###### GENERAL

2018, 67 (9): 090201.
doi: 10.7498/aps.67.20180142

Abstract +

With the rapid development of industrial manufacturing, people are stricter and stricter for measuring accuracy and demanding for measurable objects. The demand for a new generation of industrial measurement has evolved from the cooperative target toward the diffuse surface object with faster measurement speed and higher precision. Frequency modulated continuous wave (FMCW) laser ranging technology has proved to be an efficient method in the high-precision ranging fields for absolute distance measurement of a diffuse reflecting target.However, its accuracy is subjected to the stability of continuous-wave light source which cannot scan frequency linearly, which further leads to the instability of beat frequency and poor spectrum resolution. Generally, this problem could be solved by the active linearization technique and the post-processing technique. The most popular method is the non-uniform interval resampling technique, which belongs to the post-processing scheme and uses the zero-crossings or peaks of a long delay Mach-Zehnder interferometer signal as triggers for acquiring the measurement signal data. This technique is low cost, easy to be integrated into FMCW ladar system, and especially suitable for short-range small-band scanning measurements. However, in the large-bandwidth long-distance measurement cases, due to the jitter and dispersion of a long fiber, the spectrum obtained by this method is deteriorated such as the spectral broadening and distance shifting, so the range position cannot be determined precisely. To improve the precision, the fast Fourier transform, chirp Z transform and the multiple signal classification methods are used to obtain the distance spectral information. There are also other methods to solve this problem, but there is no common precision evaluation method to test the validities of these methods.In this paper, a precision evaluation method of measuring the FMCW absolute distance based on two-fiber interferometer is presented. A lower Cramer-Rao lower bound on the variance of distance parameter of the resampled signal in the presence of noise is derived. It shows that the precision of absolute distance is affected by the signal-to-noise ratio of the system and chirp bandwidth. This result is verified experimentally.Besides, the proposed method is not restricted to any distance estimation algorithm. According to this boundary, an optimal distance estimation method could be chosen. Moreover, a simulation of range precision versus signal-to-noise ratio and bandwidth is also demonstrated. When the chirped bandwidth is equal to 20 nm and the signal-to-noise ratio of absolute distance measurement interferometer is raised to more than 70 dB, the obtained precision is below 1 μm. This method can provide a theoretical reference for improving the precision of FMCW distance measurement and it could be widely used in the future.

2018, 67 (9): 090501.
doi: 10.7498/aps.67.20172505

Abstract +

Early afterdepolarization (EAD) is an important cause of lethal ventricular arrhythmias in heart failure because afterdepolarizations can promote the transition from ventricular tachycardia to fibrillation, which is related to the transition from spiral wave to spatiotemporal chaos. However, it remains unclear about how the EAD results in the breakup of spiral wave. In this paper, we explore the manner of spiral wave breakup induced by EADs under evenly distributed cells. The two-dimensional tissue is simulated with the Greenberg-Hasting cellular automaton model. The normal cells and aging cells are introduced into the model, in which the EAD only occurs in aging cells and can excite the resting cells. The numerical results show that the EAD can produce backward waves as well as forward waves. The EAD has no influence on the behavior of spiral wave in a few cases. The ratio of the number of unaffected spiral waves to the number of all tests is about 26.4%. The EAD can have various effects on spiral wave in other cases. The small influences on spiral wave are that the EAD leads to the meander, drift, and the deformation of spiral wave. The serious influences on spiral wave are that the EAD results in the disappearance and breakup of spiral wave. We find that spiral wave can disappear through the conduction block and transition from spiral wave to target wave. We observe the eight kinds of spiral wave breakups in connection with the excitation of EADs, such as symmetry breaking-induced breakup, nonsymmetry breaking-induced breakup, asymmetric excitation-induced breakup, conduction block-induced breakup, double wave-induced breakup, etc. Spiral wave generally breaks up into multiple spiral waves and spatiotemporal chaos. The ratio of the number of spiral wave breakup to the number of all tests is about 13.8%. However, the ratio of spiral wave breakup can reach about 32.4% under appropriately chosen parameters. The results are basically consistent with the survey results of arrhythmia-induced death rate. Furthermore, we also find that the excitation of EAD can prevent the spiral wave from disappearing and promote the breakup of spiral wave. The physical mechanisms underlying those phenomena are also briefly analyzed.

