Vol. 67, No. 1 (2018)
Quantum vortex phenomenon of many sub-Bose-Einstein condensations formed by expansion and superposition
2018, 67 (1): 010501. doi: 10.7498/aps.67.20171708
Based on the two-dimensional model, the formation mechanism of quantum vortex by the expansions and superpositions of the many sub-Bose-Einstein condensations (BECs) in the weak harmonic trap is studied. In the harmonic approximation, the initial wave function of the sub-BEC is Gaussian function. Once the initial wave function is known, by using the propagation method, the time evolution of the wave function for the sub-BECs could be obtained. The physical processes of the macroscopic quantum vortex formed by the symmetric distribution of the three sub-BEC expansions and superpositions are analyzed, and the law of quantum vortex with time evolution is obtained. It is found that the vortex distribution is oscillatory in the harmonic trap, and vortex and anti-vortex are mutually transformed in time. At the same time of evolution, the vortex direction is always opposite to that of the neighbor vortex, and at different evolutionary times t and t', which satisfy a relation of t+t'=T (period of harmonic trap), the position of vortex nucleus is unchanged, but the vortex is transformed into the anti-vortex. These basic phenomena of quantum vortex are explained and discussed. In particular, in this paper we also introduce the particle flow density, calculate the flow circulation of our system, and analyze the mechanism of vortex formation. The research ideas and methods in this paper are easily to be extended to the study about the vortex formed by more than three sub-BEC expansions and superpositions, and they can also be used to discuss the effects of sub-BECs with different initial phase differences. This model is also easier to implement in experiment. Therefore, the research of this paper also has enlightenment to the experimental work.
2018, 67 (1): 010601. doi: 10.7498/aps.67.20171922
Optical methods in distance measurement, which are categorized by interferometry and time-of-flight (TOF) detection, have received widespread attention in recent years. However, interferometry cannot provide absolute distance and traditional TOF measurement cannot obtain a high precision measurement result either. The TOF ranging by femtosecond lasers, a novel precise measurement approach, enabling a sub-micrometer precision for long distance absolute ranging, can solve the problems above and has a wide application prospect in aerospace, remote sensing and surface profilometry. Particularly, a dual-comb ranging approach has attracted great attention due to high update rate (~kHz) and a simple system structure (i.e., working with free running mode-locked laser system). However, the quantum limited timing jitter of mode-locked lasers will inevitably introduce uncertainty into TOF estimation due to the equivalent sampling nature of a dual-comb scheme. As a result, the distance measurement precision is significantly degraded. Even though a simple multiple averaging can be used to alleviate this problem, the measurement speed is limited to a very low level, which is unacceptable to many applications. Moreover, multiple averaging fails in the presence of more complex noise sources. Singular spectrum analysis (SSA), known as a non-parametric spectral estimation technique, has been widely used in dynamic systems to distinguish complex patterns in signals without a priori knowledge of the dynamical model. In this paper, for the first time, we apply SSA to extract distance information from a noisy time series generated by a high update rate dual-comb ranging system. Numerical simulation shows that the SSA is a powerful tool for separating distance series into signal and random noise regardless its color. Specifically, we extract a one-dimensional step profile with high precision in the presence of violet noise (density proportional to f2). In experiment, a dual-comb ranging system is built based on two home-built polarization maintaining mode-locked fiber lasers by using carbon nanotube as saturable absorber. Their repetition rates are both about 74 MHz, their difference being about 2 kHz. We measure the distance of a moving target placed at ~0.5 m away from the range finder and use the SSA for signal extraction. The direct measurement precision is 1.9968 m rms at 200 Hz update rate. The SSA successfully separates the quantum noise from the ranging time series, resulting in 0.1522 m rms ranging precision, corresponding to about 13 times ranging precision improvement. This method can be further extended to high dimension, enabling high precision and high speed profilometry for complex surfaces based on femtosecond laser ranging.
2018, 67 (1): 010701. doi: 10.7498/aps.67.20170271
Optical fibers have a wide range of applications and constitute the core of fiber-optic gyroscope which is revolutionizing the ancient inertial rotation detection. However, fiber coils in these instruments are susceptible to surrounding physical quantities, which can seriously deteriorate their accuracy. And the thermally induced parasitic effect is one of the most critical factors leading to the bias drift. This drift error is due to the nonreciprocity phase shift in the counter-propagating optical loops when a thermal gradient passes through the fiber coil as described by Shupe. The quadrupole winding patterns along with other coiling schemes have been proposed to reduce the Shupe effect by maintaining fiber parts at equal distances from the coil center beside each other. Many researchers have investigated the thermal effect on this drift on the assumption that the temperature transient propagates only radially along the fiber coil, while little attention has been paid to the case of the multidimensional thermal field. This can hardly satisfy completeness of the theory, and be applied to certain complicated working conditions. In this paper, we develop theoretical models that describe drift signals caused by radially, axially and circumferentially transmitted thermal effects on the quadrupole winding fiber coil. The obtained findings indicate that the bias error excited by the thermal flow in radial and axial directions is proportional to the weighted sum of the difference in temperature changing rate between outer and inner sides of the fiber ring. And the share of the sum linearly grows when approaching to the top surface near the input and output end (I/O end) of the fiber. Thus, it is suggested that it should be avoided to place heat sources in the neighboring area. For the circumferentially distributed temperature field, the drift depends on the symmetry of the thermal gradients on both sides of the centerline connecting the fiber midpoint and the I/O end. This circumferential thermal effect can be dominant, since it tends to cover a larger spatial scale than its counterparts in radial and axial directions. And besides making a good symmetrical design of the temperature distribution with respect to the centerline, it can be suppressed by arranging the nonuniformity of the thermal field in the opposite direction of the fiber coil to the I/O end, which is also beneficial to reducing its sensitivity to the angular change. Our results can help better understand the mechanisms for the thermal error formation and guide us in optimizing and facilitating the thermo-structure design of both fiber gyroscopes and navigation systems.
ATOMIC AND MOLECULAR PHYSICS
2018, 67 (1): 013302. doi: 10.7498/aps.67.20171464
As a new groundwater exploration method, noninvasive surface nuclear magnetic resonance (SNMR) has the benefits of direct, quantitative and uniqueness estimation of water content and relaxation time (T2*) in the near surface groundwater exploration. In practice, the earth magnetic field is difficult to be determined accurately, due to its inhomogeneity, time-varying and susceptible to ambient noise, which results in off-resonance excitation and serious decrease in accuracy of the inversion result. In this paper, based on the model of surface nuclear magnetic off-resonance (SNMOR) and the expression for the kernel function, the influences of the frequency offset on the amplitude and phase of the free induced decay (FID) signal are discussed, and a complex envelop inversion (CEI) based on automatic matching system phase and involving both the real part and imaginary part of the signal is applied. By comparing the synthetic signals generated from the SNMR and SNMOR models, it can be concluded that the phase of the FID signal significantly changes with the increase of the frequency offset, and the amplitude of the signal can be increased by 65.9% for the synthetic model in this paper. Thus when the frequency offset is greater than 2 Hz, the distribution of water content and T2* from the inversion results using the SNMR kernel will have a serious deviation from the actual model. However, using the SNMOR kernel based on the frequency offset, the inversion results are more accurate, and the maximum error of the water content and T2* are 4.2% and 39.3 ms, respectively. Moreover, synthetic data with different noise levels are inverted by the CEI method and conventional amplitude envelop inversion method (or QTI). The results show CEI obtain better performances in stability and reliability at a high noise level. Finally, a field measurement of SNMOR is conducted in Taipingchi Reservoir near Changchun City, China. The off-resonance FID signals are obviously observed by utilizing the JLMRS instrument and can be used to estimate the frequency offset. The characteristics of the FID signal with the frequency offset confirm the correctness of the SNMOR model. And the inversion result of field data using SNMOR kernel show that the distribution of water content and T2* are consistent with the known geological data from the drillings and other geophysical methods, which is much better than that using the SNMR kernel or conventional amplitude envelop inversion method. Therefore, the validities and accuracies of the SNMOR model and CEI method proposed in this paper are verified, which provides a new idea and technique for groundwater exploration in the near surface.
Simulation analyses of 100-keV as well as low and high energy protons through insulating nanocapillary
2018, 67 (1): 013401. doi: 10.7498/aps.67.20171701
In order to clearly understand the physical images of incident ions passing through the insulating nanocapillary, in this work we establish a theoretical model, in which the matlab program is combined with the Monte Carlo method, to estimate the time evolution of transmission features, such as the angular and deposited charge distribution, three-dimensional (3D) trajectories of H+ particles with proton incident energies of 10 keV, 100 keV and 1 MeV at -1 title angle. The simulation results show that the transmission mechanism of 100 keV protons is different from those of 10 keV and 1 MeV protons. After a sufficiently charging and discharging stage, 10 keV H+ particles are guided along the direction of capillary axis, indicating that the guiding force from the surface charge patches is significant, and the small-angle scattering of 1 MeV protons under the capillary inner wall is a physical process that determines the transport of H+ particles through the nanocapillary. However, for 100 keV H+ particles, the centroid angle gradually shifts from the guiding direction to the direction close to the incident beam, which is attributed to the fact that the stochastic inelastic binary collision below the surface is the main transmission mechanism at the beginning. After the charging and discharging reach an equilibrium state, the H+ particles are likely to pass through the nanocapillary, and the main transmission mechanism is the charge-patch-assisted specular scattering. This mechanism deepens the understanding of the transport behavior of protons through the nanocapillary, which will contribute to the control and application of the 100 keV proton beam.
