Vol. 65, No. 7 (2016)
2016, 65 (7): 070702. doi: 10.7498/aps.65.070702
Terahertz technology has been developed rapidly in the past 30 years. Numerous applications in medicine, biology, agriculture, materials, security, communication and astronomy have been demonstrated. Terahertz sources can be divided into narrowband (monochromatic) source and broadband source according to their spectral characteristics. From a spectral perspective, coherent broadband and narrowband terahertz sources are mutually complementary, each having its own characteristics and scope of applications. Broadband terahertz sources can be used for quick access to the hybrid spectra of rotational and vibrational molecular fingerprints or imaging in a wider spectral range. Narrowband terahertz source with good spectral resolution and sensitivity, is suitable for pump-probe, fine structure resolution of molecular fingerprints and terahertz remote detection and imaging. Therefore, developing the tunable high peak power and narrowband terahertz sources is very important for the applications in the detection and identification of molecular fingerprints. The difference frequency generation is one of the most important techniques for obtaining widely tunable, high power and narrowband terahertz sources. In this review, the recent progress of tunable terahertz sources based on the difference frequency generation in the last five years is reviewed, including the two fields of optical laser-based difference frequency sources and quantum cascade laser-based difference frequency sources. For the former class, the experimental results from reports with different difference frequency sources and several typical nonlinear crystals are classified, and the corresponding experimental techniques and results are introduced. For terahertz wave generation, different optical difference frequency sources by a dual-wavelength laser, double laser, a laser and an optical parametric oscillator (OPO), the signal and idler waves of an OPO, and double OPOs are demonstrated in increasing their tunabilities. Significant progress has been made in the nonlinear crystals used to generate terahertz wave by the difference frequency process, for example, by improving the property of inorganic crystals with ion doping, taking advantage of waveguide and PPLN structures, and especially developing novel nonlinear organic crystals. For the quantum cascade laser-based difference frequency sources, the latest advances in the techniques of difference frequency generation and wavelength tunability are presented. GaAs-based terahertz quantum cascade lasers are powerful semiconductor THz sources but cryogenic cooling is still a necessity. Recently, difference frequency generation was combined with the mid-infrared quantum cascade laser technology, thus becoming a leading room temperature semiconductor source in the terahertz range. To improve the frequency tuning range in the difference frequency terahertz quantum cascade laser, wavelength tuning techniques of the inner cavity and the external cavity have been developed. The difference frequency generation quantum cascade terahertz laser source has been the only technique workable at room temperature for the quantum cascade laser so far, which opens the door for developing the compact and widely tunable room temperature terahertz sources.
2016, 65 (7): 070201. doi: 10.7498/aps.65.070201
Light trapping has been considered as an important strategy to increase the conversion efficiency of silicon thin film solar cell. It shows that photonic crystal with feature size comparable to the wavelength, for example, the silicon nanowire array has a great potential to exceed the conventional Yablonovitch 4n2 limit. Silicon nanowire array has been designed and constructed on silicon thin film solar cell due to its excellent optical properties. Generally, silicon nanowire array is used as the antireflection coating, axial or radial p-n junction of solar cell. Different applications of the silicon nanowire arrays need different optical properties. Theoretical investigations show that the optical property is strongly dependent on the structural parameters. In this work, several structural parameters including period (P), diameter (D), height (H), and filling ratio (FR) are optimized when silicon nanowire array plays different roles. Here, by using the finite difference time domain (FDTD) method, we focus on the relations between the structural parameters and the optical properties including reflection and absorption from 300 to 1100 nm. In the FDTD simulation model, the substrate material is crystal silicon film, and the silicon nanowire array is on the surface of the substrate. In this calculation, the top and the bottom of the unit cell are air with perfectly matched layers, and with periodic boundary conditions at the side walls. When the silicon nanowire array is used as the antireflection coating, the silicon nanowire array shows a lowest reflection (7.9%) with H=1.5 m, P=300 nm, and FR=0.282. When silicon nanowire array acts as axial p-n junction solar cell (the p-n junction is formed by substrate and nanowire array), the absorption efficiency reaches a maximum value of 22.3% with H=1.5 m, P=500 nm, and FR=0.55. When the silicon nanowire array acts as the radial p-n junction solar cell, the absorption efficiency could obtain a maximum value of 32.4% with H=6 m, P=300 nm, FR=0.349. In addition, the optical properties of silicon nanowire array with random diameter and position are also analyzed here. The absorption efficiency of optimized random silicon nanowire array reaches 27.8% compared with a value of 19.9% from ordered silicon nanowire array. All of these results presented here can provide a theoretical support for the silicon thin film solar cell to increase the efficiency in the future application.
R-branch and Q-branch high rotational spectral lines of diatomic molecules using improved difference converging method
2016, 65 (7): 070202. doi: 10.7498/aps.65.070202
The accuracies of the predicted R-branch and Q-branch transitional lines of rovibrational diatomic systems for rotational states of J 100 are improved by using new analytical formulae and an improved difference converging method (DCM) in this study. The new formulae include the contributions from a higher-order energy term Hv. The improved DCM method includes a new physical converging criterion that is particularly useful in predicting unknown transitional lines. These improvements are used to study the transitional lines of the R-branch of the TiF and CO molecules and the Q-branch of the TiF molecule. The results show that the accuracies of the R-branch and Q-branch rotational lines are about one order of magnitude better than the results obtained using previous formulae; the new physical converging criterion can efficiently reduce the possible errors in the spectrum computations; the theoretical rotational lines obtained using the improved DCM method are much better than those obtained using the least-squares method.
2016, 65 (7): 070301. doi: 10.7498/aps.65.070301
The synthetic aperture radar imaging of fractal rough surface is studied. The natural surface can be very accurately described in terms of fractal geometry. Such a two-dimensional fractional Brownian motion (FBM) stochastic process provides a very sound description of natural surface. The samples of band-limited FBM process are realized by using physical Weierstrass-Mandelbrot function. The parameters of fractal rough surface are discussed and how to choose the value is analyzed. The roughness is mostly determined by the Hurst coefficient or the fractal dimension. In the actual simulation, a fractal rough surface can be seen as the superposition of finite sinusoidal tones, and any scattering measurement is limited to a finite set of scales. In this paper, the surface is described with two-scale model, i. e., locally approximated by plane facets with dimension smaller than that of resolution cells, but much larger than wavelength. Because this paper focuses on the texture of the synthetic aperture radar (SAR) image and the overall image texture is related to the macroscopic scale, the microscopic roughness superimposed on the facets is neglected. For the macroscopic scale scattering problem, a facet Kirchhoff approach is proposed. The fractal rough surface consists of many triangle facets, and the scatter field of each facet can be obtained by the facet Kirchhoff approach. The principle of dimension selection is studied. The dimension of facet must follow the principle that the surface profile is not damaged. At the same time, the facet dimension should be as large as possible in order to increase the efficiency of imaging. After establishing the fractal geometry model and obtaining the field from each facet, the SAR image can be realized through Rang-Doppler method in the stripmap mode. The results show that in the SAR image, the effects of fractal parameters on the rough surface can be obviously observed. The peaks and ravines of rough surface are obviously observed at low fractal dimension or high Hurst coefficient. However, when the fractal dimension gets higher or Hurst coefficient gets higher, the peaks and ravines disappear because the surface becomes rougher and diffuse scattering is enhanced. The effect of fractal parameter on the SAR image can be specifically expressed with entropy and angle second moment. With the increase of fractal dimension D, the texture of SAR image behaves more randomly and disorderly. So the entropy of SAR image becomes larger and the angle second moment of SAR image becomes smaller. The texture of SAR image is also related to the squint angle and frequency of incidence wave. The relative roughness will become larger when the squint angle and frequency of incidence wave become larger. The research on a complete fractal surface SAR imaging system consists of establishing the environmental model, developing the electromagnetic scattering model, and using the SAR imaging technique. The achievements show the characteristics of fractal rough surface SAR image, which have a theoretical support for natural environment remote sensing and the environment parameters inversion.
2016, 65 (7): 070501. doi: 10.7498/aps.65.070501
Weak signal detection is a vital technology in underwater acoustic communication with strong noise background. In this area, non-autonomous Duffing system is still widely used, and a lot of researches focus on enhancing the ability to detect weak signal and to find out the detection limitation of the Duffing system. Moreover, great achievements have already made. But problems still exist such as non-convergence of the periodic state of the Duffing system and its narrow detection domain. Unfortunately, researches on weak signal detection by using other systems are still rare. In order to solve the above problems, a new three-dimensional similar Liu chaotic system for weak signal detection is proposed. A thorough theoretical analysis for the similar Liu chaotic system is given, and its equilibrium point and the Lyapunov index are deduced and analyzed in detail. The major conclusion is that the variable x of the new system becomes a deformation signal when the input signal amplitude is greater than a certain critical value, the variables y and z converge to zero, and the Lyapunov exponents are less than zero at the same time. This means that no matter how strong the input signal is, the detection can be achieved by using a similar Liu chaotic system as long as its amplitude exceeds the threshold value. The periodic convergence and wide area detection of the similar Liu chaotic system are proved by the Matlab simulation, the Multisim circuit simulation, and the actual circuit test. This new system solves the two problems of the period convergence and narrow detection domain for the traditional Duffing system. The periodic state and chaotic state are easy to distinguish when detected. The periodic state can be maintained when the signal amplitude changes from short distance to long distance in a new system. The spectral signal-to-noise ratio range increases up to -46:57 dB in the similar Liu chaotic system. The characteristics of the new system are only effected by its structure and parameters. The system does not rely on the external factors, and it can be extended. By using some switching devices, the conversion between the chaotic state and periodic state can be realized in the practical engineering applications with a higher detection accuracy. The new design concept of the similar Liu chaotic system shows a very high practical value. It will lay a certain foundation for the underwater acoustic communication of the ocean internet of things in the future.
A chaotic signal detection method based on the component permutation of the incomplete two-dimensional phase-space
2016, 65 (7): 070502. doi: 10.7498/aps.65.070502
Detection and identification of chaotic signal is very important in the chaotic time series analysis. It is not easy to distinguish chaotic time series from stochastic processes since they share some similar natures. The detection methods to capture and utilize the structure of state-space dynamics can be very effective. In practice, it is very hard to obtain full information about the structure, and accurate phase-space reconstruction from scalar time series data is also a real challenge. However, the chaotic signals also show fundamental dynamical structure in the incomplete two-dimensional phase-space for the reason that they are generated by the deterministic chaotic systems or maps. Based on the fact that the distribution of chaotic signals is quite different from that of the noise signals in the incomplete two-dimensional phase-space, a novel detection method, which depends on the component permutation of the incomplete two-dimensional phase-space, is proposed. The incomplete two-dimensional phase-space is first obtained through the time series. Then, the first component is sorted in the ascending order, and the second component is permutated accordingly. The permutated component shows more structure characteristics for chaotic signals because of the relation between these two components. But this phenomenon does not appear in the noise because these components are independent of each other. And then, the permutated component is segmented into several groups properly. Finally, the sample mean and sample variance of different groups are calculated to obtain the sequence of sample mean (SSM) and the sequence of sample variance (SSV). Meanwhile, by calculating the variance of the SSM and the mean of the SSV, the test statistic is obtained. Furthermore, it is proved that this test statistic follows the F distribution under the null hypothesis of Gaussian noise. The proposed method is first adopted for detecting the several chaotic signals under different data lengths in Gaussian noise conditions. The simulation results show that the proposed method can detect chaotic signals effectively under low signal-to-noise ratio and it also has a good robustness against noise compared with the permutation entropy test. The time consumptions of the proposed method under different data lengths are evaluated and also compared with the results of permutation entropy test, showing that the proposed method can detect chaotic signals quickly, and the time complexity is much lower than that of the permutation entropy test. The theoretical analysis and simulation results demonstrate that the proposed method not only outperforms the permutation entropy test with lower complexity, but also has a better robustness against noise.
