Vol. 66, No. 1 (2017)
2017, 66 (1): 010301. doi: 10.7498/aps.66.010301
Liao Xiang-Ping et al.(Chin. Phys. B 23 020304, 2014) pointed out that the method of weak measurement and quantum weak measurement reversal can protect entanglement and improve the fidelity of three-qubit quantum state. We generalize the method of weak measurement to the case of qudit state in this paper. By using the operation of weak measurement and quantum weak measurement reversal, we investigate the evolution dynamics of fidelity and fidelity improvement for qudit state under amplitude damping decoherence. We compare two kinds of operations: one is to let the input qudit state cross the amplitude damping decoherence directly, and the other one is that we first make a weak measurement operation on the input qudit state, then through the amplitude damping decoherence, finally an operation of quantum weak measurement reversal is done with the output qudit state. We discuss the GHZ state, W state, CL state and some special separable states exactly and obtain the analytic expressions of fidelity and fidelity improvement for qudit state before and after the weak measurement and quantum weak measurement reversal operation. According to the analytic expressions we plot the evolution curves against its corresponding parameters. The effects of corresponding parameters are discussed and a susceptible protection region of the qudit state is also given in the context. The results show that the structure of qudit state is the determined factor to the effect of weak measurement and quantum weak measurement reversal. There are some different effects on the different structured qudit states. For entangled state, the fidelity of qudit GHZ state can be protected in a relatively big evolution region, most part of the fidelity improvement is in the upper part of the zero reference plane. While the fidelity of qudit W state can be improved effectively in the whole evolution region, which is a perfect protection. The evolution regulations of qudit CL state and Dick state are between evolution regulations of the GHZ state and W state. When we input some special separable qudit states which have similar structures to W state, their fidelity and fidelity improvement are almost the same as W state’s. It is demonstrated that the structure of qudit state is important for the weak measurement in a step. This work is meaningful for the quantum information process.
2017, 66 (1): 010501. doi: 10.7498/aps.66.010501
Brownian motion in the environment of the thermal fluctuations is a long-study issue in nonequilibrium statistical physics. In recent years, the directed transport properties of Brownian ratchets attract the widespread attention of scholars. When a ratchet system possesses the spatio-temporal symmetry-breaking feature, the directed transport can be produced. Although the breakthrough progress in the directed transport of the Brownian ratchet has been made, the energy conversion efficiency of feedback ratchet is not clear. Therefore, the center-of-mass mean velocity and the energy conversion efficiency of coupled ratchet under the influences of the time asymmetry of external force and the spatial asymmetry of external potential are discussed in detail. The overdamped coupled Brownian particles are investigated. Nevertheless, the optimized control of the coupled ratchet is the important for directed transport. Therefore, the closed-loop control which depends on the state of the system is adopted. The dynamic behavior of coupled particles can be described by the overdamped Langevin equation, and the equation is numerically solved by using the stochastic Runge-Kutta algorithm. Some properties of the directed transport can be obtained through this method, such as the center-of-mass mean velocity, the energy conversion efficiency, etc. It is interesting to find that the center-of-mass mean velocity can reach a maximum as the amplitude of external force increases. However, the mean velocity can show the quasi-periodic oscillations with the increase of the period of external force for different values of the spatial asymmetry of external potential. In addition, it can be found that the feedback ratchet needs strong noise to make the directed transport of the ratchet reach the maximum as the coupled strength increases. On the other hand, the energy conversion efficiencies of the feedback ratchet can achieve their corresponding maximum values with the increase of the amplitude of external force for different values of the time asymmetry, and the maximum increases as the time asymmetry increases. However, the efficiency can also show the quasi-periodic oscillations with the increase of the period of the external force for different values of the spatial asymmetry of external potential. Moreover, the energy conversion efficiency can achieve the maximum as the noise strength increases, but the maximum of the efficiency will decrease with the increase of coupling strength. From the discussion above, the optimal values of the time asymmetry, the spatial asymmetry, the period of the external force and the noise strength can promote the directed transport of the feedback coupled Brownian ratchet. These conclusions can provide some guidance in the enhancement of the energy conversion efficiency of a nanomachine.
2017, 66 (1): 010702. doi: 10.7498/aps.66.010702
Optical micromanipulation of particles based on the optical trapping effect induced by the interaction between light and particles has been successfully applied to many interdisciplinary fields including biomedicine and material sciences. When particles are trapped in three dimensions, the conventional wide-field optical microscopy can only monitor the movement of the trapped particles in a certain transverse plane. The ability to observe the particle movement along light trajectories is limited. Recently, a novel method named axial plane optical microscopy(APOM) has been developed to directly image the axial plane that is parallel to the optical axis of an objective lens. The APOM observes the axial plane by converting the axial information of a sample into that of a transverse plane by using a 45°-tilted mirror. In this paper, we propose and demonstrate that the APOM serves as an effective tool for observing the axial movement of particles in optical tweezers. By combining with a conventional wide-field optical microscopy, we show that both transverse and axial information can be acquired simultaneously for the optical micromanipulation. As in our first experimental demonstration, we observe two particles which are trapped and aligned along the optical axis. From the transverse image, only one particle is observable, and it is difficult to obtain the information along the axial direction. However, in the axial plane imaging, the longitudinal dipolar structure formed by the two particles is clearly visible. This clearly demonstrates the APOM imaging capability along the axial axis. The numerically simulations on the trapping focal spot against the position of a collimating lens agree well with our experimental APOM results. Furthermore, we directly observe the dynamic capture process of a single trapped particle in transverse plane by conventional wide-field optical microscopy as well in axial plane by the APOM, and can obtain the 3D information rapidly and simultaneously. We point out that the observable axial dynamic range is about 30 μm. Taking advantages of no requirement of scanning and data reconstruction, the APOM has potential applications in many fields, including optical trapping with novel beams and 3D imaging of thick biological specimens.
2017, 66 (1): 010701. doi: 10.7498/aps.66.010701
An innovative and practical scheme of building far-detuned optical lattice for 87Rb atoms is proposed.The disposals of aligning the lattice beams,tuning the lattice frequency and controlling the tapered amplifier for output are described in detail.Alignment of optical lattices is quite difficult in principle,for several beams are required to hit the same atomic cloud.For the relatively near-detuned one-and two-dimensional lattices,the coarse alignment is accomplished by tuning the lattice laser onto resonance with the magnetic-optic trap(MOT) frequency,and then blowing away the MOT in real time.A more precision alignment is implemented at the end of the MOT loading,the atoms are first pumped into the lower hyperfine level by turning off the repumping for some time;then,the pulsed lattice beams are turned on for a short time at some reasonably large detuning.Finally,a fluorescent image of the MOT is taken without repumping,in order to detect only those atoms which are repumped by the lattice laser.For the purpose of controlling the detuning of the lattice easily and accurately,a home-made grating wavemeter with a resolution better than 1 GHz is used.This way allows the laser to be locked at any frequency by using a software PID and is experimentally simple to implement.The intensity of the lattice is controlled directly by pulsing the current through the tapered amplifier using a function generator and a laser diode driver.This technique has already been demonstrated before by Prof.M.Kasevich's group at Stanford. Our experiment starts with a MOT capturing approximately 4107 atoms in 200 ms.The lattice loading is overlap with the end of polarization gradient cooling(PGC),after that,the molasses laser beams are extinguished, and the adiabatic expansion is accomplished in the same time by a decrease in the lattice light intensity according to release function.On the basis of MOT and PGC,the dependences of atomic loading on such parameters as the intensity and frequency detuning of optical lattice are investigated experimentally.The vibration frequency is measured by intentionally modulating the trap intensity.Experimental results show that the lattice structure facilitates the cooling with the temperature of atoms cloud being reduced to 1/3 compared with free space polarization gradient cooling.The system design,experimental results and conclusions are of definite significance and can serve as a fine reference for other kinds of lattices designs or alkali atomic plans.