Determination of heavy metal content in dust of earth's surface and dust on air conditioner filter by X-ray fluorescence analysis
2018, 67 (1): 013301. doi: 10.7498/aps.67.20171400
The dust of earth's surface and the dust on air conditioner filters reflect a certain area of air pollution in a period of time. In the present study, we investigate the dust collected from the Wangjiang campus of Sichuan University on March, 2017. The dust is divided into 9 groups according to their diameters. The dust is made into the samples by mixing the dust and analytically pure starch at a ratio of 1:2, and pressing it into slices of 1.5 cm in radius and 6 mm in thickness through using a powder compressor. Likewise, the salts (MnSO4H2O, Fe(NO3)39H2O, CuSO45H2O, ZnSO47H2O, Pb(NO3)2), are also made into standard samples of different elements (Mn, Fe, Cu, Zn, and Pb). X-ray fluorescence analyzer is used to measure the element content in each of the samples according to calibration curves measured from the standard samples. The results show that the content of each element in the earth's surface dust is lower than that in the dust on the air conditioner filter. The values of Cu, Zn, and Pb content in the dust are higher than the average content of the topsoil in Sichuan Province, China. These elements possibly originate from motor vehicle exhaust. Based on the theoretical model for the gaseous elements to change into the fine particulate matter, the change of the trace element content with the particle size can be expressed as Ci kiDn, where Ci is the content of the metal element i in the dust, ki is a scale factor, D is the diameter of the dust particle, and n is the distribution index. From the results it is concluded that the distribution indexes corresponding to various elements are approximately the same in the size range of interest to us (32.5-230 m). A recommended value of n is -0.430.06.
Calculationof isotope shift of Mg+ ion by using the relativistic multi-configuration interaction method
2018, 67 (1): 013101. doi: 10.7498/aps.67.20171817
The special mass shift coefficients and field shift factors for the atomic transitions 3s2S1/2-3s2P1/2 and 3s2S1/2-3s2S3/2 of Mg+ ion are calculated by the relativistic multi-configuration interaction method, and the isotope shifts are also obtained for the Mg+ isotopes with the neutron numbers 8 N 20. Our calculations are carried out by using the GRASP2 K package together with the relativistic isotope shift computation code package RIS3. In our calculations the nuclear charge distribution is described by the two-parameter Fermi model and the field shifts are calculated by the first-order perturbation. In order to generate the active configurations, a restricted double excitation mode is used here, the electron in the 3s shell (3s1) is chosen to be excited, another electron is excited from the 2s or 2p shells (2s22p6), and the two electrons in the inner 1s shell (1s2) are not excited. The active configurations are expanded from the occupied orbitals to some active sets layer by layer, each correlation layer is labeled by the principal quantum number n and contains the corresponding orbitals s, p, detc. The maximum principal quantum number n is 6 and the largest orbital quantum number lmax is g. According to our calculations, the normal mass shift coefficients are -586.99 GHzamu and -588.50 GHzamu, the special mass shift coefficients are -371.90 GHzamu and -371.95 GHzamu, the field shift factors are -117.10 MHzfm-2 and -117.18 MHzfm-2 for the 3s2S1/2-3s2P1/2 and the 3s2S1/2 -3s2S3/2 transitions of Mg+ ions, respectively. Then the isotope shifts for different Mg+ isotopes are obtained using the available data of the nuclear mass and the nuclear charge radii. Our results are coincident with other theoretical calculations and also with experimental results. The relative errors of our calculations are in a range from 0.13% to 0.28% compared with the latest measurements. Our calculations are the most consistent with the experimental measurements for the moment. The results provided here in this paper could be referred to for the experimental and theoretical study of Mg+ isotope shift, and they could be applied to the spectral measurement experiments of the short-lived Mg+ isotopes and also used for the research of the characteristics of exotic nuclei with Mg+ isotopes near the magic neutron numbers N=8 and N=20. The calculation method and the excitation mode used here could also be extended to other multi-electron systems with eleven orbital electrons, and the corresponding theoretical studies of the atomic spectral structures and isotope shifts could then be carried out.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2018, 67 (1): 014202. doi: 10.7498/aps.67.20171562
The quasi parallel light interference is one kind of basic ways to use the energy of interference light to interact with matter. Because the phase of each parallel light beam needs to meet the coherent condition, it is required that the phase of each light beam be controlled timely. There are some kinds of phase control methods, such as the heterodyne phase-locking method, the stochastic parallel gradient descent algorithm, the self-referred and self-synchronous phase-locking method the multi-dithering phase-locking method, etc. Among them, the multi-dithering method needs not the referenc light, it is to load multi-frequency sinusoid signals to the phase modulator, and realize the recognition of phase difference and the output of feedback voltage by multiplying circuit and integrating circuit. In view of the shortcomings of the existing methods, a scheme of filter-type multi-dithering phase control for quasi parallel light interference is proposed, in which the phase differences are identified and corrected by the modulation signals and filtering signals of different frequencies. Theoretical analysis of coherent light intensity for the scheme is made. The principle of filter-type multi-dithering phase control method is put forward, and the numerical analysis and simulation experiment for filter-type multi-dithering phase control method are carried out. In the simulation experiment, the fiber interference light path is used to simulate the light intensity of quasi parallel light interference at one point in space, and the change of photoelectric signal indicates the change of interference light intensity. The phase control feedback loop is composed of photoelectric signal amplifying circuit, bandpass filtering circuit, amplitude measuring circuit, direct current amplifying circuit and adder circuit. The results have shown that the phase difference among light beams can be recognized by the method, and the direct current voltage signal that is proportional to the phase difference of signal can be fed to control the phase modulator. The phase difference can be corrected. The control bandwidth is 2.5 kHz, and the output voltage range of phase control is 0.034.45 V. Compared with the classical multi-dithering method, the method of filter-type multi-dithering phase control has some advantages. Each multiplying circuit in the classical method needs a very small amplitude reference signal, which causes the reference signal to have a very small range of values, and the relationship between integral time and modulation period needs considering. The integral time is usually ten times longer than the modulation period, which causes the control bandwidth of the system to decrease. However, the feedback loop of the filter-type multi-dithering phase control method does not require any reference signal, so each signal does not affect each other, and the increase in the number of beams does not have a significant influence on the control bandwidth either. Therefore the filter-type multi-dithering phase control method is a useful phase-control method.
Numerical calculation and discussion on return photons of polychromatic laser guide stars by a laser beam with 330 nm wavelength
2018, 67 (1): 014205. doi: 10.7498/aps.67.20171025
The properties of return photons of polychromatic laser guide stars excited by a modeless laser with 330 nm wavelength are investigated in this paper by numerical simulation. The repetition rate, linewidth, initial diameter of laser spot and atmospheric transmittance have great influences on the return photons at 330 nm and 2207 nm from polychromatic laser guide stars. First, the laser linewidth is optimized by solving the rate equations of interaction between laser and sodium atoms. We find that the 0.6 GHz linewidth for the continuous wave laser and the 1.0 GHz linewidth for the pulse laser are beneficial to obtaining the higher excited probability of sodium atoms. Based on the fitted relation between the excitation probability of sodium atoms and laser intensity, considering the random distributions of laser intensity at the mesosphere due to the influence of atmospheric turbulence, the return photons from polychromatic laser guide stars are numerically calculated. The results show that the return photons at 330 nm excited by the continuous-wave laser are more than those excited by the pulse laser. And the return photons excited by continuous-wave laser almost do not fluctuate when laser power arriving at sodium layer is 1 W. Furthermore, effects of the repetition rate of pulse laser and the laser initial diameter on the return photons at 330 nm are studied. The two results are obtained as follows. The first result is that the increment of return photons at 330 nm will converge to a constant value when the repetition rate of pulse laser is over 50 kHz. The second result is that the initial diameter of continuous wave laser has no effect on the return photons but the effect of pulse laser is more obvious. Particularly, the atmospheric transmittance is an important factor of influence because it causes a severe loss of light power at 330 nm wavelength. Under the conditions of 5 km atmospheric visibility and 12.8 cm atmospheric turbulence coherence length, the launched power of pulse laser with 50 ns duration should be more than 34 W for obtaining enough return photons required for the effective detection of atmospheric turbulence tip-tilt with the natural stars. But for the continuous-wave laser, the launched power should be more than 20 W. In the case of 10 km atmospheric visibility, if the same return photons at 330 nm are required, the launched power of pulse laser will also be more than that of the continuous-wave laser under the same conditions. Therefore, the continuous-wave laser has more advantages than the pulse laser in exciting the polychromatic laser guide stars. We hope that the above results will be beneficial to the further experimental research.
