Vol. 65, No. 20 (2016)
Characterization of effective conductive layer of nano organic thin film using reflectance spectroscopy
2016, 65 (20): 200201. doi: 10.7498/aps.65.200201
We propose an optical approach for analyzing the formation of the conductive layer during organic thin film growth. The relationship between the properties of multi-layer film, such as thickness and optical coefficients, and the corresponding differential reflectance spectrum (DRS) is derived as math formula based on the effective medium approximation. With the deduced formula, the thickness of the deposited film, for example, electron transport layer in this paper, can be estimated according to the measured DRS data. But, in fact, the fitting error always exists. It is, on the other hand, a useful evidence to indicate the actual situation of the thin film. A concept of the normalized fitting error (NFE) is offered here to equivalently assess the fitting results of all DRS data in the growth process. The curve of NFE versus time is proposed to analyze the growth revolution of the thin film and reveal the inner physical mechanism. In order to demonstrate the performance of the proposed method, an organic field effect transistor (OFET) with a bottom-gate structure is fabricated and pentacene organic thin film is deposited by vacuum thermal evaporation, as an electron transport layer, on the top of the transistor, i. e., an insulator substrate of Si/SiO2. The reflected optical spectrum and the current between the drain and the source of the OFET device are investigated in real time in the growth process. It has been reported that pentacene has three kinds of crystal structures and their optical properties differ from each other. The actual phase of the pentacene film in our experiment is discussed at first. The fitting results show that the pentacene layer exists mainly in thin film phase here. Then, the thickness of SiO2 layer is determined to be 296 nm, which is close to the design value of 300 nm. With those parameters, a four-layer model is used to calculate the thickness of the organic film. The thickness data indicate that the film appears to be linearly growing and the growth rate is 0.2 nm/min. Next, the NFE is plot as a function of time. In this plot, the curve of the NFE increases quickly at the beginning of the growth and reaches to a positive peak at 70 min. After that, the NFE decreases and then keeps constant for a while. When the measured current-time curve is added into this plot, one finds that the increase of the current happens at the same time with the peak of the NFE. It implies that the NFE is related to the structure change of the organic film and thus linked indirectly to the electronic property. The peak of the NFE, to a certain extent, reveals the completeness of the organic conductive layer. As a result, the presented optical approach is valuable for analyzing the electronic status of the organic thin film, especially if the electronic test cannot be performed.
Extractraction of non-stationary harmonic from chaotic background based on synchrosqueezed wavelet transform
2016, 65 (20): 200202. doi: 10.7498/aps.65.200202
The signal detection in chaotic background has gradually become one of the research focuses in recent years. Previous research showed that the measured signals were often unavoidable to be contaminated by the chaotic noise, such as the radar signal detection from sea clutter wave,signal source extraction from chaotic secure communication and ECG/EEG abnormal signal detection,etc.At present,there are two methods to detect the target signal from the chaotic background.One is to detect the target signals by using the difference in geometric structure between the chaotic signal and the target signal,and the other is to regard the chaotic signal as the noise,and the target signal is extracted from the chaotic background by the time frequency analysis method,such as wavelet transform and empirical mode decomposition. The first kind of method can detect the target signal well,but it needs to characterize the chaotic system and reconstruct the phase space,which is difficult in the practical applications.The second kind of method does not need to reconstruct the chaotic phase space and can effectively extract the target signal from the chaotic background.However,the wavelet transform lacks adaption and how to select the optimal wavelet basis and decomposition layers is a difficult problem.In the empirical mode decomposition there exists the mode mixing that is very sensitive to the noise.The synchrosqueezed wavelet transform (SST) effectively improves the mixing of mode by compressing the continuous wavelet coefficients in the frequency direction,but also it has good robustness to noise.Therefore,the SST can extract the harmonic signal well from the chaotic background.In the present algorithm of abstracting harmonic signal from chaotic background by SST,the harmonic signals are extracted by using single accumulation frequency range SST (SAFR-SST) based on wavelet ridge detection.If the target signal is stable harmonic signal,whose frequency does not change with time,the SAFR-SST method can have a high abstraction precision.But if the target signal is the non-stable harmonic signal whose frequency changes with time,the SAFR-SST method is not enough nor can obtain high abstraction precision.In order to overcome the shortcomings of the SST in extracting the non-stationary harmonic signal from the chaotic background, an improved SST extracting method is proposed which is based on the adaptive optimal cumulative frequency range. Firstly,the formulas of calculating the adaptive cumulative frequency range in SST extraction are deduced according to the relationship between the wavelet coefficient of non-stationary harmonic and the interval of supporting wavelet bases.Then,the optimal values of the parameters in the adaptive cumulative frequency range formula are calculated by the minimum energy error criterion according to the integrity and orthogonality of the intrinsic mode types function.Finally,the SST adaptive extraction of the non-stationary harmonic signal is realized according to the SST inverse transform.In experiment,the different types of non-stationary harmonics are extracted from the Lorenz and Duffing chaotic background.The experimental results show that the proposed method can effectively extract the non-stationary harmonic from the noisy chaotic background.Compared with the classical SST method with single cumulative frequency range,the proposed method has good performance in both mean square error and correlation coefficient.And when the chaotic background contains different-intenity Gauss white noises,the proposed method can also effectively abstract the non-stationary harmonic from the chaos and noise interference.So,the proposed method has a good practice value.
2016, 65 (20): 200501. doi: 10.7498/aps.65.200501
As is well known, chaos synchronization has a good performance in secret communication. However, in most existing researches, chaos synchronization is realized between master chaotic system and slave chaotic system with real-time communication. A lot of network bandwidths are wasted in useless communication which may bring great economic costs and increase the likelihood of network congestion. In this paper, the synchronization for the heterogeneous master-slave system is studied. Compared with the homogeneous synchronization, the heterogeneous synchronization has a broad application prospect. Then the sampling controller based on event trigger is designed to achieve the heterogeneous master-slave synchronization which can save network bandwidth, with the control performance maintained. Because transmission time delay is universal in communication system, the delay model of the system is constructed and utilized firstly. Based on the proposed system model, the master-slave heterogeneous synchronization problem is equivalently converted into the asymptotical stability problem of a time-delay system by using the input delayed approach. Then, according to the Lyapunov stability theory and linear matrix inequality, we may rigouously prove that the synchronization of heteogeneous master-slave chaotic system can be achieved. Meanwhile, the heterogeneous master-slave system synchronization conditions and sampling controller can be given based on the constructing Lyapunov-Krasovskii functional with the Wirtinger inequality and free weight matrix method. The sampling controller actually is a stateback controller. But for the purpose of reducing the network usage rate, wheter the state can be transmitted can be determined by the event-triggered rules. The event-triggered rules are designed based on the error state between master chaotic system and slave chaotic system, and the synchronization performance index can be determined by choosing the system parameter. The designed sampling controller updates the control parameters when the event-triggered rules are satisfied. Finally, numerical simulation experiments are employed to verify the correctness and effectiveness of the proposed method. Results indicate that the hetergeneous master-slave chaotic system with transimission time delay can indeed achieve synchronization by event-triggerd sample control. Moreover, the number of communications between master chaotic system and slave chaotic system is less than before and the synchroization performance is also at an idea level.
Research and analysis on lidar performance with intrinsic fluorescence biological aerosol measurements
2016, 65 (20): 200701. doi: 10.7498/aps.65.200701
Biological aerosols which could cause diseases of human beings, animals and plants, are living particles suspended in the atmosphere. Ultraviolet laser induced fluorescence has been developed as a standard technique used to discriminate between biological and non-biological particles. As an effective tool of remote sensing, fluorescence lidar is capable of detecting concentration of biological aerosols with high spatial and temporal resolutions. Intrinsic fluorescence, one of the most important characteristics of biological aerosols, has quite a large effect on the performances of fluorescence lidar. To investigate the effects of intrinsic fluorescence on biological aerosols, we design an ultraviolet laser induced fluorescence lidar at an excited wavelength of 266 nm, with a repetition rate of 10 Hz. Fluorescence signals are collected by a Cassegrain telescope with a diameter of 254 mm, in which fluorescence spectra of 300-800 nm are mainly considered. A spectrograph and a multichannel photomultiplier tube (PMT) array detector are employed to achieve the fine separation and highefficiency detection of fluorescence signals. According to the present configuration, we perform a series of simulations to estimate the measurement range and the concentration resolution of biological aerosols, with a certain pulse energy. With a relative error less than 10%, theoretical analysis shows that designed fluorescence lidar is able to detect the biological aerosols within a range of 1.5 km at a concentration of 1000 particles·L-1. When the detection distance enlarges to 2.1 km, detectable wavelength range is limited to 300-310 nm. In addition, the lidar is capable of identifying minimum concentrations of biological aerosols with 2 particles·L-1 and 4 particles·L-1 at fluorescence wavelengths of 350 nm and 600 nm, respectively, where the induced pulse energy is set to be 60 mJ and detected range 0.1 km. With setting energies of 40 mJ and 20 mJ, minimum concentrations of biological aerosols decrease to 3 particles·L-1 and 6 particles·L-1, respectively, at a fluorescence wavelength of 350 nm. The relative error of minimum concentration resolution is about 2 particles·L-1, increasing rapidly with range. For a fluorescence wavelength of 600 nm, both the minimum concentration and the relative error show relatively high values, 5 particles·L-1 at 40 mJ and 10 particles·L-1 at 20 mJ, where the relative errors are found to be 2 particles·L-1 and 4 particles·L-1, respectively. The results prove that a shorter intrinsic fluorescence wavelength has a better effect on biological aerosol measurement. We believe that a proper intrinsic fluorescence wavelength will further improve the detection accuracy of biological aerosols.