2017, 66 (1): 010703. doi: 10.7498/aps.66.010703
Apart from neutrons, the fusion core produces gamma rays during fusion reaction. The spectrum of gamma ray can provide very important information for fusion diagnosis. However, due to the gamma energy and yield in one fusion pulse being both lower, the gamma spectrometer used should have high detection efficiency and energy resolution. The concept of a Gamma-to-electron magnetic spectrometer(GEMS) provides the idea to build up such a spectrometer to meet this requirement. Based on this concept design, four important parts of this facility are investigated. The first part is the gamma-electron converter. The main physics processes include Compton scattering of gamma ray with converter material generating electron, the electron multiple Coulomb scattering(MCS) inside the converter and the electron attenuation. Affected by the thickness of convector, these processes give a complex influence on the detection efficiency and angular-energy distribution of the electrons which are emitted from the downstream face of the convector. The Monte Carlo code Geant4 is employed to investigate theeffects of Compton scattering, MCS and converter thick on the angular-energy distribution. The second one is the collimation. The collimation is used to select the forward direction, the performances of cutoff angle of the collimator on the detection efficiency and resolutions, the correlation between electron transportation direction and energy, are also studied using Geant4 code. The third part is the dipole magnetic field. There are several geometric and magnetic parameters, therefore, a multi-thread parallelized genetic algorithm is developed to obtain the best result. Both the irregular geometric shape and dipole magnetic field strength are optimized to achieve the best energy resolution and detection efficiency. The obtained magnetic field has an intensity of less than 100 Gauss, and its performance on gathering elections is also verified by Geant4 code. The last one is the location of electron detectors. The study shows that all the electron detectors should be located not in a straight line but a quadratic curve. Then the optimized spectrometer is simulated by Geant4 to obtain the responses of gamma rays with various energies. For the gammas provided by fusion reaction, the simulation shows that when the neutron yields are about 2.5×1015 and 1.2×1016, the energy resolutions reach 0.5 MeV and 0.25 MeV, respectively, provided that different thick Be converters are employed. All in all, this optimized GEMS can be employed to measure the spectrum of gamma rays generated fom the fusion reaction.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2017, 66 (1): 014203. doi: 10.7498/aps.66.014203
The ultrafast pump-probe spectroscopy allows us to make movies of the dynamics of the carriers and vibrational excitations on the timescales shorter than the typical scattering time. In general, the temporal evolution of the reflectivity change is comprised of the oscillatory and the non-oscillatory components. The former corresponds to the coherent lattice vibration, while the latter is related to the complex cooling process of the hot carriers. To investigate the dynamics of the hot carrier and the lattice vibration, it is necessary to decouple the two parts in the detected signal. Comparatively, the manipulation of the coherent lattice vibration is easier in spite of its super-high frequency and subatomic vibration amplitude. In this work, the behavior of the coherent lattice vibration in Bi2Te3 single crystalline film with a thickness of 100 nm is studied by using the double pump-single probe ultrafast spectroscopy. Firstly, the coherent lattice vibration with the subatomic amplitude and a frequency of about 1.856 THz is simulated by a femtosecond pump pulse, and its damped oscillation signal is detected by the reflectivity change of a probe pulse. Compared with the Raman spectrum, this vibration is confirmed to be the coherent optical phonon with A1g1 symmetric vibration mode. To manipulate this lattice vibration, a pulse shaper is then installed in the pump-beam arm to generate double pump pulses with the different separation times and the intensity ratios. The resulting reflectivity change is found to be a superposition of the pulse train: the oscillation amplitude is enhanced when the separation time is matched to the period of the oscillation; if the separation time is the odd times the half-period of the oscillation, the A1g1 vibration mode can be completely cancelled out after adjusting the intensity ratio. Finally, by maintaining the same intensity ratio, the amplitudes of the oscillation signals after the second pump pulse are measured with different separation times. The results agree well with the theoretical predictions: the amplitude of the oscillation after the second pump pulse shows a cosine function of separation time with a period of about 1080 fs, which is the twice the period of the oscillation illuminated by a single pump pulse. This work suggests that the lattice vibration can be optically manipulated, thus provides an effective way to disentangle the lifetimes of the phonons and the interactions with the excited carriers in the ultrafast energy relaxation process in semiconductor, which is extremely important for a number of interesting phenomena such as the non-thermal melting and the insulator-to-metal transition.
2017, 66 (1): 014204. doi: 10.7498/aps.66.014204
The photonic absolute bandgaps have many potential applications in specific fields, and some methods to enlarge the absolute bandgaps, such as adjusting the material and the rotational symmetry, constituting a heterostructure have been explored. Recently, with the occurring of metamaterial, the photonic crystal based on metamaterial has also realized the wide absolute bandgaps. However, the metamaterial is an artificially structured material of which the construction is more complicated. In this paper, one-dimensional magnetic photonic crystal structure with wide absolute bandgaps is proposed, which is composed of two kinds of magnetic materials with the same refractive index and physical thickness but different wave impedances. First of all, the transmission properties of one-dimensional magnetic and non-magnetic photonic crystals with the same wave impedance ratio are studied by using transfer matrix method. It is shown that the normalized frequency bandwidth of magnetic photonic crystal, i. e. the ratio of the band of bandgap to its center, is 0.41, while the normalized frequency bandwidth of the non-magnetic photonic crystal is 0.14. From the results, we can conclude that the absolute bandgap of the above magnetic photonic crystal is wider than that of non-magnetic photonic crystal because the former bandgap is not sensitive to the incident angle nor polarization. Secondly, we adjust the wave impedance ratios of the two kinds of magnetic materials and make them respectively reach 2, 4 and 6, with the refractive index and the physical thickness kept unchanged. By analyzing their transmission properties, it is found that the normalized frequency bandwidths of the absolute bandgaps are respectively 0.47, 0.84 and 1.03, and the greater the difference between the two wave impedances, the wider the normalized frequency bandwidth is. Thirdly, we investigate the influence of the per-layer physical thickness of the magnetic material on the bandgap, with the other parameters remaining unchanged. It is shown that the center of the absolute bandgap shifts toward high frequency with the decrease of the per-layer physical thickness. Finally, a kind of heterostructure is constructed by the above two one-dimensional magnetic photonic crystals. The normalized frequency ranges of the first and the second absolute bandgap of one magnetic photonic crystal structure are respectively 1.18-2.85 and 5.37-6.85. The normalized frequency range of the absolute bandgap of the other magnetic photonic crystal is 2.37-5.68. The normalized frequency range of the absolute bandgap of the heterostructure can be enlarged to 1.18-6.85 and the corresponding normalized frequency bandwidth can reach more than 1.41. The wide absolute bandgaps can be applied to integrated optics, optical fiber communication and high-power laser systems, according to which we may design the polarization-independent and omnidirectional devices such as reflectors, optical switchers and optical filters.
2017, 66 (1): 014205. doi: 10.7498/aps.66.014205
In the last decade, the vortex beams have received lots of attention for their orbital angular momentum.When they are applied to optical fiber communication field,the data channels will increase and information propagation speed will be effectively improved. Recently, researchers have shown the capabilities of long length stably propagation, nonlinear frequency conversion and mode division multiplexing of vortex modes in a ring fiber. Due to the photonic crystal fiber(PCF) having very flexible design degrees of freedom, it will enable a wide range of propagation properties. In this paper, a SiO2 air-hole ring PCF is proposed for separation and propagation of optical vortex modes.By using COMSOL Multiphysics software,the vortex modes(TE01, HE21± and TM01) are simulated and calculated. The differences in effective refractive index between them are 4.59×10-4 and 3.62×10-4 respectively. One can analyze the propagation properties of vortex beams in the ring PCF by changing the size of first layer air hole radius and air hole pitch. When the incident light wavelength of TE01 mode ranges from 1650 nm to 1950 nm, this ring PCF can achieve a total dispersion variation between 44.18 to 45.83 ps·nm-1·km-1, which is tend to be flat. When incident light wavelength is 1550 nm, the nonlinear coefficient of TE01 mode vortex light is 1.37 W-1·km-1. Due to the fact that long wavelength light is easier to leakage through the cladding than the short wavelength light, the confinement loss increases with the wavelength. When incident light wavelength is 2000 nm, there is still an eight-orders-of-magnitude of the low confinement loss. Theoretically, flat dispersion and low loss vortex beams in this fiber can be beneficial to propagating stably, and the vortex modes lay the foundation for long distance propagation in the optical fiber. In the future, this ring PCF will be used in optical fiber communication field and applications in aspects such as continuous spectrum research, which can make it have immense advantage over traditional fibers.
2017, 66 (1): 014206. doi: 10.7498/aps.66.014206
Fiber Bragg grating sensor is widely used in military, construction, transportation, aviation and other fields due to its advantages in high sensitivity, high precision, high multiplexing and small volume. However, in some special fields such as ultrasonic flaw detection, high-speed vibration and aeroengine monitoring, the signals are rapidly changing, thus requiring high speed sampling. But the demodulation speed of traditional fiber Bragg grating demodulation techniques is hardly to satisfy the requirements, which seriously limits the application of fiber Bragg grating sensor in these fields. To solve this problem, in this paper we propose a dispersion compensation fiber(DCF)-single mode fiber(SMF) dual-channel demodulation method. Based on the SMF and the DCF with the characteristics of positive and negative dispersion coefficients in the anomalous dispersion region respectively, and combining with the optical time domain reflection technology, high speed and high precision demodulation of fiber grating can be realized. This system adopts the whole fiber structure without wavelength scanning, and the grating wavelength and position information can be obtained according to the pulse delay difference under a single optical pulse. There are three factors that quite influence the system accuracy and need to be solved: the grating space disturbance which is caused by the temperature change of the sensor network fiber; the dual-channel length disturbance caused by the DCF-SMF dual-channel temperature change; the dispersion disturbance caused by the inaccurate dispersion difference of the DCF-SMF. By constructing the DCF-SMF dual-channel, adopting the reference grating and introducing the dispersion difference correction model, these influence factors are solved. The case of temperature disturbance elimination is tested by the 5-75℃ temperature experiments. And the results are as follows: when the temperature of the sensor network fiber changes, the standard deviation of this dual-channel demodulation system is 16.8 pm, while only using the DCF single-channel to form the demodulation system, the standard deviation is 3614 pm. And when the DCF-SMF dual-channel is disturbed by temperature, the standard deviation is 11.9 pm. For a long time demodulation under constant temperature, the standard deviation of this system is 6.4 pm. Thus the influences of the sensor network fiber temperature change and the dual-channel temperature change on the system demodulation accuracy are effectively reduced. The feasibility and accuracy of this method are also verified by the strain experiment. Experimental results show that the highest demodulation rate of this method is 1 MHz, while the linearity can be up to 0.9998, and the accuracy is about 8.5 pm. So the system with the dispersion difference correction model has a high precision. Therefore, this novel demodulation method has advantages of high speed and high precision, good stability and large dynamic range, and it is very applicable to quasi-distributed fiber Bragg grating sensing system.
2017, 66 (1): 014201. doi: 10.7498/aps.66.014201
Moving target imaging(MTI) plays an important role in practical applications. How to capture dynamic images of the targets with high qualities has become a hot point of research in the field of MTI. In order to improve the reconstruction quality, a new MTI model based on compressed sensing(CS) is proposed here, by using a sampling protocol of the row-scanning together with a motion measurement matrix constructed by us. It is proved by the simulation and the experimental results that a relatively high quality can be achieved through this approach. Furthermore, an evaluation criterion of reconstructed image is introduced to analyze the relationship between the imaging quality and the moving speed of the target. By contrast, the performance of our algorithm is much better than that of traditional CS algorithm under the same moving speed condition. As a result, it is suggested that our imaging method may have a great application prospect in the earth observation of unmanned aerial vehicles, video monitoring in the product line and other fields.
