Vol. 66, No. 20 (2017)
All-fiber spectral compression of femtosecond pulse for coherent anti-Stokes Raman scattering excitation source
2017, 66 (20): 204207. doi: 10.7498/aps.66.204207
Coherent anti-Stokes Raman scattering (CARS) imaging of femtosecond pulses has been a research hotspot in recent years, but the wide spectrum of the femtosecond pulse limits the spectral resolution of CARS imaging. Spectral compression is considered as an effective method to solve this problem. In this work, an all-fiber chirp spectral compression method of graded-index multi-mode fiber/single-mode fiber (GI-MMF/SMF) structure based on fiber pre-chirp and self-phase modulation is presented. It can be used as a CARS excitation source to increase the spectral resolution of CARS imaging. In the section of numerical simulation, the mean group velocity dispersion value of GI-MMF is used as a numerical parameter of the chirp analysis, which is estimated by analyzing modes of GI-MMF. On one hand, the mode field distributions in GI-MMF are simulated numerically by the finite-difference time-domain method, and these different modes are divided into eight mode groups. On the other hand, the energy proportion of each mode group is regarded as a weight value. Then we can obtain a mean group velocity dispersion value of 50/125 m GI-MMF, which is -2.28710-5 fs2/nm, by calculating the sum of group velocity dispersion weight values of mode groups. The results of spectral compression with different length ratios of 50/125 m GI-MMF to 780HP SMF are also analyzed based on the generalized nonlinear Schrdinger equation and split-step Fourier algorithm. The spectral width of 2.486 nm and the compression ratio of 5.230 are calculated, when the length ratio of 50/125 m GI-MMF to 780HP SMF is 1.2. In the section of experiment, three kinds of GI-MMFs with different core diameters are used in the experiment, the influences of the core diameter and the length ratio of GI-MMF to 780HP SMF on the spectral compression are investigated. The results show that the spectral width of 2.243 nm, corresponding to the compression ratio of 5.796 is obtained, when the length ratio of 50/125 m GI-MMF to 780HP SMF is 1.2, which is consistent with the simulation result. Under the condition of the same length ratio, the use of 105/125 m GI-MMF can raise the compression ratio to 152.941, and the spectral width of output pulse is 0.085 nm. When the pulse is applied to CARS spectrum detection, the theoretical spectral resolution can be 1.386 cm-1. The experimental results show that the spectral compression way to improve spectral resolution of CARS imaging is effective. This spectral compression system is characterized by simple structure, and high and controllable compression ratio, which provides theoretical and experimental basis for the all-fiber high spectral resolution CARS excitation source research.
2017, 66 (20): 208101. doi: 10.7498/aps.66.208101
Diamond has been considered as an ultimate semiconductor, which has great potential applications in high power, high frequency semiconductor devices. Up to now, the twodimensional hole gas (2DHG) induced on the hydrogenterminated diamond surface is used most popularly to form electric conduction in diamond semiconductor at room temperature, due to the obstacle caused by lacking of easily-ionized dopants. A 200-nm-thick single crystalline diamond is grown by microwave plasma chemical vapor deposition on the type-Ib high-pressure high-temperature synthesized diamond substrate. Then the sample is treated in hydrogen plasma atmosphere to achieve hydrogen terminated diamond surface. The sample is characterized by X-ray photoelectron spectroscopy and atomic force microscope. After that, the normally-on hydrogen-terminated diamond field effect transistors are achieved. The device with a gate length of 2 μup m delivers a saturation leakage current of 96 mA/mm at gate voltage VGS=-6 V, at which, however, the gate leakage current is too large. The saturation current reaches 77 mA/mm at VGS=-3.5 V with safety. The device shows typical long-channel behavior. The gate voltage varies almost linearly. In the saturation region of the device, the transconductance (gm) increases near-linearly to 30 mS/mm with the increase of the gate voltage in a range of 5.9 V. Analyses of the on-resistance and capacitance-voltage (C-V) data show that the 2DHG under the gate achieves a density as high as 1.99×1013 cm-2, and the extracted channel carrier density and mobility are always kept increasing with VGS negatively shifting to -2.5 V. The nearlinearly increasing of gm in a large VGS range is attributed to high 2DHG density, quite a large gate capacitance (good gate control), and increased mobility. The relevant researches of improving the carrier mobility in the channel and of finding proper gate dielectrics to improve the forward gate breakdown voltage are underway.
2017, 66 (20): 208102. doi: 10.7498/aps.66.208102
A new contactless technique called Lorentz force particle analyzer (LFPA) with an array probe for detecting the flaws in metallic material is presented in this paper. Based on the principle of LFPA, the shape and size of the flaw or the direction of the crack can be obtained by analyzing the pulses of the force acting on the permanent magnet. In the LFPA system, the small Lorentz force on the magnet is measured by a laser-cantilever system with high sensitivity, which operates in a similar principle to that of an atomic force microscope. The traditional displacement detecting method in the LFPA is not suitable for the array probe presented in this paper due to its complex structure. Therefore, speckle pattern interferometry is introduced into the LPFA. The speckle pattern interferometry can measure not only the out-of-plane displacement of the multiple cantilever in the array probe, or of slopes of deformation, but also the in-plane displacement. Those advantages make the speckle pattern interferometry a useful tool in the LFPA for analysing the shapes of the flaws and the directions of the cracks. In this paper, a Michelson-type shear of graphic setup with enlarged angle of view is built to measure the displacement of the cantilever which is deformed by the flaws in the sample. Four frames of shear under several grams before and after the deformation are captured and recorded by a digital camera. The phase difference is processed for calculating the displacement with the software which is designed for the LFPA. A full-field measurement of the cantilever displacement is achieved and the relationship between the phase difference and the volume of the flaws is also obtained successfully. The utilization of the speckle pattern interferometry technique in the LFPA leads to the invention of a new real-time, online, in-situ contactless technique of detecting the shapes of the internal flaws and the directions of the cracks.
2017, 66 (20): 200201. doi: 10.7498/aps.66.200201
Neuronal firing plays a key role in the neuronal information transmission, and different neuronal firing patterns are reported, such as spiking, bursting. A number of neuron models are introduced to reproduce the firing patterns of single neuron or neuronal network. The key factors determining the firing pattern gain more and more attention in the study of neuron system, such as noise, network topology. Noise is able to induce sub-or super-threshold coherent neuronal firing easily, and a number of coherence resonances are reported in the noise induced firing. The network topology determines the synchronization of the firing patterns of the neuronal network, and the change of network topology may induce fruitful synchronization transitions. It is well known that synapses exhibit a high variability with a diverse origin during information transmission, such as the stochastic release of neurotransmitters, variations in chemical concentration through synapses, and spatial heterogeneity of synaptic response over dendrite tree. The collective effect of all of these factors might result in the notion of dynamic synapses. In reality, the neuronal network often involves time delay due to the ?nite signal propagation time in biological networks. Recently, neuronal networks with time delay have received considerable attention. Delay-sustained neuronal firing patterns may be relevant to neuronal networks for establishing a concept of collective information processing in the presence of delayed information transmission. According to the above-mentioned motivations, the firing dynamics of the single postsynapic neuron is investigated based on a simple postsynaptic neuron model by using numerical simulation and Fourier transform analysis. In this model, the postsynapic neuron receives dynamic synaptic currents from a population of presynaptic neurons. It is found that the firing rate resonance between the pre-and postsynaptic neuron determines the firing of the postsynaptic neuron. Stimulus currents in specific frequency range are easy to stimulate postsynaptic neuron firing. The random currents released from dynamic synapses determine the postsynaptic firing rate. Then the single postsynaptic neuron is extended to a neuronal network, in which 100 neurons connect to its 4 nearest neighbors regularly and receive delayed synaptic currents from connected neurons. All the neurons in the network receive the same dynamic synaptic currents from the presynaptic neurons. The results show that the synaptic coupling in the network is able to promote the neuron firing in the network, and time delay in the synaptic coupling could reinforce the promotion, but the mode of the promotion is not changed.
2017, 66 (20): 200301. doi: 10.7498/aps.66.200301
It is well known that Bell inequalities are derived under the assumptions of locality and realism. Bell inequalities impose strict constraints on the statistical correlations of measurements of multipartite systems. Violating each of them guarantees the existence of quantum correlations in a quantum state. A quantum state with non-vanishing entanglement may violate some Bell inequalities. Recent progress of the fields like quantum biology and quantum thermodynamics reveals a particular role of quantum coherence in quantum information processing. Quantum coherence is identified by the presence of off-diagonal terms in the density matrix. To quantify quantum coherence of a given state, Baumgratz et al. (Baumgratz T, Cramer M, Plenio M B 2014 Phys. Rev. Lett. 113 140401) provided several kinds of coherence measures such as l1-norm of coherence and relative entropy of coherence. In this paper, we propose to use quantum coherence to derive Bell inequalities. We construct the Bell inequalities of four-partite product states with l1-norm of coherence, relative entropy of coherence. In the Bell inequalities of four-partite correlations, measurement operators are products of local measurement operators. Each local operator is one of the two arbitrary observables. We consider the violations of the four-partite Bell inequalities by the four-partite general pure Greenberger-Horne-Zeilinger (GHZ) state, cluster states, W states with real coefficients. We also investigate the violations of the four-partite Bell inequalities by the four-partite GHZ class mixed states, cluster class mixed states, W class mixed states and Dicke class mixed states. It is shown that the four-partite Bell inequalities in terms of relative entropy of coherence are always violated by the four-partite general pure GHZ states, cluster states with the real coefficients. Hence there is non-vanishing entanglement for these states.