Design and performance of spectroscopic filter of rotational Raman temperature lidar for absolute measurement
2018, 67 (1): 014207. doi: 10.7498/aps.67.20171834
Rotational Raman temperature lidar for absolute measurement is an important method to directly detect the atmospheric temperature profile by using active remote sensing technology. Compared with the rotational Raman temperature relative measurement, the absolute measurement can avoid the systematic error caused by the calibration process, but its high-precision requirements of rotational Raman spectroscopic filter restrict the development of absolute measurement technique for atmosphere temperature. In order to achieve the absolute measurement technique of rotational Raman temperature lidar, the fine resolution of single rotational Raman line and the effective suppression 60-70 dB for the elastic scattering signal are the key factors for directly retrieving the atmospheric temperature by using the relationship between the single rotational Raman line and temperature. Based on the operational principle of grating, a two-stage parallel multi-channel Raman spectroscopic filter with one-order blazed grating and fiber Bragg grating is designed, and the parameters and optical path structure of the core cascade device (micron-level fiber array) are optimized. The optical path of the primary spectroscope is simulated, the wavelength difference between the rotational Raman lines of adjacent even rotational quantum numbers of nitrogen molecule (N2) gradually decreases from 0.4506 nm to 0.4475 nm. Compared with the average of approximately 0.4494 nm, its floating interval is -0.0012-+0.0019 nm, and the maximum centrifugal distortion of the rotational Raman spectra is approximately 0.0031 nm, which means that the centrifugal distortion ratio is 0.69%. Under the different values of incident angle , the diffraction position difference between adjacent rotational Raman lines varies from 124.43 m to 125.51 m, with a variation interval of -0.57-+0.51 m compared with a fixed value of 125 m. In order to test the matching consistency between rotational Raman spectra and the multi-channel fiber array, and to obtain the out-of-band suppression and channel coefficient of each fiber channel, an experimental system which consists of a first-order blazed grating, a convex lens and a fiber array is set up, and the atmospheric echo signal is simulated by using a broadband light-source and a semiconductor laser (LD). The experimental results show that the channel coefficient of the rotational Raman channels of the primary spectroscope is above 0.75, and the maximum deviation between the measured wavelength of extracted spectrum and the theoretical value is approximately 0.0398 nm, which means the the deviation degree is 8.86%. Each channel can provide more than 27 dB effective suppression to elastic scattering signal, and then by combining with the second spectroscope of fiber Bragg grating, the suppression at least is up to 62 dB. Therefore we can fine extract single rotational Raman line of even rotational quantum number.
Enhanced selfintegration algorithm for fiber torsion sensor based acoustically-induced fiber grating
2018, 67 (1): 014208. doi: 10.7498/aps.67.20171525
Mechanical parameter monitoring based on optical mode detection benefits from its low cross sensitivity and inexpensive instrument. The key to improving detection accuracy is to generate high-quality detection light and use efficient algorithms. We present a strain-independent torsion sensor based on acoustically-induced fiber grating (AIFG) in the dual-mode fiber (DMF) and use the enhanced self-integration algorithm to improve the sensing accuracy. By tuning the radio frequency of driving signal, the LP11 mode generated by the AIFG can be exploited to measure the dynamic torsion variations. Without the complex device such as fiber interferometers and photonic crystal fibers (PCFs), the simple structure built by mode converter and charge coupled device (CCD) can track the dynamic variations and has less cross sensitivity of strain along the transmission direction. The AIFG driven by a radio frequency as a mode converter at specific wavelength does not participate in sensing but generates the high-purity LP11 mode that accounts for more than 90% of total power. With the twist from the rotator stage, the DMF keeps rotating and CCD records the spatial distribution of mode profiles. The features of optical mode is enhanced based on matrix analysis and then the relationship between twist angle and mode features is obtained. Based on image processing, the dynamic variation of spatial beam detected by CCD can be easily tracked and quantified. In experiment, the rotation angle can be obtained by calculating the feature value of the optical mode. Our image detection algorithm is specially designed for the optical fiber mode. Compared with traditional image recognition based on feature learning, it is simple and fast because it is needless to use image segmentation and stochastic processing. Through a series of experiments on angle rotation and parallel strain, we verify the correctness of the enhanced self-integration model and analyse the computational uncertainties that influence the stability of experiment. In the 0 to 180 measurement range, the maximum range of measurement error is less than 11%. When the axial strain is between 100 and 1500 , the sensor is strain-independent. Thus, it is verified that the torsion sensor based on AIFG has high sensitivity and can overcome the cross sensitivity of strain along a certain direction. The pertinent results have significant guidance in designing the multi-parameter sensor. The optical mode detection, instead of the traditional spectrum measurement, enables the whole structure to have the potential to be rebuilt by inexpensive devices that work in visible wavelengths.
High resolution electron bombareded complementary metal oxide semiconductor sensor for ultraviolet detection
2018, 67 (1): 014209. doi: 10.7498/aps.67.20171729
High resolution and high sensitive low light level imaging sensors are crucial in many applications such as astronomical observation, high energy physics, night vision and remote sensing. The electron bombarded complementary metal oxide semiconductor (EBCMOS) sensor is a novel imager in which very high gain can be produced by hitting the semiconductor with high voltage without any noise generation. In addition, it can process high-definition image with kHz frame rate. These advatages make the EBCMOS an ideal device for ultrafast single-photon imgaing. In this article, we present an EBCMOS sensor working in the ultraviolet range by combing the technology of vacuum photocathode and back illuminated CMOS together. This EBCMOS sensor can realize very high resolution in 40 mlx light illumination environment. The achieved spatial resolution is 25 lp/mm (line paris per millimeter) when the electric field intensity is 5000 V/mm. The liner relation between electric field intensity and the resolution indicates that much better perofromance can be achieved if the electric field intensity increases to a much higher value. The EBCMOS sensor developed in this paper can be directly applied to UV weak light detection, moreover it will provide a good reference for further developing the visible and near infrared sensitive EBCMOS sensors.
2018, 67 (1): 014703. doi: 10.7498/aps.67.20171617
Multiphase jets and cavitation problems are inevitable for high-speed underwater vehicles propelled by jet engines. Unlike being injected into stagnant water, the gaseous jet behind a underwater vehicle is usually conjugated with a tail cavity. The pulsation and collapse of such cavities can seriously affect the vehicle performance. In this study, the shape character, forming mechanism and control conditions for the supersonic gaseous jet induced tail cavity at the wake of a revolution body are experimentally investigated in a water tunnel. The induced cavity is ventilated only by a convergent-divergent nozzle with a designed Mach number of 2.45. The form of the cavity is recorded through two high-speed cameras both horizontally and vertically under different Froude numbers and ventilation rates. The time averaged form is thus obtained through digital image processing to eliminate the transient characteristics of the cavity. The experiment is conducted with the Froude number ranging from 3.2 to 16.2, and the ventilation rate 0 to 0.5. Due to the high density and velocity ratio between water and gas, the structure of such flow is usually very complicated. Many novel phenomena of the jet-cavity interaction are observed. With increasing stagnation pressure of the central jet, the induced cavities evolves form foamy, intact, partially break, to pulsating foamy closure type. The foamy and intact tail cavities share the same profile and characteristics that of a supercavity. And the pulsating foamy closure type was never observed before in a traditional supercavitating flow. The outline of the pulsating foamy cavity is the same as the foamy cavity's, indicating that they have the similar forming mechanism. A comparison with the jet-cavity interaction model is made and the following conclusions are obtained:the real ventilation rate, which corresponds to the re-entrant jet gas blocked by the cavity boundary, is the key factor in controlling the cavity form. When the gaseous jet is completely blocked by the water-gas interface, an intact or foamy cavity will be formed. A partially break cavity appears only when some fraction of the jet is blocked and this is when some of the strongest interactions between jet and cavity occurs. When little gas was blocked by the interface, a pulsating foamy cavity forms. With the structure of gaseous jet considered, the transition of the induced cavity closure between different types is in favour of the prediction from Paryshev's model of cavity closure to a central jet. The variation of the cavity form, thus the interaction strength between jet and cavity, coincides with the real ventilation rate estimated through the theoretical model.