2016, 65 (20): 200702. doi: 10.7498/aps.65.200702
High-performance vertical vibration isolators are required in precision instruments and physical experiments to reduce the seismic noise, which limits the instrument performance and measurement results. For example, inertial references are needed in interferometric gravitational wave detectors and absolute gravimeters, in order to separate the useful signal from noise. Microseisms typically occur at around 0.07 Hz. The secondary microseisms occur at about 0.14 Hz. Buildings usually wobble at frequencies between 0.1 and 1 Hz. To reduce all these vibrations would require a spring-mass system with a resonance frequency lower than 0.05 Hz. The most commonly applied techniques use a passive vertical isolation system, which is easy to set up and cheap to build. However, to achieve low cut-off frequency, such as 0.05 Hz, there requires longer than 100 m static deflection for a simple passive isolator, which is impractical in most applications. An ultra-low-frequency active vertical vibration isolator, based on a two-stage beam structure, is proposed and demonstrated in this paper. Two beams are connected to a frame with flexural pivots. The upper beam is suspended from the frame with a normal hex spring. The lower beam is suspended from the upper one by a zero-length spring. The flexural pivots of the upper beam are not vertically placed above the pivots of the lower beam. With this special design, the attachment points of the zero-length spring to the beams can be moved to change the effective stiffness. A laser reflectometry is used to detect the angle between the two beams. A laser collimator, a mirror, a beam splitter and an optical detector are fixed to the upper beam, and another mirror is fixed to the lower beam. A laser beam from the collimator is directed to the detector via the mirrors and the beam splitter. The output of the detector is proportional to the angle between the two beams. The minimum detectable angle is 36 nrad. The angle signal is sent to a circuit to generate a control signal, which drives a voice coil mounted between the lower beam and the frame to maintain the angle between the two beams to a fixed value. The isolation system can achieve a natural period of 100 s by carefully adjusting the attachment points of the zero-length spring and the feedback parameters. This type of isolator has a simpler and more robust structure than the famous active vibration isolator-the super spring. The system is promising in applications such as precision instruments and experiments, especially in absolute gravimeters.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
Wide-band composite electromagnetic scattering from the earth soil surface and multiple targets shallowly buried
2016, 65 (20): 204101. doi: 10.7498/aps.65.204101
Wide-band electromagnetic scattering from multiple objects shallowly buried beneath rough earth soil surfaces has been an important research topic in recent years because of its extensive applications in detecting the buried objects such as mines, pipes, and tunnels. Due to the advantages of finite-difference time-domain (FDTD) method in simulating wide-band electromagnetic scattering from rough surface in the presence of multiple objects, the FDTD method under Gaussian differential pulse wave incidence is utilized in the present study to analyze the frequency response of rough soil surfaces with shallowly buried objects, which serves as a basis for the detection and discrimination of objects buried below rough soil surfaces. The Topp equation model that can predict the dielectric constant of soil-water mixture is adopted in the present study to properly describe the dielectric property of earth soil with water. The actual rough land surface is modeled as the realization of a Gaussian random process with exponential spectrum by using Monte Carlo method. Simulation results show that the variation of composite scattering coefficient with frequency is oscillatory. It is also shown that the composite scattering coefficient versus frequency increases with the increase of root-mean-square of soil surface, water ratio of soil, the target section height, and the separation distance of target. However, simulation results indicate that the composite scattering coefficient versus frequency decreases with the increase of target section width. In summary, the variation of wide-band scattering coefficient is very complicated and is very sensitive to the incidence angle of electromagnetic wave. However, the wide-band scattering coefficient under Gaussian differential pulse wave incidence is less sensitive to the correlation length of rough soil surface, the depth of buried objects, and the dielectric constant of target. These qualitative results relating to the frequency response of rough soil surfaces in the presence of multiple objects are potentially valuable for detecting and discriminating the objects buried below rough soil surfaces by utilizing a wide-band ground penetrating radar system, although the present study is limited to one-dimensional rough soil surface due to the severe computational burden encountered in the large-scale Monte Carlo simulations. In addition, compared with frequency-domain numerical methods, the FDTD method has significant advantages in calculating wide-band composite scattering from rough surfaces in the presence of multiple objects, and thus has extensive applications in radar imaging simulation of multiple objects below or above rough surfaces, which goes beyond the scope of this paper.
Time reversal multi-target imaging technique based on eliminating the diffusion of the time reversal field
2016, 65 (20): 204102. doi: 10.7498/aps.65.204102
Time reversal technique has the adaptive time-space focusing characteristics, which has been widely used in communication systems, imaging systems, and power combining systems. However, the ideal time reversal processing cannot be implemented in an actual imaging system and some diffusion phenomenon has been observed. In this paper, the diffusion phenomenon of the time reversal field in an imaging system is analyzed based on the time reversal cavity theory. Since the corresponding absorption source cannot be set in an imaging process, the time reversal field will continue to disperse after the convergence. Therefore, the field produced by the time reversal cavity will be similar to the sinc-function near the source. The diffusion field will result in mutual interference between the imaging targets. In a traditional time reversal multi-target imaging system, weaker targets can easily be concealed and artifacts may occur. In this paper, a multi-target imaging technique based on the elimination of the time reversal field diffusion is proposed. In order to eliminate the effect of the diffusion field, the Clean algorithm is used. The Clean algorithm is a de-convolution algorithm, which can effectively suppress the side lobe signal. By using the Clean algorithm in the time reversal imaging system, the interaction between multi-targets can be eliminated. Full-wave simulation shows a good performance of the proposed method. In practice, the time reversal mirrors are used to replace the time reversal cavity, for the fully closed time reversal cavity cannot be implemented. The effects of the time reversal mirrors have also been analyzed in this paper. The result shows that the positions of the time reversal mirrors have an significant influence on the reversed field distribution, which affects the Clean algorithm and the proposed imaging method. In order to eliminate the influence of time-reversal mirror position, an effective time reversal signal equalization algorithm is proposed. In the equalization algorithm, the amplitude of the time reversal signal in the time reversal mirrors is adjusted according to both the distance and the intensity. The proposed equalization algorithm can keep the time reversal field stable and provide effective support for the imaging method.
Dynamics of slow electrons transmitting through straight glass capillary and tapered glass capillary
2016, 65 (20): 204103. doi: 10.7498/aps.65.204103
It has been found that the transmission rate of the electrons through insulating capillaries as a function of time/incident charge is not the same as that of the ions. The question arises that by using the electrons, if the negative charge patches can be formed to facilitate the transmission of the following electrons, thereby substantiating that the so-called guiding effect works also for electrons. This study aims to observe the time evolutions of the transmission of electrons through a straight glass tube and a tapered glass capillary. This will reveal the details of how and (or) if the negative charge patches can be formed when the electrons transport through them. In this work, a set of MCP/phosphor two-dimensional detection system based on Labview platform is developed to obtain the time evolution of the angular distribution of the transmitted electrons. The pulsed electron beams are obtained to test our detection system. The time evolution of the angular profile of 1.5 keV electrons transmitting through the glass tube/capillary is observed. The transmitted electrons are observed on the detector for a very short time and disappear for a time and then appear again for both the glass tube and tapered glass capillary, leading to an oscillation. The positive charge patches are formed in the insulating glass tube and tapered glass capillary since the secondary electron emission coefficient for the incident energy is larger than 1. It is due to the fact that fast discharge of the deposited charge leads to the increase of the transmission rate, while the fast blocking of the incident electrons due to the deposited positive charge leads to the decrease of the transmission rate. The geometrical configuration of the taper glass capillary tends to make the secondary electrons deposited at the exit part to form the negative patches that facilitate the transmission of electrons. This suggests that if the stable transmission needs to be reached for producing the electron micro-beam by using tapered glass capillaries, the steps must be taken to have the proper grounding and shielding of the glass capillaries and tubes. Our results show a difference in transmission through the insulating capillary between electrons and highly charged ions.