2016, 65 (7): 070701. doi: 10.7498/aps.65.070701
For a high speed mobile communication system, Doppler shift affects its performance seriously. In the future, broad band communication based on orthogonal frequency division multiplexing which depends on the orthogonality among sub-carriers will become popular. The absence of the orthogonality due to being destroyed by Doppler shift, leads to the failure of signal demodulation. So Doppler shift must be estimated and compensated for, which is the main purpose of previous work. On the other hand, many applications have shown that Doppler shift can be utilized to acquire the direction and speed or improve the quality of a signal. In this paper, we propose a method of not only estimating and compensating for Doppler shift, but also generating multiple non-frequency shifted signals, which can be regarded as the output of a virtual antenna array. As to the method, uniform phase sampling is the key algorithm. At first, the relation between uniform time sampling and uniform phase sampling is discussed in detail. This relation shows that the equivalence between uniform phase sampling and uniform time sampling is the necessary and sufficient condition for a non Doppler shifted signal. Next, the algorithm of Doppler shift compensation and virtualized antenna array is proposed, in which 1) original Doppler shifted signal is processed with interpolation, 2) new signals are generated by uniform phase sampling and buffered, 3) buffered new signals are read out by uniform time sampling. The theory of this process and the performance improvement for a high speed mobile communications system is mathematically analyzed, and the hardware architecture model of this algorithm is also given. The diversity gain could be obtained when an antenna array is used. In order to verify that this virtualized antenna array has the same benefit, the ability to suppress the interference and the bit error rate is analyzed with numerical simulation. The number of virtual elements and the virtual element distance are two variables related to the direction pattern of virtual antenna array. The effects of these two variables are given by the simulation, showing that the more virtual elements, the narrower beam are obtained. But more virtual elements result in more complicated hardware source. In addition, the communications scenarios of two communications radiators at different sites are simulated to verify whether this algorithm can suppress interference signal. The frequency spectrum of beamformed virtual antenna array signal shows that the interference signal can be suppressed effectively. These characteristics cannot be provided by pure Doppler frequency shift compensation. Thus these results show that high speed mobile communication systems on aircrafts or high speed trains would obtain better performances when a received Doppler shift signal is processed by this method to construct a virtual antenna array.
2016, 65 (7): 070703. doi: 10.7498/aps.65.070703
In order to achieve effective process control, the fast, inexpensive, nondestructive and accurate nanoscale feature measurements are extremely useful in high-volume nanomanufacturing. The optical scatterometry has currently become one of the important approaches for in-line metrology of geometrical parameters of nanostructures in high-volume nanomanufacturing due to its high throughput, low cost, and minimal sample damage. Conventional scatterometry techniques can only obtain the mean geometrical parameter values located in the illumination spot, but cannot acquire the microscopic variation of geometrical parameters less than the illumination region. In addition, conventional scatterometry techniques can only perform monospot test. Therefore, the sample stage must be scanned spot by spot in order to obtain the distribution of geometrical parameters in a large area. Consequently, the final test efficiency will be greatly reduced. Accordingly, in this paper, we combine conventional scatterometry with imaging techniques and adopt the Mueller matrix imaging ellipsometry (MMIE) for fast, large-scale and accurate nanostructure metrology. A spectroscopic Mueller matrix imaging ellipsometer is developed in our laboratory by substituting a complementary metal oxide semiconductor camera for the spectrometer in a previously developed dual rotating-compensator Mueller matrix ellipsometer and by placing a telecentric lens as an imaging lens in the polarization state analyzer arm of the ellipsometer. The light wavelengths in the developed imaging ellipsometer are scanned in a range of 400-700 nm by using a monochromator. The spectroscopic Mueller matrix imaging ellipsometer is then used for measuring a typical Si grating template used in nanoimprint lithography. The measurement results indicate that the developed instrument has a measurement accuracy of better than 0.05 for all the Mueller matrix elements in both the whole image and the whole spectral range. The three-dimensional microscopic maps of geometrical parameters of the Si grating template over a large area with pixel-sized lateral resolution are then reconstructed from the collected spectral imaging Mueller matrices by solving an inverse diffraction problem. The MMIE-measured results that are extracted from Mueller matrix spectra collected by a single pixel of the camera are in good agreement with those measured by a scanning electron microscope and the conventional Mueller matrix ellipsometer. The MMIE that combines the great power of conventional Mueller matrix ellipsometry with the high spatial resolution of optical microscopy is thus expected to be a powerful tool for large-scale nanostructure metrology in future high-volume nanomanufacturing.
2016, 65 (7): 070704. doi: 10.7498/aps.65.070704
In order to measure the oil pollution on water surface, a fluorescence lidar model system based on laser induced fluorescence is put forward for detecting oil slick. The system model and fluorescence detecting principle are described in detail. According to the properties of detected material, wavelength of laser and filter of receiving system are adopted to ensure that the lidar system is operated at the peak wavelength. Following the development trend of miniaturization in the world, using single laser and intensified charge-coupled devices, a small fluorescence detecting system is designed. FTSS 350-50 laser made by CRYLAS company, with compact dimension, low weight and excellent energy efficiency, and PI-MAX4 intensified charge-couple devices made by Princeton Instruments company, with good time resolution characteristic, are selected to produce laser as a launch device and to inspect fluorescence lifetime and capture image as a receiving device, respectively. The laser excitation wavelength, the energy of laser, the center wavelength and bandwidth of filter, the received echo fluorescence signals, the detected concentration and distance are discussed in detail by means of the instance for oil on water surface. Through analyzing the relationship between the energy of laser single pulse and the detection concentration and by combining with the parameters of fluorescence lidar system and fluorescence lidar equation, the detecting ability of system model, signal-to-noise ratio, etc. are simulated particularly. A numerical simulation of the signal-to-noise ratio of the fluorescence particles is conducted particularly so that the detectable capacity of system designed could be described better. The results show that the signal-noise ratio of system which is operated during the night is superior to in daytime in the same single pulse energy case and that the detected range becomes gradually longer as the energy of laser improves with the same signal-noise ratio case. The required single pulse energy to support system is calculated, and further verifies the feasibility of the lidar system. The test results of the sample show that in the daytime, the design of fluorescence lidar model, with a Nd:YAG laser of 50 J single pulse energy and 355 nm wavelength serving as an excitation light source, with a collection device placed at a distance of 7 m, can satisfy the requirements for detecting oil pollution on the water surface in laboratory, and its signal-noise ratio can reach 10. In view of the actual surface fluorescence lidar detection requirements, the method of increasing the laser power is proposed. A real system with 50 mJ single pulse energy at a distance of 230 m has nearly the same performance as the laboratory lidar system, which could provide a valuable guidance for designing a real system.
2016, 65 (7): 070705. doi: 10.7498/aps.65.070705
In the paper, using the one-seed method and multiseed method separately, the hexahedral type-Ib diamonds are synthesized in a cubic anvil under high pressure and high temperature. This cubic anvil is of 550 mm hydraulic cylinder with the sample chambers of 14 mm or 26 mm in diameter under 5.6 GPa and 1200-1400 ℃. The FeNiMnCo alloy is chosen as catalyst. The high-quality abrasive diamonds each with a diameter of 0.9 mm are used as seed crystals. High purity-graphite powder (99.99%, purity) is selected as the carbon source. The effects of cavity size on the growth of hexahedral type-Ib Gem-diamond single crystal are studied carefully. The Relationship between oil pressure and synthesis pressure is obtained in our studies. When the pressure is transmitted the same distance, in the catalyst melt, the pressure loss is less than in the pressure transmitting medium. By expanding synthesis cavity size, the pressure transmission efficiency of the oil pressure increases significantly, which can be attributed to the transmission distance shortening in the pressure transmitting medium and transmission distance lengthening in the catalyst melt. Using the 14 mm synthesis cavity, by the one-seed method, the 5 mm grade diamond single crystals of cubo-octahedral shape are synthesized, but the 5 mm grade diamond single crystals of perfectly hexahedral shape could not be synthesized. Choosing the 14 mm synthesis cavity, by the five-seed method, the 3 mm grade diamond single crystals in the center each present a perfectly hexahedral shape, but each outside of the crystals exhibits a cubo-octahedral shape. According to the application requirement for the type-Ib hexahedral diamond single crystal with a size of 3.0-3.5 mm on an industrial diamond single crystal tool, the diamond single crystals of perfect hexahedral shape are synthesized by the multiseed method. Using the 26 mm synthesis cavity, many 3 mm grade diamond single crystals of perfectly hexahedral shape are synthesized in one synthesis cavity. In our studies, up to 14 diamond single crystals of perfect hexahedral shape are synthesized in one synthesis cavity by the multiseed method. We find that the uniformity of temperature field of the 26 mm synthesis cavity is better than that of the 14 mm synthesis cavity, so the 26 mm synthesis cavity is suitable for growing 3 mm grade diamond single crystals of perfect hexahedral shape by the multiseed method. In 35 h growth time, the overall growth rate of the 26 mm synthesis cavity (25.2 mg/h) synthesizing 14 diamonds in one time (9.4 mg/h) is 2.68 times that of the 14 mm synthesis cavity by five-seed method. Moreover, the Raman spectra of the synthesized high-quality hexahedral type-Ib diamond single crystals and natural diamond single crystal indicate that the structure and quality of the synthesized high-quality diamond single crystal is better than that of a natural diamond.
ATOMIC AND MOLECULAR PHYSICS
Orientation and concentration of Ag conductive filament in HfO2-based resistive random access memory: first-principles study
2016, 65 (7): 073101. doi: 10.7498/aps.65.073101
HfO2-based resistive random access memory takes advantage of metal dopants defects in its principle of operation. Then, it is significantly important to study the performance of metal dopants in the formation of conductive filament. Except for the effects of the applied voltage, the orientation and concentration mechanism of the Ag dopants are investigated based on the first principle. First, five possible models of Ag in HfO2 are established in , , , [-111] and  directions, in each of which adequate and equal dopants of Ag are ensured. The isosurface plots of partial charge density, formation energy, highest isosurface value and migration barrier of Ag dopants are calculated and compared to investigate the promising formation direction of Ag in the five established orientation systems. The formations of conductive filament are observed in , ,  and [-111] directions in the unit cell structure from the isosurface plots of partial charge density. But no filament is formed in  direction. And the highest isosurface value of Ag dopant is largest in [-111] direction. This indicates that the most favorable conductive filament formation takes place in this direction. The formation energy of Ag in the different direction is different, and the values in [-111] and  direction are minimum and close to each other, which shows that it is easy to form conductive filaments in these two directions. In addition, the smallest migration barrier of Ag in [-111] direction reveals that the [-111] orientation is the optimal conductive path of Ag in HfO2, which will effectively influence the SET voltage, formation voltage and the ON/OFF ratio of the device. Next, based on the results of orientation dependence, four different concentration models (HfAgxO2, x=2, 3, 4, 5) are established along the [-111] crystal orientation. The isosurface plots of partial charge density about those concentration models are compared, showing that the resistive switching phenomenon cannot be observed for the samples deposited in a mixture with less than 4.00 at.% of Ag content (HfAg4O2). The RS behavior is improved with Ag content increasing from 4.00 at. % to 4.95 at.%. However, the formation energy and highest isosurface value are calculated and it is found that the conductive filaments cannot be switched into a stable state when Ag content becomes greater than 4.00 at.%. Then, the total electron density of states and the projected electron density of states are also calculated for the two models. It indirectly shows that the conductive filament is mainly comprised of Ag atoms, rather than Hf atoms or oxygen vacancy. Also, it is not helpful to improve the ON/OFF ratio of the device when the Ag dopant concentration is higher than 4.00 at.%. Therefore, the best doping concentration of Ag is 4.00 at.% and it is more advantageous to change the resistance memory storage features. This work may provide a theoretical guidance for improving the performances of HfO2-based resistive random access memory.