2018, 67 (1): 014203. doi: 10.7498/aps.67.20171442
Fresnel incoherent correlation holography (FINCH) has attracted much attention because it is able to record the holograms of three-dimensional (3D) samples under incoherent illumination with just a charge coupled device (CCD) and spatial light modulator (SLM). The FINCH technology achieves the splitting and phase shifting of the incident beam by loading a phase mask on an SLM. Three holograms, whose phase factors are different from each other, are recorded sequentially by a CCD. After the three holograms are superposed in the computer, the zero order image and a twin image are eliminated, and a complex hologram is obtained. The 3D properties of the object are revealed when the complex hologram is reconstructed in the computer. Spiral phase filters (SPFs) are commonly used to produce optical vortices, which can enhance and recognize image edges. In this paper, the spiral phase modulated FINCH system illuminated by Xenon lamp is built, in which the phase-only SLM is space-division multiplexed by a helical lens (superposed by an SPF and a lens) and a conventional lens. The mathematical model of spiral phase modulated FINCH system is established based on wave optics theory. The specific forms of the point spread function (PSF) and the reconstruction distance of the system are given for the first time. Experiments are conducted by using a small aperture with a diameter of 20 nm as a point source, the point source hologram recorded by CCD and the reconstructed image are consistent with the simulated ones. When the system is used for imaging resolution target and unstained onion cells, the edge contrast enhancement effects are obtained without the loss of resolution. The results show that the spiral phase modulated FINCH system can not only improve the edge contrast of the amplitude object, but also extract the edge information or recognition of the phase objects. This method has an important application prospect in the quantitative imaging of phase objects such as in real-time monitoring cell division and deformation of living cells.
2018, 67 (1): 014204. doi: 10.7498/aps.67.20171217
Optical quantum memory plays an important role in scaling-up linear optical quantum computations and longdistance quantum communication. For effectively realizing such tasks, a long-lived and highly-efficient quantum memory is needed. The dynamic electromagnetically-induced-transparency (EIT) process can be used for completing an absorptive storage scheme in an atomic ensemble. In such a process, the quantum states of coming single photons can be coherently transformed into spin waves associated with coherences between atomic ground levels via switching off controlling light beam. For storing a single-mode optical signal, a pair of ground levels is involved. While for storing an optical polarization qubit, i.e., two orthogonal polarization modes, the coherence between two pairs of ground levels will be involved. Also, to obtain a high efficiency in the EIT optical storage, the optical-depth of the cold ensemble should be high. For prolonging the coherent time of the spin waves stored in atomic ensemble, decoherence between spin waves due to atomic motion and non-uniform Zeeman shift of ground levels should be effectively suppressed. Recently, a long-lived and highly-efficient optical quantum memory for single-mode storage in a high-optical-depth cold atom ensemble has been experimentally demonstrated via the gradient echo memory scheme (2016 Optical 3 100). While, for the optical polarization qubit storage, a long lifetime (in ms) and high-fidelity EIT storage experiment has been achieved by our group, but the storage efficiency in the experiment is very low (8%) due to lower optical depth of the cold ensemble (2013 Phys. Rev. Lett. 111 240503). The storage efficiency in long-lived storage of two orthogonal polarization modes still needs further improving. Here in this paper we demonstrate an experiment of long-lived and highly-efficient storage of two optical orthogonal polarization modes in a high optical-depth cold atomic ensemble via dynamic EIT process. For achieving a long lifetime in the storage experiment, we follow the two steps, which are used in our previous work (2013 Phys. Rev. Lett. 111 240503). 1) We make the signal and writing-reading light beams collinearly pass through the cold-atom cloud along the z direction to suppress the decoherence between the spin waves due to atomic motion. 2) We apply a moderate magnetic field (13.5 G) to the cold-atom ensemble to lift Zeeman degeneracy. So, the magnetic-field-sensitive transitions are removed from EIT system and the two optical orthogonal polarization modes are stored as two magnetic-field-insensitive spin waves. In contrast to our previous experiment, we finish the storage in the high optical-depth cold atomic ensemble. To obtain such a high optical-depth cold atomic ensemble, we expand the diameters of the trapping laser beams and use a pair of rectangular magnetic coils in a magnetic optical trap (MOT) to prepare a cigar-shaped cold atomic ensemble. The MOT magnetic field is further compressed, and then the optical-depth of the cold atomic ensemble increases up to ~11 in the present experiment, which allows us to achieve a storage efficiency of 30%, which exceeds the previous value (8%). At an MOT repetition rate of 10 Hz, the measured zero-delay storage efficiencies for the two orthogonal polarization modes are symmetric, which go up to ~30%. The 1/e-folding lifetimes of the two orthogonal polarization modes rise up to 3 ms. We also measure the dependence of the zero-delay retrieval efficiency on the MOT repetition rate F and find that the storage efficiency is still more than 20% when the repetition rate F is 50 Hz. The present results will allow one to achieve a long lifetime and highly-efficient quantum memory for photonic polarization qubit and then find applications in scaling-up linear-optical quantum computations and long-distance quantum communication.
2018, 67 (1): 014206. doi: 10.7498/aps.67.20171324
Atmospheric optical turbulence severely restricts the performances of electro-optical systems. The turbulent atmosphere causes the intensity of a light beam to fluctuate or scintillate, leads the light beam to wander and makes the images randomly displace, which directly relates to the refractive index structure parameter Cn2. Therefore the knowledge of Cn2 is essential to evaluate and to predict the effects of optical turbulence on electro-optical imagery systems. During the period from December 13, 2016 to January 2, 2017, 30 sets of sounding data, which include temperatures, humidities, pressures, wind speeds, wind directions and atmospheric refractive index structure parameters, are obtained by using a self-developed meteorological radiosonde for turbulence at Marine Meteorological Science Experiment Base at Bohe of Maoming. On the basis of the HMNSP99 outer scale model, an atmospheric optical turbulence outer scale formula of Maoming is obtained by fitting the sounding data. At the same time, the experimental data of the turbulence profiles are statistically averaged, and then based on the Hufnagel-Valley model, a statistical model is obtained, which is appropriate to the variation of the turbulence profile on the coast. According to Tatarski turbulence parameterization and the Maoming outer scale formula, the new estimated Cn2 values are compared with their experimental observations and the results from other already defined models, respectively. Statistical analysis shows that the overall correlation coefficients of log10(Cn2) between observed values and estimated values by using the new fitting Maoming outer scale formula, the HMNSP99 model, the Dewan model and the Coulman model are 0.924, 0.848, 0.763 and 0.651, respectively. Also, both the trends and magnitudes for these four outer scale models are consistent with each other. The errors of the above four outer scale models are very small:their overall average absolute errors and average relative errors are 0.514 and 2.963%, 0.627 and 3.612%, 0.943 and 5.439%, 0.766 and 4.417%, respectively, and the error of the Maoming outer scale model is smallest. The reliabilities and validities of the new outer scale and Cn2 models are further verified. In addition, it is found that the occurrence of upper air optical turbulence is closely related to wind shear and temperature gradient. The results support the prediction of the atmospheric optical turbulence profile required for electro-optical engineering on the coast.
2018, 67 (1): 014701. doi: 10.7498/aps.67.20171518
The high-resolution numerical simulations of two-dimensional (2D) turbulent Rayleigh-Bnard convection are conducted by using the Parallel direct method of DNS (PDM-DNS) with Ra=1010 and Pr in a range from 0.05 to 20. Using the flow visualization technique, the effects of Pr on the structure of plumes and large scale circulation (LSC) are investigated. With Pr decreasing, plumes become more active and the flow turns more turbulent. When Pr4.3, pronounced LSC and corner vortex exist. The thickness of thermal boundary layer varies slightly with the value of Pr changing, which obeys a scaling law. Nusselt number (Nu) increases with Pr value increasing when Pr value is low and becomes independent when Pr value is high. Furthermore, two definitions of Reynolds number (Re) are given. The Re〈u〉 angle is calculated from the fluctuation of horizontal velocity near the center of bottom plate, and the ReUmax is calculated from maximal horizontal velocity in the mean field. Both of them follow the same scaling Re~Pr0.81.
2018, 67 (1): 014702. doi: 10.7498/aps.67.20171459
Recently, how the desert lizards run, hide or swim in the sand has attracted much attention of many scientists in granular matter field, and many valuable results have been published, except for the Phrynocephalus mystaceus, a type of the desert lizard, which can embeds itself into the sand through a motion mode which is completely different from other types of desert lizards. To illuminate the roles played by the spinning-mode in the Phrynocephalus mystaceus' motion in the sand, the three-dimentional (3D) numerical simulation using the Hertz model on the system, in which one sphere is spinning in the granular matter, is carried out with the open-source code LIGGGHTS released by the Sandia National Laboratory in USA. In the numerical simulations for all the cases, the initial conditions are the same and the sphere spins around X-axis while the X-Y plane is the horizontal plan and the Z axis is the vertical direction. According to the numerical results and analyses, for the spinning sphere deeply embedded in the granular matter we can draw some conclusions. 1) The X-axis spinning motion can cause the sphere embedded in the granular to notably displace along the Z-axis and Y-axis, but the displacement along the spinning direction is smaller than the sphere diameter. 2) The friction coefficient between the sphere and the granular matter has a notable influence on the motion of the sphere in granular matter, the spinning sphere can move vertically and horizontally only when the friction coefficient between the sphere and the granular matter is larger than that of the granular matter; and the bigger the , the more violent the movement of the sphere is. This can be used to explain why most of the desert creatures each have a coarse skin. 3) On the premise that the friction coefficient between the sphere and the granular matter is larger than that of the granular matter, the spinning velocity of the sphere also has a great influence on the movement of the sphere in the granular matter. In a spinning velocity range between 10 rad/s and 640 rad/s, the larger the , the more obvious the movement of the sphere is. When the spinning velocity reaches 1280 rad/s, the movement of the sphere slightly decreases compared with when the spinning velocity is 640 rad/s. 4) For the spining sphere in granular matter, the sphere always moves upward in the Z direction, but in the Y direction the sphere may move in a positive or negative direction depending on the and . The sphere moves in the positive direction of Y axis if the and are relatively small, while it moves in the negative direction if the and are larger.