2017, 66 (20): 200501. doi: 10.7498/aps.66.200501
The electrocardiogram (ECG) has broad applications in clinical diagnosis and prognosis of cardiovascular diseases. The accurate description for the question how the ECG come from the cardiac electrical activity is helpful for understanding the corresponding relation between the ECG waveform and cardiovascular disease. Experience is the primary method of studying the ECG, but the computer simulation method makes it more convenient to explore the effect of given factor for ECG waveform. Cellular automaton is a simple and effective computer simulation method. However, the cellular automaton model considering the main structure of the heart is not yet established. Therefore, we propose a cellular automaton model for the ECG considering the atria, the ventricle, and the ventricular septum. With this model, the conduction of the myocardial electrical activation is simulated by following the field potentials under healthy and diseased conditions, and the underlying mechanisms are analyzed. Through the computer simulations and analyses the results are obtained as follows. First, the conduction process of the electrical signal in this model is the same as that in the real heart. Second, under the healthy conditions, the behavior of the field potential appears as normal ECG, in which the P wave and the QRS wave group come from the depolarization of the atria and ventricle, respectively, on the other hand, the T wave and J wave come from the repolarization of the ventricle. The computer results support the conclusion that the J wave appears just because the existence of the notch in the epicardial transmembrane potential curve. Third, the endocardium ischemia conditions result in the T wave inversion. The mechanism is that the action potential duration of the ischemic endocardial cells is shorter than that under normal conditions, which makes larger the transmembrane potential gradient between the endocardium and the subepicardium, and then contributes a more negative value to the field potential. Fourth, the epicardium ischemia leads to the higher T wave, and this is because the shorter action potential duration of the ischemic epicardial cells brings in a larger transmembrane potential gradient between the epicardium and subepicardium, which makes the field voltage larger. Fifth, the T wave appears earlier under the through-wall ischemia. The action potential durations of cells of the endocardium, the epicardium, and the subepicardium all become shorter under the through-wall ischemia, then the repolarization processes of all of these three walls are ended earlier, which leads to the earlier T wave. The cellular automaton model proposed in this paper provides a reference for the further study of ECG.
2017, 66 (20): 200701. doi: 10.7498/aps.66.200701
To achieve high-precision fiber-optic time transfer, the method of two-way transmission is usually used. Therefore in this paper we propose to develop a high-precision long-haul fiber-optic time transfer between multi stations by simultaneously transferring the 1 pluse per second signal, time code signal and 10 MHz frequency signal over single fiber with the same wavelength, and adopting the time division multi address (TDMA) as well as the purification and regeneration method at individual station. In this proposal, the equipment at each remote station has its own address, and the equipment at the local station can establish the periodic two-way time transfer with any remote station by using the TDMA method, therefore each remote station is synchronized with the local station. To avoid the superimposed effect of optical noises during propagation in fiber, the optical-electro-optical relay amplifiers are utilized. In the meantime the propagation delay of the fiber link is compensated for at each remote station. With the self-developed engineering prototypes, the experimental verifications are subsequently conducted both in laboratory and real field. In the laboratory, the experimental setup is built by cascading 11 rolls of 50 km-long fiber coils, and locating three monitoring devices at different fiber distances of 50, 300, and 550 km from the local station. The stabilities of the time transfer at these three points are achieved to be 16.7, 16.8, and 18.4 ps in standard deviation, and the time deviations are 1.78, 2.09, and 2.92 ps at an averaging time of 1000 s respectively. In the real field test, a field fiber link of 871.6 km in length is utilized, along which 11 self-developed time-frequency transceivers are set at the cascaded fiber-optic stations. Since only the 11th remote station is co-located at the local station, the performance and the time transfer between the 11th remote station and the local station are measured accurately. The time transfer is experimentally demonstrated with the time standard deviation of 29.8 ps and the time deviations of 3.85 ps/1000 s. The timing uncertainty on the field fiber link is also checked and gives a value of 25.4 ps. To further improve the long-term stability of time transfer, the more accurate thermal control of the lasers used in the system should be adopted to reduce the optical wavelength drift. By compressing the bandwidth of the phase locked loop module in each remote device, the short-term stability of time synchronization can also be better. This proposal can also be extended to the fiber networks with star-shaped and chain-shaped connections. Therefore time synchronization in even larger areas and more stations can be realized.
ATOMIC AND MOLECULAR PHYSICS
Theoretical studies of triple differential cross sections for electron impact ionization with neon and neon-like ions
2017, 66 (20): 203401. doi: 10.7498/aps.66.203401
Electron impact single ionization of atom or molecule, the so-called (e, 2e) reaction, is one of the basic collision processes between electron and atom or molecule. The triple differential cross section (TDCS) of the collision process can provide important data for gas discharge, celestial bodies, and electron-target interaction. A large number of experimental and theoretical studies of (e, 2e) reactions on atom targets have been carried out under different geometric conditions, such as coplanar symmetric geometry, coplanar asymmetric geometry, non-coplanar symmetric geometry, etc. However, few experimental researches of (e, 2e) reaction on ion target have been reported due to the low target source density. The difference in TDCS between atomic target and ionic target can provide more information about the (e, 2e) reaction. Thus the relevant researches on ionic targets are of significance. In this paper, adopting distorted-wave Born approximation (DWBA), the TDCSs of 2 p orbital for Ne and neon-like ions are calculated at different outgoing electron energies (3, 5, 7.5, 10, 15, 20, 30, and 50 eV) under the condition of coplanar symmetric geometry. The results indicate that the TDCSs decrease with the increase of outgoing electron energy and nuclear charge number Z. Except Ne, the TDCSs of other ions present a new structure at an outgoing electron angle of about 150. The intensity of the new structure increases with the increase of the outgoing electron energy in a region of 10-20 eV, while it decreases with the increase of the outgoing electron energy in a region of 20-50 eV. We propose a kind of double-binary collision process to rationalize the new structure. The incident electron ionizes the target atom and the following two outgoing electrons exit in the directions symmetric with respect to the incident electron direction. Then these two outgoing electrons are elastically scattered by the target ions and emitted in the backward directions. In order to confirm this explanation, we compare our calculation results with the previously reported experimental and theoretical results of elastic scattering between electron and Ne. Previous research results show that the elastic scattering cross section has a large intensity at a scattering angle of~150, and it reaches the largest intensity at an outgoing electron energy of 20 eV. These structural features are consistent with our calculated results, implying that our proposed process is reasonable.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2017, 66 (20): 204101. doi: 10.7498/aps.66.204101
Recently, the terahertz waves have attracted increasing attention due to the growing practical applications in astronomy, communication, imaging, spectroscopy, etc. While the metasurfaces, with extraordinary ability to control the electromagnetic waves, have been increasingly employed to tailor their interaction with terahertz waves and offer fascinating capabilities unavailable from natural materials. However, there are more and more requirements for the dynamical tune of the responses to electromagnetic components for the practical applications such as the terahertz stealth in variable environment. As such, considerable attention to terahertz frequencies has been focused on the tunable metasurfaces. Graphene has been proved to be a good candidate to meet the requirements for tunable electromagnetic properties, especially at the terahertz frequencies. In this paper, we design a tunable terahertz metasurface and achieve dynamically manipulating the scattering of terahertz waves. The metasurface is constructed by embedding double graphene layers with voltage control into the polyimide substrate of the diffuse scattering metasurface, which consists of the random array of rectangular metal patches, polyimide substrate, and metal ground. By adjusting the bias voltage on the double graphene layers, the terahertz scattering distribution can be controlled. At zero bias, the conductivity of graphene approaches to zero, and the random phase distribution is formed over the metasurface so that the reflected terahertz waves are dispersed into the upper half space with much lower intensity from various directions. With the bias voltage increasing, the conductivity of graphene increases, then the changeable range of the phase over the metasurface can be changed from 2up to up/4. As a result, the random phase distribution of the metasurface is gradually destroyed and increasingly transformed into a uniform phase distribution, resulting in the scattering characteristic changes from the approximate diffuse reflection to the specular reflection. The expected performance of proposed metasurface is demonstrated through the full-wave simulation. The corresponding results show that the terahertz scattering pattern of the metasurface is gradually varied from diffuse scattering to specular reflection by dynamically increasing the Fermi level of graphene through increasing the bias voltage. Moreover, the performance of the proposed metasurface is insensitive to the polarization of the incident wave. All of these indicate that the proposed metasurface can continuously control the scattering characteristics of terahertz wave. Thus, the proposed metasurface can be well integrated into the changing environment, and may offer potential stealth applications at terahertz frequencies. Moreover, as we employ complete graphene layers as the controlling elements instead of structured graphene layers in other metamaterial designs, the proposed metasurface may provide an example of relating the theory to possible experimental realization in tunable graphene metasurfaces.