Electronic transport properties of oligo phenylene ethynylene molecule modified by the (CH3)2 and (NH2)2 groups
2016, 65 (7): 073102. doi: 10.7498/aps.65.073102
The modification effects of the groups (CH3)2 and (NH2)2 on the oligo phenylene ethynylene (OPE) molecules with single and double S atoms connected to the two electrodes are investigated by the density functional theory and non-equilibrium Green function. The modified OPE molecule is optimized and used to build a two-probe system with Au electrodes. Then the two-probe system is fully relaxed to obtain a stable structure. The electronic transport properties of the two-probe system are also calculated with the non-equilibrium Green function method. The calculation results show that both the modified groups and the bridge atoms can lead to the switch effect, the negative differential resistance behavior, and the rectifying behavior for the two-probe system. When molecules are connected with single S atom at one end, both the (NH2)2-OPE and the (CH3)2-OPE molecules show the rectifying behavior. However, the (NH2)2-OPE also shows a switch effect at larger voltage because there is current when the negative bias is over 1.3 V, while the (CH3)2-OPE molecule demonstrates a complete rectifying behavior because it is hardly conductive in the whole positive bias area. The current of OPE molecule without modification group always increases with the increase of voltage. Therefore, it is only a resistance. These results are different from that of NO2-OPE-NH2 molecule which shows a negative differential resistance behavior. For the case of the molecule connected with S atoms at both ends, the (NH2)2-OPE(S) and (CH3)2-OPE(S) models show negative differential resistance behaviors in the negative bias range. It is found that both (NH2)2-OPE and (CH3)2-OPE molecules demonstrate the negative differential resistance behaviors when they are connected with S atoms bridge at both ends. However, the current of the molecule with one S atom at one end is about two-order magnitude lower than that of the molecule with S atoms at both ends. It is shown that S atom acting as a bridge can significantly affect the characteristic of current-voltage. The mechanisms for the various characteristics of the electronic transport properties are explored by analyzing the projection orbit electron distribution, the transmission spectrum, and the density of states under the different bias voltages. For (NH2)2-OPE molecule with a single S atom at one end in the negative bias range, only the lowest unoccupied molecular orbital (LUMO) can transfer electron with low bias, but both highest occupied molecular orbital (HOMO) and LUMO can be conductive with high bias, which results in the switch effect. In the positive bias range, both HOMO and LUMO cannot be conductive with low bias, while LUMO can be conductive with high bias, which results in the switch behavior. For the case of (NH2)2-OPE molecule with one S atom at each end, only the HOMO can transfer electron in the low bias range, while the LUMO can be conductive at high positive bias, but both HOMO and LUMO cannot be conductive in high negative bias range, which leads to the non-symmetric negative differential resistance effect in the whole bias range. A similar analysis of the difference between HOMO and LUMO can be used to understand the characteristics of the current-voltage of (CH3)2-OPE. Considering the fact that the different modification groups can lead to various current-voltage properties of OPE molecule, the modified OPE molecule is a promising candidate for designing molecule device.
2016, 65 (7): 073103. doi: 10.7498/aps.65.073103
Hyperfine-structure (HFS) of atoms results from the interactions between the nuclear magnetic dipole moment and the magnetic field generated by the electrons (related to the magnetic dipole hyperfine constant Ahfs), and between the nuclear electric quadrupole moment and the electric field gradient due to the distribution of charge within atoms (related to the electric quadrupole hyperfine constant Bhfs), so the accurate measurement of HFS is of interest in many fields, including atomic parity nonconservation, tests of fundamental physics, electron-nucleus interaction, and high resolution spectrum and so on. Generally, in order to obtain the atomic spectra, the frequency of laser needs to be scanned over the hyperfine transitions of atoms, so the nonlinear effect from the laser frequency scanning often limits the measurement accuracy of hyperfine splitting. In this paper, we solve this problem, and demonstrate a novel method to measure the hyperfine splitting of atoms. Taking cesium (Cs) for example, based on the Cs 6S1/2-6P3/2-7S1/2 (852.3 nm + 1469.9 nm) ladder-type atomic system, three sets of optical-optical double resonance (OODR) spectra are obtained in a room-temperature vapor cell, when the 852.3 nm laser is tuned to the 6S1/2 (F=4)-6P3/2 (F'=4) resonant transition, and the carriers of 1469.9 nm probe laser accompanied with1 sidebands from a phase-type electro-optical modulator (EOM) are scanned over the whole 6P3/2-7S1/2 hyperfine transitions. Owing to the Doppler effect, some of the hyperfine transitions in these three sets of OODR spectra overlap with the narrowest linewidth only when the frequency of the signal driving EOM equals the value of hyperfine splitting 7S1/2 state. Using this phenomenon which can effectively avoid the nonlinear influence on the measurement during the frequency scanning process of 1469.9 nm laser, we measure the hyperfine splitting of 7S1/2 state to be (2183.720.23) MHz, and the magnetic dipole hyperfine constant Ahfs to be (545.930.06) MHz, which are consistent with previously reported experimental results. This technique provides a robust and simple method of measuring hyperfine splitting with a high precision, which is significant to provide the useful information about atomic structure for developing a more accurate theoretical model describing the interaction within an atom.
Design and optimization of all-fiber rotational Raman spectroscope for temperature measurement based on sampled fiber Bragg grating
2016, 65 (7): 073601. doi: 10.7498/aps.65.073601
Atmospheric temperature is a key parameter to characterize the state of the atmosphere. Owing to the independence of the aerosol effect for profiling the temperture, the pure rotational Raman lidar has become one of valid tools. To achieve all-time and high-precision active remote sensing, strong background noise needs to be filtered out, and the inhibition rate outside the band of more than 70 dB is needed for Mie-Rayleigh scattering in a rotational Raman temperature measurement lidar. In this paper, a multiple cascaded light path based on sampled fiber Bragg grating (SFBG) and fiber Bragg grating (FBG) in visible spectrum is presented to obtain characteristic spectrum. All-fiber spectroscopic system with high inhibition rate for Raman thermometry is set up based on the above light path. The core device consists of single mode fibers (460-HP) to ensure the compatibility with optical fiber. The main factors affecting the inhibition rate outside the band of sampled fiber Bragg grating, including refractive index modulation depth, total length of grating, sampling period and duty, are optimally designed by using mode coupling theory and tranmission matrix model. Then the optimized parameters of spectroscope are obtained. The results show that the inhibition rate outside the band is proportional to the refractive index modulation depth and duty, when the total length of grating is a constant. However, a larger sidelobe jamming will be caused by overlarge refractive index modulation depth. The less amount and widened full width half maximun of reflectivity peak appear following overlarge duty. In the Raman spectroscopic system of this paper, the inhibition rates outside the bands of SFBG and FBG are 30 dB and 20 dB, respectively. The inhibition rate of more than 70 dB is realized for Mie-Rayleigh scattering, after passing through two FBGs and one SFBG. The simulated optimum parameters of SFBGs are the effective index of the guide mode of 1.465, the saturation index variation of 0.00005, the SFBG length of 20 mm, the sampled period of 0.4 mm, and the Bragg wavelengths of 528.51 nm and 530.76 nm. By using the American standard model and atmospheric scattering signal model, the all-time signal-to-noise ratio (SNR) and inhibition rate of Mie-Rayleigh scattering and solar background light are simulated and analyzed. The results show that the intensities of solar background light and Mie-Rayleigh scattering signal are weaker than Raman scattering signals at 40 dB and 50 dB, respectively. The detection height in daytime and night can reach up to 1.6 km and 2.6 km under the condition of SNR of more than 100, respectively. Owing to these advantages such as miniaturization, anti-interference and high stability, this spectroscope provides a viable solution for filter systems of ground-based and spaceborne lidars.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2016, 65 (7): 074101. doi: 10.7498/aps.65.074101
For potential applications of metasurfaces in lens technologies, we propose a cross circularly polarized focusing metasurface which is capable of transforming a circularly polarized wave into cross-polarized wave and simultaneously focusing electromagnetic wave. A helicity-dependent phase change is introduced into the transmission metasurface cell, which is a single layer with a thickness of 1.5 mm and can be engineered by assembling along the spatial orientation of each Pancharatnam-Berry phase element. The phase change of the Pancharatnam-Berry phase element is analyzed theoretically, and the efficiency of the designed element is simulated under the irradiation of differently polarized waves. A phase gradient metasurface with a phase difference of 60 between neighbouring cells is designed. When simulated in CST Microwave Studio, the gradient metasurface is observed to have a ability to refract right-hand circularly polarized waves in +x direction and left-hand circularly polarized waves in -x direction but with an identical refraction angle of 33.8, which is in good accordance with the angle calculated from the general refraction law. Then we design a focusing metasurface with a size of 90 mm90 mm and 1515 cells. When the focusing metasurface lens is irradiated by left-hand circularly polarized wave, the refracted right-hand circularly polarized wave is focused at a point 40 mm away from the lens center. However, when the metasurface lens is impinged by the right-hand circularly polarized wave, the refracted left-hand circularly polarized wave is diffracted. This ultimately accords with different phase responses under different polarized waves when the metasurface cell is rotated. Furthermore, the metasurface lens diffracts the incident wave when impinged by right-hand circularly polarized wave, which validates the design principle. The beam-width at the focal spot and the focal depth are also calculated. The simulation results indicate that the beam-width at the focal spot is approximately equal to three quarters of the operating wavelength. Therefore, the circularly polarized wave refraction focusing metasurface has a good performance for focusing the refracted waves. In addition, the proposed focusing metasurface is simulated separately at f=14 GHz and f=16 GHz, and the results show a good focusing effect, which demonstrates the bandwidth characteristic of the focusing metasurface lens. This designed metasurface lens is thin, single-layered, and highly effective, and it is also convenient to fabricate. Moreover, the metasurface lens has an advantage over the conventional lens, which has potential applications in manipulating electromagnetic waves and improves the performance of lens.
Reflection phase characteristics and their applications based on one-dimensional coupled-cavity photonic crystals with gradually changed thickness ofsurface layer
2016, 65 (7): 074201. doi: 10.7498/aps.65.074201
In this paper, we first improve the traditional transfer matrix method to adapt to one-dimensional photonic crystal consisting of arbitrary materials, and then use it to study the reflection phase characteristics of two kinds of photonic crystals, i.e., a simple periodic photonic crystal structure and a coupled-cavity asymmetric photonic crystal with gradually changed thickness of surface layer. For both of the structures, the reflectivity within photonic band gap is above 98% and hardly affected by the thickness of the surface layer. However, their reflection phases exhibit distinctly different properties. For the simple photonic crystal structure, the reflection phases of both TE and TM polarizations are sensitively dependent on the thickness of surface layer, but their phase difference is almost the same as the thickness of surface layer varies, which cannot change the polarization of reflected light. While for the coupled-cavity asymmetric photonic crystal structure, studies show that the degenerate defect modes within photonic band gap will split as the thickness of the surface layer varies. Moreover, around the splitting defect modes the reflection phases of both TE and TM polarizations, as well as their phase difference, are sensitively dependent on the thickness of surface layer, resulting in sensitive polarization change of reflected light. The physical reason is attributed to the dramatic phase change caused by the splitting of degenerate defect modes. The above reflection phase characteristics of coupled-cavity asymmetric photonic crystals have potential in lowering or even eliminating the coherence of lasers in some special application cases. As an example, we design a one-dimensional photonic crystal structure with two-dimensional periodic varying thickness of surface layer. After an oblique incident narrowband laser beam is reflected from this structure and then focused by a lens, various polarized light beams (including linear polarized light beams along different directions, left-hand (or right-hand) circular (or elliptical) polarized light beams) will exist simultaneously, whose superposition will produce optical field with random phase and polarizations in the focal region. These results can effectively reduce the coherence of lasers, which holds promise in many fields such as laser nuclear fusion.