2018, 67 (9): 090502.
doi: 10.7498/aps.67.20172761

Abstract +

A memristor can be used in chaotic system as a nonlinear term, and thus enhancing the complexity of the chaotic system. Fractal theory is a leading and important branch of nonlinear science, and has been widely studied in many fields in the past few decades. The fractal and chaos are bound tightly and their relevant researches are well-established, but few of them focus on the research of the possibility of combining the fractal and the chaotic system. In order to obtain a multi scroll chaotic attractor, the fractal process is novelty introduced into the memristive chaotic system. In this paper, at first, a new memristive chaotic system is proposed. Then, the dynamic characteristics of the system are discussed from the aspects of symmetry, dissipation, stabilization of equilibrium points, power spectrum, Lyapunov exponent and fractional dimension. A mapping relationship based on classical Julia fractal is established. Through this mapping relationship, a multi-scroll memristive chaotic system based on the Julia fractal is obtained. Moreover, several deformed Julia fractal processes are applied to the memristive chaotic system, and abundant chaotic attractors are obtained. For example, the square term of the Julia fractal expression is multiplied by a coefficient, and according to the difference in coefficient, the resulting chaotic attractors have the same shape but different sizes. The exponent of the square term in the Julia fractal is changed into a variable, and the chaotic attractor of different scroll numbers is obtained according to the difference in power exponent. In addition, a rich multi-scroll chaotic attractor is obtained by using the fractal expression in the form of weighted sum polynomial. Finally, the influence of a complex constant in the fractal process on the system is discussed. The simulation results show that the combination of fractal process and chaotic system can obtain rich chaotic attractors, such as multi-scroll chaotic attractors. In general, compared with the single-scroll chaotic attractor, the multi-scroll chaotic attractor has a higher complexity and more adjustability. In addition, compared with other multi-scroll chaotic system, the proposed multi-scroll chaotic system is easy to adjust the number of the scrolls. To summarize, this work not only provides a new method of generating multi-scroll chaotic attractors, but also makes up for the lack of smoothness of the chaotic system caused by using functional methods.

2018, 67 (9): 090701.
doi: 10.7498/aps.67.20180150

Abstract +

Dual-comb spectroscopy is becoming a highlighted topic in broadband spectrum measurement techniques because of two outstanding advantages. One is its highly stable output frequency, which leads to an appealing resolution, and the other is the omitting of moving parts, which helps achieve extreme fast sampling rate. Utilizing the traditional radio frequency linked combs, however, obstructs the dual-comb spectroscopy reaching satisfied performance because the phase noise of the radio frequency standard causes the dual-comb mutual coherence to severely degrade. Specifically, traditional frequency comb stabilizes the carrier envelope offset at a radio frequency by a self-reference system, and the order number of each output comb tooth is over a hundred thousand. Thus, the phase noise of the radio frequency reference is significantly multiplied in output optical frequency by the same order of magnitude as the tooth order number. In this paper, we demonstrate an optical frequency linked dual-comb spectrometer where the two combs are locked to a common narrow linewidth laser. In this configuration, the two combs are synchronized at an identical optical frequency, which means that the carrier envelope offset of the two combs are changed to an optical frequency and the order number of the output comb teeth are reduced by two orders of magnitude. Therefore, not only the complex and costly self-reference system can be removed but also the phase noise of the optical frequency of each comb tooth is effectively reduced, which leads to lower mutual frequency jitters and better mutual coherence. To prove the performance, we measure the 1+3 P branch of 13C2H2 molecular and the results accord well with the reported line positions and reveals a spectral resolution of 0.086 cm-1. The average signal-to-noise ratio exceeds 200:1 (62.5 ms, 100 times on average) and the noise equivalent coefficient is 6.0106 cm-1Hz-1/2. This work provides a solution for pragmatic dual-comb spectroscopy with high resolution and low-cost configuration.

###### ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

2018, 67 (9): 094102.
doi: 10.7498/aps.67.20172292

Abstract +

Polarizing beam splitter (PBS) can separate the propagating directions of two incident orthogonally polarized light beams. However, conventional PBS and multi-layered metamaterial structures are complicated and neither of them can meet the requirements for broadband characteristics due to their resonant characters. In this paper, an anisotropic beam splitter based on metal slit array of the metal-dielectric structure is proposed in order to simplify the structure and improve the beam splitting efficiency. Because of the transverse momentum generated by the inhomogeneous interface, the transverse magnetic (TM) wave is negatively reflected from the surface of the gold film after it has entered into the slit with the waveguide mode of the plasma. When the free electrons on the metal surface oscillate, the transverse electric (TE) wave parallel to the grating direction can cause electrons to oscillate along the grating direction, so that the TE light cannot enter into the slit, resulting in specular reflection. The finite element method is used to study the effects of TM and TE polarized light such as negative reflection (NR) and specular reflection (SR). The results show that when the incident angle of the polarized light is set to be in a range from 20 to 70, the incident TM light has a strong NR of about 0.9, but the TE light is weakly reflected and decreases sharply with the increase of the wavelength. The ideal NR points of the beam splitter and the perfect symmetrical response of the reflection surface are calculated, and the ideal NR point satisfies P=/(2sin 0). When the incident light angle changes, the variations of the wavelength of the negative and zero order reflection peak are different from those of TM and TE wave, which is more conducive to the tuning of the interaction between light and grating structure. The NR and SR spectral reflectance of different polarized light beams are calculated by rigorous coupled-wave analysis, and the extinction ratios in the two cases are both 106. In addition, those designs of plasmonic splitters will pave the way for the practical applications of plasmonic devices in data storages and optical holography.

2018, 67 (9): 094201.
doi: 10.7498/aps.67.20172341

Abstract +

White light emitting diode has become the next generation of light source because of its high illuminance efficiency, low power consumption, and long life, and it has also been adopted in the application of indoor visible light communication (VLC) system. The VLC has great development potentials, however, there is a lack of research on optical receiving antenna which is a key component of VLC. Therefore, in this paper we design a novel two-stage optical receiving antenna for indoor VLC system. In the designed antenna, the lens wall of a compound parabolic concentrator with a certain rotation angle and thickness is obtained through rotating the parabolic reflector cross-section reference curve. Furthermore, a novel two-stage optical receiving antenna is designed by taking advantage of lens-wall compound parabolic concentrators and hemispherical lenses. This significantly increases the view angle by nearly 20 in the case of gain retention. The analytical model of the optical antenna in a 5 m5 m3 m open room is established by using a software of TraceProTM. The indoor VLC system is also modelled and implemented by using a software of MatlabTM. The results show that the growth rate of average received power is 757.2%, which is 5.62 times that of the compound parabolic concentrator, and the signal-noise-ratio is increased by 28.07%, on average, which is 1.67 times that of the compound parabolic concentrator. The optical gain of the two-stage optical receiving antenna is 11.49, which is 2.81 times that of the compound parabolic concentrator. The spot radius is only 2.5 mm, which is reduced by nearly 37.5% compared with the spot radius of the compound parabolic concentrator, and the energy concentration is evenly distributed at the same time. This further confirms that the designed two-stage optical receiving antenna is suitable for indoor VLC system. Finally, the performance analysis and experimental verification of the new two-stage optical receiver antenna are also given.