2016, 65 (20): 204201. doi: 10.7498/aps.65.204201
The irradiance uniformity on target plane is a key issue in laser-driven inertial confinement facilities. In the typical schemes of one-dimensional smoothing by spectral dispersion (1D-SSD) and the “star” grating, the stripe pattern inside the focal spot appears inevitably, besides, the fabrication of the “star” grating is relatively difficult. Thus, a new spectral dispersion smoothing scheme based on a hybrid grating is proposed, which not only achieves the better smoothing effect, but also exhibits some specific advantages in the fabrication and the dispersion way. According to the different direction of the spectral dispersion, the hybrid grating is divided into inner and outer dispersion areas. That is, the dispersion direction of the inter dispersion area is in the horizontal or vertical direction, and the dispersion direction of the outer dispersion area is in the azimuthal direction. When the laser beam with the temporal phase modulation propagates through the hybrid grating, the dispersion directions of the laser beam in the inner and the outer dispersion areas are different, leading to the redistribution of the speckles inside the focal spot in the resultant direction of the translation and rotation on the target plane. Consequently, the focal spot on the target plane achieves the beam smoothing in radial and horizontal or vertical direction. In the present paper, the theoretical model of the hybrid grating scheme based on the spectral dispersion smoothing is built up. Using the theoretical model, the smoothing effect of the hybrid grating scheme is analyzed, and compared with those of the typical schemes of 1D-SSD and the “star” grating. The contrast and the fractional power above the intensity (FOPAI) are used to evaluate the smoothing characteristic of the focal spot. In addition, the influences of the area ratio and groove density in the different dispersion areas of the hybrid grating on beam-smoothing effect are also discussed. Results indicate that when the inner dispersion area accounts for the 0.3-0.5 of the total dispersion area, the hybrid grating scheme can effectively suppress the stripe intensity modulation both in the radial direction and the vertical direction. With increasing the groove densities of the inner and the outer dispersion area in a certain range, the irradiance uniformity of the focal spot is further improved. However, considering the actual processing of the hybrid grating, the appropriate groove density should be selected. Compared with the typical scheme of the 1D-SSD, the scheme of the hybrid grating can achieve the better smoothing effect with the multi-direction spectral dispersion smoothing. Furthermore, the fabrication of the hybrid grating is relatively simple and the irradiation uniformity on the target plane is also good compared with those of the “star” grating scheme.
2016, 65 (20): 204202. doi: 10.7498/aps.65.204202
Ionizations of atoms and molecules in strong laser fields are fundamental processes of ultrafast physics. Compared with atom ionization, molecular ionization is very complex due to the existence of multi Coulomb centers. As a simplest molecule, H2+ has been widely used to explore new phenomena of molecules in strong laser fields. One of the notable processes in H2+ ionization is charge resonance enhanced ionization (CREI), in which the ionization rate is enhanced substantially when the internuclear distances are around 6 a.u. and 10 a.u. CREI has been extensively studied by numerically simulating the time-dependent Schrödinger equation. While quantum calculations provide accurate ionization rates, the mechanism governing the CREI is not revealed in such ab-initio calculations. On the contrary, the calculations based on the classical trajectories Monte-Carlo assembly may offer an intuitive picture for CREI though some quantum information is not included. In this paper, we revisit the CREI of H2+ in a strong infrared laser field by Monte-Carlo simulation. By initializing ten-thousand classical points whose initial positions and velocities satisfy the field-free Hamiltonian of H2+, we solve the classical Newtonian equation and obtain the trajectories of all particles, from which one may analyze the particle velocities, energies, etc. We count the ionization events by diagnosing the particle energy after the laser interaction. If the sum of the kinetic energy and potential energy is larger than 0, we set it as an ionization event. The ionization rate is calculated by collecting all ionization events and normalizing it with the total particle number involved in the calculation. By setting the internuclear distances to be different values, we obtain the ionization rate as a function of internuclear distance. Our simulation shows that the ionization probability is greatly enhanced when the internuclear distance is about 5 to 6 a.u. by employing a 1064 nm, 4×1013 W/cm2, five cycles laser pulse. By tracing the particle trajectory, we find that the electron usually gains the energy from the laser field by circulating one nucleus, then passes through the interatomic barrier and moves around the other nucleus before being ionized. By looking into the relationship between the ionization probability and the laser-distorted Coulomb potential at different internuclear distances, we find that the ionization probability is maximum when the energy difference between the ground state and the interatomic Coulomb barrier, or between the ground state and the saddle value of the laser-distorted potential, is minimum. The classical calculation of the ionization of H2+ interacting with intense laser field reproduces the qualitative features of the corresponding quantum-mechanical calculation. It offers an intuitive physical picture of the tunneling ionization of molecules through investigating the classical trajectories and provides a new perspective to inspect the intriguing phenomena in quantum systems.
Two broadband chaotic signals generated simultaneously by semiconductor ring laser with parallel chaotic injection
2016, 65 (20): 204203. doi: 10.7498/aps.65.204203
Recently semiconductor ring laser (SRL) as a novel device has received much attention, for its special cavity allows the output light to propagate in two opposite directions, namely the clockwise mode and counterclockwise mode. SRL does not require gratings or cleaved facets for optical feedback and can be a candidate for small sized photonic integrated circuits which have been developed for secure data transmission, with chaotic carriers and high rate random bit generated. In this paper, we propose a method to obtain two broadband chaotic signals with high unpredictability degree by utilizing injected slave SRL and further explore the physical mechanism and injection conditions. Based on a conventional master-slave configuration, the proposed method obtains two modes of chaotic signals by master SRL with external cross feedback, which are injected in parallel to a slave SRL correspondingly. According to the well-known Lang-Kobayashi rate equations, we establish rate equations and numerically investigate the influences of frequency detuning and injection strength on bandwidth and unpredictability degree. We adapt the given definition of bandwidth and the normalized permutation entropy to respectively evaluate bandwidth and unpredictability degree of chaotic signals. Furthermore, we reveal the underlying physical mechanism of bandwidth enhancement and asymmetric bandwidth-enhancing region by analyzing the radiofrequency and optical spectra of intensity time series. The results show that two chaotic signals have similar routes to enhancing the bandwidth in frequency domain. In the unlocking injection area, two broadband and unpredictability-enhancing chaotic signals generated by slave SRL are simultaneously achieved by choosing appropriate control parameters. Analyses of optical spectra reveal that high-frequency periodic oscillation generated between injection chaotic signals and slave light via beating is the physical mechanism of bandwidth enhncment. The bandwidthenhancing domains of two chaotic signals are asymmetrical due to redshift of master SRL frequency, with external chaotic signals injected. Bandwidth-enhanced chaotic signals are easier to obtain in the domain of negative frequency detuning. The asymmetrical injections contribute to reducing the locking region and extending the bandwidthenhancing region under high injection strength. This conventional master-slave configuration composed of two SRLs can be easily implemented on chip and save other optical devices. The slave SRL subjected to parallel injection signals from master SRL can be used as a wideband unpredictability-enhancing chaotic source, which is extremely useful for the high capacity security-enhancing multiple chaotic communications, as well as for the potential applications of high speed random number generators.
Fractional Fourier transform of astigmatic sine-Gaussian beams and generation of dark hollow light beams with elliptic geometry
2016, 65 (20): 204204. doi: 10.7498/aps.65.204204
In this work, we develop a novel method of creating dark hollow beam with vortex by converting a sine-Gaussian beam (SeGB) with edge-dislocation and astigmatism through using fractional Fourier transform (FrFT) optical system. On the basis of the definition of the FrFT, an analytical transformation formula is derived for an astigmatic SeGB passing through such a transform system. By use of the derived formulae, the changes of the intensity distribution and the corresponding phase properties associated with the transforming astigmatic SeGBs are analytically discussed in detail. It is found that for an input SeGB without astigmatism, there is still a dark line or an edge dislocation associated with the intensity distribution of the FrFT beam along the initial dislocation line, similar to that of the input SeGB. However, when the input SeGB astigmatically passes through an FrFT optical system, the dark line of the intensity distribution of the input SeGB can be converted into a solitary zero point, or in other words, a dark hollow beam with a single-charge vortex can be produced by SeGB with an edge dislocation. The results reveal that the astigmatism plays a critical role in transforming a SeGB into a dark hollow one through the FrFT optical system. Furthermore, some numerical calculation results based on the derived formula are presented and discussed graphically. It is shown that for appropriate beam parameters and carefully adjusting the transform angle of FrFT, dark hollow beams with single-charge vortex and elongated elliptic geometry can be realized with astigmatic SeGBs. The influences of the beam parameters and the transform angle of FrFT optical system on the generation of perfect dark hollow beams are also investigated. The results demonstrate that the linear eccentricity of the dark hollow beam, which is roughly defined as the ratio of semi-minor axis to semi-major one of the intensity pattern, mainly depends on the Fresnel number. And the optimal linear eccentricity may be relatively large under carefully selecting the beam and optical system parameters. Moreover, optimal parameter values corresponding to perfect dark hollow beam configurations which can be experimentally accessed are presented. As is well known, there are two types of pure phase defects or dislocations in the optical fields:one is screw dislocation or vortex and the other is edge-dislocation. Due to their important applications, the propagation dynamics of optical vortices or edge dislocations are extensively studied both theoretically and experimentally. The vortex-edge dislocation interaction is investigated in detail. However, there are fewer reports on the direct conversion between a single edge dislocation and a vortex. Therefore, the results obtained in this paper represent a significant step forward in understanding the transformation dynamics between beams with pure edge dislocation and vortex, and also opens possibilities for their potential applications, e.g., in generating dark hollow beams with elliptic geometry using FrFT systems.