2017, 66 (1): 014302. doi: 10.7498/aps.66.014302
The secondary Bjerknes force between bubbles in an acoustic field is a well-known acoustic phenomenon. The theoretical researches of the secondary Bjerknes force mainly focus on the case of two spherical bubbles. The secondary Bjerknes force between two spherical bubbles, calculated based on the linear equations, is very small and negligible. Therefore these theoretical researches donot give a good explanation for the phenomenon, such as “streamer formation” and multi-bubble sonoluminescence(MBSL). Experiments of sonoluminescence show that the shapes of the bubbles in a sound field are not entirely spherical. Nonspherical effects have an important influence on the secondary Bjerknes force when two bubbles come close to each other in a strong acoustic field(>1.0×105 Pa). How the shape distortion of a nonspherical bubble causes the secondary Bjerknes force between two bubbles to change, and how the secondary Bjerknes force affects the oscillations and movements of bubbles are major problems which we are to solve in the present research. The expression of the secondary Bjerknes force between a nonspherical bubble and a spherical bubble is obtained by considering the shape oscillation of a nonspherical bubble. We numerical simulate the secondary Bjerknes force between a nonspherical bubble and a spherical bubble based on the nonlinear oscillation equations of two bubbles, and compare the secondary Bjerknes force between a nonspherical bubble and a spherical bubble with the secondary Bjerknes force between two spherical bubbles in the same condition. We discuss the influence of nonspherical effects on the secondary Bjerknes force between two bubbles. The results show that when the amplitude of driving pressure is greater than the Blake threshold of a nonspherical bubble and makes the bubble oscillate stably, the secondary Bjerknes force between this nonspherical bubble and a spherical bubble is different from the secondary Bjerknes force between two spherical bubbles in direction and magnitude. The secondary Bjerknes force between a nonspherical bubble and a spherical bubble is much bigger than that between two spherical bubbles. The interactional distance of the secondary Bjerknes force between a nonspherical bubble and a spherical bubble is longer than that between two spherical bubbles. The secondary Bjerknes force between a spherical bubble and a nonspherical bubble depends on the radii of two bubbles, distance between two bubbles, shape mode of the nonspherical bubble and the amplitude of driving pressure. Our research is closer to the actual bubbles in liquid. We also prove that big mutual interaction between bubbles is the main cause for froming a stable structure between bubbles. For bubbles, big mutual interaction causing the cavitation becomes easier. These results are important for explaining the phenomenon in an acoustic field, such as “streamer formation” and MBSL.
2017, 66 (1): 014306. doi: 10.7498/aps.66.014306
An ocean ambient noise model is established considering source depth distribution. The model is used to analyze the effect of source depth on the vertical characteristics of ambient noise field. The analyses are explained and validated by normal mode theory. The energy of normal mode excited changes with source depth. Effects of different order normal modes are different. The high order modes raise up the equivalent seabed reflection loss, whereas the low order modes depress it. It is found that the seabed sound speed, density and source depth all have significant influences on equivalent seabed reflection loss at large grazing angles. So the source depth should be taken into account and the model is used in geo-acoustic inversion. Two sets of experimental data in a bandwidth of 200-525 Hz are used to obtain geo-acoustic parameters. The results show that the geo-acoustic parameters inverted from ocean ambient noise and from sound propagation data are similar. The mean value of inverted source depth tends to be smaller as frequency increases, which demonstrates that wind waves become dominant over ship noise. The average of inverted source depth values in the band(>400 Hz) is very small when sea state is higher than grade 3, which is consistent with the result from the Monahan's bubble theory.
2017, 66 (1): 014501. doi: 10.7498/aps.66.014501
Intense particle pulsation during discharging may lead to the vibration of silo, even the failure of silo structure. To date, the studies related to particle pulsation have mainly concentrated in the following aspects: the noise caused by vibration of silo, the minimum decisive height to produce silo music and the factors affecting particle pulsation. However, the above studies cannot in depth analyze the motion state nor the flow law of all particles in silo. To explore the pulsation characteristics of particles, in this paper we simulate the discharging tests of ellipsoidal particles in deep silo with different half-cone angles based on the discrete element method, in order to reveal the mechanisms of particle pulsation and variation of contact force among the particles in the silo. In each simulation discharging test, the cylinder section of silo is divided into 4 fixed areas where flow behavior and the motion characteristics of particles are analyzed. The simulation results show that the velocity fluctuation of particles exists in the whole discharging process. At the early stage of discharging, the cyclical pulsation with large amplitude appears while irregular fluctuation with small amplitude occurs in the later stages. The study also finds that the dynamic characteristics of the axial force among particles are the same as those of velocity pulsation in the corresponding areas. Besides, the amplitude of particle pulsation shows an increase trend and the contact force of particles presents more periodic pulsation along the negative direction of outlet. The pulsation characteristics(velocity pulsation and force pulsation) of adjacent particle layers are similar, including similar waveform and identical cycle. During the intense pulsation stage, each minimum of the axial force of particles in the top layer is close to the gravity, indicating that the contact force among these particles disappears. Furthermore, the periodic pulsation of particles causes the contact force among particles to periodically disappear. It is noted that the stability of discharging, frequency, amplitude and duration of the intense pulsation increase with the decrease of the half-cone angle. In order to evaluate the fluctuation degree of the velocity pulsation, the standard deviation of particle velocities is used. Note that the particle velocities are no longer subjected to the influence of rising trend, which result is obtained by the finite difference method. The results show that the standard deviation gradually increases with the decrease of half-cone angle. This is because the increase of half-cone angle causes the time and amplitude of stable fluctuation to decrease. This numerical study of particle pulsation will provide the reference for safety design of discharging devices.
2017, 66 (1): 014702. doi: 10.7498/aps.66.014702
Graphene, as a classical two-dimensional material, has various excellent physical properties, which can be further transferred into its nanocomposite. Under external fields, the nonspherical nanoparticles in liquid environment will exhibit various deterministic movements, among them is the orientation behavior. By realizing the orientation control of nanoparticles, we can, on one hand, increase the thermal conductivity of the system along the oriented direction, and on the other hand, fabricate novel nano-devices based on the nanoscale self-assembly, which may become the key components in NEMS and Lab-on-a-chip architectures. However, current studies mainly focus on the orientations of one-dimensional rod-shaped particles, like carbon nanotubes. For a two-dimensional nanoparticle, like graphene, the situation is more complex than the one-dimensional one, because two unit vectors should be defined to monitor the orientation behaviors. As far as we know, this part of research has not been extensively carried out. Thus, in this paper, the molecular dynamics method is used to study the orientation of a single uncharged rectangular graphene in water, induced by DC electric fields. We track the orientations of the normal and long-side vectors of graphene. The results show that at a relatively high electric strength of 1.0 V/nm, the graphene is preferred to orient its normal vector perpendicular and its long-side vector with a small angle(located between 0° and 30°) with respect to the electric direction, respectively. With the increase of the electric field strength, the orientation preference of the normal vector along the electric direction is increased. To explain this phenomenon, we calculate the orientation distribution of water molecules in the first hydration shell. The dipoles tend to be parallel to the electric direction, and the surfaces of water molecules tend to be parallel to the surface of graphene. These two combined effects result in the above orientation behavior of the normal vector. Another interesting phenomenon is that the decrease of the length to width ratio of graphene will cause both the orientation preferences of the normal vector and the long-side vector to decrease. By utilizing the Einstein relation, we can obtain the rotational diffusion coefficients of graphene around the normal vector and long-side vector. The qualitative results show that the orientation orders of the normal vector and long-side vector respectively have negative correlations with the rotational diffusion coefficients of the rotation around the long-side vector and the normal vector. The orientation behavior of the platelike graphene actually comes from the competing effects between its rotational Brownian motion and the external field. Increasing the strength of the external field or reducing the rotational diffusivity will both lead to an increased orientation order of the nonspherical nanoparticle.
2017, 66 (1): 014703. doi: 10.7498/aps.66.014703
The prediction and control of the laminar-turbulent transition are always one of the most concerned frontiers and hot topics.Receptivity is the initial stage of the laminar-turbulent transition process in the boundary layer,which decides the physical process of the turbulent formation.To date,the researches of receptivity in the three-dimensional boundary layer are much less than those in the two-dimensional boundary layer;while most of the real laminar-turbulent transition in practical engineering occurs in three-dimensional boundary layers.Therefore,receptivity under the threedimensional wall local roughness in a typical three-dimensional boundary layer,i.e.,a 45° back swept infinite flat plate, is numerically studied.And a numerical method for direct numerical simulation (DNS) is constructed in this paper by using fourth order modified Runge-Kutta scheme for temporal march and high-order compact finite difference schemes based on non-uniform mesh for spatial discretization:the convective term is discretized by fifth-order upwind compact finite difference schemes;the pressure term is discretized by sixth-order compact finite difference schemes;the viscous term is discretized by fifth-order compact finite difference schemes;and the pressure equation is solved by third-order finite difference schemes based on non-uniform mesh.As a result,the excited steady cross-flow vortices are observed in the three-dimensional boundary layer.In addition,the relations of three-dimensional boundary-layer receptivity with the length,the width,and the height of three-dimensional wall localized roughness respectively are also ascertained.Then, the influences of the different distributions,the geometrical shapes,and the location to the flat-plate leading-edge of the three-dimensional wall local roughness,and multiple three-dimensional wall local roughness distributed in streamwise and spanwise directions on three-dimensional boundary-layer receptivity are considered.Finally,the effect of the distance between the midpoint of the three-dimensional wall localized roughness and the back-swept angle on three-dimensional boundary-layer receptivity is studied.The intensive research of receptivity in the three-dimensional boundary-layer receptivity will provide the basic theory for awareness and understanding of the laminar-turbulent transition.