2018, 67 (9): 094204.
doi: 10.7498/aps.67.20172396

Abstract +

The squeezed light field is a kind of important continuous variable quantum resource.It has wide applications in precision measurement and quantum information processing.Quantum storage is the foundations of quantum repeater and long distance quantum communication,and alkali metal atoms are an ideal quantum storage medium due to long ground state coherent time. With the rapid development of quantum storage technology in atomic medium,the preparation of the squeezed light which resonates with alkali metal atoms has become one of the research hotspots in the field of quantum information.In this paper,we report the generation of squeezed vacuum at 795 nm (resonant on the rubidium D1 transition line) by using an optical parametric oscillation based on a periodically poled KTiOPO4 crystal. The generated squeezed light field is detected by a balanced homodyne detector,and the squeezing of-3 dB and anti-squeezing of 5.8 dB are observed at a pump power of 45 mW.By using a maximum likelihood estimation,the density matrix of the squeezed light field is reconstructed.The time-domain signals from the balanced homodyne detector are collected to acquire the noise distribution of the squeezed light under different phase angles.The likelihood function is established for the measured quadrature components.An identity matrix is chosen as an initial density matrix,and the density matrix of the squeezed field is obtained through an iterative algorithm.The diagonal elements of the density matrix denote the photon number distribution,which includes not only even photon number states but also odd photon number states.The occurrence of odd photon number states mainly comes from the system losses and the imperfect quantum efficiency of detector.The Wigner function in phase space is calculated through the density matrix,and the maximum value of the Wigner function is 0.309.The standard deviation of the squeezed component is 64.4% of that of the vacuum state,corresponding to the squeezing degree of-3.8 dB.The standard deviation of the anti-squeezing component is 1.64 times that of the vacuum state,corresponding to the anti-squeezing degree of 4.3 dB.We theoretically calculate the photon number distribution and the Wigner function of the vacuum squeezed field,and compare the results obtained by theoretical calculation with those obtained by maximum likelihood reconstruction.The probability of vacuum state|0 obtained by maximum likelihood reconstruction is greater,and the probability of photon number state|n(n=1,2,) is smaller than the corresponding theoretical calculation results.From the theoretical calculation,the maximum value of Wigner function is 0.231,and the short axis and long axis of noise range deduced from the contours of the Wigner function are larger than the results from the maximum likelihood reconstruction.The possible reasons for the discrepancy are as follows. 1) The phase scanning is nonuniform during the measurement of the quadrature components.2) The low-frequency electronic noise is not completely filtered out in the datum acquisition process.3) The datum points of measured quadrature components are not enough.In conclusion,we produce a vacuum squeezed field of 795 nm,and obtain the photon number distribution and the Wigner function in phase space through maximum likelihood estimation and theoretical calculation,respectively.This work will provide an experimental basis for generating the Schrodinger cat state.

2018, 67 (9): 094205.
doi: 10.7498/aps.67.20172050

Abstract +

The evolution of two-level atomic system, in which the initial state is excited state, is investigated by adjusting the structural parameters of the dynamic and static ideal photonic band-gap environment reservoir. In a static state (no modulation), we study the effects of half width, center resonant frequency, and specific gravity on the evolution of energy level population. The results show that when the half width or the specific gravity decreases, in the atomic system there happens decoherence, and the energy dissipation to the outside becomes slower. When the center resonant frequency increases, there exists no resonance between the library central resonant frequency and the atom transition frequency, then the attenuation suppression effect occurs, and the time of atomic attenuation to ground state is longer. An actual quantum system is not isolated, so it is inevitable that it interacts with its ambient environment. Owing to the influence of environment, in the system there appears an irreversible quantum decoherence phenomenon. Therefore, how to effectively suppress the decoherence of quantum system becomes an important problem in quantum information science. Linington and Garraway (2008 Phys. Rev. A 77 033831) pointed out that the evolution process of a two-level atom quantum state can be manipulated by a dynamic dissipative environment. So, we use the dynamic cavity environment to control the evolution of spontaneous emission from an excited two-level atom. The dynamic modulation form for the center resonant frequency of the ideal photonic band-gap environment reservoir includes the rectangular single pulse, rectangular periodic pulse, and slow continuous period. Owing to the periodic modulation, the atoms are affected by different environments. On this basis, the influence of dynamic modulation form on the atomic population evolution is discussed. It is found that no matter what form the dynamic modulation is in, the attenuation inhibition in the evolution of atomic system is evident. These conclusions make the idea of using the environmental change to modulate the coherent evolution of atomic system become true.