Double resonant sum-frequency generation in an external-cavity under high-efficiency frequency conversion
2016, 65 (7): 074202. doi: 10.7498/aps.65.074202
In recent years, more than 90% of the signal laser power can be up-converted based on the high-efficiency double resonant external cavity sum-frequency generation (SFG), especially when the whole system runs under the undepleted pump approximation scheme. Therefore it is difficult to directly achieve an error signal with a high signal-to-noise ratio through the signal laser to lock its frequency to the cavity mode. In this paper a novel method, based on the frequency modulation of signal laser and demodulation of the SFG laser, is used to obtain the error signal to realize the cascade frequency locking between the two fundamental lasers and the external cavity. In this experiment, 1064 nm laser is the pump laser and 1583 nm laser is the signal laser. They are coupled into a ring cavity inside which a 5% MgO-doped PPLN (25 mm1 mm0.5 mm) is used to produce the SFG laser of 636 nm. When the pump laser is resonant with the external cavity, a circulating power of 14.3 W is obtained with its input power of 1.3 W. The reflectivity of the input coupling mirror of signal laser is 10% to restrain the impendence mismatch. The temperature of PPLN is set at 68.5 ℃ to reach the optimum SFG temperature. In order to keep the signal laser resonance inside the external cavity, one needs to lock its frequency to the cavity mode. A 28.5 kHz sinusoidal voltage is used to modulate the frequency of the signal laser so that the frequency of 636 nm laser is modulated simultaneously. Then 5% of the output 636 nm laser power is sent into a Si photodiode detector the signal of which is demodulated at the modulation frequency by a lock-in amplifier. Finally the demodulated signal is feedback to the frequency control port of signal laser. Under these conditions, 73% of 1583 nm signal laser power can be converted into 636 nm laser power when the incident power varies from 10 W to demodulation of the transmitted cavity mode of 1583 nm when the incident signal laser power is below 12 mW. When the signal laser power increases from 50 mW to 295 mW, the conversion efficiency linearly drops to 60%, which is mainly caused by depleting the 1064 nm pump laser power. Finally a 440 mW of 636 nm laser is generated with an incident signal laser power of 295 mW. This scheme can realize a high-efficiency SFG with a low input signal laser power or poor single-pass SFG efficiency.
2016, 65 (7): 074203. doi: 10.7498/aps.65.074203
The mechanism of eliminating the aerosol haze particles by destroying the force balanced system with laser gradient force is proposed in terms of the force balanced system exerted on the haze particles in the atmosphere, namely, rotary lift force balances gravity, and haze particles are balanced by repulsion between particles and particles. First of all, nonlinear equations about forces exerted on haze particles are obtained according to the Newton's second law, and these main forces (air drag force, van der Waals repulsion, rotary lift force) on the haze particles are solved by integrating these nonlinear equations by the Runge-Kutta method. The facts that the concentrations of particles with diameters in a range from 0.5 m to 0.835 $m increased obviously in the haze particles test on December 17-25, 2013 and February 20-26, 2014 in Xi'an are verified. Secondly, the laser gradient force is analyzed and calculated in the homogeneous medium of haze particles. It is found that the system balanced by the force exerted on haze particles is destroyed completely by laser gradient force, because the magnitude of laser gradient force is always larger than any other forces exerted on haze particles. Therefore, it is feasible to eliminate these haze particles by laser gradient force. It is undoubted that the proposed way of eliminating haze particles is of great significance to establish harmonious surroundings for human.
2016, 65 (7): 074204. doi: 10.7498/aps.65.074204
By using nonlinear Schrdinger equation including Raman gain and self-steepening but ignoring fiber loss situation, the propagation characteristics of Airy pulse are simulated and analyzed in the single-mode fiber. Simulations show that Airy pulse can be converted into soliton and its propagation direction is skewed due to the effects of Raman gain and self-steepening under a certain condition. In time domain, the number of small peaks of Airy pulse reduces rapidly. Airy pulse becomes a peak structure containing a main peak and sub-peaks where the energies can be ignored by changing the coefficient a reasonablely, which is approximated as the soliton structure. Therefore, Airy pulse is regarded as transforming into soliton. Meanwhile, in the case of small values b, there exists a significant difference in shape between Airy pulse and soliton. With the value of parameter b increasing slowly, the shape of Airy pulse is very close to soliton's, therefore Airy pulse can transform into soliton by changing value b reasonablely. Compared with by changing b value, Airy pulse convered into the soliton is stable by changing the a value reasonablely. Simultaneously, with the increases of values of coefficient a and amplitude b, the time-shift of Airy pulse increases. However, the time-shift of Airy pulse would decrease when Raman gain and Self-steepening become strong, no matter what the values of a and b are. Further, the acceleration properties of Airy pulse are investigated. It is found that Airy pulse autoacceleration is not a stable value at the beginning but it gradually stabilizes with the increase of transmission distance. The reason is that the energies of secondary peaks exert a tremendous influence on the main lobe of Airy pulse at the beginning, however, secondary peaks diffuse fast with the increase of transmission and then the influence can be ignored to a certain extent. So, the main peak gradually stabilizes with the increase of transmission distance.
Theoretical and experimental study of atmospheric turbulence measurement using two-aperture differential scintillation method
2016, 65 (7): 074205. doi: 10.7498/aps.65.074205
We report the basic theory and first horizontal results of a method called two-aperture differential scintillation method which is aimed at monitoring the vertical profile of atmospheric optical turbulence strength. The method is based on irradiance fluctuation of active light source, but can extract the optical turbulence information in the single-passage path. In this paper, the theoretical principle of two-aperture differential scintillation method is derived in detail. A concise expression is proposed for irradiance fluctuation structure function with differential aperture in the Rytov approximation under a weak fluctuation regime based on the cross-path theory. The mathematic relationship between irradiance fluctuation structure function and atmospheric optical turbulence strength is then developed. The effects of beacon aperture and beacon altitude on path weighting function of this method are analyzed for Kolmogorov turbulence. In order to test the validity of the new method, the experiments are conducted to compare the two-aperture differential scintillation method and single-aperture scintillation method in atmospheric boundary layer over 2 km horizontal single-passage path. In this arrangement, we employ a differential image motion monitor system to measure differential scintillation. Simultaneously, a large aperture scintillation instrument is placed 5 m away at the same altitude to measure the single-aperture scintillation. It is shown that the results of atmospheric refractive index structure constant deduced from the two methods are in good agreement. The measurements of atmospheric coherence length for spherical wave corresponding to the two methods indicate a linear correction factor (R2) of 0.96, in a slope of 0.98 with an offset of -0.09 cm. Feasibility and effectiveness of two-aperture differential scintillation method are thus verified experimentally. The novel method can separate single-passage scintillation information of active beacon double-passage propagation, thereby providing an accurate technique for measuring the atmospheric turbulence of active beacon.
2016, 65 (7): 074206. doi: 10.7498/aps.65.074206
In this article, a high-quality fiber optical turbulence sensing array is arranged on the ocean-going instrumentation ship-Yuan Wang for 37 day continuous measurement. It is the first time that the spatial multipoint simultaneous measurement of maritime atmospheric optical turbulence has been realized. According to the measurement, the essential characteristics and quantitative data of the atmospheric optical turbulence on the open sea are obtained. Then the Greenwood spatial correlation function model is used for evaluating the outer scale of optical turbulence by a nonlinear fitting algorithm. Finally, the optical turbulence spectrum is divided into two sections, and the power spectrum scaling exponent is given by a section algorithm. Results show that the intensity of maritime atmospheric optical turbulence is a little smaller than that in the near ground layer and has no obvious diurnal variation characteristic. The value of outer scale is small and in a range of 0.2-0.3 m. It is easily found that the probability that the turbulence spectrum scaling exponent conforms to -5/3 is about 25% near the surface of open sea, which is smaller than that in the near ground layer.
Influence of laser linewidth on the conversion efficiency of single-pass frequency doubling with a PPMgO: LN crystal
2016, 65 (7): 074207. doi: 10.7498/aps.65.074207
We report on the generation of 2.06 W of tunable cw light at 780 nm by a single-pass frequency doubling in a PPMgO:LN crystal with a seeded high-power fiber amplifier. A 1560 nm distributed feedback (DFB) diode laser, a Littman-type grating external-cavity diode laser (ECDL) and a DFB-type erbium-doped fiber laser (DFB-EDFL) were separately used as a seeding laser source of an erbium-doped fiber amplifier (EDFA). Here we use the EDFA which has a narrow linewidth option, the fundamental frequency light will not be obviously broadened. The influence of laser linewidth on the conversion efficiency of frequency doubling is investigated. The injection power and output power of EDFA must be consistent. So when temperature of the PPMgO:LN crystal is fixed, the conversion efficiency for different seeding resources can be obtained as follows. When the fundamental power is 12.42 W, using the DFB as seeding resource yields 1.36 W of 780 nm doubling output, and the corresponding conversion efficiency is 11.0%. Using the ECDL as seeding source yields 1.78 W of 780 nm doubling output, and the corresponding conversion efficiency is 14.3%. While using the DFB-EDFL as seeding source yields 2.06 W of 780 nm doubling output, and the conversion efficiency is 16.6%. The measured laser linewidths of the three seeding sources are 1.2 MHz, 200 kHz, and 600 Hz for the DFB, ECDL, and DFB-EDFL, respectively. The experimental results show that the narrower laser linewidth, the higher doubling efficiency, and the experimental results agree with our theoretical analysis.
2016, 65 (7): 074208. doi: 10.7498/aps.65.074208
Plasma filling can significantly improve the efficiency and power of a vacuum device. In this paper, we first analyze the dispersion properties of a plasma-filled metal-photonic-crystal slow-wave structure (SWS), and then investigate the interaction procedure between a relativistic electron beam and the Cherenkov radiation in the plasma-filled metallic-photonic-crystal by the particle in cell method. We pay our attention to the influences of plasma density, cathode voltage, and guiding magnetic field on output frequency and power. The results show that the electric field strength in the SWS increases obviously at a fixed plasma density of 50 mTorr (1m mTorr=0.133 Pa). The device works at a stable single TM01 mode due to the good mode properties of the metal photonic crystal even if plasma is filled in it. The maximum value of Ez field along the z axis of the device increases from 46.34 MV/m without plasma to 79 MV/m with plasma. The value along the x axis increases from 136 MV/m without plasma to 185 MV/m with plasma. The working frequency (35.5 GHz) of the device, obtained from simulation, is consistent with the theoretical estimation (35.4 GHz). The power increases with the cathode voltage between 500 kV and 600 kV while the frequency increases only a little. When the magnetic field B increases, the output power first increases and then decreases. But the frequency is not affected due to the dispersion property. The output power of the device increases 20% when the air pressure increases from 0 to 100 mTorr. However, there is a pretty distribution of the field Ez along the angular direction only in an appropriate plasma density around 50 mTorr. According to the theory and simulation, the output power and efficiency can be improved in an appropriate range of plasma density. These results provide a basis for developing the plasma-filled vacuum devices.
2016, 65 (7): 074209. doi: 10.7498/aps.65.074209
In recent years, metamaterials (MMs) have been widely investigated for their exotic electromagnetic characteristics which cannot be achieved in nature. However, one of the main limitations in traditional metallic-film MMs is a high level of radiation loss in metal and insertion loss of the high-permittivity or thick substrate. Fortunately, all-dielectric MMs with high refractive-index dielectric structures show significantly less material loss than their metallic counterparts. In this paper, an all-dielectric grating is fabricated on a 100-m-thick silicon wafer by using direct-laser-writing technique, and the properties of its Mie resonances are investigated by THz time-domain spectroscopy. Then we measure the spectral response of the all-dielectric grating under the optical modulation by a near-infrared pump-THz probe method. The modulation light source is an 808 nm continuous semiconductor laser with a maximum power (10 W). To give an insight into the underlying mechanisms of the Mie-type resonance effects on the arrayed, silicon pillars, the transmission of the all-dielectric grating is investigated numerically by the finite-element simulations through using CST Microwave Studio. In our experiment, the incident THz magnetic field is along the grating lines. The research results show that three typical Mie resonances are excited from 0 to 1 THz in the all-dielectric structure, and all the three resonant modes are different in the distributions of electric field and magnetic field. Furthermore, it is found that the resonance intensities of these three resonance peaks appear to be weakened variously with the increase of the optical power, and the first resonant peak modulation amplitude maximally reaches more than 50%. Combining the simulation results, we prove that the decrease of Mie resonance intensity under photo-excitation is caused by the absorption and the scattering of the incident THz wave by photo-generated carriers. Besides, we estimate the conductivity values of the all-dielectric grating under different optical excitations and find that the conductivity values reach 1000 S/m and 1500 S/m corresponding to 5 W and 10 W optical excitation, respectively. The estimated conductivity data will play an important role in the prospective optical modulation simulation. All the results mentioned above will provide an important reference for researches on the resonance properties of the all-dielectric metamaterials and the development of related functional devices.