Passively Q-switched mode-locked low threshold Tm, Ho: LLF laser with an single walled carbon nanotubes saturable absorber
2018, 67 (1): 014201. doi: 10.7498/aps.67.20171748
Employing single walled carbon nanotubes (SWCNT) grown by the vertical growth method as a saturable absorber for the initiation of the pulse generation, and designing a low threshold resonant cavity, we demonstrate a stable passively Q-switched mode-locked (QML) Tm, Ho:LiLuF4 solid-state laser with low threshold for the first time. With wavelength tunable Ti:sapphire solid laser operating at 785 nm as a pumping source, continuous-wave (CW) absorbed pump thresholds of 52, 59 and 62 mW are obtained by using 1.5%, 3% and 5% output coupled mirrors respectively. In this case, the maximum output powers are 645, 828 and 940 mW respectively, whose corresponding slope deficiencies are 31.02%, 39.16% and 43.78%, respectively. When the SWCNT-SAs is inserted in the cavity, the cavity loss is further increased, so the laser threshold is improved. Employing the 1.5% output mirror, a laser threshold is obtained to be as low as 85 mW, but the maximum laser output power is only 70 mW, corresponding slope efficiency is 3.42%; employing the 3% output coupling mirror, the laser threshold is obtained to be as low as 99 mW, the maximum output power is 154 mW, and the corresponding slope efficiency is 8.47%; employing the 5% output mirror, owing to the loss in the cavity being too large, the QML operation cannot be achieved. The output power of the 3% output mirror is twice higher than that of the 1.5% output mirror, but the laser threshold difference is only 14 mW. With a comprehensive analysis, we use the 3% output mirror. In this case, a stable QML operation with a threshold of 250 mW is obtained. When the absorption pump power is 1.85 W, the maximum output power is 154 mW with a typical Q-switched pulse envelope width of 300 s, which is corresponding to a 178.6 MHz of the mode-locked frequency. The modulation depth in Q-switching envelope is close to 100%. According to the definition of the rise time and considering the symmetric shape of the mode locked pulse, we could assume the duration of the pulse to be approximately 1.25 times more than the rise time of the pulse. So the width of the mode locked pulse is estimated to be about 663 ps. The results show that the SWCNT is a promising SA for QML solid-state laser with the 2 m wavelength. In the later stage, we increase the pump power, optimize the quality of the SWCNT material, and compensate for the dispersion in the cavity. It is expected to achieve a stable continuous mode-locking operation, and obtain a femtosecond mode-locked ultrashort pulse output. The mode-locked mid-infrared pulses have a lot of potential applications such as ultrafast molecule spectroscopy, the generation of mid-IR pulse, laser radar, atmospheric environment monitoring, etc.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2018, 67 (1): 015203. doi: 10.7498/aps.67.20171124
The soft X-ray laser shadow imaging technique is a good tool for diagnosing shadow profiles near the critical surface of high-temperature dense plasma. The short-pulse plasma X-ray laser, driven by high-power laser, is used as the backlight, which spreads freely approximately 500 mm far, passes through the plasma to be diagnosed, and changes its optical path by using a multi-layer spherical lens and multi-layer plane mirror, is attenuated by filters, and is recorded by a soft X-ray charge-coupled device (CCD). The plasma to be diagnosed can be driven by one or multiple laser beams, according to the needs of the physical research being conducted, and is imaged onto the CCD surface through a multilayer spherical lens. The shadow profile image of the plasma to be diagnosed at a particular time is obtained by using the instantaneous photographic mode of short-pulse soft X-ray laser backlight imaging. Compared with the traditional keV hard X-ray backlight technique, the soft X-ray laser shadow imaging technique has two distinct advantages. One is the appropriate wavelength of the probe light, which makes it possible to diagnose plasma near the critical surfac, and the other is a better spatial resolution because of the use of mature multilayer optical elements for near-normal incidence imaging. However, there has been no systematic study on the extent to which the spatial resolution of the imaging technology can be achieved. In this study, a careful analysis is carried out considering three aspects:the optical path geometry, the diffraction limit, and the imaging aberration. The results show that a spatial resolution of approximately 2 m can be achieved. An experiment is carried out to measure the Rayleigh-Taylor instability of plasma from the lateral direction, by using the soft X-ray laser shadow imaging technique. Some microfluids with a width of several microns can be clearly distinguished in the experimental shadow image, indicating that the diagnostic technique has a good spatial resolution. Further analysis reveals that the main factor that limits the spatial resolution is the optical path geometry. It is possible to achieve a spatial resolution of up to 1 m by increasing the magnification, selecting CCDs with smaller receiving units, etc.
2018, 67 (1): 015201. doi: 10.7498/aps.67.20171225
Pulsed inductive thruster, which employs pulsed inductive magnetic field to ionize propellant and accelerate a bulk of plasma, is accompanied with complicated phenomena such as plasma physics, magnetohydrodynamics and the strong coupling effect between the drive-circuit and plasma load. Simulations employing a snowplow circuit model or present magnetohydrodynamic model might be insufficient to capture these important phenomena simultaneously and self-consistently. Therefore the validity of currently existing numerical models remain to be verified. In this paper, a novel circuit-coupled magnetohydrodynamic model is proposed. The flow process of the plasma in the acceleration channel and the discharge process of the circuit are solved simultaneously in a bi-directionally coupled method by calculating the voltage drop across the drive-coil according to the drive-coil geometry and the temporal electric field distribution. The magnetohydrodynamic field is solved with Navier-Stokes equations coupled with Maxwell equations, while the plasma thermodynamic parameters and transport parameters are calculated by employing the local thermal equilibrium model. And the circuit process is solved with a set of circuit equations based on Kirchhoff's law. All the physics fields are computed by the finite element method in COMSOL MultiphysicsTM. Numerical simulation for American TRW Inc.'s MK-1 thruster successfully reproduces its working process. The numerical magnetic field distribution in plasma, the time-dependent collective Lorentz force and the specific impulse and efficiency of the thruster under varying working voltages agree well with the corresponding experimental data. Numerical results imply that a compact azimuthal plasma current sheet is established in the initial 1-2 s in the near-face region of the drive-coil. This plasma current sheet, which entrains the majority of the propellant, is excluded and accelerated by the Lorentz force derived from the drive-coil magnetic field. Most of the propellant acceleration is accomplished within the first half period of the circuit current, which is about 7-8 s. Furthermore, the bi-directional coupling effect is quantitatively analyzed with the current model. Numerical results indicate that the coupling plasma load generally tends to increase the effective resistance and reduce the effective inductance of the drive-circuit. Moreover, this effect changes as the plasma structure varies. When the plasma current sheet moves away from the drive-coil, the mutual inductance between plasma load and drive-coil decreases monotonically. That implys that the plasma current sheet decouples gradually from the dirve-circuit in the process. In conclusion, bidirectional coupling effect between plasma load and drive-circuit plays an important role in the operation of the thruster. This model could be used to predict the performances of pulsed inductive thrusters and might be helpful in designing a more effective thruster.