2017, 66 (20): 204102. doi: 10.7498/aps.66.204102
Dealing with potential applications of metasurface in lens technologies, we propose a focusing metasurface with high polarized purity based on anisotropic elements, and then put it into application of high gain antenna with high polarized purity. Firstly, we design a metasurface cell with the polarization filtering characteristic, which is capable of transmitting the x-polarized waves efficiently while reflecting the y-polarized waves completely. By changing the metallic patch size, we can modulate the phase shift for the x-polarized transmitting waves. Then by imposing a hyperboloidal phase profile onto the surface, we design a metasurface lens with 105 mm105 mm in size, 2121 in cell number, and 30 mm in focal length. According to the principle of reversibility of light path, the spherical waves emitted from the patch antenna can be converted into plane waves by the focusing metasurface lens, which is used to improve the antenna gain. As for the experiment, we tend to obtain the metasurface lens impinged by differently polarized waves in order to study the lens response to differently polarized waves. The results show that the metasurface acts as a lens when impinged by the x-polarized waves but serves as a reflector when illuminated by the y-polarized waves. That is to say, the y-polarized waves are mostly filtered out while the x-polarized waves are efficiently transmitted and focused, which is in good accordance with the designed principle. Assuming that a patch antenna emits the x/y-polarized waves at the focal point, we obtain not only the antenna gain improved remarkably but also polarized isolation above 25 dB in the operating bandwidth of the designed metasurface. The results of the antenna application give a further proof of the designed lens which eventually contributes to the high gain and high polarized purity of the lens antenna.
Interface of photonic crystal heterostructure for broadening bandwidth of unidirectional light transmission
2017, 66 (20): 204103. doi: 10.7498/aps.66.204103
An all-optical diode (AOD) is a spatially nonreciprocal device that in the ideal case and for a specific wavelength allows light to totally transmit along the forward direction but totally inhibits light to propagate along the backward direction,yielding a unitary contrast.AODs are widely considered to be the key components for the next-generation all-optical signal processing,and completely analogous to electronic diodes which are widely used in computers for processing electric signals.Most of AOD designs suffer some serious drawbacks which make them not suitable for commercial and large-scale applications.Relatively large physical sizes are often needed,the balance between figure of merit and optical intensity is usually inadequate,and in some cases cumbersome structural designs are necessary to provide structural asymmetry.Among different approaches,the AOD based on two-dimensional (2D) photonic crystal (PC) heterostructure has shown significant advantages due to the capability of on-chip integration with other photonic devices.However,current PC heterostructure AOD (PCH-AOD) is based on the mismatch of directional bandgaps,which shows poor performance as a result of the relatively low forward transmittance (0.40) and contrast ratio (0.75) with a narrow bandwidth (about 10 nm).In order to improve the performance,here we propose a new PCH-AOD design based on the total reflection principle,which is able to achieve high forward transmittance and contrast ratio within a broad wavelength range.Our design is composed of two rectangle lattice 2D PC structures,in which periodically distributed air holes are embedded in silica (PC1) and silicon (PC2) materials,respectively.The two PCs are combined with an inclined interface along the -M direction of both PCs.In this way,the total reflection condition is satisfied when light propagates from silicon to silica material.The forward and backward propagating optical waves are incident along the -X direction of both PCs,in which direction there are transmission bands for TE mode centered at 1550~nm wavelength.A commercial software (R-soft) based on the finite-difference time-domain (FDTD) method is used to study the unidirectional transmission performance of the PCH-AOD.The results show that the forward propagating optical waves (from PC1 to PC2) can transmit efficiently through the device.In addition,we further improve the forward transmittance by exploiting the self-collimation effect of PCs and optimizing the coupling from PC1 to PC2.In the meantime,the light propagating along the backward direction (from PC2 to PC1) is blocked at the total reflection interface with near-zero transmittance.In this way,the unidirectional transmission is achieved without the reliance on the directional bandgap mismatch,and thus broad bandwidth is achieved.The AOD has a forward transmittance of 0.64 and a transmission contrast of 0.97 with a bandwidth of 553 nm at 1550 nm.The equal frequency contours (EFCs) of the PCs is plotted to demonstrate the working principle of the PCH-AOD.Finally,considering the experimental fabrication of the AOD device,we analyze the unidirectional transmission performance of a planar PCH-AOD with a finite thickness of 1500 nm.Despite a small reduction (12.3%) in the forward transmittance,the transmission contrast is maintained at about 0.97,and the unidirectional transmission bandwidth is increased to 600 nm.Therefore,our design can be implemented in practice and our work provides a theoretical framework for designing high performance PCH-AOD.In addition,our design allows an unprecedented high forward transmittance,contrast ratio and broad working bandwidth of the device at extremely low operational optical intensity,due to the total reflection condition,and the optimized forward propagation and coupling condition.The proposed device has a small footprint that is promising for next-generation on-chip applications.
2017, 66 (20): 204201. doi: 10.7498/aps.66.204201
Like the theoretical pattern of non-diffracting Bessel beams, ideal non-diffracting Mathieu beams also carry infinite energy, but cannot be generated as a physically realizable entity. Mathieu-Gaussian beams can be experimentally generated by modulating ideal Mathieu beams with a Gaussian function, and thus they are a kind of pseudo-non-diffracting beams with finite energy and finite transverse extent. The research of Mathieu-Gaussian beam propagating characteristics in free space is of great significance. In order to analytically study the propagation of Mathieu-Gaussian beams, the Mathieu function is expanded into the superposition of a series of Bessel functions in polar coordinates based on the superposition principle of light waves. It means that the Mathieu-Gaussian beam can be converted into accumulation of the infinite terms of the Bessel beams with different orders. According to the properties of the Bessel function, the free-space propagation properties of Mathieu-Gaussian beams can be studied in the circular cylindrical coordinates. Thus, a group of virtual optical sources are introduced to generate the odd Mathieu-Gaussian beams of the first kind, i.e., (2n+2)th-order, which is a family of Mathieu-Gaussian beams. Using the virtual source technique and the Green function, we derive the rigorous integral formula for the odd Mathieu-Gaussian beams of the first kind. Taking for example the first three orders with non-paraxial corrections, the analytical solution of the on-axis field of odd Mathieu-Gaussian beams of the first kind is further obtained from the integral formula. The axial intensity distribution of the odd Mathieu-Gaussian beams of the first kind is numerically calculated by the integral formula. The simulation results show that the calculation results obtained with the paraxial theory and the rigorous integral expressions of non-paraxial Mathieu-Gaussian beams are obviously different when the propagation distance of the odd Mathieu-Gaussian beams of the first kind is small. The calculation results of the two methods are coming closer and closer with the increasing propagation distance. The results indicate that the correct results can be obtained with the paraxial theory when we study the propagation of Mathieu-Gaussian beams in the far-field, but the non-paraxial theory must be used to obtain correct results when we study the propagation of Mathieu-Gaussian beams in the near-field. Owing to the complexity of the non-paraxial theory, it is difficult to obtain the exact analytic solutions of Mathieu-Gaussian beams in the near-field with the classical diffraction theory. Based on the superposition principle of light waves, by introducing the virtual source technique and the Green function, the complex Mathieu-Gaussian function can be expanded into the superposition of a series of simple Bessel functions, and the axial intensity distributions of Mathieu-Gaussian beams in the far-field and the near-field can be studied well. It will also provide a feasible method to study other complex beams propagating in free space.
2017, 66 (20): 204202. doi: 10.7498/aps.66.204202
Sheared-beam imaging technique is a non-conventional imaging method which can be used to image remote objects through atmospheric turbulence without needing any adaptive optics. In this imaging technique, the target is coherently illuminated by three laser beams which are laterally sheared at the transmitter plane and arranged into an L shape. In addition, each beam is modulated by a slight frequency shift. The speckle intensity signals scattered from the target are received by a detector array, and then the image of target can be reconstructed by computer algorithm. By far, most of studies in this field have focused on two-dimensional imaging. In real conditions, however, the surface of targets we are concerned about reveals that different depths introduce various phase delays in the scattering signal from target. This delay causes the phase-shift errors to appear between the ideal target Fourier spectrum and the Fourier spectrum received by detector array. Finally, this would result in poor image quality and low resolution. In this study, a three-dimensional target imaging model is established based on the two-dimensional target imaging model. The influence of modulated beat frequency between sheared beam and reference beam is studied on the objects with depth information, and the result shows that large beat frequency may have an adverse effect on reconstructed images. The simulation we have developed for this three-dimensional imaging model uses three targets with different shapes. Each target is divided into several sub-blocks, and we set different depth values (within 10 m) for these blocks. Then beat frequencies are increased from 5 Hz to about 1 MHz, respectively. At each pair of frequencies, the reconstructed image is recorded. Srehl ratio is used as the measure of the imaging quality. Computer simulation results show that the Srehl ratio of reconstructed images descends with the increase of beat frequency, which is fully consistent with the theory of three-dimensional target imaging proposed before. Meanwhile, we find that the depth distribution of target also has an effect on imaging quality. As for actual space targets, the maximum depth is usually not more than 10 m. Compared with the influence caused by beat frequencies, the effect produced by depth distribution is negligible. Therefore when a space target is imaged, beat frequencies play the major role in reconstructing high-quality image. The results presented in this paper indicate that in order to achieve better imaging quality in the practical application, it is necessary to select the smallest beat frequency according to the detector performance and keep the candidate frequencies away from the low-frequency noise of the detector.