2016, 65 (7): 074210. doi: 10.7498/aps.65.074210
Channeled modulated polarimetry imaging (CMPI) is a novel detection technology which can acquire full-Stokes parameters of each pixel of the sensor. Compared with the other imaging polarimetric technologies, CMPI has advantages in compact, high spatial resolution and acquiring full-Stokes information simultaneously. It has been widely used in remote sensing, military reconnaissance and biomedical diagnosis. However CMPI can only be used for quasi-monochromatic light during full-Stokes imaging, which leads to low signal-to-noise ratio in many cases especially under the condition of low light. Expanding the imaging spectral bandwidth of the CMPI is of great urgency. In order to expand the bandwidth, the limitation factors and conditions of the imaging bandwidth should be clearly understood first. So an imaging bandwidth criterion is deduced in this paper for the researchers to estimate the limitation bandwidth of the CMPI. We analyze the factors which might affect the fringe visibility based on a Savart plate (SP) CMPI and obtain the conclusion that carry frequency (CF) is the main factor which restricts the bandwidth. Then, according to the definition of CF, = /(f), in which is the shearing distance of SP, is the imaging wavelength, and f the focal length of imaging lens, we investigate how these factors influence the CF. It turns out that is the main factor which causes the fringe to arise in a certain CPI system while would add an error to CF within 5% in visible light domain. To investigate how the wavelength influences the imaging spectral bandwidth, we deduce the total irradiance on the image plane under broadband light and use Fourier transform for it to obtain the distribution of the spatial frequency of the image plane. And the conclusion is obtained that the CF bandwidth be expressed as (20-1/(2L), 20 + 1/(2L)) referred to as the Rayleigh criterion, in which 0 is the central CF and L is the range of the imaging plane. After substituting the relevant parameters into the CF bandwidth, we can obtain the imaging spectral bandwidth criterion equation as = 2D02/(4D2-02) , in which is the maximum imaging bandwidth, D is the maximum optical path difference, and 0 is the central wavelength of the CMPI system. To validate the accuracy of the spectral bandwidth criterion, some simulations are conducted to generate a maximum imaging spectral bandwidth while the visibility of the fringes decreases to 0.5 for the fringes which cannot be distinguished when the visibility is less than 0.5. The results show that the error between the simulated spectral bandwidth and the calculated spectral bandwidth is less than 1 nm. This criterion value fits the test well for the SP CMPI system. In addition, it can also be used for estimating the maximum imaging bandwidth of the other CMPI system whose shearing distance is independent or quasi-independent of wavelength.
Evolution of the ultrasonic resonance modes in a three-layer structure with change of material and interface adhesion properties
2016, 65 (7): 074301. doi: 10.7498/aps.65.074301
The quantitative non-destructive evaluation (NDE) of interface adhesion has long been a challenge for the safe use of bonding structures. It is difficult to predict the adhesion resistance force between adhesive and adhered material without performing destructive testing. Ultrasonic approach seems to be the only potential way for its NDE based on the reason of mechanical nature of the problem. Different ultrasonic techniques, such as bulk wave echography, reflection resonance, and Lamb guided waves, have been used to evaluate the interface adhesion strength. But no direct relation between the interfacial bonding strength and the ultrasonic measurement has been established. The most used compression wave echography and resonance at normal incidence are less sensitive to the interface condition, except for a disbond. It is essential that the interface should be excited with a shear stress component to increase the measurement sensibility. But it is not easy to excite the interface by using shear waves in experiment, while the use of guided waves will encounter the problems of high attenuation and mode selection as all modes are not sensitive to a certain interface in a bonded structure. A previous study has shown that the V (z) inversion technique can be used to perform a multimode measurement on a layered structure, where both compression and shear stress resonance occur. This method has the advantage in using a simple experimental setup working at the normal incidence with a focus transducer of large angular aperture. The inversed angular-frequency reflectance function R(; f) gives the resonance modes which are equivalent to the Lamb type guided modes, while it is a local determination of the wave mode, thus the difficulty in guided wave measurement above mentioned can be avoided. The first part of the paper contains the development of the theoretical model for wave propagation in a multilayered structure where three-layer sandwich bonded structures can be considered as a particular case. A weak interfacial adhesion is described by two interface compression and shear stiffness parameters, namely km and kt. By integrating the transfer matrix formalism under the non-ideal boundary conditions, the plane wave angular (incident angle) and frequency reflection coefficient function R(; f) for a liquid immersed asymmetric metal-adhesive-metal three-layer and its dispersion curves of guided mode waves with or without charge are calculated. It is confirmed that the evolutions of the reflection zeros (mode resonances) correspond to the dispersion curves of the guided waves of the same structure without charge. Furthermore, the resonance modes observed in R(; f) can be considered as a combination of the respective Lamb modes of the top and bottom single metal layers coupled through the modes conditioned by the middle adhesive layer and the its interface conditions. The second part of the paper shows the behaviors of the resonance modes by changing the parameters related to the bonding strength. The acoustical impedance, the mass density and the thickness of the adhesive layer, which are related to the cohesive property, and the shear interfacial stiffness coefficient kt which conditions the adhesive property, are changed respectively to observe the resonance mode evolutions. The mode evolutions due to each parameter are analyzed and differentiated. It can be concluded that the change in the adhesion strength of the bonding structure does not affect significantly the modes belonging to those inherent to the two adhered aluminum layers, while the coupling modes will be shifted in frequency and exchange with or replace the said inherent modes. It is expected that the obtained results in this study will be of significance for quantitatively characterizing the interfacial properties of an adhesively bonded layered structure by using the V (z) inversion technique.
2016, 65 (7): 074701. doi: 10.7498/aps.65.074701
As one of the major causes of cardiovascular diseases, the formation mechanism and the external factors of blood embolism are always the concerned problems of medical field, biological and physical field. Owing to thrombotic formation and structure being complicated, the difficulty in curing thrombosis greatly increases. Pulsation flows have a positive effect on dredging blood embolism. Owing to the blood viscosity and the inertia of red blood cells, waveform, amplitude and frequency of pulsating flow will influence the effect of dredging blood embolism. The research in this paper is mainly based on the lattice Boltzmann method. In conical pipe with embolism, in order to explore the influences of triangle wave pulsating flow waveform, the effects of differential pressure and frequency on vascular thrombus are studied by calculating the effect of dredging blood embolism. Calculation shows that the effect of dredging blood embolism is not obvious under the condition of low frequency and low differential pressure. On the contrary, the effect is good under the condition of high frequency. Appropriately increasing the differential pressure can also improve the frequency of the triangular wave of the bolt.
2016, 65 (7): 074702. doi: 10.7498/aps.65.074702
Convective speed of a vortex structure in mixing layer is an important physical quantity for correcting aero-optics caused by the flowfield as a beam passes; however, knowledge about the dynamic characteristics of convective speed of a vortex structure in mixing layer is limited because the convective speed calculated from isentropic model, which is widely used at present, is a constant. Based on the large eddy simulation and ray tracing method, the optical path length (OPL) profile over the mixing layer flowfield as beams pass through the flowfield is calculated and compared with the instantaneous vorticity contours at the same time instant. The analysis of the relationship between the local minimum of OPL in the OPL profile and the position of vortex core shows that the point of the local minimum of OPL just corresponds to the center of the vortex core. Based on this corresponding relation, the position extraction of vortex core, which is a quantitative method of calculating the instantaneous convective speed of a vortex structures in mixing layer, is proposed and validated with the data obtained from direct geometry measurement. Using this quantitative method, the instantaneous convective speeds of vortex structures with different sizes, two vortexes in the process of vortex pairing and merging, and vortex structures in the strongly compressive flowfield are calculated quantitatively and analyzed. Our quantitative results clearly present the characteristics of convective speed of vortex structures in mixing layer as follows. 1) The instantaneous convective velocity of a single vortex structure in the mixing layer flowfield varies with time, that is the fluctuation characteristics, and the fluctuation amplitude also varies with the size of a vortex structure and the compressibility of the flowfield. Specifically, the amplitude is proportional to the size of a vortex and the compressibility of the flowfield. 2) In the process of vortex pairing and merging, the variation ranges of instantaneous convective speeds of the two vortex structures are large. Specifically, the maximum value of instantaneous convective speed is close to the speed of the high-speed layer and the minimum value of instantaneous convective speed is close to the speed of the low-speed layer, and the profile of instantaneous convective speed of each vortex structure in this process approximately shows a shape of sinusoidal curve. 3) The mean value of instantaneous convective speed of a vortex structure in mixing layer is not consistent with the theoretical convective speed of vortex structure, which is calculated from the isentropic model, and the deviation between instantaneous convective speed and theoretical convective speed varies with the size of a vortex structure and the compressibility of the flowfield. In addition, the physical reasons for explaining the characteristics of instantaneous convective speed of the vortex structures in mixing layer are also presented.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2016, 65 (7): 075201. doi: 10.7498/aps.65.075201
Owing to high efficiency for delivering thermal radiation from Z-pinch plasma to an inertial fusion capsule, Z-pinch dynamic hohlraum (ZPDH) is a promising indirect-drive inertial confinement fusion (ICF) approach. ZPDH is created by accelerating an annular tungsten Z-pinch plasma radially inward to an internal low density convertor. The collision launches a radiating shock traveling inward. Radiations emitted from the shock, after being trapped and thermalized by the optically thick tungsten plasma, drive the internal fusion capsule to implode. In our previous experiments, shock propagating process has never been imaged or even never been formed, due to low drive current (about 1.3 MA). In this paper, the ZPDH has a load of single tungsten wire array embedded in a cylindrical 16 mg/cm3 C15H20O6 foam, and the tungsten wire array is explored using JuLong-1 facility (also named PTS facility) driven by current with a peak value of 7-8 MA and rising time of 60-70 ns (from 10% to 90%). Several results are presented for improving the understanding of the physics of the shock propagating and hohlraum forming. For the high optical depth in tungsten plasmas around the foam, radially directly diagnosing hohlraum radiation distribution along axis is impossible. The most convenient way to diagnose the radiation symmetry and the shock evolution is to take the end-on X-ray images. The time-resolved X-ray images of annular radiating shock evolution, which are performed with a 10-frame time-gated X-ray pinhole camera located at 0 with respect to the Z-pinch axis, are obtained for the first time in China. By analyzing the radial X-ray emission power waveform and intensity distribution of end-on radiation image, the process of wire array plasma impacting on the foam convertor and properties of dynamic hohlraum radiation are discussed. The shock emission structures are found to be circular, similar to the results predicted theoretically. The shock velocity which seems to be constant in the whole process of inward propagating is linearly fitted to be (14.21.7) cm/s. The annular width of shock emission is 0.8-0.9 mm, which is inferred from the full width at half maximum of radial lineout of end-on X-ray image at time t=-11.9 ns and the blurring effect of shock velocity. The radiation symmetry is assessed by statistic property of mean intensity of 36 sectors of end-on X-ray image evenly divided by 10. The standard deviation of azimuthal shock emission intensity is 10% while that of hohlraum region prior to shock impact is 4.2%. The azimuthal symmetry improvement from shock emission to hohlraum radiation is a piece of exciting news for ZPDH driven ICF.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2016, 65 (7): 076101. doi: 10.7498/aps.65.076101
Elastic properties and phase stabilities of L12-A13Sc precipitate phase in Al-Sc alloys have been topics of experimental and theoretical research over the past years. However, these properties of off-stoichiometric L12-A13Sc have not been investigated. Firstly, in combination with Wagner-Schottky model, the first-principles total energy calculations based on density functional theory are performed to study point defect concentrations of intermetallic L12-A13Sc each as a function of temperature and alloy composition. We calculate the point defect formation enthalpies and plot the point defect density curves of stoichiometric and off-stoichiometric L12-A13Sc at 1000 K. The results show that within the whole temperature range (300-1200 K), Al and Sc vacancies dominate on stoichiometric L12-A13Sc but with low concentrations (~10-6 even at 1200 K); on the Al-rich side of off-stoichiometric L12-A13Sc, the Al anti-site and the Sc vacancy concentrations dominate, and their concentrations are comparable, however, on Sc-rich side of off-stoichiometric L12-A13Sc, the Sc anti-site defect dominates. Furthermore, the lattice constants and the elastic constants of stoichiometric and off-stoichiometric L12-A13Sc are calculated, and it is worth noting that 222 supercell models with a point defect are used for off-stoichiometric L12-A13Sc in the calculation. Then employing calculated elastic constants, the values of Youngs modulus, shear modulus, bulk modulus, anisotropic index, G/B ratio, Cauchy pressure, and Poisson ratio of stoichiometric and off-stoichiometric L12-A13Sc are computed. And lastly, combining these data with point defect concentrations of off-stoichiometric L12-A13Sc at 1000 K, the comprehensive effects of four point defects on elastic properties of L12-A13Sc are evaluated. The four point defects coexist in L12-A13Sc as we know from the calculations of equilibrium point defect density. The conclusions are as follows. 1) The point defects can cause off-stoichiometric L12-A13Sc lattice distortion. On the Sc-rich side, lattice constant appears to be an increasing tendency, from 4.105 to the biggest value of ~4.13 (~0.5% growth), while on the Al-rich side, it shows an opposite trend, from 4.105 to the smallest value of ~4.10 (~0.24% fall). Although there is the lattice distortion in off-stoichiometric L12-A13Sc, off-stoichiometric L12-A13Sc can still keep stable crystal structure for the value of xAl in a range of 0.72-0.78. 2) The point defects also affect elastic constants of off-stoichiometric L12-A13Sc. Specifically, on the Sc-rich side, elastic constant c11 decreases with the increase of deviation degree of stoichiometric ratio, and the maximal reduction is ~9% at xAl = 0.72, while elastic constants c12 and c44 show the opposite variation trends, and the maximal increase is ~8% at xAl = 0.72. On the Al-rich side, there are little changes for elastic constants c11, c12 and c44. 3) The point defects obviously increase the elastic anisotropy of off-stoichiometric L12-A13Sc, and especially on the Sc-rich side, the notable increase is found, which jumps from 1.610-6 to 0.04. 4) The values of Youngs modulus, shear modulus, and bulk modulus of off-stoichiometric L12-A13Sc decrease due to point defects, with the maximal reduction being 3%-4%. These elastic modules fall first rapidly and then slowly on the Sc-rich side, while they present approximately a linear downward trend on the Al-rich side. In addition, weak influences are exerted on brittleness and toughness of off-stoichiometric L12-A13Sc by the point defects, compared with the other elastic effects mentioned above. In summary, in the scope of xAl = 0.72-0.78, the point defects can not only reduce Youngs modulus, shear modulus, and bulk modulus of off-stoichiometric L12-A13Sc, but also increase the anisotropies of the elastic properties of off-stoichiometric L12-A13Sc. However, the point defects have weak influences on the brittleness and toughness of off-stoichiometric L12-A13Sc.