Luminescence selective output characteristics tuned by laser pulse width in Tm3+ doped NaYF4 nanorods
2016, 65 (20): 204205. doi: 10.7498/aps.65.204205
The variations in material composition, phase and structure can provide a useful tool for tuning emission colour, but the controlling of the emission colour in a material, with a composition fixed, remains to be a daunting challenge. In this work, we systematically study the luminescence selective output characteristics of Tm3+ doped NaYF4 nanorods, and also the dependences of fluerecence output on pulse duration, excitation wavelength, pump power, and ambient temperature. The results show that the color of output light is strongly dependent on laser pulse duration compared with other factors. The temperature dependent luminescence of the nanorods shows very different behaviors with short-pulse laser excitation from those of continuous wave (CW) laser. When the pulse laser at 656 nm is employed, the emission spectra from NaYF4:0.5 mol% Tm3+ nanorods at the different temperatures are dominated by near-infrared (NIR) luminescence about 800 nm accompanied with weak blue luminescence, giving rise to nearly spectrally-pure NIR emissions at 20 K. When the pulse laser is replaced by CW laser, blue double emissions at 453 and 478 nm with the same order of magnitude of NIR luminescence can be clearly detected at room temperature. The key mechanism responsible for colour-tunable emission can be explained in terms of the population process of luminescence level, in which the different luminescence level populations need different time intervals. Considering excited-state absorption (ESA) for a particular 1D2 energy level, there needs an extra step of 3F2, 33H4 multiphonon nonradiation relaxation process to populate the 3H4 state and subsequently pump its 1D2 state for blue emission. Therefore, the pulse width should be longer than nonradiation relaxation time of 3F2, 33H4 to comply with the ESA, while the nonradiation relaxation time can further be tuned by controlling ambient temperature. We show that the variation of the excitation power leads to interesting change in the upconversion (UC) decay curve. We focus our attention on the excitation wavelength dependences of 3H4 and 1D2 emission lifetimes in order to validate the population mechanism of luminescence level. We demonstrate that the 3H4 luminescence time depends on excitation wavelength, while 1D2 emission lifetime nearly keeps constant when varying the excitation wavelength. Based on multi-phonon relaxation theory and time-resolved photoluminescence studies, it is indicated that the UC luminescence under short-pulse laser excitation mainly originates from the ions at/near the surface, while downconversion is mainly from the ions in the core for NaYF4:Tm3+ nanorods. The single-band NIR luminescence output by changing the pulse width and excitation wavelength provides an insight into the controlling of the population processes of luminescent levels and offers a versatile approach to tuning the spectral output.
Experimental study of diode-pumped Nd, Y:CaF2 amplifier for inertial confinement fusion laser driver
2016, 65 (20): 204206. doi: 10.7498/aps.65.204206
In a conventional laser-driven inertial confinement fusion (ICF), Nd-doped phosphate glass is used as a gain medium. However, the repetition frequency operation of such a laser system is restricted by the low thermal conductivity of the phosphate glass. To attain a high ICF performance, the laser driver must be able to operate at a repetition frequency of no less than 10 Hz. Typically, an Nd-doped laser glass operates at a repetition frequency well below 10 Hz. In this paper, an Nd, Y:CaF2 crystal is taken as a gain medium for the laser amplifier, and experiments are carried out to demonstrate the capability of Nd, Y:CaF2 crystal to act as a gain medium for ICF laser driver. A laser-diode plane-array five-direction horizontal-side-pumped Nd, Y:CaF2 laser amplifier 5 mm70 mm is developed and an experimental study is carried out. The absorption spectrum and emission spectrum of Nd, Y:CaF2 crystal and the fluorescence distribution of the amplifier are measured. The Nd:CaF2 co-doped with Y3+ ions results in a broad absorption band, which makes the laser diode pumping more efficient. The strongest excitation band peak is centered around 796 nm. The small signal gains of Nd, Y:CaF2 and Nd:Glass working respectively at repetition frequencies of 10 and 1 Hz under the same pump power are measured. The small signal gain of Nd, Y:CaF2 amplifier reaches 6.12 under a pump power of 9.63 kW, which is 1.5 times that of Nd:Glass amplifier. The measurements of the spectrum of the seed beam and the spectrum from Nd, Y:CaF2 amplifier show that the signals have no change before and after being amplified. Most likely the Nd, Y:CaF2 crystal is a promising laser material for repetitive ICF laser drivers.
Extraordinary transmission of light enhanced by exciting hybrid states of Tamm and surface plasmon polaritions in a single nano-slit
2016, 65 (20): 204207. doi: 10.7498/aps.65.204207
Extraordinary optical transmission (EOT) through a metallic nano-slit or nano-slit arrays has become an efficient method to manipulate the light on a subwavelength scale. While a variety of nano-devices based on surface plasmon polaritons (SPPs) could be an ideal candidate for the next-generation ultra-compact integrated photonic circuits, this EOT phenomenon is also generally attributed to the excitation of SPPs in the nano-slit. Thus, due to its being compact in structure and amenable to integrate with other nano-devices, single nano-slit can be implemented to construct an optical source in the nano-device based on SPPs. However, the transmission through an isolated nano-slit is too low to be practically used. The main reason is that the excitation efficiency of SPPs in the nano-slit is not high enough. In fact, one of the key issues is how to enhance the excitation efficiency in a nano-slit. In this paper, a novel method and the related structure are proposed to effectively enhance the EOT in a single nano-slit by improving the excitation efficiency of SPPs. This structure is made up of a silver film on a distributed Bragg reflector (DBR), where a single nano-slit is imbedded in the silver film. Under the illumination of a TM polarized light from the DBR side of this structure, the Tamm plasmon polaritons (TPPs) at the interface between the silver film and the DBR and the SPPs in the nano-slit can be excited simultaneously. The TPP is another surface mode, which describes how an electromagnetic field is localized at the boundary of silver film and the DBR. In this structure, coupling between the TPPs and the SPPs leads to the appearance of a TPP-SPP hybrid state. When the wave-vectors between the TPP and the SPP modes are matched, due to the local field enhancement of the TPP mode, the excitation efficiency of SPPs can be improved significantly. Furthermore, utilizing the quasi Fabry-Pérot (F-P) resonance in the nano-slit, where a single nano-slit can be regarded as an F-P cavity with two open ends, a high light transmission through the single nano-slit can be achieved. In the present paper, the transmission properties of the “DBR-silver nano-slit” structure are analyzed with the finite element method and the transfer matrix method. After optimizing the structure parameters, with a thickness of the silver film of 100 nm and a width of the nano-slit of 11 nm, the light transmission through the single nano-slit in this structure can be increased by about 16 times, in comparison with the light transmission through a single nano-slit in a silver film on the TiO2 substrate (without DBR). This method of enhancing the light transmission through a single nano-slit by exciting TPPs mode and utilizing its local field enhancement property, has potential applications in the polariton lasers, the nano-scale photonic integration, the near-field imaging and sensing, and other relevant areas.
2016, 65 (20): 204208. doi: 10.7498/aps.65.204208
The superimposed gratings have attracted considerable interest because they can extend the potential applications of gratings. Superimposed gratings are fabricated by inscribing multiple gratings at the same section of the fiber, and they can demonstrate various features simultaneously. A number of optical devices based on superimposed gratings have been reported, such as multi-wavelength filters, beam shapers, ultrahigh repetition rate optical pulse generators, etc. Photonic crystal fiber (PCF) can bring new optical characteristics by changing the sizes, spacings and arrangements of the air holes in the fiber. In this paper, we present the spectra of the superimposed gratings inscribed in a photonic crystal fiber. A numerical mode is proposed based on the V-I transmission matrices. The traditional cosinoidal variation of refractive index is replaced with a square-type refractive index variation, and the scattering occurs at a localized discrete location. According to the simulations, the reflection spectra and time delays of a superimposed Bragg grating and superimposed chirped Bragg grating are analyzed. A superimposed Bragg grating and a superimposed chirped Bragg grating are fabricated in the single mode photosensitive PCFs under the irradiation of a 193 nm ultraviolet laser. The superimposed Bragg grating is composed of four subgratings with resonance wavelengths at set spacings. And under a phase mask displacement of 1.03 mm, the superimposed chirped Bragg grating has a periodic resonance with a period of 0.82 nm. The results show that the spectrum of superimposed Bragg grating can be flexibly customized by the parameters of each subgrating. Superimposed chirped Bragg gratings have good linear group delays and flat periodic resonance amplitudes, and the resonance period can be adjusted by displacing the phase mask. The grating spectra obtained from experiments are in good agreement with the theoretical analyses. The research results in this paper provide an important basis for designing, fabricating, and applying the superimposed PCF gratings.
