the Jeffery equation. Periodic boundary conditions are used for the fiber motion to ensure that the fibers keep staying in the computational area. Two cases i. e., two cylinders rotate in the opposite directions with the same speed and only the mandrel cylinder rotates, are considered. Physical quantities, such as velocity and pressure, for each case are obtained. For the first case, the velocity and pressure are completely symmetric about the mid-line of the computational area and the absolute values of the maximum and minimum velocity are equal due to the fact that both the casing and mandrel cylinders rotate at the same speed. The absolute values of the maximum and minimum pressure are not equal because the radii of the two cylinders are different. For the second case that only the mandrel cylinder rotates, the symmetries of the velocity and pressure about the mid-line of the computational area can also be found although the absolute values of the maximum and minimum velocity are not equal because of the different velocities of the two cylinders. Fiber motions and orientations at different times for both cases are captured. The twisting of fibers (matrix) can be observed vividly. For the case that the casing and the mandrel cylinders rotate in the opposite directions, fibers move and orientate in a two-layer structure. While for the case that only the mandrel cylinder rotates, fibers move and orientation in a single-layered structure. For both cases, the fibers have a strong tendency to align along the stream lines of the field. The influence of the slenderness ratio of fibers on fiber orientation is also studied. A stronger tendency to align along the stream lines of the field can be found as the slenderness ratio of fibers increases.

2015, 64 (11): 118501.
doi: 10.7498/aps.64.118501

Abstract +

Junction temperature, as one of the most important properties of light-emitting diodes (LEDs), has great impact on LEDs’ power efficiency, luminosity, reliability, life-time, and so on. Precise measurement of junction temperature for LED device is quite important in the research of chip’s fabrication, device packaging and related applications. In this paper, we propose a new approach to measure the junction temperature of LEDs by using temperature-dependent capacitance. The capacitance of white LEDs at room temperature is measured and found to be decreased first and then increased with an increasing reverse bias. Equivalent model using vertical and horizontal capacitances connected in parallel is proposed to qualitatively explain the variation of capacitance under different reverse bias. Result obtained from the model fitting agrees well with the experimental result. The capacitance-temperature (C-T) curve of white LEDs under different reverse bias is measured and analysed. Results show that the capacitance of LEDs is sensitive to temperature at all biases. Under a reverse voltage of 0.5 V, the capacitance has the maximal response of 1.971 pF/℃ and a good linear temperature-dependent property. The C-T curve is used as the calibration for the measurement of junction temperature. By monitoring the change of capacitance of the working LEDs and comparing it with the C-T curve, the junction temperature of the LED device is successfully measured. The junction temperature of a white LED obtained by the proposed C-T method is compared with that by tranditional forward voltage method, and they are in good agreement. The C-T method is also used to measure the real-time junction temperatures of white LEDs under a constant current of 350 mA and a constant voltage of 3.2 V, respectively. In both conditions, the junction temperature of an LED needs approximately 110 sec to rise from room temperature to a steady value, and subsequently needs approximately 500 sec to fall back to room temperature after the LED is turned off. Compared with traditional methods, C-T method only needs to measure one calibration and this calibration can be applied to LEDs working at any current and voltage. Therefore, C-T method is a simple and flexible alternative to the existing technique of temperature measurement in electronic device.

2015, 64 (11): 118502.
doi: 10.7498/aps.64.118502

Abstract +

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

2015, 64 (11): 118503.
doi: 10.7498/aps.64.118503

Abstract +

For long line uncooled infrared detectors, the non-uniformity of different detecting elements is the key parameter in measuring the circuit performance. So far there have been few research reports in this area. Most uncooled infrared detector circuits require corresponding blind detector for readout circuit design, which increases the complexity of uncooled infrared detector. In addition, the performances of these circuits need to be further improved in practical applications. In order to achieve high performance readout of the long line uncooled infrared detectors, a kind of 160 element readout circuit based on current mirror is designed in this paper. The readout circuit is composed of current mirror input part, capacitor feedback transimpedance amplifier (CTIA), and correlated double sampling (CDS) output circuit. The circuit is fabricated by using the 0.5 micron technology. The non-uniformity of circuit is obviously improved by reasonable parameter setting and current mirror circuit layout. Transconductance amplifier CTIA with capacitance negative feedback is used in the circuit. The integral capacitor consists of three capacitors whose capacitances are 10 pF, 20 pF and 20 pF respectively, thus the circuit can realize different integration capacitances, which forms different magnifications. The circuit can meet different response rates of uncooled detectors. Folded-cascode structure is adopted as the CMOS differential amplifier. The open loop gain is over 80 dB. This single-state folded-cascode construct can overcome the two-stage amplifier’s disadvantages, which easily leads to oscillations. The CDS N SF (source follow) and P SF are adopted as the circuit output, the output swing can easily be greater than 2 V. On average, the CDS N SF and P SF power consumptions are very low. So the total power consumption of 160 line circuit is lower than 100 mW. In the test, the non-uniformity of the readout circuit decreases from 10% to 1%. This result is in accordance with simulation result on non-uniformity. The other test results of total power consumption and the output amplitude also agree with simulation results. The readout circuit has good noise characteristics and the output noise is lower than 1 mV. When the readout circuit and uncooled infrared detector are connected, the infrared signal can be well read out. When the integration time is 20 μups, the device response is 0.294 mV/Ω. The overall system performance is very good. This circuit design based on current mirror has laid the technical foundation for developing readout circuit of the very large scale uncooled infrared detector in the future.

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

2015, 64 (11): 119101.
doi: 10.7498/aps.64.119101

Abstract +

The optical-absorption and refractive-index properties of (Mg0.97, Fe0.03)O ferropericlase crystals without and with Mg and O ionic divacancy point-defect under the pressure of the Earth’s lower mantle are investigated using the first-principles calculations. Optical-absorption data show that the perfect-crystal results are similar to the predictions from the crystal-field theory:the pressure-induced spin transition of iron in ferropericlase causes a large blue-shift in its optical-absorption spectrum, leaving the near-infrared region transparent. However, when there are point defects in ferropericlase, the calculated optical-absorption results are completely inconsistent with predictions from the crystal-field theory, the spin transition causes the enhancement in the optical absorption in the near-infrared region. Refractive-index data of defect crystal indicate that the effects of pressure, wavenumber, and spin-transition on the high-pressure refractive-index of (Mg0.97, Fe0.03)O ferropericlase are obvious, but perfect-crystal results show that those effects should be relatively weak. The ～15%-20% iron-bearing ferropericlase is currently considered as an important mineral in the Earth’s lower mantle. Due to similar characteristics of the observed high-pressure optical-absorption spectrum in ferropericlase with different iron content, we suggest that:(1) the above-mentioned calculated results is conducive to the understanding of high-pressure optical properties of lower-mantle ferropericlase and the exploring of the origin of discrepancies in its high-pressure optical-absorption spectrum between experiment and crystal-field theory; (2) the high-pressure optical-absorption spectrum measurements may be a good approach for probing iron spin state.