2016, 65 (7): 076102. doi: 10.7498/aps.65.076102
Enhancing low dose rate sensitivity (ELDRS) in bipolar device is a major problem of liner circuit radiation hardness prediction for space application. ELDRS is usually attributed to space-charge effect. A key element is the difference in transport rate between holes and protons in SiO2. Interface-trap formation at high dose rate is reduced due to positive charge buildup in the Si/SiO2 interfacial region (due to the trapping of holes and/or protons) which reduces the flow rates of subsequent holes and protons (relative to the low-dose-rate case) from the bulk of the oxide to the Si/SiO2 interface. Generally speaking, the dose rate of metal oxide semiconductor (MOS) device is time dependent when annealing of radiation-induced charge is taken into account. The degradation of MOS device induced by the low dose rate irradiation is the same as that by high dose rate when annealing of radiation-induced charge is taken into account. However, radiation response of new generation MOS device is dominated by charge buildup in shallow trench isolation (STI) rather than gate oxide as older generation device. Unlike gate oxides, which are routinely grown by thermal oxidation, field oxides are produced using a wide variety of deposition techniques. As a result, they are typically thick (100 nm), soft to ionizing radiation, and electric field is far less than that of gate oxide, which is similar to the passivation layer of bipolar device and may lead to ELDRS. Therefore, dose-rate sensitivities of n-type metal oxide semiconductor field effect transistor (NMOSFET) and static random access memory (SRAM) manufactured by 0.18 m complementary metal oxide semiconductor (CMOS) process are explored experimentally and theoretically in this paper. Radiation-induced leakages in NMOSFET and SRAM are examined each as a function of dose rate. Under the worst-case bias, the degradation of NMOSFET is more severe under the low dose rate irradiation than under the high dose rate irradiation and anneal. Moreover, radiation-induced standby current rising in SRAM is more severe under the low dose rate irradiation than under the high dose rate irradiation even when anneal is not considered. The above experimental results reveal that the dose-rate sensitivity of deep sub-micron CMOS process is not related to time-dependent effects of CMOS devices. Mathematical description of the combination between enhanced low dose-rate sensitivity and timedependent effects as applied to radiation-induced leakage in NMOSFET is developed. It has been numerically found that non time-dependent effect of deep sub-micron CMOS device arises due to the competition between enhanced low dose-rate sensitivity in bottom of STI and time-dependent effect at the top of STI. The high dose rate irradiation is overly conservative for devices used in a low dose rate environment. The test method provides an extended room temperature anneal test to allow leakage-related parameters that exceed postirradiation specifications to return to a specified range.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2016, 65 (7): 077101. doi: 10.7498/aps.65.077101
TiAl alloy has attracted significant attention as a candidate material with high melting temperature, low density, relatively high hardness and excellent corrosion resistance, good oxidation and creep resistance at high temperatures. The inherent brittleness at low temperatures is by far the greatest hurdle that prevents it from being widely used in industries. Doping has long been considered as an effective way to improve the performance of alloy. The properties of TiAl alloy are highly dependent on the third alloying element. Although the mechanical properties of TiAl alloy are improved to a certain extent by adjusting the composition, to date the physical mechanism has been still unclear. In this paper, from the microscopic electronic structure the influence of metal element X (X represents V, Nb, Ta, Cr, Mo and W) doping on the mechanical properties of TiAl alloy is studied by first-principle method. The first-principle calculations presented here are based on electronic density-functional theory framework. The ultrasoft pseudopotentials and a plane-wave basis set with a cut-off energy of 350.00 eV are used. The generalized gradient approximation refined by Perdew and Zunger is employed for determining the exchange-correlation energy. Brillouin zone is set to be within 888 k point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is at 5.010-7 eV/atom. The supercell (222), (221) and (121) are selected as a computational model. According to the calculated structural parameters of the doped systems, we find that the lattice constant ratio c/a decreases with the increase of doping ratio, correspondingly the anisotropy of crystal reduces. The interactions between Ti and Al atoms are enhanced. Under the same pressure, the influences of doping concentration and type of doping element on volume are different. According to the obtained elastic constants, bulk moduli and shear moduli of doping systems, we find that with a doping concentration of 6.25%, Cr, Mo and W doping can improve the toughness of TiAl alloy more than V, Nb and Ta doping. For a doping concentration of 12.5%, the toughening effect of Mo is the strongest in all the six doping elements. The strong s-s, p-p and d-d electron interactions exist between the Ti and Mo atom, which is verified by the results of partial electron density of state and charge density. The strong interaction caused by doping restricts effectively the migration of Ti and Al atom. It is beneficial to enhance the stability and strength of the TiAl alloy. In summary, starting from the microscopic electronic structure we find that doping can effectively reduce the anisotropy of TiAl alloy, enhance the interaction between Ti and Al atoms, weaken covalent bond energy, enhance metal bond energy and then promote the plastic deformation of TiAl alloy. The results can provide theoretical support for improving the performances of TiAl based alloys.
Influence of power dissipation and case temperature on thermal resistance of AlGaN/GaN high-speed electron mobility transistor
2016, 65 (7): 077201. doi: 10.7498/aps.65.077201
The junction temperature is a main factor affecting the device performance and reliability. The thermal resistance is usually used to calculate the junction temperature. However, the thermal resistance is not constant under different operating conditions. In this work, we examine the high-speed electron mobility transistor (HEMT) from the CREE Company to investigate its thermal resistances under different case temperatures and dissipation powers. To avoid the self-oscillating phenomenon of the HEMT device, a circuit is designed to prevent the self-oscillating in experiment. First, the temperatures of the active region of the GaN HEMT device are measured by the infrared image method under different dissipation powers (including 2.8, 5.6, 8.4, 11.2, and 14 W) and different case temperatures, respectively. Then according to the result of infrared image method, the simulation model is set up by using the Sentaurus TCAD. From the final optimized model, we extract the device junction temperature and calculate the thermal resistance. It is expected to ascertain the characteristic of the thermal resistance and compare it with the result from the infrared image method. It is found that as the device case temperature increases from 80 ℃ to 130 ℃, the thermal resistance changes from 5.9 ℃/W to 6.8 ℃/W, i.e., it is increased by 15%. When the power increases from 2.8 W to 14 W, the thermal resistance changes from 5.3 ℃/W to 6.5 ℃/W, i.e., it is increased by 22%. This phenomenon is mainly attributed to the changes of the thermal conductivity of device materials. According to the formula for the coefficient of the thermal conductivity of nonmetallic material SiC, the phonon scattering rate becomes larger with the increase of temperature. Thus, the phonon mean free path can decrease by reducing the average freedom time. Finally, the coefficient of thermal conductivity becomes smaller. It was reported by Kotchetkov et al. (Kotchetkov D, Zou J, Balandin A A, Florescu D I 2001 Appl. Phys. Lett. 79 4316) that the coefficient of thermal conductivity of GaN becomes smaller under high temperature. All of these have an effect on the heat dissipation of the device, which will cause the thermal resistance to increase. Based on the result from the infrared image method and TCAD simulation, the changing characteristic of the thermal resistance is obtained, thereby reducing the errors in the calculation of the junction temperature.
First-principles study of the electronic properties and magnetism of LaMnO3/SrTiO3 heterointerface with the different component thickness ratios
2016, 65 (7): 077301. doi: 10.7498/aps.65.077301
Using first-principles calculations based on density functional theory and projector augmented wave method, we investigate the thickness ratio dependences of the ionic relaxation, electronic structure, and magnetism of (LaMnO3)n/(SrTiO3)m heterostructure. Polar and nonpolar oxide interfaces have become a hot point of research in condensed matter physics; in this system, polar discontinuity at the interface may cause charge transfer to occur at interfaces between Mott and band insulating perovskites. Here, we consider two types of interfaces, namely n-type (LaO)+/(TiO2)0 and p-type (MnO2)-/(SrO)0 interfaces. The results show that the different thickness ratios and interface-types lead to different degrees of ionic relaxation, inducing charges of different concentrations to transfer. The distortions of the oxygen octahedra are found to vary distinctly with the component thickness ratio (n:m), which is consistent with recent experimental results. Furthermore, both n and m are found to strongly affect the charge transfer. When the thickness of LaMnO3 reaches a thickness of critical layers of 6 unit cells, the Mn-eg electrons are transferred to the Ti-dxy orbitals of SrTiO3, which is caused by the interface polar discontinuity. Two-dimensional electron gas with high mobility is formed in an n-type (LaMnO3)n/(SrTiO3)2 interface region. Meanwhile, spin polarization of interface-layer Ti atoms becomes more obvious, which induces Ti magnetic moment to be close to 0.05B. We find that Mn magnetic moment of 3.9B is a larger value at the n-type interface than at the p-type interface. The above studied heterostructure favours ferromagnetic spin ordering rather than the A-type antiferromagnetic spin ordering of bulk LaMnO3. Whether n-type or p-type (LaMnO3)2/(SrTiO3)8 interfaces consist of ultrathin LaMnO3 layer and thicker SrTiO3 layer, there is no structure distortion at the side of SrTiO3 basically, which is in agreement with experimental results. Stronger interface-layer polar distortions for p-type interface prevent the electron transfer from occurring, and spin polarization of Ti cannot occur either. In addition, it is found that the two types of interfaces possess 2 eV potential difference by comparing the average electrostatic potential, thus charge transfer is more difficult to occur in the p-type interface than in the n-type interface.