High speed demodulation method of identical weak fiber Bragg gratings based on wavelength-sweep optical time-domain reflectometry
2016, 65 (20): 204209. doi: 10.7498/aps.65.204209
The identical weak reflection Fiber Bragg gratings (FBGs) with large capacity has become one of the central issues of optical fiber sensing field in the engineering application.Currently,wavelength division multiplexing (WDM) and time division multiplexing (TDM) are two major multiplexing techniques.For a WDM system,the maximum number of FBGs is limited by the spectral bandwidth of laser.So the identical weak FBGs are proposed to break through the limitation of the multiplexing capacity.For large-capacity multiplexing of identical weak FBGs,TDM technique is commonly used.In a TDM system,the spectral information of all FBGs can be obtained by some pulsed light with different wavelengths.However,with increasing the number of identical weak FBGs in TDM system,some problems such as complex demodulation process and slow response time are highlighted in the current various demodulation methods. Thus in this paper we propose a new high-speed demodulation method combined with wavelength-sweep optical timedomain reflectometry (WSOTDR) which is different from the pulsed light in optical time domain reflectometry (OTDR), namely a continuous wavelength-sweep light source is used in WSOTDR.In this method,the reflected signals of identical weak FBG at each position will be distinguished from others in time domain through optical delay effect,hence the location information of each FBG could be acquired,and meanwhile the wavelength information of all the identical weak FBGs could be obtained through high-frequency periodical wavelength-swept spectrum in just one wavelength scanning period.In order to calibrate the error of FBG demodulation which is caused by optical delay at high-speed wavelength sweep,we propose a self-calibration method in which two different wavelength-sweep rates are used to obtain the inherent delay parameters of each FBG.In practical application,we use this self-calibration method in the initial stage of demodulation because the inherent delay parameters are usually stable after the layout of an identical weak FBGs network.So the demodulating speed at the working stage of this system is not affected by this self-calibration method. In this paper,by setting up a Fourier domain mode locking laser as an output of continuous wavelength-sweep and highspeed (3.27×106 and 2.72×106 nm/s) light,an identical weak FBG sensing network which consists of 18 FBGs is tested in three experiments.In the initial calibration experiment,we use the self-calibration method to calibrate the inherent delay parameters of each FBG and to verify the accuracy of the system by comparing with the measurement result of spectrum analyzer.In the temperature experiment,the wavelength of each FBG is demodulated from 30 to 100 ℃ in order to test the demodulation linearity of the system.Then in the vibration experiment,a dynamic measurement of 3.6 kHz vibration of FBG is demonstrated with a demodulating speed as fast as 120 kHz,and a 0-60 kHz frequency spectrum is analyzed to prove the speed.The experimental results show that the demodulation error is less than 15 pm, the resolution is 1pm,the linearity is above 0.998,and the demodulating speed reaches 120 kHz.
2016, 65 (20): 204701. doi: 10.7498/aps.65.204701
The droplet dynamic in a bifurcating micro-channel, as one of the basic multiphase problems, is frequently encountered in the fields of science and engineering. Due to its great relevance to many important applications and also its fascinating physical phenomena, it has attracted the increasing attention in the past decades. However, this problem is still not fully understood since it is very complicated:the droplet behaviors may be influenced by several physical factors. To clearly elucidate the physics governing droplet dynamics in a bifurcating micro-channel, a detailed numerical study on this problem is conducted. The present investigation is based on our recently developed phase-field-based lattice Boltzmann multiphase model, in which one distribution function is used to solve the Cahn-Hilliard equation, and the other is adopted to solve the Navier-Stokes equations. In this paper, we mainly focus on the effects of the surface wettability, capillary number and outlet flux ratio on the droplet dynamics, and the volume of the generated daughter droplet is also presented. The numerical results show that when the capillary number is large enough, the droplet behaviors depend critically on surface wettability. For the nonwetting case, the main droplet breaks up into two daughter droplets, which then completely suspend in the branched channels and flow towards the outlet. While for the wetting case, the main droplet also breaks up into two daughter droplets at first, and then different behaviors can be observed. The daughter droplet undergoes a secondary breakup, which results in part of droplet adhering to the wall, and the remaining flowing to the outlet. The volume of the generated daughter droplet is also measured, and it is shown that it increases linearly with contact angle increasing. When the capillary number is small enough, the droplet remains at the bifurcating position, which does not break up. Finally, we also find that the outlet flux ratio affects the rupture mechanism of the droplet. When the outlet flux ratio is 1, the droplet is split into two identical daughter droplets. When the outlet flux ratio increases, an asymmetric rupture resulting in the generation of two different daughter droplets, will be observed. However, if the outlet flux ratio is larger enough, the droplet does not breakup, and flows into the branched channel where the fluid velocity is larger. Here we define a critical outlet flux ratio, below which the droplet breakup occurs, and above which the droplet does not break up. The relationship between the capillary number and the critical outlet flux ratio is examined, and it is found that the critical outlet flux ratio increases with capillary number increasing.
2016, 65 (20): 204702. doi: 10.7498/aps.65.204702
The parallel direct method of direct numerical simulation (PDM-DNS) for Rayleigh-Bénard (RB) convection is used in this paper. The differences and similarities in flow characteristic between two-dimensional (2D) and three-dimensional (3D) turbulent RB convection are studied using mean field for Ra=109, 1010, 5×1010, and Pr=4.3. Each of 2D and 3D cases has a large-scale circulation and corner rolls. The shape of large-scale circulation becomes round and the size of corner roll turns small as Ra increases. In 2D RB convection, there are four corner vortices at the corner of the square cavity and a stable large-scale circulation which is elliptical. For spanwise averaged 3D RB convection with two corner vortices, large-scale circulation reveals spindle shape. Due to the characteristic of the corner roll, the region plume dominating is wider in 2D RB convection than in the spanwise-averaged 3D case. Further, the Ra-dependence of thermal boundary layer properties is also studied. The thermal boundary layer thickness is scaled with Ra and the scaling exponents of λθ with Ra in the 2D and 3D cases are very similar.
The present study aims to address the effect of sphere temperature on water-entry cavity. For this purpose, an experiment on vertical water-entry cavity of a heated sphere is conducted by utilizing a high-speed video camera. The temperature of the sphere ranges from 17℃ to 800℃. The complex flow phenomena of water entry, produced by a change in temperature of a sphere, is obtained for the first time. According to the finding, cavity is not formed around the room temperature sphere under the condition of the impact velocity of 1.5 m/s. When the temperature of the sphere is 300℃, the cavity appears, while it disappears when the temperature reaches up to 400℃. Interestingly, cavity appears again as the sphere is heated to a temperature of 700℃. The degrees of drag reduction of the sphere are different in various temperature conditions. Based on the theory of heat transfer and fluid dynamics, we analyze the mechanism for the influences of temperature and velocity on the forming of cavitation. The results show that the heat-transfer efficiency and heat-transfer mode between sphere and water change with the increase of temperature. Meanwhile the turbulent characteristic around the sphere, the surface roughness and hydrophobicity of the sphere are affected by the bubbles and vapor layer. In consequence, these characteristics influence the formation of cavity. The results of the effect of impact velocity on water-entry cavity reveal that the heat transfer performance plays a significant role in the forming of cavity, while the heat transfer efficiency is improved by the increase of impact velocity. The water-entry characteristics are similar to those in flow field under high temperature at low impact velocity as well as under low temperature at high impact velocity. The flow field of water entry looks similar under 330℃ at high impact velocity as well as under 400℃ at low impact velocity. Thus, an abnormal phenomenon appears. That is to say, the cavity size first decreases, and then disappears with the increase of impact velocity for the sphere at 330℃. The heat transfer performance can determine whether a cavity forms under the conditions of the impact velocity ranging from 1.5 m/s to 3.8 m/s. Meanwhile, the impact velocity itself can merely affect the cavity shape. The pitch-off time of the 300℃ sphere is irrelevant to impact velocity, which shows a good consistency with the literature result. Also, this research will be conductive to gaining an insight into the complex flow of water-entry with a heated sphere.