2017, 66 (1): 014202. doi: 10.7498/aps.66.014202
The spontaneous emission from a V-type three-level atom embedded in an isotropic photonic crystal with dynamic photonic band edge is studied. We consider the situation where the atom interacts with all possible radiation modes, and calculate numerically the evolution of atomic population without using Markov approximation. The calculation method can be used in related researches. In the present paper, we mainly discuss the effects of modulation parameters and the quantum interference on spontaneous emission when the band edge is modulated with step function or triangle function. We hope that the results can contribute to the applications in the dynamic photonic crystal environment in controlling the spontaneous emission via the quantum interference. The results show that in the step-modulated situation, the number of the photon-atom bound dressed states after the modulation has happened depends on atomic transition frequencies and the band edge frequency at that time, and is identical to the one in the unmodulated situation with the same parameters. The long-time evolution of the atomic population is affected by the time when the modulation happens. Depending on the system initial state, after the modulation has happened, the quantum interference can weaken the probability amplitude components corresponding to the photon-atom bound dressed states, and cause the upper-level population to decay quickly from a great value to a value near zero; or on the contrary, it can strengthen the bound dressed states, and make the upper levels retain a high population. In the modulated situation with trigonometric functions, after long enough time, the total upper-level population presents a decaying quasi-periodic oscillation behaviour. And the evolution of the total upper-level population tends to synchronize with the modulation, so the frequency of the quasi-periodic oscillation is approximately equal to the modulation frequency. But, the quantum interference can destroy the synchronization under some conditions. The decay rate of the total upper-level population is affected by the modulation frequency, and also by the initial state of the system and the angle between two dipole moment because of the quantum interference.
2017, 66 (1): 014301. doi: 10.7498/aps.66.014301
The ocean ambient noise field experiences a stochastic process of many such noise sources and the respective interactions of their wave fields with the waveguide boundaries. At frequencies of about 1 kHz and higher, forward scattering from surface wave can strongly affect shallow water sound propagation. However, most of the available ambient forecasting models do not consider the effects of multiple forward scattering from surface wave. Therefore, there is a need for an accurate method of predicting ambient noises at middle and high-frequency which can account for surface scatterings. Aiming at such a requirement, a propagation model based on transport theory method is described which yields the second-order moment of the acoustic field. Monte Carlo simulations of acoustic propagation loss are employed to validate the transport theory method. The mode number dependence of mode coupling phenomenon is demonstrated at 1000 Hz via the competing effects of mode coupling and attenuation ranges. Low and middle propagating modes are seen to have a smaller coupling range than the attenuation range, allowing mode coupling effects to take precedence over attenuation effects. The mode energies and the coherences are also examined, and it is found that the mode coupling rate for surface wave is significant, but strongly dependent on mode number. Mode phase randomization by surface waves is found to be dominated by coupling effects. On the basis of transport theory propagation model, connecting with the properties of ambient noise sources, a spatial characteristic model for ambient noise under surface wave is presented. Further, the effects of surface wave on ambient noise intensity, vertical correlation and vertical directionality are analyzed. Simulation results show that the surface wave may result in energy transfer from medium modes to low modes and high modes, the rate of energy transfer depends on the mode energy difference. Since the medium mode plays an important role in noise intensity, the noise intensity decreases with the increase of surface wave. In addition to noise intensity, the vertical correlation of ambient noise also decreases due to mode phase randomization by surface wave. Besides, mode coupling can also lead to a change of vertical beam intensity distribution, positive high-angle beams associated with direct, surface, and bottom-surface-bounced rays become weaker, while negative high-angle beams associated with bottom bounced rays become stronger. Since the vertical directionality is sensitive to surface wave, the model can be applied to ocean surface parameter inversion. In summary, the model provided in this paper is closer to actual ocean waveguide and has future prospect in ocean acoustic engineering application.
Noise source identification by using near field acoustic holograpy and focused beamforming based on spherical microphone array with random unifrom distribution of elements
2017, 66 (1): 014303. doi: 10.7498/aps.66.014303
With the development of techlology, noise controlling has received wide attention in recent years. Noise source identification is the key step for noise controlling. Spherical microphone array, which can locate the noise source of arbitrary direction in three-dimensional space, has been widely used for noise source identification in recent years. Conventional methods of locating noise source include spherical near field acoustic holography and spherical focused beamforming. The acoustic quantities are reconstructed by using spherical near field acoustic holography method to realize the noise source identification, while the noise source can also be located by using focused beamforming based on spherical harmonic wave decomposition. However, both these methods have their own limitations when they are used in identifying the noise source. Spherical near field acoustic holography has low resolution at high frequency with a far distance from noise source to measurement array for noise source identification, whereas the spherically focused beamforming has low localization resolution at low frequency. Noise source identification is discussed here, and a 64-element microphone spherical array with randomly uniform distribution of elements is designed. The combination methods of noise source identification by using spherical near field acoustic holography and mode decomposition focused beamforming are investigated. The performance of the proposed combination method is simulated, and an experiment on noise source identification is carried out based on the designed spherical microphone array to test the validity of proposed method. Research results show that the high-resolution noise source identification can be achieved by using near field acoustic holography when reconstruction frequency is 100-1000 Hz with a distance 0.3-0.45 m from noise source to the center of spherical array, while high resolution of noise source localization can be achieved by using spherical wave decomposition beamforming when signal frequency is 1000-5000 Hz with a distance 0.5-3 m from noise source to the center of spherical array. Spherical array with random uniform distribution of elements maintains stable identification ability in all bearings. The spherical near field acoustic holography has high-resolution distinguishing ability in near field and at low frequency, while the focused beamforming method has high-resolution distinguishing ability in far field and at high frequency. Therefore the noise source can be efficiently identified by using the proposed combination method of near field holography and focused beamforming with less elements and small aperture spherical microphone array.
2017, 66 (1): 014304. doi: 10.7498/aps.66.014304
Acoustic environment has low signal-to-noise ratio(SNR); hence, array signal processing is always used for reducing noise and enhancing signal. Because the delay-and-sum beam forming method is robust, so it is almost widely used, but the array gain is limited by the array aperture. The actual underwater ambient noise is complex, which includes uncorrelated noise and correlated noise. The noise powers of array elements are unequal to each other. The noise covariance matrix is not a scaled identity matrix. Consequently, the performance of array signal processing method decreases obviously. Aiming at these two problems, a diagonal reducing method of the covariance matrix in the complex noise field is proposed. Firstly, a reducing matrix, which is defined as a diagonal matrix with unequal diagonal elements, is subtracted from the covariance matrix so as to reduce the noise, and a new matrix is obtained. Secondly, the delay-and-sum beamforming is done by using the new matrix to obtain the beaming output. The analytic solution and approximate solution of reducing matrix are obtained under the constraint condition that the output SNR attains its maximum. Thirdly, the estimation of the reducing matrix is determined by minimizing the function that is defined as the error between the covariance matrix and the estimated covariance matrix. This minimization problem is accomplished in an iterative method. Fourthly, if the noise is uniform white noise or the nonuniform white noise, this proposed method performs well. While, under the complex noise field the performance of the proposed method may be deteriorated. So the effects of the correlation of the noise field and the input SNR on the estimated error are analyzed. In fact, the weaker the correlation is, or the larger the input SNR is, the smaller the estimated error is. Lastly, the simulation experiment and the lake trial are implemented. The simulation results show that the diagonal reducing method of the covariance matrix reduces some ambient noises, the noise output power decreases, the output SNR increases, and the proposed method improves the performance of array signal processing. The experimental results show that the output SNR of the target by using the proposed method is increased by about 14 dB. The diagonal reducing method of covariance matrix has definite value for engineering application, and is computationally attractive.
2017, 66 (1): 014305. doi: 10.7498/aps.66.014305
With using the effective medium theory to describe acoustic scattering from bubble clouds, one of the underlying assumptions shows that the probability of an individual bubble located at some position in space is independent of the locations of other bubbles. However, bubbles within the clouds that naturally occur are usually influenced by the motion of the fluid, which makes them preferentially concentrated or clustered. According to Weber's method, it is a useful way of introducing the spatial correlation function to describe this phenomenon in bubble cloud. The spatial correlation function is involved in acoustic scattering and it is important to notice that the spatial correlation should be dependent on the position and radius of each bubble due to the “hole correction” or the effect of the dynamics of the fluid. Because of these reasons, it is hard to invert the spatial distribution of bubble clouds by using the spatial correlation function in acoustic scattering. A method is described here in which bubble clouds are separated into many small subareas and the conception, called effective spatial correlation function which is the statistic of spatial correlation function, is used to describe the correlation between subareas of bubble clouds. Since the effective spatial correlation function is independent of bubble radius and positions, the bubble clouddistribution and the trend of clustering can be inverted by using this function. The simulation indicates that the effective spatial correlation function can precisely trace the position of the clustering center, even the clustering center covered by other bubble clouds can be detected. With using the multi-bean sonar for measuring the bubbly ship wake generated by a small trial vessel, the method is used to invert the spatial distribution and clustering centers of bubble field in the ship wake. The results show that the effective spatial correlation function accurately inverts the distribution and clustering centers of bubbles in ship wake. Furthermore, the method presented in this paper could distinguish between the bubble clouds caused by different reasons and detect upper ocean bubble clouds covered by other bubbles generated by wave breaking as well.