2017, 66 (20): 204203. doi: 10.7498/aps.66.204203
With the development of the technology for fabricating high-quality synthetic diamond and diamond waveguide structures, more and more researchers are being involved in exploring the particular optical properties of diamond for different applications. Because of its high refractive index and nontoxicity to biological species, diamond can be used to make micro-ring resonator to detect the concentration of liquid or gas. In this paper, a single micro-ring resonator model with diamond serving as the core layer is proposed. In the model, the vertical-section of the waveguide adopts a five-layer ridge-type waveguide structure based on As2S3, SiO2 and diamond, i.e. As2S3-SiO2-Diamond-SiO2-As2S3. To investigate the optical properties of the resonator, the vertical-section of the single straight waveguide, the coupling region of the direct waveguide, and the ring waveguide are simulated with the adopted operating wavelength =1550 nm based on the coupling mode theory and micro-ring resonance theory. In addition, the distribution of the field strength for the micro-ring is described at a resonant wavelength of 1543 nm. It is very important to explore the field intensity distribution of the micro-ring for understanding how the light transmits. The transmission characteristics of the micro-ring with the change of the distance between the straight waveguide and the ring waveguide in the coupling region are also simulated. The quality factor and the influence of the coupling coefficient change on the output spectrum are studied by the transfer matrix method and the micro-ring loss is discussed. It is shown that the micro-ring resonator designed with the diamond material has good transmission characteristics. When the resonant wavelength is 1543 nm, the resonant peak reaches more than -12 dB. The quality factor is about 1.54105. When the coupling coefficient k is 0.01, the free spectral range is about 40 nm. The coupling coefficient k is determined by the distance S of the coupling region. The results show that when S is equal to 50 nm, the output spectrum has a good extinction ratio and is better compared with the other values. The error of material processing is mainly affected by size, so the output spectrum near the distance S=50 nm is studied. The result shows that in the tiny change scope, the spectral output peak is linearly related to S. The structure we suggested in this paper expands the application scope of diamond in the field of optics, and provides some guiding significance for developing the optical integrated chips.
Measuring spectral parameters of water vapor at low temperature based on tunable diode laser absorption spectroscopy
2017, 66 (20): 204204. doi: 10.7498/aps.66.204204
Accurate and reliable spectral line parameters of gas are very important for measuring gas concentration and temperature.The mainstream spectrum database (e.g.HITRAN) includes the values from theoretical computation based on different models,which have some inevitable deviations from the corresponding actual values.To address this problem,we develop a low-temperature spectral experimental platform for simulating low temperature and low pressure environment so as to accurately measure gas absorption spectral parameters.The spectral experimental platform uses the static cooling technology combined with the Dewar insulation system to maintain the quartz cell at a constant temperature.Through adjusting the electric heating and liquid helium refrigeration,we can achieve temperature change and stability.Temperature of the low temperature absorption cell can be adjusted in a range of 100-350 K with a precision lower than 0.3 K and the temperature gradient in the cell is lower than 0.01 K/cm.The length of quartz cell is 100 cm,and a reflector can be used to increase optical path for absorption.The window diameter is 76 mm,and the spectral resolution is better than 0.001 cm-1.We use a tunable diode laser spectrometer to measure absorption spectra of pure water vapor with the platform at different temperatures (230-340 K) and different pressures (10-1000 Pa).Voigt profile is the leastsquares fit to the measured spectra by using a multi-spectrum fitting routine.A filter is used to reduce electronic noise of detector signal.As spectral lines in the band of 7240-7246 cm-1 are often used in low temperature wind tunnel flow field measurements,a distributed feedback (DFB) diode laser with a wavelength of 1381 nm is used in the experiment, and five water vapor lines are selected and measured.Firstly,from the linear fitting of line area and the full width at half maximum of collisional broadening (or pressure broadening) we obtain line strengths and self-broadening half-width coefficients at different temperatures.Secondly,from nonlinear fitting of line strengths and self-broadening half-width coefficients at different temperatures we obtain the values of line strengths and self-broadening half-width coefficients at the reference temperature (296 K).In the end,comparison between our experimental results and HITRAN2012 database values shows that the maximum discrepancy between the HITRAN database and the experimental result is 10.96%.A transparent uncertainty analysis is given for the measurement values.Uncertainties of our measured line strengths are in a 1.11%-2.98% range (95% confidence level,k=2),which is smaller than those of HITRAN2012 database values (uncertainties are in a range of 5%-10%).The accurate spectral parameters are obtained experimentally,and of great significance for improving the spectrum measurement accuracy of water vapor in low temperature environment in the future.
Attosecond X-ray generation driven by the relativistic laser pulse based on the semi-analytical self-consistent theory
2017, 66 (20): 204205. doi: 10.7498/aps.66.204205
A semi-analytical theory of the interaction between a relativistic laser pulse and the overdense plasma to generate an attosecond X-ray source is presented.The physical parameters such as plasma oscillation trajectory,surface electric field and magnetic field can be given by this model,and the high-order harmonic spectrum is also calculated accurately from the solution of the plasma surface oscillations,the obtained result is consistent with the result from the PIC simulation program.This model can be valid for arbitrary laser duration,solid densities,and a large set of laser peak intensities (1018-1021 W/cm2).In addition,the model is not applicable for the small laser focal spots (less than ten times the laser wavelength),although two-dimensional effects such as the pulse finite size may significantly change the movement progress of the electrons,the laser spot can be larger than ten times the laser wavelength under the general laboratory conditions. In this model,the laser energy absorption is small,and the electron kinetic pressure is also small.Due to the radiation pressure of the laser pulse,the electrons are pushed into the solid,forming a very steep density profile.As a result,the relevant forces makes the electrons ponderomotive and the longitudinal electric field is caused by the strong electric charge separation effect.This semi-analytical self-consistent theory can give us a reasonable physical description, and the momentum equation and the continuity equation of the electric and magnetic field at the boundary allow us to determine the plasma surface oscillations.The spatiotemporal characteristics of the reflected magnetic and electric field at the boundary can allow us to determine the emitting characteristics of the high order harmonic. Our results show that the radiation of the attosecond X-ray source is dependent on the plasma surface oscillation. The plasma surface oscillates with a duration about twice the laser optical cycle,and the high-order harmonics also emit twice the laser optical cycle,thus an attosecond pulse train driven by the multi-cycle laser pulse can be formed.By using a few-cycle laser field,the smooth high-order harmonics can be obtained,which leads to a single attosecond pulse with high signal-to-noise ratio.In a word,our calculation results show that the time evolution progress of plasma surface can be controlled by changing the carrier envelope phase of the few-cycle laser pulse,and then the radiation progress of the high-order harmonics can be influenced as result of a single attosecond X-ray pulse.
Tuning upconversion fluorescence emission of -NaLuF4:Yb3+/Ho3+ nanocrystals through codoping Ce3+ ions
2017, 66 (20): 204206. doi: 10.7498/aps.66.204206
Rare-earth-doped up-conversion (UC) fluoride materials have been widely used in phosphors, color displays, optical storages, solid-state lasers, solar cells and biomedical imaging, due to the fact that their low phonon energy can effectively suppress the nonradiative multiphonon relaxation process. In this work, the NaLuF4:Yb3+/Ho3+ nanocrystals are successfully synthesized by a facile solvothermal method. The crystal structure and morphology of the NaLuF4 nanocrystals are characterized by the X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) respectively. The diffraction peaks are well consistent with those of high-purity hexagonal NaLuF4 (JCPDS No. 77-2042, P63/m space group). The TEM image reveals that the product is composed of monodisperse hexagonal rods with an average length of about 170 nm and an average diameter of 30 nm. The crystal structure and morphology do not present obvious change with the increasing Ce3+ ion concentration, which is due to the similarity in ion radius between Ce3+ and Lu3+. Under 980 nm excitation, the UC emissions of -NaLuF4:Yb3+/Ho3+ nanocrystals with different Ce3+ codoping concentrations are carefully studied. The strong green and red UC emissions of Ho3+ ions are observed in -NaLuF4 nanocrystals. It can be found that the UC emission of Ho3+ ions is tuned from green to red in -NaLuF4 nanocrystals through increasing Ce3+ ion concentrations from 0 to 12%, and the red-to-green (R/G) ratio is enhanced from 0.34 to 8.44. According to the level structure of Ho3+ ions, the red UC emission originates from the excited state 5F5. However, the population of the 5F5 excited state mainly depends on the two nonradiative relaxation processes of 5S2/5F45F5 and 5I65I7 transitions. In fact, the two nonradiative relaxation processes are very difficult to occur according to multiphonon nonradiative relaxation rate. When Ce3+ ion is introduced into the system, the red UC emission intensity and R/G ratio of Ho3+ are increased, because the energy gap from the excited state 5F7/2 to the ground state 2F5/2 is about 3000 cm-1 for Ce3+ ions, which is similar to the gaps of 5S2/5F45F5 and 5I65I7 transitions of Ho3+ ions. According to the energy conservation law, the two inefficient nonradiative processes from the 5S2/5F4 and 5I6 states of Ho3+ ions are substituted in order by resonant cross relaxation (CR) processes 5S2 (5F4) (Ho3+) + 2F5/2 (Ce3+5F5 (Ho3+) + 2F7/2 (Ce3+) and 5I6 (Ho3+) + 2F5/2 (Ce3+)5I7 (Ho3+) +2F7/2 (Ce3+) between Ho3+ and Ce3+ ions. These two resonant CR processes can transfer populations from the 5S2/5F4 state and 5I6 state to the 5F5 state and its intermediate 5I7 state, respectively. The resonant modality and the strong interaction between Ho3+ and Ce3+ ions are employed to enhance the red emission and suppress the green emission. The occurrence of CR process between Ho3+ and Ce3+ ions is further proved by the down-conversion emission spectra of Ho3+ ions under 532 and 980 nm laser excitation, respectively. We demonstrate that the highly efficient red UC emission of -NaLuF4:Yb3+/Ho3+/Ce3+ nanocrystals offers opportunities as desired optical materials for color displays, anticounterfeiting techniques and multiplexed labeling applications.