Abnormal behaviors in lock-in transition of the vortices in melt textured growth of YBa2Cu3O7- crystals
2016, 65 (7): 077401. doi: 10.7498/aps.65.077401
The magnetization behavior of the layered anisotropic high-Tc superconductor in the mixed state Hc1 H Hc2 has a feature that when the angle between the applied magnetic field H and the CuO plane (a-b plane) is less than a critical value ( L), the vortex lattice is converted from three-dimensional structure into two-dimensional structure, forming a phenomenon so called the lock-in transition, where the flux lines are completely parallel to the a-b plane, and the vertical component of the magnetic induction B丄 (perpendicular to the a-b plane) is consequently zero. So far, there have still existed the differences in the physical explanation of the lock-in phenomenon. For the lock-in phenomenon occurring in the region between the CuO planes, it can be considered to be caused by the transverse Meissner effect. However, for the one occurring in other extended correlated defect areas, such as twin boundaries in YBa2Cu3O7- (YBCO) crystal, this phenomenon is believed to be the results of the energy linearization of the vortices trapped in the defect channels. Many theoretical and experimental studies have revealed the existence of the lock-in behaviors related to the microstructure properties of the superconductor crystals. Therefore, the research of the lock-in transition behavior will be helpful to understand the intrinsic pinning properties of the layered anisotropic superconductors, and the phase transition process in the vortex system. In this paper, we systematically measure the magnetic torque signal in melt texture growth YBCO (MTG-YBCO) bulk and observe an abnormal lock-in transition behavior in the vortex system. The critical angle of the lock-in transition is found to be directly proportional to the strength of the magnetic field, which is contrary to the observations in the common cases. According to the framework of the Ginzburg-Landau theory and the kink structure model of the vortex line, we discuss the abnormal phenomenon, and propose that there is a type of extend-correlated defect structure, which is parallel to the a-b plane, in the MTG-YBCO crystal. The relationship between the critical angle of the lock-in transition to the temperature and the magnetic field is established theoretically, and the theoretical results coincide well with the torque measurements.
2016, 65 (7): 077402. doi: 10.7498/aps.65.077402
Searching new superconducting materials and understanding their superconducting mechanisms are the important research directions in the condensed matter physics study. The recent discovery of aromatic hydrocarbon superconductors, including potassium-doped picene, phenanthrene and dibenzopentacene, has aroused considerable research interest of physicists and materials scientists. In this work, potassium-doped p-terphenyl is grown by sealing potassium and p-terphenyl with a mole ratio of 3 : 1 in high-vacuum glass tube and then annealed at 170 ℃ for 7 days or at 240 and 260 ℃ for 24 h. The crystal structure, molecular vibration, and magnetic property are characterized by using X-ray diffraction, Raman scattering, and superconducting quantum interference device. The combination of X-ray diffraction spectrum and Raman spectrum shows that besides potassium-doped p-terphenyl and KH, there exist C60 and graphite in annealed sample, which are found for the first time in the metal-doped aromatic hydrocarbon. Owing to the presence of potassium with high chemical activity, the C-H bond can be broken, resulting in dehydrogenated p-terphenyl with dangling bonds. Consequently, the recombination of dehydrogenated p-terphenyl will form graphite and C60. In addition, the red-shifts of partial peaks of p-terphenyl in Raman spectrum demonstrate that 4 s electron of doped potassium is transferred to C atom. For the samples annealed at 170 and 240 ℃, Curie paramagnetic behaviors are observed in the whole temperature region. On the other hand, in one of the samples annealed at 260 ℃, there exist three anomalous sharp decreases respectively at 17.86, 10.00 and 6.42 K in the zero-field cooling magnetic measurement. Previous studies indicated that the superconducting transition temperatures of potassium-doped C60 and potassium-doped graphite are about 18 K and 3 K. Therefore, it is reasonable to attribute the anomalous sharp decrease at 17.86 K to being produced by potassium-doped C60, while the anomalous sharp decreases at 10.00 and 6.42 K, which have not been reported yet, may be produced by potassium-doped p-terphenyl. The first principles calculations show that potassium-doped p-terphenyl lies in the metallic state, which can form superconductivity due to the electron-phonon interaction. Our results are useful for understanding the crystal growth and physical properties of metal-doped aromatic hydrocarbon organic superconductors. Furthermore, our findings provide a new routine to synthesizing C60 and graphite at low temperature.
2016, 65 (7): 077701. doi: 10.7498/aps.65.077701
Epoxy resin is widely used as a polymeric insulating material in power equipment, such as gas-insulated switchgear and gas-insulated lines. The motions of molecular chains or segmental chains in a polymeric insulating material can affect the material properties, such as dielectric relaxation, charge transport, breakdown, and glass transition temperature. Molecular or segmental chains may form dipoles, and their motions can contribute to dielectric relaxation properties. Molecular or segmental chains with different scales have different relaxation time constants. Their motions affect dielectric relaxation processes in different frequency ranges. The motions of molecular or segmental chains are also affected by temperature, since the magnitudes of motions are restricted by free volume in a polymeric insulating material. However, the effects of motions of molecular or segmental chains in epoxy resin on electrical properties have not been very clear to date. Therefore, it is important to investigate the relations between the motion of molecular or segmental chains and dielectric relaxation properties, the temperature and molecular scale dependence of the motions, and their effects on charge transport of epoxy resin. In this paper, the properties of dielectric relaxation and glass transition of epoxy resin are measured. Before the experimental tests, samples of pure epoxy resin are prepared by using epoxy raw materials supplied by Pinggao Group, and the curing temperature is 130 ℃. The glass transition temperature is around 105 ℃ measured by a differential scanning calorimetry (DSC). As for the dielectric relaxation measurement with Novocontrol broadband dielectric relaxation spectroscopy, the sample is processed into a disk with a diameter of 50 mm and a thickness of 1 mm. The measurement temperature and frequency are in ranges of 100-180 ℃ and 10-1-107 Hz, respectively. The results reveal that there are two relaxation processes at high temperature. In addition, above glass transition temperature, a relaxation peak occurs at high frequencies due to the motions of molecular chains or segmental chains, and a direct current (DC) conductivity resulting from the migration of charge carriers appears at low frequencies. Besides, molecular chains with different scales have different relaxation times. It is found that epoxy resin has a very broad distribution of relaxation times. The distributions of relaxation times at various temperatures are calculated. The results show that the temperature dependence of molecular relaxation and DC conductivity satisfy Vogel-Tammann-Fulcher equation. Through fitting the experimental results, the Vogel temperatures and strength parameters of molecular relaxation and DC conductivity are obtained. From the Vogel temperatures, the glass transition temperature is estimated to be 102 ℃, which is consistent with the DSC result. It means that free volume in epoxy resin increases with the increase of temperature, which facilitates the motions of molecular chains and the migration of charge carriers.
Numerical extraction of electric field distribution from thermal pulse method based on Monte Carlo simulation
2016, 65 (7): 077702. doi: 10.7498/aps.65.077702
Thermal-pulse method is a powerful tool for measuring space charge distributions in polymer films. The data analysis for thermal-pulse method involves the Fredholm integral equation of the first kind, which requires an appropriate numerical procedure to obtain a solution. Various numerical techniques, including scale transformation and regulation method, are proposed. Of those numerical methods, the scale transformation (ST) is the simplest and the most widely used method. However, it presents a high spatial resolution only near the sample surface. Monte Carlo (MC) method is one of the recently proposed ways to solve the equation numerically and has been successfully applied to the analysis of laser intensity modulation method data, which also involves the Fredholm integral equation of the first kind. In this paper we attempt to analyze thermal-pulse data in frequency domain with the MC method and discuss its effectiveness based on some numerical simulations. The simulation results indicate that the electric field profiles can be effectively extracted by the MC method. The computed profiles by the MC method consist well with the supposed distributions in the entire thickness of the sample, while the profiles reconstructed by the ST method fit very well to the supposed one at the vicinity of the target surface and distort sharply along the direction of the thermal pulse propagation in the sample bulk. On the other hand, the oscillations in the computed results by the MC method could deteriorate its accuracy in this study. The influence of noise level on the analysis based on the MC method is also tested by the use of the simulated data. The results show that the computed profiles would become more fluctuant as the noise level increases. This problem can be solved by selecting a larger value of tolerance during the singular value decomposition procedure. Thus, the value of tolerance is considered to be one of the key parameters in this algorithm, which is actually hard to determine. Additionally, the experimental data obtained from a polypropylene film under applied electric field are analyzed to illustrate the feasibility of MC method to be applied to the thermal-pulse experimental data. The results also show that the spatial accuracy by the MC method in the entire sample thickness is higher than by the ST method, which verifies that the MC method is more suitable for detecting the electric field distribution in the deep bulk of the sample. Owing to noise and error, the accuracy of MC calculation depends on the chosen tolerance value, which is now considered to be an obstacle in applying this method to the practical thermal-pulse measurement.
Influences of InGaN/GaN superlattice thickness on the electronic and optical properties of GaN based blue light-emitting diodes grown on Si substrates
2016, 65 (7): 077801. doi: 10.7498/aps.65.077801
GaN based light-emitting diodes (LEDs) are subjected to a large polarization-related built-in electric field in c-plane InGaN multiple quantum well (MQW) during growth, which causes the reduction of emission efficiency. To mitigate the electric field, a superlattice layer with a numerous good characteristics, such as a small thickness, a high crystalline quality, is embedded in the epitaxial structure of LED. However, the effect of the superlattice thickness on the properties of LED is not fully understood. In this paper, two blue-LED MQW thin film structures with different thickness values of InGaN/GaN superlattice inserted between n-GaN and MQW, are grown on Si (111) substrates by metal-organic chemical vapor deposition. Electronic and optical properties of the two kinds of samples are investigated. The obtained results are as follows. 1) Comparing two samples, it is observed that more serious reverse-bias leakage current exists in the one with thicker superlattice; 2) Room temperature electroluminescence (EL) measurement shows that the emission spectrum peak between two samples is blue-shifted to different extents as the injection current increases. With superlattice thickness increasing, the extent to which the peak is blue-shifted decreases. Nevertheless, there is no obvious discrepancy in the EL intensity between two samples with different thickness values at 300 K. In addition, the V-shaped pit characteristics including density and size, and the dislocation densities of two samples are studied by high-resolution X-ray diffraction, scanning electron microscope, and transmission electron microscope. The experimental data reveal that the reason for a tremendously different in reverse-bias leakage current between two samples is that there are larger and more V-pits in the superlattice sample with a large thickness. Whereas, V-pits also act as preferential paths for carriers, resulting in the fact that the thicker superlattice suffers more serious reverse-bias leakage current. According to reciprocal space X-ray diffraction intensity around the asymmetrical (105) for GaN measurement, the relaxed degree of InGaN quantum well on GaN is proportional to the superlattice thickness. On the other hand, it is useful for increasing superlattice thickness to reduce a huge stress in c-plane InGaN. Owing to joint effects of above factors, the EL intensities of the superlattice sample with different thickness values are almost identical. Our results show the functions of superlattice thickness in electronic and optical characteristics. What is more, the conclusions obtained in the present research indicate the practical significance for improving the performances of LED.