A broadband low-frequency sound insulation structure based on two-dimensionally inbuilt Helmholtz resonator
2017, 66 (1): 014307. doi: 10.7498/aps.66.014307
Helmholtz resonator(HR) has already been demonstrated both theoretically and experimentally to be a metamaterial with negative mass density and negative bulk modulus simultaneously. The HR can resonate at a frequency corresponding to a wavelength much longer than its geometrical parameters. At this time, the incident acoustic energy can be located. Therefore, the HR structures are considered to be good choices for controlling low-frequency sound waves. Furthermore, existing results indicate that the wide forbidden band could be formed by a one-dimensional structure shunted with detuned HRs. Based on these aforementioned theories, a man-made acoustical structure with broadband low-frequency sound insulation effect is designed by circularly inbuilt HRs. Beyond this structure's surface, a two-dimensional quiet zone can be created. With the same simulated model, an experimental structure is fabricated based on PVC plastic material. The structure consists of five layerd circular plates. In the top four plates, two kinds of holes are drilled. The smaller holes in the top plate act as shot necks of the HR, while the bigger holes in the middle three plates serve as the cavities of the HR. They can construct 60 resonators with different resonant frequencies. Experiments are carried out to study its sound insulation properties. In the experiments, three kinds of HRs with resonant frequencies 785, 840 and 890 Hz from inner loop to outer loop, respectively, are formed. The experimental results are very coincident with the simulation results from the software of COMSOL Multiphysics based on finite element method, which shows that this structure has an excellent sound insulation effect in a frequency band of 680-1050 Hz, and the maximum insulation sound pressure level can reach 41 dB. Meanwhile, the distribution of the two-dimensional sound field is measured. The results point out that the range of the insulation area can be changed with the incident frequency. In addition, the sound insulation effect is sensitive to the resonant state of the HRs. When all of the resonators at the same loop resonate simultaneously, the insulation sound pressure level will be higher. On the contrary, the insulation sound pressure level will be lower because of the energy leaking through the positions where the HRs do not resonate with the others. This work will be of help for designing new sound protection devices for low-frequency sound waves.
Variations of the electrical conductivity and the Fermi velocity of epitaxial graphene with temperature
2017, 66 (1): 014701. doi: 10.7498/aps.66.014701
The atomic anharmonic vibration and the electron-phonon interaction are considered, and then a physical model about the metal-based epitaxial graphene is built. Variations of the electrical conductivity and the Fermi velocity with temperature for the metal-based epitaxial graphene are given based on the solid state physics theory or method. The alkali-metal epitaxial graphene is selected as the substrate, and then the influences of substrate material, electron-phonon interaction and the anharmonic vibration on the electrical conductivity and the Fermi velocity of epitaxial graphene are discussed. Some results are shown as follows. Firstly, at zero temperature, the electrical conductivity and the Fermi velocity of the alkali-metal-base epitaxial graphene increase with the number of the atoms in substrate material increasing. Secondly, the electrical conductivity of epitaxial graphene decreases with temperature rising. Furthermore, the variation rate also decreases with temperature rising. Generally, the electrical conductivity originates mainly from electrons and phones. The electronic contribution to the electrical conductivity varies with temperature slowly, but the phone contribution to electrical conductivity varies with temperature evidently. Therefore, the contribution of phonons to electrical conductivity is much larger than that of electrons. Furthermore, the contribution increases with the number of atoms in basal elements. The phonon contribution to conductivity decreases with temperature rising, but it is unrelated to the basal elements. Thirdly, the Fermi velocity of the epitaxial graphene increases with temperature slowly. The variation of the Fermi velocity with temperature decreases with the increase of interaction between the graphene and the basal atoms. However, it increases with the number of atoms of the basal materials. The anharmonic effect causes important influences on the electrical conductivity and the Fermi velocity. Under the harmonic approximation the velocity is constant. However, the conductance increases rapidly with temperature. With considering the atomic anharmonic terms, the Fermi velocity increases with temperature. The variation of the electrical conductivity with temperature increasing becomes slower. If the temperature is higher, the anharmonic effects become more evident.
2017, 66 (1): 014704. doi: 10.7498/aps.66.014704
Many researches of a dense droplet impacting on a flat surface have been reported in the literature. However, the mechanism of a hollow droplet impacting on a flat surface has not yet been well addressed. A mathematical model is developed in the present research to resolve this impacting process. The model couples level set and volume of fluid method, and considers heat transfer and contact resistance between the droplet and surface. The validation of the model is carried out by comparing simulation results with experiment data. Different impact behaviors are observed in the impacting processes of both the dense droplet and the hollow droplet on a flat surface, obtained from the simulation result. The hydrodynamics and heat transfer behaviors of the hollow droplet impacting on a flat surface and the formation of central jetting are also explored. The effects of impact velocity and surface wettability on the impacting behavior of the hollow droplet are also analyzed. The results show that in the impacting process, the hollow droplet presents a spread and central jetting pattern, accompanying liquid shell contraction and breakup, while only spread and liquid shell contraction are observed in the dense droplet impacting process. It is also observed that the central jetting of the hollow droplet peels off the surface in the final impacting stage. The dimensionless spread factor for the hollow droplet is less than that of the dense droplet with the same initial kinetic energy in spread stage. The pressure gradient inside the hollow droplet is the main factor resulting in the spread and central jetting. The temperature distribution in the liquid shell and the surface is more uniform than in the central jetting, which is caused by the secondary breakup of the liquid shell. The spread factor of the hollow droplet remains unchanged as the impact velocity increases but is closely related to the surface wettability. The spread factor of the hydrophilic surface is larger than that of the hydrophobic surface. The effects of the surface wettability on the spread factor gradually reduce with the increase of the impact velocity. The effects of the impact velocity on the dimensionless jet length and the average wall heat flux are significant, while the surface wettability plays a negligible role in them. Improving the impact velocity increases the dimensionless length of the central jetting and the average wall heat flux, but this influence diminishes under a high impact velocity condition. Neither the dimensionless time spans of reaching the maximum spread factor nor the maximum average wall heat flux for the hollow droplet is influenced by the impact velocity and surface wettability and the development of the spread falls behind the heat transfer. Furthermore, the maximum spread factor increases with Reynolds number, and when Reynolds number is higher than 500, the increase in the maximum spread factor is no longer significant.
CONDENSED MATTER:STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Simulation study of effect of cooling rate on evolution of microstructures during solidification of liquid Mg
2017, 66 (1): 016101. doi: 10.7498/aps.66.016101
Magnesium metal and its alloys are widely used in industry,especially,as biodegradable materials are highly suitable for biomedical applications.Since macroscopic properties and service behaviors of materials are mainly determined by their microstructures,it is very important to in depth understand the melting structure of pure magnesium and its evolution process in solidification process.In this work,a molecular dynamic simulation studyis performed with embedded atom method potential at different cooling rates to investigate the rapid solidification process of liquid magnesium,and the microstructure evolution and phase transition mechanisms are systematically analyzed by using E-T curves,pair distribution function g (r),Honeycutt-Anderson (HA) bond-type index method,cluster-type index method (CTIM-3) and three-dimentional (3D) visualization method,respectively.It is found that the cooling rate plays an important role in the evolution of microstructures,especially;from HA bond index method,CTIM-3 and 3D visualization method,the microstructure details of crystalline or amorphous structures in the system are displayed quite clearly with temperature decreasing.Meanwhile,it can be easily found how some basic clusters interconnect to form a larger one in the system. For short,some local configurations under different conditions at four typical temperatures are also given to show the difference in microstructure on a relatively large scale.At a lower cooling rate of 11011 K/s,the evolution of metastable bcc structure is obviously consistent with the Ostwald's step rule in the system,meaning that the bcc structure is first formed preferentially and then dissociated largely,and eventually the stable crystalline structures are formed mainly with the predominant hcp structure and fcc structure,and coexisting along with remaining partial bcc structure.At a middle cooling rate of 11012 K/s,the crystallization process is slower,the bcc initially is formed at lower temperature, suggesting that the crystalline process is postponed,and the coexisting structures is still formed with the predominant hcp structure and fcc,bcc structures,but lacking in the larger grains,due to the competitions among the hcp,fcc and bcc structures.Finally,for a higher cooling rate of 11013 K/s,amorphous magnesium is formed with basic amorphous clusters characterized by 1551,1441 and 1431 bond types and there is not a predominant structure,although a small number of medium or long range orders come out.In addition,there surely exists a critical cooling rate for forming amorphous structures in a range of 11012-11013 K/s.From the evolution of bcc,it is also suggested that short range orders in super-cooling liquid give birth to bcc structure and the process can be avoided by simply speeding up the cooling rate to a critical one.
2017, 66 (1): 016102. doi: 10.7498/aps.66.016102
InSb infrared focal plane array(IRFPA) detector, active in 3-5 m range, has been widely used in military fields. Higher fracture probability appearing in InSb infrared focal plane arrays(IRFPAs) subjected to thermal shock test, restricts its final yield. In order to analyze and optimize the structure of InSb IRFPAs, it is necessary to create the three-dimensional structural model of InSb IRFPAs, which is employed to estimate its strain distribution appearing in the different fabricating processes. In this paper, the curing model of underfill is described by its volume contraction percentage combined with the elastic modulus of the completely cured underfill. Thus, both the von Mises stress and the Z-components of strain accumulated in the curing process of underfill are calculated. When InSb IRFPAs is naturally cooled to room temperature from the curing temperature of underfill, the Z-component of strain distribution appearing on the top surface of InSb IRFPAs is obtained with our structural model, which is identical to the deformation distribution on the top surface of InSb IRFPAs measured at room temperature. In the following thermal shock simulation, we find that the maximal von Mises stress appears at 100 K and the maximal Z-component of strain appears at 150 K, these two temperature points are located in the second half of the thermal shock process, these results indicate that the fracture of InSb chip happens more easily in liquid nitrogen shock test. This inference is consistent with the fact appearing in liquid nitrogen shock test. All these findings suggest that the proposed model is suitable to estimate the deformation distribution of InSb IRFPAs and its changing rule in its different fabricating stages.
Effects of the doping of Al and O interstitial atoms on thermodynamic properties of -Al2O3:first-principles calculations
2017, 66 (1): 016103. doi: 10.7498/aps.66.016103
Al particles are widely used as a metal reductant in the thermite, and a native Al2O3 film always forms on the particle surface as a passivating oxide shell. The diffusions of Al and O atom through the oxide shell will influence the structure and thermodynamic properties of Al2O3, and thus the ignition process of the thermite. In this work, the thermodynamics properties of -Al2O3, -Al2O3 doped by Al interstitial atom and -Al2O3 doped by O interstitial atom under high pressure and temperature are comparatively investigated by the first-principles calculations based on density-functional theory and quasi-harhmonic Debye model. The effects of the doping of Al and O interstitial atoms on the thermodynamic properties of -Al2O3 are discussed. The results indicate that the doping of the Al and O interstitial atoms will reduce the bulk modulus, and increase the volume thermal expansion coefficient and constant volume heat capacity of -Al2O3. Therefore, the diffusions of Al and O atom will make the oxide shell more ductile, and adverse to the spallation during the ignition of Al particles.