2017, 66 (20): 204301. doi: 10.7498/aps.66.204301
Seabed is an important part of the marine environment and it has a significant influence on sound propagation. Considering the fact that geoacoustic parameters are directly acquired with difficulty and complexity, a lot of researchers have focused on the inversion of them. The seabed attenuation coefficient is insensitivity to the matching field. However it has great effects on the transmission loss, mode amplitude ratios, etc. It can be inverted from measurements of these quantities. In this paper, we present an inversion scheme based on warping transform technique for estimating the seabed attenuation coefficient. It utilizes an equivalent seabed model which is constructed by using a prior and posterior knowledge. The dispersion characteristics of normal modes can be observed using the time-frequency analysis of the explosive signal recorded. The dispersion curve can be used to invert the seabed sound speed and density. The results presented by other scholars in the same circle are cited in this paper that focuses on how to obtain the seabed attenuation. Warping transform technique is used to separate and extract the normal modes. The main advantage of warping transform is that it can transform the time-frequency spectrogram into linear relationship which makes it easier to extract the normal modes. The feature of this paper lies in determining the distance normalized normal mode transmission loss. If the depths of receiving hydrophone and the explosion source are constant, the plot of normalized normal mode transmission loss versus distance is a straight line from the normal modes theory, which can be used to obtain the attenuation factor of real part of pressure. Then the seabed attenuation coefficient of the shallow water acoustic model can be calculated. In order to verify the effectiveness of this method, the warping transformation technology is used to separate and extract the first three modes from the simulated Gaussian pulse signal which is obtained in a simulated environment which is similar to the real marine environment. The extracted results are completely consistent with the numerical results. After that, the impulsive signal data collected in the Yellow Sea are analyzed according to the scheme process, and the relationship between the seabed attenuation and frequency is =0.581fk1.86(dB/m) in a range from 150 Hz to 550 Hz. The results are in good agreement with those obtained by other scholars in the same circle. On the other hand, the inversion results of seabed attenuation from different modes can be used for comparison at the same frequency, which can be a good support for the result.
Comparative studies on constructal optimizations of discrete heat generation components based on entransy dissipation minimization and maximum temperature minimization
2017, 66 (20): 204401. doi: 10.7498/aps.66.204401
A three-dimensional (3D) turbulent heat dissipation model of cylindrical discrete heat generation components is established on a conductive basis. The whole solid section is set in a square channel with adiabatic walls, and the components, cooled by clean air flowing through the channel, are arranged in a line with equal spacings. The influences of the heat conductivities of the components, intensities of heat sources and velocity of fluid flow on the maximum temperature (MT) of components, the equivalent thermal resistance (ETR) based on entransy dissipation of the heat dissipation system, and the averaged Nu number are investigated with the constructal theory considering variable properties, compressibility and viscous dissipation of air. The total heat generation rate and the total heat conductivity of heat sources are fixed as the constraint conditions. The circumstances in which heat generation rates and heat conductivities of heat sources are unequal are considered. The results show that for the fixed total heat generation rate of heat sources, despite MT or ETR that is taken as the performance index for thermal design, there exists an optimal intensity distribution of heat sources for the best thermal performance of the system. In fact, for different objectives, the optimal intensity distributions of heat sources are corresponding to the best match between the distributions of heat sources and the distributions of temperature gradient. There are different optimal distributions for different velocities of the fluid flow and different optimization objectives. Besides, the averaged Nu number increases with the increase of intensity difference in heat sources, which means that the convective heat transfer is enhanced, but this phenomenon is relatively weak when the velocity of fluid flow is low. For the fixed total heat generation rate of heat sources, when the intensities of heat sources are equal and the thermal conductivities of heat sources are lower than that of the conductive basis, increasing heat conductivities of the heat sources can evidently improve thermal performance of the system; the MT can be lowest when the conductivities of heat sources increase along the fluid flow; and the ETR is lowest when the conductivities of heat sources are equal. Both the MT and the ETR decrease with the increasing velocity of fluid flow. The results can provide some theoretical guidelines for the practical thermal design of the electronic components with different materials and different heat generation rates.
Simulation of effect of bottom heat source on natural convective heat transfer characteristics in a porous cavity by lattice Boltzmann method
2017, 66 (20): 204402. doi: 10.7498/aps.66.204402
The influence of bottom heat source on natural convective heat transfer characteristics in a two-dimensional square cavity fully filled with a homogeneous porous medium is numerically studied by the lattice Boltzmann method. In this physical model, the upper wall of porous cavity is set to be a cold heat source, and the bottom wall is designed as a local hot heat source. Both the left wall and the right wall are set to be adibatic. Specifically, the effects of both the position and size of bottom heat source on the properties of natural convective heat transfer are analyzed. The numerical results show that the position and size of bottom heat source have great influences on the characteristics of natural convective heat transfer, and there also exist the best position (a=4/16) and optimal size (b=0.75) of the bottom heat source for the maximal convective heat transfer intensity (Numax 10.35) and heat exchange capacity (Qmax 5.69).
2017, 66 (20): 204501. doi: 10.7498/aps.66.204501
In order to make it easier to investigate some problems such as the mechanism of Janssen effect and the stress distribution in granular medium, we simplify a granular column into a lattice system, in which a lattice point represents a small lump of granular medium and only neighbor interactions are considered. To study the disordered granular columns, a force propagation lattice model determined by the absorption coefficient p and the lateral transfer coefficient q is proposed, and this model is analyzed from the theoretical view. Firstly, the equation of force propagation in the matrix form is given, and this equation is determined by a tridiagonal matrix A(p,q) that is called transfer coefficient matrix. Based on the force transfer equation, the bottom force distribution varying with the top force distribution and the layer of lattice system is deduced, and its analytical solution refers to the similarity diagonalization of matrix A(p, q). Then, a method based on the second order difference equations is proposed to obtain the eigenvalues and eigenvectors of the transfer coefficient matrix. The eigenvalues and eigenvectors of A(p, q) can be rigorously deduced for a typical case, and with these results the pressure distribution relationship between the top and the bottom of the container is given. Based on these theoretical expressions, the relationship between the effective mass and the total mass of granular medium is deduced, and it means that the force propagation model and the Janssen model can lead to similar results. Moreover, the bottom stress distribution is calculated without the top load. Calculations show that the stress distribution reaches a maximum at the center bottom and drops down to either side. Finally, numerical calculations are performed to investigate the effects of parameters p and q on the relation between bottom pressure and packing height. Numerical results show that the saturated value of pressure decreases while parameter p or q increases.
2017, 66 (20): 204701. doi: 10.7498/aps.66.204701
In this paper, a stability analysis is given to study the unstable mechanism of the Richtmyer-Meshkov flow of explosion-driven copper interface. The Richtmyer-Meshkov flow refers as an interfacial instability growth under shockwave incident loading. Numerical investigations are performed to check the applicability of the two-dimensional hydrocode, which is named AFE2D, and the physical models of detonation waves propagating in the high explosives, equations of state and the constitutive behaviors of solids in the analysis of Richtmyer-Meshkov flow problems. Here we theoretically analyze the two key issues of the unstable mechanism in Richtmyer-Meshkov flow in solids. The unstable mechanism includes temperature related melting mechanism and the plastic evolution related tensile fracture mechanism. In the analysis of the temperature related unstable mechanisms, the calculated temperature increase during the shockwave compression from the shock Hugoniot data in the shockwave physics is not enough to melt the material near the perturbed interface. On the other hand, the temperature increase from the translation of plastic work during perturbation growth which relats to the distribution of the cumulative effective plastic strain is also not enough to supply the thermal energy which is needed to melt the crystal lattice of solid, either. Therefore, the temperature related melting mechanism is not the main factor of the unstable growth of copper interface under explosion driven. In the analysis of the plastic tensile fracture related unstable mechanism, a scaling law between the maximum cumulative effective plastic strain and the scaled maximum amplitude of spikes is proposed to describe the relationship between the plastic deformation of material and the perturbation growth of interface. Combined with a critical plastic strain fracture criterion, the unstable condition of the scaled maximum amplitude of spikes is given. If the spikes grow sufficiently to meet the unstable condition, the interfacial growth will be unstable. Numerical simulations with varying initial configurations of perturbation and yield strength of materials show good agreement with the theoretical stability analysis. Finally, a criterion to judging whether the growth is stable is discussed in the form of competition between the temperature related unstable mechanism and the tensile fracture unstable mechanism.