2018, 67 (1): 015202. doi: 10.7498/aps.67.20171728
During reentry of hypersonic spacecraft into the atmosphere, a break in the radio communication due to the presence of a plasma sheath on the spacecraft can occur. The break is commonly known as reentry communication blackout. Normally, for high density plasma, it is difficult for the electromagnetic waves of L and S bands to penetrate through. They may be decayed rapidly or reflected. That is why reentry communication blackout happens. In recent years, initiative methods are used to reduce the effects of reentry communication blackout such as by designing ideal shape for reentry vehicle, sprinkling special substances on the surface of the vehicle to improve efficiency of electromagnetic wave, adding magnetic field within the blackout area, etc. However, these methods not only fail to fully resolve the problems caused by blackout but also bring some new ones. Therefore, to resolve the problems, transmission mechanism of electromagnetic waves in plasmons should be analyzed. In this paper, we use the finite difference time domain, consider the mechanism of electromagnetic waves in a structure consisting of high-density plasma rods, and refer to the two-dimensional (2D) photonic crystal and surface wave local coupling theory. A new type of high-density plasma micro-rod cavity structure is designed. The special structure, consisting of metal cavity, high-density plasma rod, and dielectric medium filled within the cavity, is quite different from traditional 2D sub-wavelength plasma rod arrays. This kind of design takes advantage of cavity structure to couple electromagnetic wave within the plasma rod so that the surface wave diffraction transmission mode can be changed into a local coupling enhancement penetrating mode. In this paper, we investigate the plasma micro-rod cavity structures with two shapes:cylinder and square, respectively. It is found that electromagnetic waves of L and S bands can have unusual transmission properties in certain frequency ranges, such that electromagnetic waves can pass through the interior of the high-density plasma rod.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Theoretical and experimental study of average reflection optical path length of diffuse cubic cavity
2018, 67 (1): 016102. doi: 10.7498/aps.67.20171808
The most direct and efficient method to improve the sensitivity of gas sensor is to increase the effective optical path length (Leff) of gas cell according to the Beer-Lambert law. Moreover through experimental research and analysis, the diffuse cubic cavity, as a kind of gas cell, can effectively increase the value of Leff, which is crucial to the study of the reflection law of light in the diffuse cubic cavity. Based on the analysis of the reflection law of light in the diffuse cubic cavity, the theoretical value of the single reflection average optical path length (Lave) is obtained, the theoretical approximation model of the light reflection in the diffuse cubic cavity is established, and the simulation values are obtained by the finite element method. The tunable diode laser absorption spectroscopy (TDLAS) is a perferred gas dection technique with high selectivity, fast response and non-contact measuring. We develop diffuse cubic cavities of different sizes and study the reflection law and characteristics of the light in the cavities. We obtain the Leff values of the cubic cavities using TDLAS, with that and the theoretical formula between Leff and Lave, which in relation to the side length a, the diffuse reflectivity of coating and port fraction f, the experimental values of the Lave are obtained. The accuracies and stabilities of the theoretical approximation model and the simulation results by the finite element method are verified. According to the relationship between the Lave and the number of reflections established by the finite element method, the relative errors between the simulation values and the theoretical values of Lave are less than 3.6%, when each inner surface of the diffuse cubic cavity is divided into 10001000 or more small patches. It shows that the finite element method has a satisfactory effect on the cubic cavities with different sizes, and the error range is less than 0.1%. The TDLAS is used to measure the Leff values of three different cubic cavities with side lengths of 5 cm, 8 cm, and 12 cm, and the corresponding experimental values of the Lave are calculated indirectly. A comparison among the theoretical values, simulation values and experimentical values of the Lave, shows that these three values are well consistent with each other, which indicates that the simulation of the reflection law of light in the diffuse reflection cubic cavity has a significant reference value for the experimental study. Also, the present study of the diffuse cubic cavity will provide a technical support for studying the diffuse cavity of arbitrary shape in the future.
Formation and control of bubbles during the mitigation of laser-induced damage on fused silica surface
2018, 67 (1): 016103. doi: 10.7498/aps.67.20171839
Surface damage on fused silica optics initiated by high fluence 351 nm laser is one of the major bottlenecks for the high power laser systems, such as, Shenguang Ⅲ (SG-Ⅲ) laser facility. Generally, the CO2 laser, which is strongly absorbed by fused silica and thus can effectively heat fused silica above melting temperature, is used to locally mitigate the damages, called the non-evaporative mitigation method. However, subsurface bubbles may be introduced in the damage mitigation process by CO2 laser melting. Unfortunately, the mitigated damage sites with subsurface bubbles can be easily re-initiated upon subsequent laser shots. In this article, in order to eliminate the subsurface bubbles, we systematically investigate the influences of mitigation protocols in different ways of laser irradiation preheating on the formation and control of subsurface bubbles. Based on the simulated results of the temperature distribution and structural changes under CO2 laser irradiation, two CO2 laser-based non-evaporative mitigation methods are proposed, which are adopted for the mitigation of surface damage sites ranging in size from 150 m to 250 m, and systematically investigated to assess the effect of eliminating subsurface bubbles. The process of mitigation method I is that multiple laser irradiations with short time and increasing power are initially used to preheat the damage site and then a higher power laser irradiation is adopted to mitigate the damage site. The process of mitigation method Ⅱ is that a long time, low power laser irradiation is first used to preheat the damage site and then a high power laser irradiation is adopted to mitigate the damage site. The detailed morphologies of the mitigation sites and subsurface bubbles produced by the two mitigation methods are measured by optical microscope with high magnification. A large number of small subsurface bubbles are observed in mitigation method I. While, less subsurface bubbles are observed in mitigation method Ⅱ. The statistical results indicate that among the thirty-four mitigated sites, only eight have no surface bubbles in method I. In contrast, among the fifty-four mitigated sites, forty-nine have no surface bubbles in mitigation method Ⅱ. The experimental results suggest that the formation probability of subsurface bubbles is effectively suppressed by the mitigation method Ⅱ. The mechanism of eliminating subsurface bubbles in the mitigation method Ⅱ is discussed based on the structural changes of the fused silica in the mitigation process. It is found that the fused silica is not melted by the long time, low power laser irradiation, which means that a long time preheating without melting could provide enough time to effectively reject air and impurities enwrapping in cracks, and thus reducing the formation probability of subsurface bubbles in the form of the crack closing due to rapid melting. With the mitigation method Ⅱ, the probability of mitigated sites without subsurface bubbles can reach 98%.
Thermal transport and microscopic dynamics in filled skutterudite YbFe4Sb12 studied by ab initio molecular dynamics simulation
2018, 67 (1): 016301. doi: 10.7498/aps.67.20171406
Filled skutterudite is a typical thermoelectric material with high thermoelectric figure of merit at intermediate temperatures. One of the important features is the low lattice thermal conductivity (L) caused by the low frequency vibrations of filler atoms in the oversized void cages. In the past decades, it has been still under debate whether the underlying phonon scattering mechanism should be considered to be resonant scattering or enhanced three-phonon process. To clarify the role played by the filler atoms in reducing the lattice thermal conductivity, we study the microscopic dynamical process of filler and related interactions by means of ab initio molecular dynamics (AIMD) and temperature dependent effective potential (TDEP) technique. Firstly, we simulate the dynamical trajectories of fully filled skutterudite YbFe4Sb12 at different temperatures through AIMD. In this approach, the nonlinear guest-host interactions at finite temperatures are taken into consideration naturally from dynamical trajectories. Then, we extract the effective temperature-dependent harmonic and anharmonic interatomic force constants (IFCs) by TDEP method through the statistical analyses of both trajectories and forces. The atomic participation ratios and lifetimes of phonon modes are calculated based on the effective IFCs. The results demonstrate that the local vibration modes of Yb couple with acoustic branches and reduce the lifetimes of the lattice phonons significantly. However, the calculated L, which is on the assumption that the filler interacts with lattice phonons through three-phonon collision, still deviates from the experimental result. In order to rationalize the discrepancy, we analyze the correlation properties between different Yb atoms by velocity coherence in atomic dynamical motions. The localized and independent vibration characteristic of Yb is found in this analysis. This implies that the motions of Yb atoms deviate from the periodic and collective vibration excitation paradigm of phonon. Therefore, the mechanism for how filler atoms scatter lattice phonon and enhance thermal resistance is beyond three-phonon scattering process. We thus introduce resonant scattering into the lifetimes of Yb-dominant localized vibration modes, and so-calculated L is in a good agreement with the experimental data. Overall, we come to a conclusion that both the phonon-phonon interaction and the resonant scattering due to the localized oscillators cause the low lattice thermal conductivity of YbFe4Sb12.