## EDITOR'S SUGGESTION

2018, 67 (9): 094207.
doi: 10.7498/aps.67.20172503

Abstract +

Optical frequency comb (OFC) is a new type of high-quality laser source. The visible and near-infrared OFCs have become mature, and it has been widely used in optical frequency metrology, time/frequency transfer, precision laser spectroscopy and other fields. Since the mid and far-infrared spectral regions contain a large number of baseband absorption lines for molecules and the absorption intensities are several orders of magnitude higher than those in the visible and near-infrared spectral region, one has made great efforts to develop the mid and far-infrared OFCs in recent years. Although a variety of approaches to achieving infrared OFCs directly have been proposed, the method of difference frequency generation (DFG) infrared OFC based on the optical rectification technique is still more efficient. DFG infrared OFCs with widely tuning ability have been demonstrated based on fiber lasers so far. However, how to obtain the broadband spectrum for a DFG infrared OFC with widely tuning ability still needs to be solved. In this paper we report a fiber-type DFG infrared OFC by using the femtosecond pulses from a mode-locked erbium-doped fiber laser as the fundamental light. Based on the self-developed mode-locked fiber laser oscillator with repetition rate locked, the two-color fundamental pulse trains with the central wavelengths of 1.5 and 2.0 m are respectively achieved after the chirped pulse fiber amplification and all-fiber supercontinuum (SC) generation techniques have been utilized. With a time-domain synchronous detection system based on the intensity autocorrelation principle, the accurate synchronization with the fundamental two-color pulses is obtained by optimizing the OFS compensated fiber length and adjusting a tunable optical delay line. Finally, by using the optical rectification technique, a fiber-type DFG infrared OFC is successfully generated with the help of a suitable designed GaSe nonlinear crystal. Our experimental results also show that the spectral location of the DFG infrared OFC can be tuned by controlling the spectral shape of the SC combined with the adjustment of the phase-matching for the nonlinear crystal. The measured tuning range of the DFG infrared OFC is from 6 to 10 m, and the maximum spectral width is 1.3 m. This fiber-type DFG infrared OFC may play an important role in the molecular spectroscopy, the atmospheric environmental monitoring, and other fields.

2018, 67 (9): 094208.
doi: 10.7498/aps.67.20172638

Abstract +

The existing theoretical equations cannot provide an excellent guidance for developing four-wave mixing (FWM)-based optical logic devices, though the experiments have been done in several researches. The optimization of noise figure performances of such devices should be further investigated. In the paper, the universal analytic expressions for the amplitude and phase of the idler in degenerate or non-degenerate FWM process under pump depletion are derived in detail from the nonlinear coupled-mode equations for guiding optical waves propagation in highly nonlinear fiber. The universal analytic expressions are obtained by the first-and the third-kind of elliptic integrals. By using equivalent infinitesimal to calculate the limit of phase sensitive amplification, we find out the initial phase relationship between the idler and the input guided wave for phase-independent amplification, which is crucially important for explaining the operating principles of the FWM-based adder and subtracter. As an example, the configuration of non-degenerate FWM-based hybrid arithmetic device with three logic functions of A+B-C, A+C-B, and B+C-A for QPSK signals is presented, and then the noise transfer characteristics in terms of signal-to-noise ratio (SNR) and error vector magnitude (EVM) are taken into account by adjusting the fiber length, input wavelength, and optical power. The calculation results show as follows. 1) This kind of arithmetic device has a noise figure of about 1.1 dB and an input SNR of more than 24 dB is necessary for the symbol error rate of 10-3 without forward error correction, corresponding to an output EVM of 23.2%. 2) The length of highly nonlinear fiber used in the hybrid arithmetic device may be taken flexibly, provided that the variation of FWM conversion efficiency is controlled in a range of 1 dB relative to the maximum, with an EVM fluctuation of less than for the idlers. 3) The hybrid arithmetic device has an operating optical bandwidth of about 16 nm for the SNR degradation of 1.3 dB. 4) The output EVM increases with the increase of input power, and the allowable input power should be no more than 100 mW for an input SNR of 28 dB, noting that the larger the input SNR, the higher the allowable input power is.