2017, 66 (20): 204702. doi: 10.7498/aps.66.204702
Boundary-layer receptivity is the initial stage of the laminar-turbulent transition process, which plays a key role in the transition, especially for the case of three-dimensional boundary-layer flow. The research of the three-dimensional boundary-layer receptivity is theoretically significant for further understanding of the mechanisms of laminar-turbulent transition and turbulence formation. A numerical method is used to study the three-dimensional boundary-layer receptivity under the interaction of the free-stream turbulence and the three-dimensional localized wall roughness. Then whether a new cross-flow instability mode can be found in the three-dimensional boundary layer is studied. Subsequently, investigated are the conditions under which the steady or unsteady cross-flow instability mode can be induced in the three-dimensional boundary layer, the influences of the intensity, spanwise wave number and normal wave number of the free-stream turbulence, and the size and structure of the three-dimensional roughness on the three-dimensional boundary-layer receptivity under the free-stream turbulence interacting with the three-dimensional localized wall roughness, and the instability mode that can be induced and its role in the three-dimensional boundary-layer receptivity. The numerical results show that when the turbulence intensity is low, the steady cross-flow vortex excited by the three-dimensional localized wall roughness dominates the three-dimensional boundary-layer receptivity; on the contrary, when the turbulence intensity is high, the unsteady cross-flow vortex excited by the free-stream turbulence dominates the receptivity; additionally, when the interaction between the three-dimensional localized wall roughness and the free-stream turbulence is existent, three kinds of instability modes are all produced at the same time, namely, the steady cross-flow vortex, the unsteady cross-flow vortex and the new unsteady cross-flow vortex whose dispersion relation is equal to the linear combination of the positive and negative spanwise wave numbers of the first steady cross-flow vortex and the second unsteady cross-flow vortex. The in-depth research on the three-dimensional boundary-layer receptivity under the interaction of the free-stream turbulence and the three-dimensional localized wall roughness is of benefit to accomplishing the hydrodynamic instability theory and the turbulence theory, and providing the theoretical foundation for the prediction and control of the laminar-turbulent transition.
Validation and analysis of gas-kinetic unified algorithm for solving Boltzmann model equation with vibrational energy excitation
2017, 66 (20): 204703. doi: 10.7498/aps.66.204703
With the increase of temperature in flow field,gas molecules possess not only rotational degree of freedom,but also vibrational energy excitation.In order to simulate and study the influence of internal energy excitation on polyatomic gas flow with high temperature and high Mach number,according to the general Boltzmann equation,we consider the rotational and vibrational energy modes as the independent variables of gas molecular velocity distribution function.It is assumed that the rotational and vibrational energy modes are described by continuous distribution with degree of freedom and temperature.Based on the Borgnakke-Larsen collision model used in direct simulation Monte Carlo (DSMC) method, the collision term of Boltzmann equation with internal energy excitation is divided into elastic and inelastic collision terms.The inelastic collision is decomposed into translational-rotational energy relaxation and translational-rotationalvibrational energy relaxation according to a certain relaxation rate obtained from the reciprocalities of rotational and vibrational collisions numbers per one elastic collision.Then a kind of Boltzmann model equation considering the excitation of vibrational energy is constructed.For showing the consistency between the present model equation and Boltzmann equation,the conservation of summational invariants and the H-theorem of this model are proved.When solving the present model equation with numerical methods,because of the continuous energy modes,it is difficult to simulate this model equation directly.In this paper,three control equations are derived and solved by the LU-SGS (lower-upper symmetric Gauss-Seidel) method,and the cell-centered finite volume method with multi-block patched grid technique in physical space.As a result,these gas-kinetic unified algorithm (GKUA) with vibrational energy excitation has been developed.Results are presented for N2 with different Knudsen numbers around cylinder from continuum to rarefied gas flow by using the present Boltzmann model equation,GKUA with simple gas model,and DSMC method. Very good agreement between the present model and DSMC results is obtained,which shows that the accuracy and reliability of the present model.Comparing the translational,rotational,vibrational,and total temperatures computed by different methods,the effects of the rotational and vibrational degrees of freedom are demonstrated.For the simple gas model,the translational temperature is much higher than those for the other two models with internal energy excitation. At the same time,the distance from shock wave to wall for the simple gas model is about twice those for the other two models.On the other hand,the obtained aerodynamic force coefficients of the cylinder are increasing according to the sequence from the simple gas model to the rotational energy excitation model to the vibrational energy excitation model, but the variation range is very small.By reducing the gas characteristic vibrational temperature,the temperature after the shock wave is much lower,and the heat flux declines evidently at the stagnation point with the same temperature as the wall temperature.This implies that with the wall temperature increasing the heat flux declines.
2017, 66 (20): 204704. doi: 10.7498/aps.66.204704
Fluid flow and heat transfer in a nanochannel may depart from the traditional behavior due to the scale effect, and the velocity slip and temperature jump at the fluid-solid interface must be taken into account. A lot of papers about fluid flows in nanochannels with the same wettability at two surfaces have been published. It is necessary to investigate fluid flow and heat transfer in nanochannels with the asymmetric wettability by the molecular dynamics method. The fluid velocity and temperature distributions, interfacial velocity slip and temperature jump in a rough nanochannel are evaluated. The effects of asymmetric wettability on the velocity slip, temperature jump and internal fluid heat transfer are analyzed. The results indicate that the velocity of the fluid flow under an external force in a nanochannel in a bulk region is of a parabolic distribution, but the parabolic distribution is not centrosymmetric because of the centrosymmetric density profile. The difference in density distribution can affect the fluid flow. Viscous dissipation due to shear flow will increase the fluid temperature. The range that is affected by the interaction between solid and liquid is small. So the wettability of the cold wall hardly affects the velocity of the fluid near the hot wall, and the slip velocity is almost constant. At this time, the negative slip will take place at the fluid-solid interface near the hot wall. But the velocity of the fluid near the cold wall comes up with the increasing hydrophobicity of the cold wall, and the slip velocity increases. The temperature jump on both sides of interface increases with the increasing hydrophobicity of the cold wall, but the degree of temperature jump at a liquid-cold solid interface is higher than that at a liquid-hot solid interface. Then the fluid temperature near the cold wall gradually exceeds the fluid temperature near the hot wall. The internal heat flow of the fluid will be reversed. The inverted temperature profile of the fluid will appear. The inverted temperature profile becomes more obvious when the degree of asymmetric wettability increases.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2017, 66 (20): 205201. doi: 10.7498/aps.66.205201
The protons accelerated by ultra-high intensity laser have been extensively studied. The most commonly used detectors for measuring laser-driven proton are Tomspon parabola ion energy analyser (TP) and filtered nuclear track detectors, such as radiochromic films (RCF). The TP uses a parallel magneto-electric field to distinguish ions. This conventional technique can precisely identify the species and energy spectra of ions. However, the strong electromagnetic field produced by the laser-plasma interaction has an effect on TP, which results in no spatial resolution of TP. The RCF can give the spatial integration spectrum of proton, but it is easy to be saturated and cannot be reused anymore. In this paper, we present a method based on the traditional charged particle activation analysis and the gamma-gamma coincidence measurement to measure the spectrum of protons accelerated by ultra intense lasers. In this method, a copper plate stack is placed in the proton emission direction. Colliding with MeV proton converts 63Cu in the copper plates into radionuclide 63Zn whose decay can be easily observed and measured. Proton spectrum is then recovered from 63Zn decay counts from layers in the copper stack. The layout of diagnostics and the method to solve proton spectrum are discussed in detail and a self-consistent test is given. This spectrum analysis method is used in a laser-driven proton acceleration experiment carried out on XG-Ⅲ laser facility. The results show that protons up to 18 MeV are obtained, and the spatial integrated spectrum and a laser-proton conversion efficiency of 1.07% are achieved. In conclusion, our method has some advantages as a laser-driven ion diagnostic tool. It has no saturation problem and is not affected by strong electromagnetic fields. The basic principle of charged particle activation analysis is based on nuclear reaction, and can be extended to the measuring of other charged particle beams besides protons, such as deuterons, helium ions produced by ultra-high intensity laser.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMALPROPERTIES
2017, 66 (20): 206101. doi: 10.7498/aps.66.206101
Semi-insulating cadmium zinc telluride (CdZnTe or CZT) is an excellent material candidate for fabricating room-temperature nuclear radiation semiconductor detectors due to its high resistivity and good carrier transport behaviors. It is widely used in nuclear security, nuclear medicine, space science, etc. Nevertheless, the traditional CdZnTe planar detector is subjected to the effect of hole trailing on its hole transport characteristic, where its energy resolution and the photoelectric peak efficiency both decrease, and thus deteriorating the detection performance. In order to eliminate the effect of hole capture, the electrode with pixel structure for CdZnTe detector is designed for detecting single carriers that are only electrons. In this paper, a 10 mm10 mm2 mm wafer cut from an In doped Cd0.9Zn0.1Te single crystal, grown by the modified vertical Bridgman method, is employed to fabricate a 44 CdZnTe pixel detector, which is composed of 16 small pixel units with an area of 2 mm2 mm. Each of the pixel units is linked up with ASIC multichannel preamplifier and shaping amplifier by flip chip technology. Finally, the signal is treated by an integrated sensing chip. In the first case, the electrical properties and carrier transport properties of CdZnTe pixel detector are characterized by current-voltage (I-V) measurement via an Agilent 4155C semiconductor parameter analyzer and ray energy spectrum response via a standard Multi Channel Analyzer 6560 spectra measurement system, respectively. In the second case, the differences between CdZnTe planar detector and 44 pixel detector in the detection performance are discussed in detail. The results indicate that the bulk resistivity of CdZnTe pixel detector is determined to be about 1.7310 cm by a linear fit of I-V curve. The maximum leakage current of a single pixel is less than 2.2 nA for a bias voltage of 100 V. Furthermore, the carrier transport behaviors are evaluated with the mobility-lifetime product for electron in CdZnTe detector, which is 5.4110-4 cm2V-1 estimated by ray energy spectroscopy response under various bias voltages from 50 to 300 V at room temperature. The energy resolutions of the two CdZnTe detectors can reflect the ability of them to distinguish different energy gays during operation. The best energy resolution of a single pixel in CdZnTe pixel detector for 241Am@59.5 keV ray increases up to 5.78% under a 300 V bias voltage, whereas that of CdZnTe planar detector is only 6.85% in the same conditions. As a consequence, the detection performance of 44 CdZnTe pixel detector is better than that of the planar detector.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2017, 66 (20): 207801. doi: 10.7498/aps.66.207801