2018, 67 (1): 016104. doi: 10.7498/aps.67.20172025
Nd2Fe14B rare earth and Sm2Co17 type permanent magnets have been widely used in the third generation of synchronous radiation light source and free electron laser facility in undulators and other components of particle accelerators. In addition, the permanent magnets are used in the radiation treatment system for cancer as a beam line component. Compared with Sm2Co17 type permanent magnet, Nd2Fe14B rare earth permanent magnet has the characteristics of large magnet energy product, rich starting materials and low price. Although its Curie point and coercive force are lower than those of Sm2Co17 type of permanent magnet, Nd2Fe14B rare earth permanent magnet is still widely used. As an important part of the accelerator, the magnetic loss phenomenon appears when permanent magnet is used in long-term irradiation environments, which affects the stability and quality of the beam. Therefore, it is important to investigate the magnet demagnetization induced by photon irradiation. Recently, there have appeared many researches of the phenomena of demagnetization for the permanent magnets under the irradiation of various kinds of particles. By using different research methods and experimental conditions, single particle irradiation is performed and then the effect of irradiation on magnetic loss is investigated by comparing the macro magnetic properties (such as magnetic flux loss rate, saturation magnetization, etc.). However, there are not any available reports on the microstructure investigations of permanent magnets after irradiation. Microstructure affects macroscopic magnetic properties. In order to discuss the microscopic demagnetization mechanism, the transmission electron microscope is used to characterize and analyze the microstructure evolutions of Sm2Co17 type permanent magnet and Nd2Fe14B rare earth permanent magnet before and after proton irradiation. The evolution of the number density of nanocrystal and its size distribution induced by proton irradiation are calculated. Moreover, the effect of microstructure evolution on macroscopic magnetic loss is discussed. The results indicate that the microstructure of permanent magnet transforms from single crystal structure to polycrystalline structure with the increase of the proton irradiation damage level. Nanocrystal and the matrix of permanent magnet have the same crystal structure. With the irradiation damage level increasing, the nanocrystal density of Nd2Fe14B first increases and then decreases, while the particle size distribution first increases and then keeps constant; the number density of nanocrystal of Sm2Co17 type permanent magnet gradually decreases, while particle size gradually increases, and comparing with Sm2Co17 type permanent magnet, the crystal structure of Nd2Fe14B permanent magnet shows an obvious tendency to be amorphous in 2 dpa irradiation damage level.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Prediction of chaotic time series based on the fractional-order maximum correntropy criterion algorithm
2018, 67 (1): 018401. doi: 10.7498/aps.67.20171803
Recently, adaptive filters have been widely used to perform the prediction of chaotic time series. Generally, the Gaussian noise is considered for the system noise. However, many non-Gaussian noises, e.g., impulse noise and alpha noise, exist in real systems. Adaptive filters are therefore required to reduce such non-Gaussian noises for practical applications. For improving the robustness against non-Gaussian noise, the maximum correntropy criterion (MCC) is successfully used to derive various robust adaptive filters. In these robust adaptive filters, the steepest ascent method based on the first-order derivative is generally utilized to construct the weight update form. It is well known that the traditional derivative can be generalized by the fractional-order derivative effectively. Therefore, to further improve the performance of adaptive filters based on the MCC, the fractional-order derivative is applied to the MCC-based algorithm, generating a novel fractional-order maximum correntropy criterion (FMCC) algorithm. Under the non-Gaussian noises, the proposed FMCC algorithm can be applied to predicting the chaotic time series effectively. In the proposed FMCC algorithm, the weight update form is constructed by using a combination of the first-order derivative based term and the fractional-order derivative based term. The Riemann-Liouville definition is utilized for calculating the fractional-order derivative in the proposed FMCC algorithm. The order of the fractional-order derivative is a crucial parameter of the proposed FMCC algorithm. However, it is difficult to obtain the optimal fractional order for different nonlinear systems theoretically. Therefore, the influence of the fractional order on the prediction performance is determined by trials for different nonlinear systems. The appropriate fractional order corresponds to the optimum of prediction accuracy, and can be chosen in advance. Simulations in the context of prediction of Mackey-Glass time series and Lorenz time series demonstrate that in the case of non-Gaussian noises the proposed FMCC algorithm achieves better prediction accuracy and faster convergence rate than the least mean square (LMS) algorithm, the MCC algorithm, and the fractional-order least mean square (FLMS) algorithm. In addition, the computational complexity of different filters is compared with each other under the example of the prediction of Marckey-Glass time series by using mean consumed time. It can be found that the computational complexity of FMCC algorithm is higher than those of the MCC and the LMS algorithms, but only slightly higher than that of the FLMS algorithm. As a result, comparing with other filters, the FMCC algorithm can improve the prediction performances of chaotic time series at the cost of the increasing computational complexity.
Comparative demonstration of multimode steady-state theory for the gyrotron traveling-wave tube based on a distributed loss-loaded metal cylindrical waveguide
2018, 67 (1): 018402. doi: 10.7498/aps.67.20171831
Gyrotron traveling-wave tube (gyro-TWT) is capable of generating high-power microwave radiation in a millimeter wave range. It is one of the most promising candidates for the applications in the millimeter wave radar, communication systems, and environmental monitoring. The gyro-TWT can work at high frequency and produce high power output with high order modes. Although the high mode gyro-TWT can work at high frequency and produce high power output, the instability problem is a main factor to prevent the gyro-TWT performance from further improving and hinder this device from being put into the practical application. The earlier research of the instability primarily concentrated on the single-mode situation, which cannot be used to analyze the mutual effects between the other oscillation modes and the operating mode. Hence, it is important for academic study and engineering application to solve the mode competition problem. In this paper, based on lossy uniform/periodic dielectric-loaded metal cylindrical waveguide usually used in the international academic analysis and engineering research, a multimode steady-state beam wave interaction theory for gyro-TWT is established, which can consider the mutual effects between the other oscillation modes and the operating mode. As application examples, under the same condition of geometrical and electrical parameters, the theoretical results of the beam wave interaction for the TE01 fundamental mode gyro-TWTs are compared with the experimental results reported by NRL and IECAS for Ka band and those simulated with Magic code for W band in order to demonstrate the rationality of the theory. The results show that the theoretical results are in good agreement with the experimental and simulated ones. For the NRL design, when the velocity spread is 9.6%, the maximum output power from the theory is 127 kW at 34.09 GHz with a gain of 47.4 dB, an efficiency of 17.6%, and a -3 dB bandwidth of 1.01 GHz, and an NRL measured maximum output power is 130 kW at 34 GHz with a gain of 47.5 dB, an efficiency of 18% and a -3 dB bandwidth of 1.0 GHz. The maximum difference between the theory and the experiments occurs near the frequency of 34 GHz, the measured power by NRL is 127 kW and the calculated power is 118 kW, the relative error between the theory and the experiment is 8.5%. For the IECAS design, the simulated maximum output power from the theory is 113.73 kW at 33.85 GHz with a -3 dB bandwidth of 1.72 GHz when the velocity spread is 7%. The measured peak output power by IECAS is 110 kW at 33.88 GHz with a -3 dB bandwidth of 1.75 GHz. For a W band TE01 fundamental mode gyro-TWT design, the saturated output power is 112 kW at a frequency of 94.5 GHz with a gain of 34.28 dB and -3 dB bandwidth of about 4.1 GHz, and the saturated output power calculated with Magic code is 106.7 kW with a gain of 34.11 dB and 3 dB bandwidth of 3.9 GHz, the maximum relative errors between the theory and experiment are both about 5% for the output power and the bandwidth.
2018, 67 (1): 018501. doi: 10.7498/aps.67.20171551
Based on diamagnetic levitation, the micro-vibration energy harvester is proposed, which has advantages such as low friction, low mechanical damping, low-frequency response and free of maintenance. The floating magnet is one of the most important parts in the vibration energy harvester. The dynamic properties of the floating magnet directly determine the output characteristics of the energy harvester. In order to study the vibration properties of the floating magnet, the force characteristics of the floating magnet are investigated in the vibration energy harvester. The magnetic and diamagnetic forces exerted on the floating magnet are simulated using finite element analysis software COMSOL Multiphysics. Then the dynamic characteristics of the floating magnet are further analyzed by MATLAB. In the case of the present study, when the gap between the two pyrolytic graphite plates is smaller than 7.7 mm, the floating magnet works in a monostable state. At the same time the floating magnet runs in a bistable state when the gap between the two pyrolytic graphite plates is larger than 7.7 mm. The two working states are in accordance with the experimental results. The results prove that the theoretical analysis and experimental results are in good agreement. Furthermore, the dynamic response of the energy harvester is studied in the two working states. When the coils are open-circuited and the energy harvester is in a monostable state, it is found that the dynamic response can be equivalent to that of a linear system with a nonlinear disturbance. So, the amplitude-frequency curve is right-skewed. We also analyze the influence of the gap between the two pyrolytic graphite plates on the amplitude-frequency curve. It is found that with the increase of the gap between the two pyrolytic graphite plates, the nonlinear disturbance becomes stronger, leading to a stronger right-skewed phenomenon in the amplitude-frequency curve. When the coils are open-circuited and the energy harvester is in a bistabtle state, the dynamic response is very complex, which includes double period, 4-time period and chaos. It is because the change of the amplitude of external excitation affects relative strength between the linear and nonlinear parts in the energy harvester system, resulting in the change of vibration characteristic of the floating magnet. When the coils are linked to load and the energy harvester is in a bistabtle state, the frequency of the energy harvester is consistent with that of the external excitation. This study can serve as a reference for designing the structure of the vibration energy harvester with using diamagnetic levitation. And it provides a theoretical guidance for improving the performance of the energy harvester and expanding the working bandwidth of the harvester. The energy harvester has vast application potential in wireless sensor networks and portable electronic devices.