2016, 65 (20): 204704. doi: 10.7498/aps.65.204704
The manipulation of double emulsion droplet via shear flow field is widely encountered in microfluidic devices. However, the interface evolution and hydrodynamics behavior of double emulsion droplet in shear flow is less understood till now. In this paper, a theoretical model of double emulsion droplet in a Couette flow device is developed and numerically analyzed to characterize the interface behavior of incompressible double emulsion droplet, which is also verified by a visualization experiment. Based on this model, the mechanisms underlying the steady deformation of double emulsion droplet under shear are investigated, and the effects of radius ratio of inner droplet to the outer one and viscosities of working fluids on the steady deformation are discussed. The results indicate that the steady deformation of double emulsion droplet in the shear increases with the rise in capillary number, and the deformation resistance of inner droplet is larger than that of the outer droplet. With increasing the radius ratio of inner droplet to the outer one, the interaction between the inner and outer droplets becomes great and thus the deformation degree of the inner droplet is increased. In addition, the effect of big deformation resistance by the inner droplet tends to be obvious, leading to decreasing the deformation degree of outer droplet. The viscosities of both inner and outer droplets provide a resistance for the deformation of double emulsion droplet. With the rises in viscosities of inner and outer droplets, the deformation degree of integral double emulsion droplet decreases. In addition, the effect of outer droplet viscosity on the steady deformation is more obvious than that of the inner droplet.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2016, 65 (20): 208101. doi: 10.7498/aps.65.208101
Electrons are an important constituent part of radiation space of charged particles and could damage microelectronic devices. Therefore, effective radiation protection is very important for the electronic device. So, a radiation protecting material with lightweight, high performance and low cost is urgently needed. Polyethylene (PE) with high hydrogen content/carbon nanotube (CNT) composite as a space shielding material is very promising for spacecraft application in the future. In order to meet the requirement of space applications for PE/CNT composite, it is of important academic value and practical significance to explore melting and crystallization behaviors of LDPE/MWCNT composites. In this paper, melting and crystallization behaviours of the irradiated LDPE/MWCNT composites irradiated by 110 keV electrons are studied by differential scanning calorimetry (DSC), synchrotron radiation X-ray small angle scattering (SAXS) and wide angle diffraction (WAXD). Experimental results show that the irradiation by 110 keV electrons does not affect the thermal characteristics of the LDPE, but can enhance the initial melting temperature and the melting-terminating temperature of LDPE/2% MWCNT composites during melting. Also, the radiation by 110 keV electrons could reduce the initial crystallization temperature and the crystallization-terminating temperature during crystallization. MWCNTs could enhance the initial melting temperature and reduce the melting-terminating temperature of LDPE/2% MWCNT composites during melting. Moreover, MWCNTs could enhance the initial crystallization temperature and crystallinity, and reduce the crystallization-terminating temperature during crystallization. SAXS and WAXD analyses show that with increasing the temperature, long periods of the irradiated/unirradiated LDPE increase during melting. Compared with that of the unirradiated LDPE, at a given temperature, long period of the irradiated LDPE is small. During crystallization, with reducing the temperature, long period of the irradiated/unirradiated LDPE begins to appear and gradually decreases. At the same temperature, long period of the irradiated LDPE is larger than that of the unirradiated one. For LDPE/2% MWCNT composites, long periods of the irradiated/unirradiated samples during melting and crystallization do not exist. The 110 keV electron irradiation mainly influences LDPE matrix of LDPE/2% MWCNT composites during melting and crystallization. The 110 keV electron irradiation can slow down the amorphous region expansion and the initial melting of lamellae of the LDPE matrix during melting. The 110 keV electron irradiation can slow down the amorphous region shrinkage and inhibit crystal from growing up during crystallization. During melting, MWCNTs can hinder the amorphous and crystalline molecular chains of LDPE from moving, which hinders the LDPE matrix from initially melting, but promotes the melting process after the initial melting has begun. During crystallization, MWCNTs could promote the formation of crystal of the LDPE matrix and inhibit the crystal from growing up.
2016, 65 (20): 208102. doi: 10.7498/aps.65.208102
Surface-enhanced Raman scattering has a high sensitivity in the detections of complex biological systems, and it has a lot of potential applications in food inspection, biological imaging and biosensors in biochemistry, etc. Here, we investigate the surface Raman enhancements on gold films of different morphologies and further simulate the enhancements by using the finite difference time domain. To prepare the substrates with different morphologies, polymethyl methacrylate (PMMA) is spin coated 2000 rpm in one minute on a silicon wafer, followed by annealing at 180℃ for 5 min. Then, PMMA is etched by a 20 kV electron beam lithography. With the PMMA used as a soft imprint template, polydimethylsiloxane (PDMS) is dropped on the template then removed gently from the template after drying at 60℃ for 4 h. Finally, a gold thin film is prepared on the PDMS by magnetron sputtering with a current of 10 mA for 15 min. We design two kinds of morphologies:a four-way grid and a square morphology. The dimension of the four-way grids is 40 m and the grid width is 4 upm. The dimension of the square is also 4 upm. The cystine and melamine solutions with concentrations of 50, 100, 200 and 400 ppm are deposited on the surfaces of the gold thin film, respectively. The Raman spectra of cystine and melamine solutions are measured on the substrates with four-way grids and dot arrays. The Raman spectra of cystine on two kinds of substrates show no obvious difference. Due to the relatively small enhancement of melamine, the Raman peaks of melamine solutions of concentrations 50 and 100 ppm on the substrate of square morphologies are not easy to detect. On the contrary, all of the Raman spectra of melamine on the substrate of four-way grid morphologies are clear. The result indicates that the substrate with four-way grids has better sensitivity and enhancement performance. To verify the influence of the morphologies of the substrates on surface Raman enhancement and understand the mechanism of the enhancement, we simulate the scattering spectra and field distributions of different morphologies on gold thin films by using the finite difference time domain method. It is indicated that more complex the structure, the more obvious the enhanced Raman spectra will be. The calculations show that the enhancements of four-way grid morphologies are better than those of square morphologies. The predicted results of the surface enhanced Raman scattering are consistent with the measurements. These results will provide guidance and theoretical basis for further applications of surface enhanced.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2016, 65 (20): 209401. doi: 10.7498/aps.65.209401
The traditional simulation model of sulfur hexafluoride ionosphere release is a simple point-source model and the simulation precision is not high. The three-dimensional refined simulation model of rocket SF6 release is established in this paper, in which the rocket pose and velocity, gas injection velocity and flow, and wind velocity are all taken into account in the diffusion equation. Meanwhile, the influences of geomagnetic inclination and the field diffusion on artificial disturbance form are considered in the plasma diffusion equation, and the two-dimensional plasma diffusion equation is extended to three-dimensional case. The ray tracing method is used to study the influence of ionospheric artificial disturbance on the short wave propagation path. The research results of the ionosphere kinetics process, ionospheric uneven body generation mechanism and evolution are of great significance.
A method of suppressing vibration for high precision broadband laser frequency scanning interferometry
2016, 65 (20): 209501. doi: 10.7498/aps.65.209501
In the paper we study the method of reducing environmental influence in broadband laser frequency scanning interferometer. Target displacement caused by vibration will result in Doppler shift in measurement beat frequency. The extent of frequency shift is usually much larger than the actual target displacement. So the direct calculating of the target distance will cause ranging precision to decrease. In this paper, we establish a model for the influence of environmental vibration on the measurement and analyze the influence of the vibration on ranging result. To suppress the vibration effect, the Kalman filter is combined with the overlapping Chirp Z transform to estimate the measured data. The general process is described as follows. Firstly, the tuning nonlinearity will lead to the frequency spectrum broadening, so this paper we use the frequency sampling method to correct the frequency modulation nonlinearity of the laser. The frequency sampling method has the advantages of high speed and high precision. Secondly, the measurement system has the dispersion mismatch effect due to the use of broadband frequency swept laser. To solve this problem, the influence of the dispersion on the measurement is reduced by using the method of dispersion chirp slope calibration. Thirdly, because of the long frequency sweep period of the external cavity swept frequency laser, the vibration process of the target cannot be recorded in real time by single sweep, so in this paper we propose segmenting the measurement signal of single sweep and conducting Chirp Z transform to calculate target distance at different times. Compared with FFT algorithm, Chirp Z transform can achieve arbitrary narrow band spectrum subdivision, with the advantages of high accuracy and fast frequency measurement. Lastly, the Chirp Z ranging result is further combined with the method of Kalman filter to estimate the state of the target distance information. The experimental results indicate that the measurement standard is reduced from 185.4 μm to 9 μm by the proposed method. Without changing the absolute distance measuring device of broadband laser frequency scanning interferometer, this method provides a solution for further improving the ranging accuracy in the vibration environment, and reduces the complexity and cost of the device.