A new ion beam induced luminescence (IBIL) measuring setup, equipped with a custom-made heating/cooling sample stage (the attainable temperature ranges from 80 K to 900 K), has been established on the GIC4117 tandem accelerator in Beijing Normal University. As the yield of back scattering ions is proportional to the beam flux, an Au-Si surface barrier detector is employed to count the back scattering ions synchronously with collecting the IBIL spectra under the multi-channel scaler (MCS) mode of the multichannel analyzer, making it possible to online monitor the beam current. Then, the yield of back scattering ions is used to correct the intensity of the IBIL spectrum and calculate the ion fluence, for eliminating the influence of the beam current fluctuation. IBIL spectra of pure lithium fluoride (LiF) at different temperatures (100, 200, 290, 450, 550 K) under the 2 MeV H+ irradiation are acquired and the significant influence of temperature on luminescence centers is observed. The emission bands relating to exciton recombination (296 and 340 nm) and impurities (400 nm) are more prominent at low temperatures and present quite lower intensities at high temperatures. Moreover, these luminescent intensities decay with ion fluence increase obviously at high temperatures after initially increasing in the early period of irradiation. The initial increase of the disturbed exciton peak at 296 nm can be attributed to the strained bonds produced by nuclear elastic scattering at a low fluence, which was not observed in previous IBIL measurements under high ionization energy density or high ion beam flux. This observed increase indicates that the emission feature may also originate from the emitting centers relating to point defects, not just from exciton transition near lattice or impurities. The luminescent intensities of F2 color centers (peaked at 670 nm) are dominant at all temperatures, while the luminescent intensities of F3+ color centers (peaked at 540 nm) are not obvious at low temperatures and the luminescent intensities of F3-/F2+ color centers (peaked at 880 nm) are weak at high temperatures. The luminescent intensities of these F-type centers reach saturated values at lower fluences at high temperatures. The different evolution behaviors under different temperatures can be due to the influence of temperature on the vacancy migration rate and the non-radiative recombination. In addition, the surface charge accumulation may lead to the luminescent intensities of color centers reaching saturated values at higher fluences, compared with the previous IBIL measurements of LiF. The self-absorption effect would reduce the intensities of F3+ color centers because of the absorption of F-type centers at low temperatures, while the effect is weak at high temperatures due to the degradation of F-type centers.
Comparison between the 1st and 3rd order mode temporal characteristics of two-sided multipactor discharge in cavity
2017, 66 (20): 207901. doi: 10.7498/aps.66.207901
For investigating the influence of high order two-sided multipactor discharge on the accelerator field-building process, the temporal characteristics of the 3rd order two-sided multipactor discharge in oxygenfree copper cavity are studied numerically. The particle-in-cell and Monte-Carlo methods are used in the simulation and the characteristics of the 1st order mode are also studied for comparison. The numerical results can be concluded as follows. In the multipactor discharge evolution, the electron number, discharge current, deposited and discharge power increase exponentially and tend to be saturated. At the saturation stage of the 3rd order mode, the values of electron number, discharge current, deposited and discharge power are lower than at the saturation stage of the 1st order mode. Meanwhile, the rising time of waveform in the 3rd order mode is longer than in the 1st order mode. There is a time-delay phenomenon in the waveform of discharge current, which results in a partial charging process in multipactor discharge. The average value of the discharge power is equal to the average deposited power. The value of discharge power in the 3rd order mode is about 1% of that in the 1st order mode. Therefore, the 3rd order mode is not significant in accelerator field-building process compared with the 1st order mode. The characteristic of the 1st order two-sided multipactor discharge is the accelerated motion of single electron beam, while that of the 3rd order is the complex accelerated-decelerated-accelerated motion of multi-electron beams. When the multipactor discharge enters into the saturation stage, the space charge effect of the 3rd order mode is not stronger than that of 1st order mode.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCEAND TECHNOLOGY
2017, 66 (20): 208201. doi: 10.7498/aps.66.208201
Using solid electrolyte instead of liquid electrolyte is regarded as an important measure to solve the safety problems of lithium ion batteries, and has attracted wide attention of researchers. Among many solid electrolytes, Li1.3Al0.3Ti1.7(PO4)3 (LATP) is considered to be one of the most commercially available solid electrolytes for its high ionic conductivity. However, as a replacement substitute of for liquid electrolyte, the LATP solid electrolyte has an ionic transport property of LATP solid electrolyte that still needs to be improved. In this paper, LATP solid electrolyte used for lithium ion batteries is successfully prepared by solid reaction process, and the influences of different sintering temperatures and addition of flux B2O3 and or LiBO2 on the ionic conductivity of LATP solid electrolyte are discussed. The structures, element content, morphologies, and ionic conductivities of the sintered samples are investigated at room temperature by X-ray diffraction, energy dispersive spectrometer, electrochemical impedance spectrum and scanning electron microscopy. It is found that pure phase LATP ceramic solid electrolyte can be obtained at the sintering temperatures between 800 and 1000℃. And the ionic conductivities of the samples first increase first and then decrease with the increasing sintering temperatures increasing. The sample with a highest ionic conductivity of 4.1610-4 S/cm can be obtained at the a sintering temperature of 900℃. Further research shows that the ionic conductivities of the sintered samples can also be effectively improved by using B2O3 instead of LiBO2 as flux. Moreover, the ionic conductivities of the samples first increase first and then decrease with the increasing amount of the flux increasing. And the highest ionic conductivity of 1.6110-3 S/cm is obtained with the sampleby adding B2O3 with a mass fraction of 2% into the sample. The results indicate that the elevating of sintering temperature and the adding of flux B2O3 and or LiBO2 can both decreasing reducing the grain boundary impedances of the LATP samples, so as to thereby improve improving their ionic conductivities. However, when the sintering temperature is higher than 900℃ or the amount of flux B2O3 and or LiBO2 exceeds the mass percentage of 2%, the ionic conductivities of the LATP samples will drop. In addition, the ionic conductivities of the samples used using B2O3 as flux are higher than that those of the samples used LiBO2 as flux. These results also indicate that the increases of ionic conductivities of LATP samples with flux is are closely related to their densities density and compactness, and is irrespective of no matter whether or not the flux contains lithium ion.
Method of optimizing secondary impedances for magnetically-insulated induction voltage adders with impedance under-matched loads
2017, 66 (20): 208401. doi: 10.7498/aps.66.208401
The magnetically-insulated induction voltage adder (MIVA) is a pulsed-power accelerator widely used in the X-ray flash radiography and -ray radiation simulation. The operating impedance of magnetically-insulated transmission line (MITL) on the secondary side of MIVA will produce significant influence on the power coupling between the pulsed-power driving source and the terminal load. Therefore, optimizing the secondary impedance of MIVA to maximize the electrical-power or radiated output of load is critical for the design of MIVA facility. According to whether the MITL operating impedance is smaller than the load impedance, MIVAs can be divided into two different types, i.e., the impedance-matched case and impedance undermatched case. For the impedance-matched MIVA, because the MITL of MIVA operates at the minimal current point or self-limited flow, the output of MIVA just depends on the MITL operating impedance and is independent of load. Correspondingly, the circuit analysis is relatively easy. However, for MIVA with impedance undermatched load, the analysis method is more complicated. Based on the classical Creedon theory of the magnetic insulation equilibrium and the sheath electron re-trapping theory, a circuit method is established for MIVA with impedance under-matched load. The analysis process consists of two steps. Firstly, the working point of the forward magnetic insulation wave is solved by the minimal current theory on the assumption that the MIVA is terminated by impedance-matched load. Then, the actual operating point after the re-trapping wave has passed is solved, in which the characteristic impedance of the re-trapping wave is treated as a vacuum impedance. And the relationship between the output parameters of MIVA, e.g., the output voltage, the cathode and anode current, and the electrical power, and the undermatched extent of load is obtained numerically. Based on the analysis method, a method to optimize the secondary impedance of MIVA with ten-stage cavities stacked in series to drive X-ray radiographic diodes is developed. This optimization method aims at maximizing the radiated X-ray dose rate of the diode loads on the assumption that only the cathode current is available for the X-ray radiographic diode. The optimization secondary impedance, Zop*, varying with the scaling factor, , is achieved, where is the power exponent between the dose rate and the diode voltage (Ḋ Ud). is usually determined by the diode type, geometrical structure, and operating characteristics. It is found that the optimization secondary impedance Zop* decays exponentially with the increase of value , i.e., the increase of the diode-voltage-dependent degree of the radiated X-ray dose rate. And the larger the load impedance, the larger the value of Zop* is. The circuit analysis method and the impedance optimization method developed in this paper are specially useful for the applications of MIVA in the flash radiographic fields.