2017, 66 (1): 016801. doi: 10.7498/aps.66.016801
The growing demand for the energy conversion and storage of miniaturized system has promoted extensive researches aiming at fabricating solid-state ionic devices in thin-film form. Recent developments in the field of thin-film growth technologies have controlled the films at an atomic level of deposited layers, thus opening new perspectives in the field of engineering of multilayers and heterostructures based on complex oxides. This work focuses on the characterizations of the low-temperature properties of Ce0.8Sm0.2O2-/Y2O3:ZrO2(SDC/YSZ)N superlattice films.(SDC/YSZ)N superlattice electrolytic films with various periods(N=4, 6, 10 and 20) are fabricated on monocrystal MgO substrates by the pulsed laser sputtering method. Here, SiTrO3(STO) is used as a buffer layer, SDC and YSZ are deposited alternately in the whole process. The total thickness values of samples are all fixed at 400 nm no matter how many periods the samples have. The surface morphologies, phase structures and electric properties of the as-deposited samples are characterized by scanning electron microscopy(SEM), X-ray diffraction and alternating current(AC) impedance spectroscopy. It is indicated that the films have excellent superlattice structures after STO has been used as a buffer layer and the substrate temperature has heated to 700℃. The interface between two layers are clearly observed by SEM. Moreover, neither cracks nor snaps are found at the interface. The grains uniformly grow on the surfaces of films and are arranged into cylinder structures, leading to compact films. Through AC impedance analysis, the samples which have more periods exhibit smaller activation energies. With increasing the number of interfaces, the activation energy of film decreases whereas the ionic conductivity increases. When the number of periods reaches 20, the activation energy is measured to be approximately 0.768 eV. The conductivity enhancement of(SDC/YSZ)N superlattice electrolyte film can be attributed to the large lattice mismatch near the interface between two different layers. That is to say, the interface between the highly dissimilar structures stabilizes a disordered oxygen sublattice with an increased number of oxygen vacancies, which promotes oxygen diffusion to increase the ionic conductivity of sample. Furthermore, the ionic conductivity of the(SDC/YSZ)20 film with a thickness ratio of m SDC: YSZ of 2:1 is much higher than that of the film witha thickness ratio of 1:1. Finally, it is noted that the STO buffer layer provides the proper lattice match for CeO2, inducing the good epitxial growth of superlattice electrolyte film(SDC/YSZ)20. And the conductivity enhancement could be attributed to the increase of SDC thickness in a bilayer. Therefore,(SDC/YSZ)20 superlattice electrolyte film is more ideal low-temperature fuel cell electrolyte material due to higher ionic conductivity.
2017, 66 (1): 017101. doi: 10.7498/aps.66.017101
Resistive switching of molecular film incorporated with nanoparticles(NPs) has become a hot topic in the information storage industry, which is systematically reviewed from the aspects of electrodes, film structure, NPs, switching mechanism and mechanical properties. There are three sorts of structures i.e., layered, core-shell and complexed films, in which the film thickness affects the device charge transport and switching performance to a large extent. Usually, higher on/off ratio and lower threshold voltage can be expected for device with less-conductive active layers than that with more conductive ones. As a key factor, the interfaces of electrode/organic and molecule/NPs may largely affect the switching performance. It is shown that the type, size and distribution of NPs and molecular structure govern the interfacial behaviors, which in turn influences the switching mechanisms including filament formation/ rupture, charge trapping/ detrapping or charge transfer. For the case of filament theory, it may be ascribed to metallic, oxygen vacant or carbon-rich model. The as-embedded NPs can be classified as metal, metal oxide and/or carbon-like materials such as Au, Ag, Al, ZnO, TiO2, or graphene etc. The Au NPs show distinguishing features of little diameter, high chemical stability and large work function. On the other hand, the metal oxide NPs may form deep interfacial barrier with the target molecules and thus improve the switching characteristics. Small molecular-weight organics are also studied as embedding materials complexed with polymers as to strengthen the switching properties, and charge transfer is believed to be responsible for such an enhancement. Except for concentration and diameter of the NPs, their distribution in the active layer critically influences the memory behavior. The NPs can be made onto the molecular layer in-situ by vacuum thermal evaporation of different metals or sputtering deposition of various metal oxides. In such cases, the thickness of the deposition layer is a key parameter to obtain good switching performance. Although great progress has been made for static devices in small-scale, it is crucial to develop roll-to-roll manufacturing, precise NPs' distribution and dynamic mechanical properties in order to fabricate large-scale, low-cost and flexible memory devices. It still needs hard work on understanding the switching mechanism and engineering the interfacial properties of molecule/electrode and molecule/NPs, especially under bending conditions. New techniques should be developed to fabricate organic memory films embedded with NPs so as to avoid the problems of pinhole, effects of solvent and dust normally existing in traditional spin-coating films.
CONDENSED MATTER:ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2017, 66 (1): 017501. doi: 10.7498/aps.66.017501
SiC with d0 ferromagnetism is thought to be one of the most important materials in the spintronics field, and it has received widespread attention. In this paper, Al: SiC magnetic powder is fabricated by high temperature calcination method with the protection of Ar gas. X-ray diffraction results show that the obtained powder is of 6H-SiC phase, and Al is proposed to enter into the 6H-SiC crystalline. Raman results show that Ar gas plays a crucial role in impeding the SiC from decomposing at high temperature. With the protection of Ar gas, it maintains round shape after calcination about 2200℃, no any other peakis detected in the Raman spectrum. Without the protection of Ar gas, SiC particle would decompose into graphite, and the instinct peak of graphite is detected in the Raman spectrum. Energy dispersive spectrometer results show that there is 0.96 at% Al in the powder. The obtained powder shows magnificent magnetic hysteresis loop and large coercive force. Its saturation magnetic moment reaches 0.07 emu/g after calcination at 1800℃. Its coercive force reaches a maximum after calcination at 2000℃, while the saturation magnetic moment is 0.012 emu/g. With the rise of calcination temperature, the magnetism of the powder changes from diamagnetism to ferromagnetism. But when the calcination temperature rises to 2200℃ or more, it would change back to diamagnetism. The phenomenon of ferromagnetism disappearing is similar to that in ZnO as reported. The total quantity of magnetic impurities(Fe, Co, Ni) is evaluated to be less than 5 ppm. Saturation magnetic moments arising from these impurities can be calculated to be less than 10-5 emu/g according to the reported results, which is impossible to affect the accuracy in the experiment. Thus it is proposed that the ferromagnetism originates from the doping of Al in SiC powder. To understand the origin of the observed magnetism, we carry out first principles calculations based on spin polarized density functional theory. All the calculations are performed by using the generalized gradient approximation in the form of the Perdew-Burke-Ernzerhof function, which is implemented in the Viemma ab initio simulation package. A supercell consisting of 331 unit cells of 6H-SiC containing one AlSi-VSi, corresponding to a defect concentration of 0.93 at%, is built for calculations. The origin of its ferromagnetism is studied, and its spin situation in the space is mapped. The results show that the combination of Al and vacancy leads to a local magnetic moment of 1.0 B, and magnetic coupling is steady in the c axis direction. It is found that the p electron of carbon is the origin of the net spin.
Large enhanced perpendicular magnetic anisotropy and thermal stability in Ta/CoFeB/MgO films with excess boron
2017, 66 (1): 017502. doi: 10.7498/aps.66.017502
The discovery of perpendicular magnetic anisotropy(PMA) in Ta/CoFeB/MgO film and the demonstration of high performance perpendicular magnetic tunnel junction(p-MTJ) based on this material system have accelerated the development of the next-generation high-density non-volatile memories and other spintronic devices. Currently it is urgently needed to improve the interfacial PMA and thermal stability of the CoFeB/MgO system for practical applications. So far, the perpendicularly magnetized CoFeB/MgO films and the corresponding p-MTJs have been extensively explored with the B content of the CoFeB layer mostly fixed at about 20 atomic percent. In this paper, four sets of multilayered films Ta/(Co0.5Fe0.5)1-xBx/MgO(x=0.1, 0.2, 0.3) and MgO/(Co0.5Fe0.5)0.7B0.3/Ta with different CoFeB thickness are deposited on thermally oxidized Si substrates by magnetron sputtering at room temperature, and subsequently they are annealed in high vacuum at different temperatures ranging from 573 to 623 K. The room temperature magnetic properties of the annealed samples are characterized by using vibrating sample magnetometer and superconducting quantum interference device magnetometer. With normal B content of 20% for the CoFeB layer, the Ta/CoFeB/MgO structure annealed at 573 K shows perpendicular magnetization when the CoFeB layer is no thicker than 1.2 nm. As the B content decreases to 10%, it has been found that PMA is achieved only in the sample with a 0.8 nm CoFeB layer under the same annealing condition. The result shows that the interfacial PMA appreciably falls off when the B content is reduced by half. On the other hand, when the B content of the CoFeB layers increases from 20% to 30%, the Ta/CoFeB/MgO structure annealed at 573 K exhibits PMA with the CoFeB layer as thick as 1.4 nm and the interfacial PMA(Ks) increases from 1.710-3 Jm-2 to 1.910-3 Jm-2 together with slightly improved thermal stability. Most remarkably, the MgO/CoFeB/Ta structure with 30% B shows optimum annealing temperature of about 623 K, at which Ks reaches 2.010-3Jm-2 and PMA is realized in the samples with the CoFeB thickness up to 1.5 nm. In contrast, the same structure with 20% B is magnetically destroyed completely under this annealing temperature. The present results suggest that the CoFeB layer with excess B can effectively improve the perpendicular magnetic properties and thermal stability for the Ta/CoFeB/MgO system, and one should take into account the B content effect to optimize the spintronic devices based on the perpendicularly magnetized CoFeB/MgO system.