Barrier and well thickness designing of InGaN/GaN multiple quantum well for better performances of GaN based laser diode
2016, 65 (7): 077802. doi: 10.7498/aps.65.077802
The effects of barrier and well thickness in InGaN/GaN (with in content of 15%) multiple quantum well (MQW) on the performances of GaN based laser diode (LD) are investigated by using LASTIP software, and the relevant physical mechanisms are discussed. It is found that when the barrier-thickness in InGaN/GaN MQW is fixed to be 7 nm, for the well thickness values of 3.0, 3.5, 4.0, 4.5, and 5.0 nm, the threshold currents of LD are 76.31, 67.96, 57.60, 64.62, and 74.59 mA, and the output light powers of LD are 12.05, 15.64, 24.70, 18.21, and 11.35 mW under an injection current of 100 mA, respectively. It indicates that too thick or too thin well may lead to a higher threshold current and a lower output power of GaN based LD. A high performance device can be obtained by using an optimized well thickness of around 4.0 nm. It is found that the LD performance is degraded by using too thin well in the device structure mainly due to the high leakage current, while strong polarization will lead to the decrease of overlap integral and luminescence intensity if the well layer is too thick, and thus a poor performance is obtained. It is found that the LD performance can be improved obviously by appropriately increasing barrier thickness from 7 nm to 15 nm. When the barrier thickness in InGaN/GaN MQW is fixed at 15 nm and the well thickness values are 3.0, 3.5, 4.0, 4.5 and 5.0 nm, the threshold currents of LD are 59.54, 52.42, 52.17, 51.38, and 58.99 mA, and the output light powers of LD are 36.12, 39.69, 40.79, 40.27, and 33.19 mW under an injection current of 100 mA, respectively, i.e., LD device parameters are improved. It suggests that the higher performances of GaN based laser diode can be realized by appropriately increasing the thickness of barrier when the thickness of well is optimized to be around 4 nm.
2016, 65 (7): 077803. doi: 10.7498/aps.65.077803
Bulk tungsten and tungsten transmission electron microscopy (TEM) lamella are implanted with 15 keV helium ions at about 873 K to study the microstructure evolution. The samples are implanted to about 11017 He+/cm2. The projected range of the helium ion in tungsten is about 43.9 nm, calculated with the stopping and range of ions in matter program (the SRIM code). The density of pores with diameters ranging from 90 nm to 430 nm is detected on the surface of helium implanted bulk tungsten by field emission scanning electron microscopy. Blistering is also observed on the surface of helium implanted bulk tungsten. The TEM results indicate that fuzz microstructure is formed in helium implanted tungsten TEM lamella, and stacking faults and micro-pores are observed in the fuzz structure. Besides, the density of nano-scaled helium bubbles is detected around the mirco-pores.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2016, 65 (7): 078801. doi: 10.7498/aps.65.078801
Recently, ternary bulk-heterojunction (BHJ) polymer solar cells (PSCs) occur as an attractive strategy with simple fabrication technology to extend the spectrum of wide bandgap polymers into the near infrared region by adding a narrow bandgap sensitizer. In this paper, a series of cells including binary BHJ-PSCs with P3HT:PCBM as the active layer (control cell) and ternary BHJ-PSCs with different PTB7-Th doping concentrations are fabricated to investigate the effect of PTB7-Th on the performance of PSC. The short-circuit current density (Jsc) and fill factor (FF) of the ternary PSCs are simultaneously improved by adding a small amount of PTB7-Th into P3HT:PCBM. The champion photoelectric conversion efficient of ternary PSCs (with 15 wt% PTB7-Th) is 3.71%, which is larger than 2.71% of the control cell. In a ternary device, the absorption region shows a distinct red-shift and the relative absorption intensity from 650 nm to 800 nm is gradually enhanced with the incrtease of PTB7-Th doping concentration. The increased photon harvesting in the solar spectral range results in an increased short-circuit current density. However, despite the fact that the photoluminescence (PL) spectrum of P3HT has a large overlap with the absorption spectra of PTB7-Th, which makes it possible for Frster resonance energy to transfer between P3HT and PTB7-Th, the PL intensity of P3HT at 650 nm is quenched with the increase of PTB7-Th doping concentration while the photoluminescence remains almost the same in the long wavelength region, which suggests that the main mechanism between PTB7-Th and P3HT is photo-induced electron transfer from P3HT to PTB7-Th (hole transfer from PTB7-Th to P3HT), not energy transfer. The PSCs with P3HT:PTB7-Th (1:1) as an active layer display a large Jsc compared with the P3HT-based one. When the concentration of PTB7-Th decreases and the concentration of P3HT is unchanged (P3HT:PTB7-Th 1 : 0.5), the Jsc can be further enhanced. The increased Jsc value of P3HT: PTB7-Th (1:0.5) PSCs confirms that the photo-generated excitons can be dissociated into free charge carriers at the P3HT:PTB7-Th interface and reinforce the charge transfer between P3HT and PTB7-Th. In P3HT:PCBM binary organic solar cell, the photo-generated excitons only can be directly dissociated into free charge carriers at the P3HT:PCBM interface and then transported to the respective electrodes, while incorporating PTB7-Th, the interaction between P3HT and PTB7-Th also makes the photo-generated excitons dissociated at the interface of P3HT:PTB7-Th, and at the interface of PTB7-Th:PCBM. The increasing of excitons dissociated leads to a higher FF. The present study is the first report on utilizing PTB7-Th in P3HT:PCBM PSC.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Three-dimensional finite volume simulation of the response of azimuth electromagnetic wave resistivity while drilling in inhomogeneous anisotropic formation
2016, 65 (7): 079101. doi: 10.7498/aps.65.079101
The azimuth electromagnetic wave resistivity while drilling is a new type of well logging technique. It can real-time detect the formation boundary, realize geosteering and borehole imaging in order to keep the tool always drilling in the some meaning reservoir. For effectively optimizing tool parameters, proper explanation and evaluation of the data obtained by azimuth electromagnetic wave resistivity while drilling, the efficient numerical simulation algorithm is required. In this paper, we use the finite volume algorithm in the cylindrical coordinate to establish the corresponding numerical method so that we can effectively simulate the response of the tool in various complex environments and investigate the influences of the change in formation and tool parameters on the tool response. Therefore, according to the typical coil architecture of the instrument of azimuth electromagnetic wave resistivity while drilling, we first introduce the electrical and magnetic dyadic Green's functions in inhomogeneous anisotropic formation by the electrical current source in the cylindrical coordinate. Through superposition principle, we derive the integral formula to compute the electric field intensity excited by tilted transmitter coils and the induction electrical potential on tilted receiving coils both mounded on the drill collar. Then, we use the coupled electrical potentials of the dyadic Green's functions to overcome the low induction number problem during modeling the electrical fields in inhomogeneous anisotropic formation. Furthermore, we use Lebedev grid in both and z directions to reduce the number of grid nodes, and the standard method to compute the equivalent conductivity in heterogeneous units for enhancing the discrete precision. On the basis, by the three-dimensional finite volume method, we discrete the equations about the coupled electrical potentials in the cylindrical coordinates and obtain the large sparse algebraic equation sets about the coupled electrical potentials field on the Lebedev grid. A combination of incomplete LU decomposition with the bi-conjugate gradient stabilization is used to solve the numerical solution. Finally, we validate the algorithm by comparing the numerical results obtained by two different methods, study the effects of the drill collar, anisotropy, the tilted angles of both coil, and borehole on the instrument response in inhomogeneous anisotropic formation. The numerical results show that the tool response obtained by the three-dimensional finite volume algorithm in the cylindrical coordinate system in anisotropic formation accord with that those obtained by other algorithms. The drill collar, inhomogeneous anisotropic n the formation will lead to both the smaller amplitude ratio and the smaller phase difference. In addition, when the coils of both transmitting and receiving coils are tilted, the amplitude ratio and phase difference of the tool are more sensitive to the change in formation parameter.
2016, 65 (7): 079501. doi: 10.7498/aps.65.079501
The continuous improvement in diversification, new-type orientation and low cost of navigation control system, the accurate measurement of the spinning aircraft flight attitude parameters has becomes a more and more urgent task. In view of the above problems, a novel attitude estimator for the spinning aircraft is proposed by using earth infrared radiation field. The attitude estimation system possesses several key advantages over the current designs in low cost, no need of moving parts, and being free from reliance on GPS or other state feedback. Firstly, the mechanism of earth infrared radiation field is described in detail, and an 8-14 m atmospheric window is selected as the study object. The land surface infrared radiation is calculated by the land surface temperature and emissivity. The sky infrared radiation is calculated through layered atmosphere by combing with the sky emissivity and infrared atmospheric transmittance. According to the calculations of land surface infrared radiation and sky infrared radiation, the mathematical model of earth infrared radiation field is established by combining with propagation law of infrared radiation in the atmosphere. Then the measurement model of thermopile sensors is derived, after analyzing the motion feature of spinning aircraft during the flight. The thermopile sensors convert the observed infrared radiation into an electrical signal well suited for onboard data acquisition. To explore the inner link between the thermopile sensor output and the spinning aircraft attitude information, the characteristics of the sensor output under different attitude angles and fields of view are studied. When the thermopile sensor characteristics are included, the fully developed model can be used to generate accurate sensor output as a function of attitude angle. Finally, the installation of the thermopile sensors on the spinning aircraft is designed, and the measurement model of onboard thermopile sensor is established. In order to improve the accuracy of attitude measurement, an extended Kalman filter is developed, which enables the estimating of real-time attitude angles and roll rate by using solely three-axis thermopile sensors as feedback. The result indicates that by using this high accurate algorithm, the pitch angle estimation error is within 0.02, the roll angle estimation error is within 0.1 and the roll rate estimation error is within 1 rad/s. The detection system is simple and practical, works stably, and can meet the requirements for spinning projectile attitude measurement. The attitude estimation system will provide a new method and theory for further developing the spinning aircraft state detection.
Analysis of measurement accuracy of ephemeris parameters for pulsar navigation based on the X-ray space observation
2016, 65 (7): 079701. doi: 10.7498/aps.65.079701
The pulsar ephemeris that maintains the time-space benchmark for pulsar navigation is an important part of Xray pulsar navigation system. The parameters of pulsar timing model which are contained in the pulsar ephemeris can influence directly the accuracy of pulsar navigation. Some studies have shown that 100 m target of X-ray pulsar navigation should need 1 mas angle position and 100 ns pulse time of arrival, the high-level precision of parameters of some pulsars can be reached by ground radio observations with large-diameter telescope. Owing to the development of high-performance X-ray detector and stable space observation platform, the technology that the parameters of pulsar ephemeris are measured by space X-ray observations may be achieved, so the feasibility of this technology is studied in this paper by reconstructing the analysis process. The process includes mainly three parts. Firstly, the methods of simulating X-ray pulsar signals, replicating pulse profile and getting the time of arrival between the observed pulse profile and the standard one from analyzing observation data of the RXTE and Chandra satellite are studied, then the accuracies of X-ray space observations for four pulsars are estimated by using the large sample duplication events. Secondly, the process of fitting model for ephemeris parameters is established and realized by computer program in C++ language. Finally, the relationships between the accuracies of ephemeris parameters and those of the following factors are analyzed: the observation accuracy, the observation duration, the observed frequency. Those results of four pulsars (Crab, B1937+21, B1821-24 and B1509-58) are concluded below. 1) The X-ray space timing precisions of Crab pulsars in the observation durations of 1000 s and an hour are 1.41 s and 0.83 s respectively, the ones of other 3 pulsars in three different observation durations of 1000 s, an hour, and a day are also gained. 2) The ephemeris parameters of four pulsars are achieved by the X-ray space simulation observations, which are similar to the result of ground pulsar radio timing, the precision of right ascension is better than that of declination. 3) The precisions of ephemeris parameters can be improved by increasing the times of observation. 4) If each pulsar can be observed for 1000 s by space satellite every half an month with a 1 m2 effective area detector, the precisions of the estimated parameters (RA, DEC and Period) for Crab pulsar are 23.4 mas 806.0 mas, 8.121310-8 s, those of other three pulsars are gained and analyzed. However, owing to the low-flux radiation characteristics of millisecond X-ray pulsar and the demand for light and efficient large detector, the high-precision ephemeris parameters can be achieved difficultly by using the space X-ray observations, but can be established and maintained well by the ground radio observation technology. The suggestion for promoting the construction of some large-diameter telescopes is made, and the method that the behavior of X-ray emissions from pulsar is predicted by the ground radio observations still needs be studied.