2017, 66 (20): 208501. doi: 10.7498/aps.66.208501
Traditional complementary metal-oxide-semiconductor (CMOS) technology has reached nanoscale and its physical limits are determined by atomic theory and quantum mechanics, which results in a series of problems such as deteriorated device reliability, large circuit interconnection delay, and huge static power dissipation. In the past decades, with the discovery of giant magnetoresistance effect and tunnel magnetoresistance effect, spintronics has become a research hotspot in this field. Specially, spin transfer torque effect has been experimentally verified that the magnetization of a ferromagnet layer can be manipulated using spin polarized current rather than an external magnetic field. Spintronics is a new type of electronics which utilizes spin rather than charge as state variable for electrical information processing and storage. As an example, all spin logic (ASL) devices, which stores information by using the magnetization direction of the nanomagnet and communication by using spin current, is generally thought to be a good post-CMOS candidate. Compared with the typical metal material, the graphene material has a large conductivity, long spin relaxation time, and weak spin-orbit interaction. Therefore, the dissipation of spin current in the graphene material is weaker than the counterpart in typical metal when the injected current is identical. In this paper, the switching characteristics of all spin logic device comprised of graphene interconnects are analyzed by using the coupled spin transport and magneto-dynamics model. The results show that comparing with ASL device comprised of copper interconnects, the magnetic moment reversal time of ASL with graphene interconnection is short and the spin current flows into the output magnet is large under the condition of same applied voltage and device size. Meanwhile, the switching delay and the energy dissipation are lower when the interconnects are shorter and narrower. When the critical switching current which is required for the magnetization reversal is applied, the reliable working length of graphene interconnection is significantly longer than that of copper interconnection. So the graphene is the more ideal interconnect material than metal material. Moreover, the switching delay and power dissipation could be further reduced by properly selecting the interconnection dimension. These results mentioned above provide guidelines for the optimization and applications of ASL devices.
Theory and verification of a microwave transmission method of measuring sheet resistance of metallic thin film
2017, 66 (20): 208801. doi: 10.7498/aps.66.208801
Metallic thin films deposited on non-conductive substrates are widely used in areas like microwave absorbers, photovoltaic, packaging, electromagnetic shielding, and integrated circuits. From scientific and engineering point of view, measuring sheet resistance of metallic thin films is important. In this study, we develop a theory of evaluating sheet resistance by using transmission coefficient of a rectangular waveguide (RG) and verify it with sputtered silver films of various thickness values. According to the field distribution of RG working under the fundamental mode and corresponding electromagnetic boundary conditions, we first analytically derive the transmission coefficient of an RG with the metallic thin film exactly occupying its cross section. Comparing existing theory, we take the effect of the non-conductive substrate supporting the metallic thin film into consideration. According to this derivation, we establish a method to calculate the sheet resistance of metallic thin films from the amplitude of RG transmission coefficient. To verify our derivation, we also conduct full-wave simulations of a standard WR-75 RG used for characterizing the metallic thin film at 13.65 GHz. Both the analytical derivations and full-wave simulations show that the amplitude of the transmission coefficient depends on the logarithm of the sheet resistance in a linear manner. It is also demonstrated that the substrate effect may not be ignored. To facilitate measurement, we propose a sandwiched structure by placing the metallic thin film between two waveguide flanges. This modification removes the stringent requirements for sample preparation. Simulations of this sandwiched structure indicate that it is possible to realize non-contact measurement if the air gap between metallic thin film and waveguide flange is below 0.1 mm. Through full-wave simulations, we also show the feasibility of metallic thin film evaluation by using such transmission lines as dielectric filled RG, circular waveguide, and coaxial line. Finally, we prepare various silver films with sheet resistances ranging from 20 m/square to 1 /square (measured by the four-point probe technique) on the top of high resistance silicon and glass substrates, respectively. We measure the amplitudes of transmission coefficient of these metal films in RG by using vector network analyzer. The obtained experimental results are well consistent with the derivation and simulation results, thereby verifying the proposed method. It is recommended that the proposed method is suitable for conductive films with sheet resistances ranging from 0.05 /square to 0.5 /square. The results of this study are of potential value for characterizing the conductive thin films in micro/nano fabrication and relevant areas.
An improved evaluating method of node spreading influence in complex network based on information spreading probability
2017, 66 (20): 208901. doi: 10.7498/aps.66.208901
How to evaluate the node spreading ability and how to find influential nodes in complex networks are crucial to controlling diseases and rumors, accelerating or hindering information from diffusing, and designing effective advertising strategies for viral marketing, etc. At present, many indicators based on the shortest path, such as closeness centrality, betweenness centrality and the (SP) index have been proposed to evaluate node spreading influence. The shortest path indicates that the information transmission path between nodes always selects the optimal mode. However, information does not know the ideal route from one place to another. The message does not flow only along geodesic paths in most networks, and information transmission path may be any reachable path between nodes. In the network with high clustering coefficient, the local high clustering of the nodes is beneficial to the large-scale dissemination of information. If only the information is transmitted according to the optimal propagation mode, which is the shortest path propagation, the ability to disseminate the node information would be underestimated, and thus the sorting precision of node spreading influence is reduced. By taking into account the transmission rate and the reachable path between a node and its three-step inner neighbors, we design an improved method named ASP to generate ranking list to evaluate the node spreading ability. We make use of the susceptible-infected-recovered (SIR) spreading model with tunable transmission rate to check the effectiveness of the proposed method on six real-world networks and three artificial networks generated by the Lancichinetii-Fortunato-Radicchi (LFR) benchmark model. In the real data sets, the proposed algorithm can achieve a better result than other metrics in a wide range of transmission rate, especially in networks with high clustering coefficients. The experimental results of the three LFR benchmark datasets show that the relative accuracy of ranking result of the ASP index and the SP index changes with the sparseness of the network and the information transmission rate. When the information dissemination rate is small, SP index is slightly better than the ASP index. The reason for this result is that when the transmission rate is small, the node influence is close to the degree. However, when the transmission rate is greater, the accuracy of the ASP index is higher than those of other indicators. This work can shed light on how the local clustering exerts an influence on the node propagation.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2017, 66 (20): 209401. doi: 10.7498/aps.66.209401
A lot of electromagnetic anomalies observed by satellites before earthquakes indicate that there is interrelation between earthquake and ionosphere.China seismo-electromagnetic satellite (CSES) is the first Chinese space-based platform of three-dimensional earthquake monitoring system.The scientific payload of plasma analyzing package (PAP) aboard CSES is designed to study the possible influence of the seismic activity on the ionospheric plasma and thereby to monitor the earthquakes from space.The PAP is made up of three sensors,retarding potential analyzer (RPA),ion drift meter (IDM),and ion capture meter (ICM).The main objective of IDM is to detect the ion bulk velocity from-3 to 3 km/s with a precision better than 20 m/s,perpendicular to the sensor-look direction. The IDM sensor consists of six-layer grids and a collector.The grid is made of beryllium copper,plated with gold.Polyimide is used to achieve electrical insulation between grids.The grid transmission rate of signal layer is designed to be 82.64%,and total transmission rate of six layers is 31.85%.To ensure the performance of IDM,the side length of the square aperture and the depth of the sensor are designed to be 40 and 20 mm,respectively.The radius of segmented planar collector is 50 mm.The arrival angle of the ions is determined by measuring the ratio between the currents from the different electrically isolated collector segments.Accordingly,velocity perpendicular to the sensor-look direction is calculated,based on arrival angle and ion velocity parallel to the sensor-look direction which is measured by the RPA. In addition,a wide-range and high-precision current measurement circuit is designed to measure the current of IDM.The preamplifier circuit has three measurement ranges,providing different amplification factors.The right measurement range is chosen automatically by the field programmable gate array (FPGA).The test results show that the circuit provides a total measuring dynamic range from 20 pA to 6A with an accuracy better than 0.4%. Finally,the method of testing in the plasma environment and the measurement results are discussed.The plasma environment test of the IDM flight model is carried out in the Institute for Space Astrophysics and Planetology,National Institute of Astrophysics (INAF-IAPS).Since the plasma source is fixed to a large volume vacuum tank,the arrival angle of the plasma with respect to the sensor-look direction is changed by horizontally or vertically mounted IDM on the rotating platform in the vacuum tank.As the platform rotates,the performance of IDM is proved by testing different ion arrival angles in vacuum tank.The ion velocities along the Y and Z axes of the spacecraft are validated by testing the horizontal arrival angle and the vertical arrival angle respectively.The IDM test data are consistent with those obtained under the setting angle of the rotating platform.The experimental results show that the detector has good performance and will fulfill the mission goal of monitoring the bulk velocity of ion,perpendicular to the sensor-look direction.