2017, 66 (1): 017201. doi: 10.7498/aps.66.017201
Spin noise spectroscopy is a non-demolition technique to detect the spin dynamics, and it is a good way to realize spin property under thermal equilibrium. Since spin noise arises from spin fluctuation at thermal equilibrium, it is a weak signal, therefore, various methods are used to enhance the signal-to-noise ratio(SNR) of the measurement system. To study the influence from different factors on the quality of spin noise spectroscopy, we report spin noise spectroscopy measurements in Rubidium vapor with three methods: a commercial frequency analyzer, a data acquisition card(DAC) with fast Fourier transform(FFT) done by a computer, and a DAC with real-time FFT based on FPGA(field-programmable gate array), respectively. According to the experimental results, we discuss several parameters and their influences on the SNR of the spectrum, including spectrum accumulation time, measurement efficiency and acquisition resolution. We find that the accumulation time is the most important factor for achieving high-quality spectrum. Measurement efficiency indicates how a good quality of the spin noise spectroscopy can be achieved in a finite time period, and we make a comparison of measurement efficiency among three methods. However, improvement of acquisition resolution does not make much more contribution to the quality of spin noise spectroscopy. Taken all into account, the DAC with real-time FFT performs best due to its bigger data utilization ratio, higher measurement efficiency and the multiplex advantage, thus it is more helpful for spin noise spectroscopy measurement in the study of spin dynamics.
2017, 66 (1): 017301. doi: 10.7498/aps.66.017301
To improve the efficiency of transmission, in this paper, we propose a structure of the surface plasmon polariton embedded in a sliver circular resonator with a sliver nanoellispod(-shaped resonator), and also investigate its optical properties by the finite element method. Firstly, we study the optical properties of -shaped resonator at a=120 nm and =0 with different values of b. The results show that the -shaped resonator structure has the narrow transmission peaks, and the transmittance spectrum can be tuned by modifying the structure parameters. So this nanostructure would find applications in the designing of the novel filter. Secondly, compared with the former Fano resonance which results from the localized plasmon resonance coupling, the Fano resonance which results from the resonance of the surface plasmon polaritons coupling is represented by this structure. When the symmetry of -shaped resonator is broken, the Fano resonance will be observed clearly. Like the Fano resonance which results from the localized plasmon resonance coupling between the bright mode of metallic nanostructure and the dark mode of metallic nanostructure, the results show that the dipolar, quadrupolar, and octupolar Fano resonances of -shaped resonator structure occur, which are caused by the destructive interference between the bright dipolar mode and the dark dipolar mode, quadrupolar mode, and octupolar mode. When we take the rotation angle as 0 and 90, 15 and 75, 30 and 90 respectively, the Fano asymmetric transmittance spectra of -shaped resonator are similar, which result from the same degree of asymmetry. The larger the degree of asymmetry of the -shaped resonator structure, the more obvious the Fano resonance is. Thirdly, the size of this structure has significant effects on the transmission peak positions, line width, and intensity of the Fano resonance, in particular, in the case that =0 corresponds to the generation of FR(FR on) and in the case corresponding to the vanishing of FR(FR off). therefore, this phenomenon of -shaped resonator will provide a new strategy for the surface plasmon polariton Fano switch. We hope that this nanostructure has potential applications in designing filter, biological sensors, and Fano switch.
Enhancement of tandem organic light-emitting diode performance by inserting an ultra-thin Ag layer in charge generation layer
2017, 66 (1): 017302. doi: 10.7498/aps.66.017302
White organic light-emitting diodes (WOLEDs) have attracted both scientific and industrial interest in the solidstate lighting and display applications due to their exceptional merits,such as high luminances,low power consumptions, high efficiencies,fast response times,wide-viewing angles,flexibilities and simple fabrications.The power efficiency of WOLED has been step-by-step improved in the last 20 years,however,the lifetime of WOLED is still unsatisfactory, which greatly restricts the further development of WOLED.In general,the tandem structure can be used to obtain high-efficiency and long-lifetime WOLED.One of the most important features of this kind of structure is that the different-colors emitting units can be connected by the charge generation layer.Therefore,the key to achieving a highperformance tandem device is how to design the charge generation layer.In this paper,we first develop a tandem green OLED by using an effective charge generation layer with an ultra-thin Ag layer between 4,7-diphenyl-1,10-phenanthroline:CsCO3 and hexaazatriphenylenehexacabonitrile,achieving high luminance,low voltage,high efficiency and long lifetime.The green tandem device with ultra-thin Ag layer (device C) obtains a highest luminance of 290000 cd/m2,which is 1.4 and 1.9 times higher than those of the tandem devices without ultra-thin Ag (device B) and singleunit device (device A),respectively.The driving voltage of device C is 7.2 V at 1000 cd/m2,1.4 V lower than that of device B.Besides,the maximum current efficiency of device C is 60.4 cd/A,which is 2.4% and 220% higher than those of device B (59 cd/A) and device A (18.7 cd/A),respectively.The power efficiency of device C is 26 lm/W,which is 21% higher than that of device B (21.5 lm/W).Moreover,the lifetime (T80) of device C reaches 250 h at an initial luminance of 10000 cd/m2,which is nearly 100 times higher than that of device B (2.7 h).Finally,we fabricate a white tandem device with the optimized charge generation layer,achieving a current efficiency and power efficiency of 75.9 cd/A and 36.1 lm/W at 1000 cd/m2,respectively.In addition,the lifetime (T80) is 77 h at an initial luminance of 10000 cd/m2.All the excellent performances are ascribed to the introduction of the ultra-thin Ag layer into the charge generation layer, which can effectively block the charge generation layer from diffusing.This exciting discovery can provide an effective way to design efficient and stable WOLED,which is beneficial to the solid-state lighting and display markets.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
An in-situ real time study of the perovskite film micro-structural evolution in a humid environment by using synchrotron based characterization technique
2017, 66 (1): 018401. doi: 10.7498/aps.66.018401
Humid environment plays a vital role in affecting the performance stability of the organic metal halide perovskite solar cells. Therefore, in situ monitoring the micro-structural evolution of perovskite film in real time will help to reveal the micro-mechanism for the device performance decay induced by humidity. A device providing a controllable humid environment has been set up at X-ray diffraction beamline of Shanghai Synchrotron Radiation Facility, which is used to monitor in situ the perovskite film micro-structural evolution in real time in a humid environment by using grazing incidence X-ray diffraction(GIXRD). After a perovskite film is exposed to the environment with a relative humidity of 60%±2%, a new component emerges near the perovskite(110) diffraction peak in the early stage of the exposure, which is observed for the first time. This new component is attributed to the perovskite intermediate phase structure transformed from the gradual degradation of the perovskite crystalline. Meanwhile, UV-Vis absorption measurements show that humidity causes the absorption of the film to decrease slightly with the blue shift of the absorption edge at ~770 nm, which indicates a reduced amount of perovskite crystalline or a decrease of perovskite crystallinity. Scan electron microscope further demonstrates that the film after the humid exposure presents a worse morphology with a lower coverage, bigger pores, and larger voids between crystalline than the pristine film. The current-voltage(J-V) measurements of the solar cells fabricated on the perovskite films before and after the humid exposure show that both the filling factor and the power conversion efficiencydecrease by over 30% due to the humidity. The present work demonstrates that the close relationship between the device performance and the perovskite film microstructure as well as their morphologies can be studied very well by in-situ synchrotron based characterization technique. The present study could lay a good foundation for the understanding of the degradation mechanism for the organic metal halide perovskites.
2017, 66 (1): 018501. doi: 10.7498/aps.66.018501
In order to analyze the influence of frequency on thermal breakdown in semiconductor device, the influences of frequency on heat generation and heat conduction in the hot zone are introduced into the theoretical model. The heat transfer equation is solved by the Green's function method, and the error function is approximated. Then, the expressions of temperature in the hot zone and failure power of semiconductor device including frequency and pulse width are derived. The change rules of failure power with the increasing of pulse width under different frequencies and with the increasing of frequency under different pulse widths are obtained. The result shows that the expression for center temperature in hot zone caused by the failure power is divided into four time regions, i.e., regions I-IV, by three thermal diffusion times ta, tb, and tc. The three diffusion times ta, tb, and tc are related to the side lengths a, b and c(c≤b≤a) of the hot zone represented by a rectangular parallelepiped, respectively. In region I(0≤t≤tc), the relation between failure power Pf and failure time t is Pf∝t-1. In this region, the failure time is short and little heat is lost from the surface of hot zone so that the adiabatic term(t-1) dominates. In region Ⅱ(tcttb), the relation between failure power Pf and failure time t is Pf t-1/2. In this region, it is indicative of heat loss from the hot zone to its surrounding medium. In region Ⅲ(tb≤t≤ta), the relation between failure power Pf and failure time t is Pf∝1/ln t. In region IV(t >ta), the failure power Pf is constant. In this region, the failure time is very large and thermal equilibrium can be established so that the steady state term dominates. The relation between failure power and frequency is divided into two parts. In part one, the failure power increases with the increasing of frequency; in part two, the failure power is nearly constant with the increasing of frequency. Meanwhile, the physical interpretation of the influence of frequency on failure power is given. From region I to region IV, each heat transfer rate increases with pulse width. The lower the frequency, the more the injection energy during region I or region Ⅱ is, when the total injection energy is constant. The heat transfer rate is slower in region I or region Ⅱ, so the energy converted into heat will be more and the temperature in the hot zone will be higher, thus the device is burned out easily.