2016, 65 (20): 209601. doi: 10.7498/aps.65.209601
The interaction between the solar wind plasma and the bias electric field of long conducting tethers is the basic operation mechanism of the electric sail thruster. A two-dimensional (2D) full particle model is established to investigate the momentum transfer process between the solar wind plasma and parallel conducting tethers, while normal incidence and oblique incidence of the solar wind are taken into account. To ensure the accuracy and stability of the present PIC method, we take a grid space step of 2.5 m that is smaller than the Debye length and a time step of 162.5 ns that is limited by the plasma frequency. The main features including the spatial electric potential and ion number density distribution are represented under the influences of tether distance and solar wind incidence angle, in addition, the effect of the bias voltage on momentum transfer process is analyzed. At a steady state, the number of electrons is slightly higher than that of ions, owing to the attraction of the positive potential of tethers. Different tether distances (i.e., from 15 m to 85 m) are taken and show that a high potential bias voltage of tethers can slow down, cease, reflect and deflect a large number of ions, resulting in a plasma cavity in the vicinity of the tethers. An ion trap forms and captures many ions, owing to the interaction between the sheaths of the two conducting tethers. In general, a bias voltage of 1 kV produces a thrust of 30 nN/m with two tethers, on the assumption that the solar wind incomes normally. If we increase the distance between two conducting tethers, both trap captured ions and thrust show a first increase and then decrease trend. Furthermore, the investigations of the solar wind oblique incidence show that the thrust of the electric sail is determined by its attitude and is separated into force components in two directions:a horizontal force that is along the solar wind and a lift force that is perpendicular to the solar wind. We conclude that the present work first shows that the lift force is less than zero when the tether plane leans to the right, and greater than zero if the tether plane turns left. The increasing of the pitch angle leads to a variation of the thrust from -40° to 40°. The presented dependence of the thrust on the attitude of the tether plane provides an important reference for the optimal design of the orbit dynamics of the electric sail spacecraft.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2016, 65 (20): 205201. doi: 10.7498/aps.65.205201
Low density gas conical implosion can produce high density plasmas with a volume compression ratio of 106 to 109, and a temperature over 10 eV. As the temperature is limited by the plasma-wall interaction, to further increase the density and temperature and confine the plasma energy, we use a theta pinch at the top of the imploding cone. A conical magnetohydrodynamic simulation method is used to calculate the properties of the plasma, in which three cases, i.e., pure conical fluid compression, pure theta pinch, and synergic compression of fluid compressing and theta pinch, are calculated in a two-dimensional conical geometry. Simulation shows that synergic compression can improve the energy confinement and efficiently raise the temperature of the plasma. Different parameter combinations are calculated to find the optimum performance.
CONDENSED MATTER:STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
First-principles studies of multiple trapped impurity C by Ni vacancy and temperature effects in NiAl intermetallics
2016, 65 (20): 206101. doi: 10.7498/aps.65.206101
By using a first-principles pseudopotential method based on the density functional theory and Vienna ab initio Simulation Package (VASP), we investigate the multiple trapping of C by Ni vacancy (VNi) and its temperature effects in NiAl intermetallics. A single C atom is energetically and favorably situated at the Ni-rich octahedron interstitial site that surrounds Ni vacancy, which is shown via calculating the formation energy of C atom in NiAl with Ni vacancy system. Single C atom prefers to interact with neighboring Ni atom and Al atom to form a covalent bond. In NiAl intermetallics, C atoms prefer to be trapped in the Ni vacancy in the sequential way, thus easily forming the CnVNi (n=1, 2, 3, 4) clusters, in which the C4VNi clusters are most energetically favorable. It is interesting to find that when C atoms are trapped by Ni vacancy, all the C atoms themselves prefer to be combined with each other to form a bond, surrounding Ni vacancy. With the C atoms further added, both the charge density and the deformation charge prefer to bind with each other despite the Ni or Al environment and the intrinsic bonding properties of CC bond contain obvious covalent characteristics. Furthermore, using first-principles calculations combined with statistical model, we quantitatively predict point defect concentration as a function of temperature in NiAl intermetallics. It is concluded that the concentration of intrinsic Ni vacancies (VNi) will obviously increase as temperature increases. With the increase of temperature, the concentration of C atoms in the CnVNi cluster is higher than that at the intrinsic position. Besides, it indicates that most of C atoms in NiAl intermetallics are trapped by Ni vacancy, which is due to the larger binding energy of the CnVNi clusters and most of the C atoms are trapped directly by vacancies at room temperature or high temperature to form CnVNi clusters. Since the formation of CnVNi clusters is a process of heat releasing which will further increase the temperature of the NiAl system and produce more and more Ni vacancies, we can conclude that much more vacancies are created as a result of the presence of C impurity in NiAl intermetallics. However, the Ni vacancies exist in the form of CnVNi clusters from our calculation in a certain temperature range (less than 700 K). The existence of this kind of CnVNi cluster can effectively restrain the generations of cracks in the vacancies, which will produce some influences on the mechanical properties of NiAl intermetallic compound. Consequently, our results will provide a valuable reference for understanding the effects of C and vacancy on the mechanical properties of the NiAl intermetallics.
A simulation study of structural and optical properties in Cu ions implantation single-crystal rutile
2016, 65 (20): 206102. doi: 10.7498/aps.65.206102
TiO2 is a versatile functional material in consumer products, such as fabrication of solar cells, light hydrolysis of hydrogen production and optical coating. Technologically, the absorption edge of TiO2 is in the ultraviolet (UV) region, which restrics its applications. Cu doping can solve the crucial problem and extend the absorption edge from the UV to the visible region. The first-principle calculation based on density functional theory with generalized gradient approximation and ultra-soft pseudo-potentials is carried out to investigate the defective rutile TiO2 through using the constructed 222 supercells in which all atoms are allowed to relax. The plane-wave cutoff energy is 340 eV by selecting 223 of k-point in Brillouin zone. O vacancy, Ti vacancy, Cu interstitial, Cu substitutional for Ti and compound defects are all considered. After the structural relaxation, the lattice host is slightly distorted with a little change of the lattice parameters, with out affecting the crystalline phase of rutile. The results show that the valence bands are mostly O 2p states while the conduction bands have mainly Ti 3d properties. The defect of Cu interstitial can bring about two new impurity levels in the energy gap because of Cu 3d states, and the defect of Cu substituted for Ti can also induce two new impurity levels while they are next to the valence band due to the interaction between Cu 3d and nonbonding orbits of O 2p. Ti vacancy can cause the Fermi level energy to lower and produce a new impurity level at the top of the valence band, which will narrow the energy gap. O vacancy can enhance the Fermi level energy and produce a new level at the bottom of the conduction bands, which shows the n-type semiconductor properties. The higher the concentration of Cu substituted for Ti, the larger the band gap is. It is due to the strong interaction between Ti 3d and Cu 3d, which makes the conduction band move to higher energy. Different compound defects have different influences. Cu interstitial and O or Ti vacancies induce new impurity levels within the band gap, which narrows the gap. Meanwhile, interstitial Cu and vacancies can also interact with each other. The hybridization between Cu 3d and nonbonding orbits of O 2p will induce new levels in the rutile with Ti vacancy structure, while nonbonding orbits of Cu 3d develop new levels by itself in the rutile with O vacancy and Cu interstitial. The Analysis the band structure of rutile with compound defects, shows that the rutile with O vacancy and Cu interstitial effectively affects influenced the absorption edge in visible light range. Cu interstitial, Cu substituted for Ti, O vacancy, Ti vacancy and compound defects can all narrow the band gap and produce a new absorption peak in the visible spectral range. It indicates that rutile with defects will improve the absorption in the visible range and achieve the goal of expanding the absorption range of single-crystal rutile.
2016, 65 (20): 206103. doi: 10.7498/aps.65.206103
The new generation ESA SEU Monitor is first applied to beam verification of Beijing HI-13 Tandem accelerator according to the popularization need of Europe Space Agency and at the desire of contrast of domestic acceleraor with international accelerator. Heavy ion single event cross section of ESA SEU Monitor obtained at HI-13 is compared with those from European facilities. Beam homogeneity is also analyzed based on the SEU physical bitmap. The accuracy of heavy ion beam monitoring technique at HI-13 accelerator is verified. Through combining the heavy ion testing result with the data from the other test sites, it can be observed that the differences between SEU cross sections with different heavy ion energies of the same LET value can reach 1-3 orders of magnitude in the sub-threshold zone of single event upset cross section curve below the direct ionization LET threshold. The geometrical structure, critical charge and collection efficiency of sensitive volume are constructed on the basis of testing data and process information. The physical mechanism of energy effect on single event upsets in ESA SEU Monitor is revealed through using the Monte-Carlo calculation. Nuclear reactions between incident heavy ions and material atoms can account for single event upsets below the direct ionization LET threshold. The differences in nuclear reaction type and cross section between the diferent energy heavy ions and material atoms are the root cause of the difference among heavy ion SEU cross sections with different energies at the same LET value. On the other hand, SEU bitmap nonuniformity among different blocks in memory array and different orientation dices in ESA SEU Monitor is first reported when the heavy ion is incident at a tilting angle at the low LET value. The analysis of device layout and calculation verification can account for this phenomenon. The material of interlayer dielectric with the tilting ion passing through is different when heavy ion reaches the sensitive volume of memory blocks with different data. This leads to the difference between efficient LET values inside the sensitive volume for different data blocks. Eventually the sensitivity difference in single event upset among blocks with different data occures. The applications of ESA SEU Monitor in beam calibrating, tuning of domestic accelerator and single event effect test can be broadened further. Prediction method of space single event upset rate including heavy ion energy dependence and special angular dependence based on full-physical simulation should be developed in the future.