2018, 67 (1): 018701. doi: 10.7498/aps.67.20171571
Since 1970, the trapping of the small objects in space by optical radiation pressure, such as nano particles and other atomic living cells, has been successfully developed and used in the applied physics, life sciences and other fields. As the optical radiation pressure is very weak, the use of radiation pressure on the particle will be strictly limited by the particle size. Also, the manipulated particles can move particle with only hundreds of microns. Therefore, it is not suitable for trapping and long-distance transporting particles with large size (micron). In recent years, with the development of the manipulation technology for large particles, a new control force-photophotetic force has gradually entered into people's vision field. Compared with the optical radiation pressure, the photophoretic force is much large under the same light intensity. Therefore, the photophoretic force makes it possible to manipulate and trap the large particles. With the development of laser beam-shaping technology, the species of laser beams become more and more abundant, which makes it more attractive to study particle manipulation based on the photophoretic force. For example, a hollow beam is used to capture and guide carbon nanoclusters in the air. A tapered optical fiber is used to trap, migrate and separate SiO2 particles. A Bessel Gaussian beam is used to trap and manipulate magnetic particles. An airy beam is used to trap glass carbon particles of absorption type. In this paper, a trapping and guiding scheme for large-size particles by using the photophoretic force of the hollow beams generated by nonlinear ZnSe crystals is proposed and analyzed theoretically. Our calculated results can be concluded as follows. 1) For the cases of two-dimensional particle trapping and one-dimentional particle guiding using a hollow beam generated by a single nonlinear ZnSe crystal, the magnitude of the longitudianl optical force is proportional to the ratio between particle size and hollow beam size to the fourth power and is proportional to the power of the hollow beam, and the direction is the same as that of the beam propagation. The closer to the hollow beam size the particle size, the greater the transverse optical force is. The results show that the photophoretic force can achieve the two-dimensional trapping of large-size particles, and a long distance (in a meter region) guiding. 2) For the case of three-dimensional particle trapping using a localized hollow beam generated by two nonlinear ZnSe crystals, the dependence of transverse photophoretic forceand that of longitudinal photophoretic force on the system parameters are similar to the scenario for the particles trapping in the hollow beam produced by a single nonlinear crystal. The difference is that under this condition, the direction of the longitudinal photophoretic force points to the center of the beam. So this scheme can achieve the effective three-dimensional trapping of large-size particles. Above all, the hollow beams generated by nonlinear ZnSe crystals can be used as an effective noncontact controlling tool for large-size particels, and might have potential applications in modern optics and biomedicine.
2018, 67 (1): 018901. doi: 10.7498/aps.67.20171630
In dynamical networks, usually there are time delays among nodes during their communication. Different pairs of nodes generally have different time delays (i.e., having non-uniform communication delays). It has more practical significance to study the successive lag synchronization on dynamical networks with non-uniform communication delays. So, in this paper we construct a dynamical network model with non-uniform communication delay. Then, by designing linear feedback control and adaptive feedback control, and by using the Lyapunov function method, we obtain sufficient conditions for guaranteeing the stability of successive lag synchronization. Finally, in the numerical simulation, we choose the Chua's circuit as the local nonlinear dynamic and two kinds of topological structures for dynamical network to verify the effectiveness and correctness of obtained results.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Magneto-induced polarization enhancement and magneto-dielectric properties in oxygen deficient La0.67Sr0.33MnO3-/BaTiO3 composite film
2018, 67 (1): 017701. doi: 10.7498/aps.67.20172019
Magnetoelectric composite film is an important type of multiferroic materials, which is usually composed of typical ferromagnetic and ferroelectric materials. For the ferroelectric layer, BaTiO3 (BTO) attracts much attention due to its lead-free characteristic. For the ferromagnetic layer, doped manganite (R1-xAxMnO3) has been a good candidate for designing the advanced multiferroic films. Multiple interactions among the freedom degrees of charge, orbital, spin and lattice inside the doped manganite bring many additional properties into the manganite based composite films. At present, most of researches of manganite/BTO focus on the stoichiometric oxygen ion in manganite. Considering the fact that the oxygen deficiency can remarkably adjust the properties of manganite itself and relevant heterostructure by the interface effect, abnormal magnetoelectric properties are expected in an oxygen deficient manganite/BTO composite film. In this work, a composite film composed of BTO and oxygen deficient La0.67Sr0.33MnO3- (LSMO) is deposited on LaAlO3 001 substrate by the pulsed laser deposition method, and the effects of magnetic field on the properties of polarization and dielectric in a temperature range of 20-300 K are investigated. The X-ray diffraction pattern reveals good epitaxial growth of this bilayer film. The upper LSMO film exhibits semiconductive characteristic (dR/dT 0) in a temperature range of 20-300 K. Magnetization curves indicate that the LSMO keeps ferromagnetic state without any magnetic phase transition in this temperature range. When applying a magnetic fields of 0.8 T, the resistance in LSMO is observed to decrease. The changing rate MR=|R0.8 T-R0 T|/R0 T decreases from 45.28% at 30 K to 0.15% at 300 K. This composite film exhibits remarkable temperature-dependent magneto-induced ferroelectric and dielectric change. It is found that the remanent polarization (Pr) and coercive electric field (Ec) are enhanced by the 0.8 T magnetic field. The maximum changing rates of Pr and Ec are 111.9% and 89.6% at the temperatures of 40 K and 60 K, respectively. The magnetic field enhances the dielectric constant , but suppresses the dielectric loss tan . The maximum changing rates of and tan both occur at 60 K with the values of 300% and 50.9%. The temperature at which appear the maximum magneto-induced relative changes of polarization and dielectric parameters is accordant with the temperature at which occurs the peak value of magnetoresistance, which indicates a charge-based coupling in this heterojunction. A potential mechanism is that the magnetic field promotes the degree of parallelism of local spin magnetic moment of Mn ion, and produces an indirect effect on BTO layer by the spin-obital coupling and interface effect. Our findings make the oxygen deficient LSMO/BTO heterojunction promising for the design of multiferroic devices.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2018, 67 (1): 019401. doi: 10.7498/aps.67.20171620
The artificial release of electron adsorbing material can cause electron density to be depleted in the ionosphere, forming the ionospheric holes rapidly. At the same time, the shell structure of the electron density enhancement around the hole is produced, owing to the extrusion of background plasma caused by rapid expansion of the release. The coexistence of ionospheric hole and enhancement structure is the significant characteristics of the early time effects. In this paper, the early time effects of neutral chemicals released into ionosphere are studied, and a physical model of spatiotemporal evolution about early time electron density is set up. At t=1 s, the maximum electron density in the enhanced region is 2.46106 cm-3, approximately 2.8 times as great as background electron density, then the electron density at the boundary gradually decreases. At t=30 s, the maximum electron density is 1.58106 cm-3, which is about 1.7 times the background electron density. At t=120 s the maximum electron density in the enhanced region is 1.12106 cm-3, which is 1.2 times the background electron density. Within 120 s after release, the size of the ionospheric cavity increases gradually; at t=5 s the distribution range of the released chemical material is of a sphere of about 10 km in diameter; at t=120 s the distribution diameter of the released chemical material is more than 70 km, and at the same time, the depletion depth of the ionospheric hole decreases slowly. At t=1 s, the depletion depth of the ionospheric hole is about 100%, and at t=120 s the depletion depth of the ionospheric cavity decreases to 95%. The effects of different-frequency radio waves propagating through ionospheric disturbance at t=10 s and t=120 s are simulated by the ray tracing. At t=10 s, the effect of electron density enhancement is remarkable, and the thickness of the enhancement is about 10 km, and the electronic density enhancement area can reflect the radio wave signal at a frequency as high as 14 MHz. At t=120 s, the phenomenon of electron density enhancement becomes weak, the thickness of the enhanced area continues to increase, and the radio wave signal that the electronic density enhancement area could reflect decreases to 11 MHz. The radio waves at a frequency range between 9 MHz and 12 MHz each have a complex diffraction, focusing and dispersing effect in the disturbed area. Furthermore, according to the working principle of ionospheric vertical measurement instrument and ray tracing theory, the vertical ionization detection figures are obtained through inversion. The results are consistent with previous experimental results of rocket exhaust, which testifies the correctness of proposed model.
Research progress and application prospect of Fe-based soft magnetic amorphous/nanocrystalline alloys
2018, 67 (1): 016101. doi: 10.7498/aps.67.20171473
Amorphous alloy is a kind of metallic materials prepared by rapidly cooling the alloy melt through hindering crystallization in cooling process. Due to the unique structure of atomic random packing, Fe-based amorphous alloys exhibit not only structural and property isotropy, but also small structural correlation length, small magnetic anisotropic constant, and then small coercivity Hc. Like crystalline Fe-based alloys, Fe-based amorphous alloys also possess high saturation induction Bs. As a result, research on engineering applications of Fe-based amorphous alloys has been promoted by their excellent soft magnetic properties. Now Fe-based soft magnetic amorphous/nanocrystalline alloys have been produced and applied to various areas on a large scale. Here in this paper, the processes of discovery, development and application of Fe-based soft magnetic amorphous alloys are reviewed, and the effects of chemical composition, structure and preparation technology on the soft magnetic properties are introduced and discussed. The obtained theoretic results and the technological innovation show that the great contributions have been made to the development and application of Fe-based soft magnetic amorphous/crystalline alloys. Based on the progress of structure and soft magnetic property and our understanding, the development process of the fundamental research and the application progress of Fe-based soft magnetic amorphous alloys could be divided into three periods. In addition, the present challenge topics in their researches and applications are proposed.