2017, 66 (1): 018701. doi: 10.7498/aps.66.018701
There exist some problems in a grating-based X-ray differential phase contrast imaging system, such as complex imaging system, low imaging efficiency and high requirements for step precision. The phase information extraction method of imaging system has been developed into an existing two-stepping phase shift method from the original phase stepping method, which improves the imaging efficiency and reduces the imaging radiation dose and imaging time. However, the method of two-stepping phase shift still needs to move the grating, and the requirement for accuracy of the step position is also very high. According to the problems mentioned above, in this paper we propose a dual energy multi-line X-ray source and a dual energy analysis grating. The dual energy multi-line X-ray source can emit two different levels of X-ray structure light, which can replace the X-ray source and source grating. The dual energy analysis grating is composed of two different types of scintillator materials, which are in staggered distribution. One is scintillator material that can transform high energy X-ray into visible light, and the other one can convert low energy X-ray into visible light. The dual energy analysis grating can replace traditional analysis grating and the conversion screen of X-ray CCD detector. By using the dual energy multi-line X-ray source and dual energy analysis grating in grating-based X-ray differential phase contrast imaging system, a dual energy grating-based X-ray phase contrast imaging system is proposed in this paper. In addition, in this paper we show the structure and imaging principle of the imaging system. The imaging system can achieve high and low energy X-ray imaging without moving grating. Two levels of X-ray imaging are equivalent to the analysis grating displacement π phase, which is in line with the traditional two-stepping method of two image phase shift requirements. Therefore, after the normalization processing of the two kinds of energies, the phase information can be extracted by the traditional two-stepping phase shift method. In order to validate the correctnesses of the imaging principle of the proposed imaging system and extraction method of phase information, the imaging system is simulated. The simulation is performed on the assumption that an X-ray beam passes through a polymethyl methacrylate sphere as a phase specimen, and the method is adopted by using the proposed dual energy X-ray about left and right lumbar imaging to extract phase information. The simulation result shows that the imaging system can realize normal imaging, and the first-order derivative distribution of the sphere phase extracted by the dual energy X-ray method is consistent with the experimental result.
2017, 66 (1): 018201. doi: 10.7498/aps.66.018201
In recent years, the static and the dynamical properties of polymer confined in nano-channels have become a hot topic due to its potential applications in technology, such as genome mapping, DNA controlling and sequencing, DNA separation, etc. From the viewpoint of polymer physics, the properties of polymer confined in nano-channels are affected by many factors, such as the channel size, the channel geometry, the polymer-channel interaction, etc. Consequently, many researches have been extensively performed to uncover the underlying physical mechanisms of the static and the dynamical properties of polymer confined in nano-channels. Although many conformations are forbidden as polymer is confined in channels, the static properties of polymer are found to be still complicated. For the simplest case, i.e., homo-polymer confined in homogeneous solid channels, there are several scaling regimes, in which polymer adopts different conformation modes and the extension of polymer shows different scaling relations with the channel diameter, the polymer length, the persistence length, etc. In addition, the dynamical properties of polymer, such as the diffusivity and the relaxation, have also been extensively studied. Though the properties of polymer confined in homogeneous channels have been well studied, we know little about those of polymer inside compound channels. It is found that the dynamics of polymer in compound channels is quite different from that of polymer in homogeneous channels, and compound channel could be useful for DNA separation and DNA controlled movement.In this work, the diffusion of diblock copolymer(ANABNB) in periodical channels patterned alternately by part and part with the same length lp/2 is studied by using Monte Carlo simulation. The interaction between monomer A and channel is attractive, while all other interactions are purely repulsive. Results show that the diffusion of polymer is remarkably affected by the length of block A(NA), and the diffusion constant D changes periodically with NA. Near the peaks of D, the projected length of block A along the channel is an even multiple of lp/2, and the diffusion is in consistence with that of homo-polymer in homogenous channels. While near the valleys of D, the projected length of block A is an odd multiple of lp/2, and polymer is in a state with long time trapping and rapid jumping to other trapped regions in the diffusion process. The physical mechanisms are discussed from the view of polymer-channel interaction energy landscape.
Uncertainty quantification in the calculation of keff using sensitity and stochastic sampling method
2017, 66 (1): 012801. doi: 10.7498/aps.66.012801
The sensitivity and uncertainty analysis(S/U) method based on the first order perturbation theory is commonly employed to calculate the uncertainties in-nuclear reactor's integral parameters, such as the neutron effective multiplication factor(keff), due to uncertainties in nuclear data. However, this method is only theoretically suitable for the linear model because of its first order approximation. Moreover, S/U method is difficult to incorporate into a neutronics code, because the adjoint angular flux is needed to obtain the sensitivity coefficient of an integral parameter to nuclear data. Meanwhile, the sampling approach based on parametric random sampling of input parameters, an easy implemented method, evaluates the uncertainties in the integral parameters by performing a set of neutronics simulations inputted with a set of stochastic nuclear data sampled from a multinomial normal distribution with nuclear cross section mean values and covariance data. The sampling approach is considered as a more exact method, as linear approximation is not needed. With the increase of computational power, the sampling methods with consuming more time are now possible. The sampling approach is incorporated into SURE, a sensitivity and uncertainty analysis code developed in IAPCM, as a functional module. A careful verification of the new function is necessary before it is used to analyze complicated problems, such as multi-physical coupling calculations of nuclear reactor. Two simple fast criticality benchmark experiments, namely Godiva(HEU-MET-FAST-001) and Jezebel(PU-MET-FAST-001), are selected to verify the sampling module of SURE. The uncertainties in nuclear data are given by multigroup covariance matrices processed from ENDF/B-VⅡ. 1 data. The uncertainties in the computed value of keff resulting from uncertainties in the nuclear data are calculated with both S/U and sampling methods. The uncertainties due to reaction cross sections for each nuclide in two benchmarks given by two methods with the multigroup covariance matrices are in good agreement. Since the S/U module of SURE code is verified extensively, the correctness of the sampling function of the code is confirmed as well. The distribution of the keff from the sampling approach obeys the normal distribution pretty well, which indicates that keff varies linearly with the nuclear data under its uncertainty range, since the nuclear data used in calculations are assumed to be normal distribution in the sampling method. The results from the sampling method also support the S/U method with linear approximation as a suitable uncertainty quantification method for keff calculation.
2017, 66 (1): 012901. doi: 10.7498/aps.66.012901
As a positionsensitive detector, laser quadrant photodetector(QPD) is widely used in the areas such as laser guidance, laser radar and space optical communication. In the echoed laser pulse detection mode, the laser pulse signal arrived at the QPD photosensitive surface is changed in pulse amplitude, pulse width and pulse waveform due to the influences of target characteristic, atmospheric transmission and other complex factors. In addition, there are random noises in QPD itself and the signal processing circuit. These factors will have an uncertainty effect on the angle measurement accuracy of the QPD. However, the study on the statistical distribution of digital angle measurement of pulsed laser QPD has not been carried out so far. To investigate this angle measurement statistical distribution, the channel of laser angle measurement circuit and the echoed laser spot on QPD photosensitive surface should be modeled first. A measurable signal model in one quadrant of QPD processing circuit channel is established based on the type of random noise and the type of desired ideal signal. The random noise model is considered to be a Gaussian distribution, and the ideal laser pulse signal is considered to have the Gaussian or inverted parabolic distribution in the time domain. Taking into account the QPD symmetry, the statistical distributions of angle measurement value αy for five different spot centers are calculated by the Monte Carlo simulation method within the range θ0∈[0,π/4], under the conditions of different signal distribution types in the time domain, different total peak powers of the spots, different ideal signal widths at half maximum, and different standard deviations of equivalent noise voltage probability density. Simulation results show that the statistical distribution of the measured angle αy value is a normal distribution, and is influenced by the above-mentioned conditions, especially by the signaltonoise ratio in one quadrant. QPD possesses higher angular accuracy as the spot center is closer to the axis center. While the spot center is not closer to the axis center, the mean of statistical distribution of the QPD measurement angle αy is always less than the ideal angle measurement value. Therefore, in order to improve the angle measurement accuracy of the pulsed laser QPD for digital purpose, laser pulse transmit power should be increased, or the noise of each circuit channel of QPD should be reduced, or the laser pulse width should be increased by modulating appropriately.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Theory of very low frequency/extra low frequency radiation by dual-beam beat wave heating ionosphere
2017, 66 (1): 019401. doi: 10.7498/aps.66.019401
At high latitudes, the proposed dual-beam beat wave method has become the focus of research because of its relative amplitude modulation method. The very low frequency/extra low frequency(VLF/ELF) radiation intensity does not depend on the background of the current, and can be used as an importantparameter of VLF/ELF radiation in poor background conditions. As for the background of weak natural current in middle and low latitude regions, the amplitude modulation method stimulates the VLF/ELF radiation effect poorly, therefore, at the low latitudes, the relative amplitude modulation method, dual-beam beat waves method may be more effective. In this paper, according to the pondermotive nonlinear heating theory in the upper ionosphere, a simulation model is presented about VLF/ELF radiation generated by dual-beam heating of the ionosphere via beating waves. This model is validated by calculations and the available experimental parameters. Based on this model, a comprehensive analysis is performed, involving the dependences of radiation intensity on various parameters such as latitude, effective radiation power(ERP), heating frequency, polarization, experimental times, and beating frequency difference. Then, we compare the dual-beam beat wave method with the amplitude modulation in stimulating VLF/ELF signals in the low and moderate latitude regions. Several conclusions are drawn as follows. First, the increases of geomagnetic declination and ERP of the heating facility may lead to a considerable improvement in radiation efficiency. Second, the X-mode polarization is more efficient for radiation than the O-mode polarization. Third, the most remarkable radiation effect may appear at winter night. The optimal heating frequency and the beating wave difference could be found under certain conditions. Stimulations of VLF/ELF radiation with the dual-beam beating wave method are more effectivethan the available amplitude modulation, the difference in VLF/ELF radiation intensity between the two methods is about 10 dB.