Theoretical studies of electronic, mechanical and thermal properties of Ti3(SnxAl1-x)C2 solid solutions
2016, 65 (20): 206201. doi: 10.7498/aps.65.206201
Available experimental and theoretical studies demonstrate that Ti3AlC2 and Ti3SnC2 compounds exhibit excellent mechanical properties at high temperatures,and thus are rendered a promising candidate of high-temperature structural materials.However,these compounds each have a relatively low hardness,Young's modulus,and poor oxidation resistance compared with other MAX phases.In order to overcome these limits,solid solutions on the M,A and/or X sites of the MAX phase compound are considered as a promising strategy to further improve the mechanical properties. Very recently,the solid solutions of Ti3(SnxAl1-x) C2 have been synthesized.However,no theoretical work has focused on the Ti3(SnxAl1-x) C2 solid solutions so far.Therefore,in this work,we perform first-principles calculation to study the microstructures,phase stabilities,electronic,mechanical and thermal properties of Ti3(SnxAl1-x) C2 solid solutions. Particularly,the effects of Sn concentration (x) on the properties are discussed for the Ti3(SnxAl1-x) C2 solid solutions by varying x from 0 to 1.0 in steps of 0.25.All the present ab initio calculations are carried out based on density-functional theory method as implemented in the Cambridge Serial Total Energy Package (CASTEP) code.The electron-ion interaction is described by Vanderbilt-type ultrasoft pseudo-potential with an exchange-correlation function in the generalized gradient approximation (GGA-PW91).The equilibrium crystal structure is fully optimized by independently modifying lattice parameters and internal atomic coordinates,and we employ the Broyden-Fletcher-Goldfarb-Shanno minimization scheme to minimize the total energy and inter-atomic forces.For the reciprocal-space integration,a Monkhorst-Pack grid of 16164 is used to sample the Brillouin-zones for Ti3AlC2 and Ti3SnC2 compound,and 882 for 221 supercell Ti3(SnxAl1-x) C2(x=0.25-0.75) compounds.The present calculated results of the enthalpy formation energy and mechanical stability criteria indicate that all the Ti3(SnxAl1-x) C2(x=0-1.0) solid solutions are thermodynamic and elastically stable.Moreover,mechanical properties (including bulk modulus B and shear modulus G),the ductile and brittle behavior and the anisotropic factors of Ti3(SnxAl1-x) C2 solid solutions are investigated,and the results indicate that all these compounds are identified as brittle materials and isotropic in nature.On the other hand,the MAX phases are good thermal materials due to their high thermal conductivities varying from 12 to 60 W/(mK) at room temperature.As for the thermal conductivity,it has become one of the most fundamental and important physical properties of the MAX phase material,especially for applications at elevated temperatures.Therefore,the lattice thermal conductivities,the minimum thermal conductivities and temperature dependences of the lattice thermal conductivity of Ti3(SnxAl1-x) C2 solid solutions are studied.Furthermore,Debye temperatures and melting points of the Ti3(SnxAl1-x) C2 compounds are also reported.Present results predict that each of all Ti3(SnxAl1-x) C2 compounds has a relative high Debye temperature and melting point,indicating that each of all Ti3(SnxAl1-x) C2 compounds possesses a rather stiff lattice and good thermal conductivity.
CONDENSED MATTER:ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2016, 65 (20): 207301. doi: 10.7498/aps.65.207301
Silicon carbide (SiC) is a wide band-gap, high-temperature-resistant, and radiation-resistant semiconducting material, which can be used as a radiation detector material in harsh environments such as high radiation background and high temperatures. Schottky barrier diode radiation detectors are fabricated using 100 upm-thick n-type 4H-SiC epitaxial layers for low energy -ray detection. The spectrum responses of 4H-SiC Schottky barrier detectors are investigated by irradiation of -ray from 241Am source. Schottky diodes are prepared by magnetron-sputtering 100 nm-thick nickel on epitaxial surface (Si face) to obtain Schottky contact and Ni/Au on substrate surface (C face) to obtain Ohmic back contact, respectively. Room temperature current-voltage (I-V) and capacitance-voltage (C-V) curves are measured to study the properties of Schottky diodes. Ohmic characteristic measurement shows that the Ohmic contact is formed after annealing in a temperature range of 900-1050℃, and the lowest specific contact resistivity of 2.5510-5 cm2 is obtained after annealing at 1050℃. The forward I-V curve reveals that the Schottky barrier height and the ideality factor are 1.617 eV and 1.127, respectively, indicating that the main current transportation process is the thermal electron emission. From the C-V curve, besides the net dopant concentration being inferred to be 2.9031014 cm-3, the profile of the free carrier concentration in epitaxial layer is also studied. A comparision of the reverse I-V curves of SiC Schottky diodes with different epitaxial layer thickness shows that the diode with 100 upm-thick epitaxial layer has a constant reverse leakage current when the bias voltage is less than 400 V, showing good rectification characteristics. By applying a reverse bias of 500 V, the diode has a leakage current of 2.11 nA, exhibiting a relatively high breakdown voltage. The depletion layer width of SiC detector is calculated to be 94.4 m at 500 V, indicating that the epitaxial layer is almost fully depleted. The signal of SiC detector through preamplifier displays a relatively low amplitude pulse (15 mV). A typical -ray spectrum response from SiC detector shows 9.49% (5.65 keV) energy resolution for 59.5 keV with a reverse bias of 300 V. The potential causes of poor count rate and energy resolution of fabricated detectors are analyzed in this article. The lower count rate is mainly caused by the narrow depletion layer, resulting in fewer photons deposited in sensitive region which can generate carriers. The poor energy resolution of SiC detector can be attributed to the electronic noise of read-out circuit, the pre-match amplifier circuit for detector needs to be improved, in addition, the extra defects existing in detector caused by increasing thickness of epitaxial layer can also deteriorate the detector performance.
2016, 65 (20): 207801. doi: 10.7498/aps.65.207801
In this paper, we propose one-step and two-step process under atmospheric pressure condition for synthesizing the CaAlSiN3:Eu2+ red phosphors by using nano-sized EuB6 and Eu2O3 as raw doping and activator materials. Moreover, the crystal structures, morphologies and luminescence properties of different-doped-Eu-concentration (2%-10%) samples are characterized in detail. According to energy dispersive spectrometer and X-ray diffraction (XRD) results, the cell volume and B content will gradually increase with the increase of the Eu concentration (2%-10%) for the sample prepared by two-step process. In contrast, the cell volume decreases with increasing the Eu concentration for the one-step prepared sample. Meanwhile, B content in the sample is less than that in the sample mentioned above and O content becomes larger. In addition, under the 460-nm blue light excitation, the two-step synthesized samples (nano EuB6 doped) has the highest emission peak in the 652-680 nm range, however, the sample by one-step synthesis (nano Eu2O3 doped) has strong emission peak only in the 630-637 nm range. Moreover, the intensity of fluorescence of the former one is stronger than that of the latter one. Both XRD and fluorescence spectra show that boron element can be introduced into the matrix by using two-step methods under atmospheric nitrogen. The introduction of boron not only reduces the oxygen content in the matrix but also changes the crystal field around Eu ions to adjust CaAlSiN3:Eu2+ phosphor luminescence peak position. Combining XRD and fluorescence spectral analysis, it is believed that boron element is introduced into the host by the two preparation methods of atmospheric nitrogen. The introduction of boron not only reduces the oxygen content in the matrix but also changes the crystal field environment of Eu2+ ions, and thus adjusting the luminescence peak position of Ca0.94AlSiN3:Eu2+ phosphor. Blue LED excitation of combined green-emitting phosphor and Ca0.94AlSiN3:0.06Eu2+ phosphor doped with nano EuB6 can yield white LED device with a color rendering index of 91 at a corresponding color temperature of 3364 K. This work has adopted a simple method to avoid expensive and complex pressure sintering equipment, and also reduces gas sintering equipment. Therefore, it is has a good prospective in industrial application and reducing the production cost.
2016, 65 (20): 207802. doi: 10.7498/aps.65.207802
The optical properties of nanoparticles and their array are closely related to their surface plasmon resonance of the particle and periodic structure parameters. In this paper, optical response features of single Ag nanosphere and periodical two-dimensional structure arrays are theoretically studied. The Mie theories and the multipole resonance theory are employed in the simulation. For Ag spheres each with a radius of less than 40 nm, one extinction peak can be observed and attributed to electric dipole resonance. When the radius of Ag sphere is more than 40 nm, apart from the peak contributed by the electric dipole, there is a peak of extinction at short wavelength, caused by resonance of the electric quadrupole. Generally, the frequency of multipole resonance decreases with increasing particle radius. The simulated results are in accord with the experimental data. For an infinite two-dimensional Ag-nanosphere arrays, two resonance peaks come from the dipole resonance of single particle and the Wood-Rayleigh anomalous diffraction. The frequency of multipole resonance can be controlled by tuning the size and the periodicity distribution of arrays. This paper provides a significant method to design advanced nanostructures with particular optical properties.
2016, 65 (20): 207501. doi: 10.7498/aps.65.207501
An anisotropic Heisenberg ferromagnetic spin chain model is studied by using Holstain-Primakoff representation. In the semiclassical limit, the exact solutions for bright and dark solitons are found by using the coherent-state method combined with the Holstein-Primakoff bosonic representation of spin operators. These results show that the solutions can be expressed in terms of the elliptic integrals in different parameter regions. Some solutions for dark solitons are the innovation points in this paper.