2018, 67 (9): 094301.
doi: 10.7498/aps.67.20172665

Abstract +

Hydrogen is an important energy carrier, and it is widely used due to its extraordinary advantages, such as high heat, clean fuel, being large-scale and renewable. The detection of hydrogen is essential in practical application. Therefore, many researches have focused on monitoring the hydrogen concentration over the past years. Acoustic relaxation theory based on molecular relaxation process is a very promising method of detecting hydrogen gas. However, the existing acoustic relaxation models for gas detection are developed from the vibrational relaxation of gas molecules, and thus they are not applicable for hydrogen and its mixture. In this paper, we present a model for the rotational relaxation process of hydrogen. Firstly, the molecular relaxation process of hydrogen is different from those of other gases due to its large spacing of rotational energy-level and special molecular physical structure. Acoustic relaxation process of hydrogen is mostly determined by the molecular rotational relaxation. Hydrogen molecule is made up of one quarter of para-hydrogen and three quarters of ortho-hydrogen at normal temperature. There is three-rotational-level model for hydrogen rotational relaxation, such as rotational level in states with J=0, 2, 4 (J is rotational quantum-number) for para-hydrogen and J=1, 3, 5 for ortho-hydrogen. Secondly, we introduce effective specific heat into one-mode rotational relaxation at constant pressure, and then extend it to multi-mode rotational relaxation. Upon periodic perturbation of acoustic waves, the temperature and the number of molecules in each rotational level change periodically in the relaxation process. On the basis, we obtain the relaxation equations in a matrix form and calculate effective specific heat at constant pressure for rotational relaxation process. With the relationship between the complex wave number and the effective thermodynamics acoustic speed, we calculate the frequency-dependent acoustic speed and relaxation absorption, and then discuss the difference between the rotational relaxation and the vibrational relaxation. Thirdly, we compare the predicted acoustic speed and absorption spectrum with their corresponding experimental data and investigate the influences of rotational characteristics on absorption spectra in hydrogen and its mixtures. The simulation results show that acoustic speed and relaxation absorption curves calculated by the proposed model are in good agreement with their corresponding experimental data. The model is not only applicable to pure hydrogen gas but also can be used to obtain the acoustic relaxation spectra of gas mixtures with multiple vibrational modes. This model provides a theoretical foundation for the acoustic detecting of hydrogen gas mixtures.

2018, 67 (9): 094701.
doi: 10.7498/aps.67.20172613

Abstract +

The thin shell (layer) configuration is adopted in inertial-confinement fusion (ICF) implosions. The weakly nonlinear deformation of the thin shell significantly influences the performances of implosion acceleration and fusion ignition, which is an important issue for the study of ICF physics. Based on the thin layer model of Ott (Ott E 1972 Phys. Rev. Lett. 29 1429), an improved thin layer model is proposed to describe the deformation and nonlinear evolution of the perturbed interface induced by the Rayleigh-Taylor instability (RTI). Differential equations describing motion are obtained by analyzing the forces of fluid elements (i.e., Newton's second law), which are then solved by numerical method. Then the position of the perturbed interface with an initial perturbation can be obtained. The linear growth rate obtained from our thin layer approximation agrees with that from the classical RTI. For fixed Atwood number (wave number), the total amplitudes of the bubble and spike obtained from the improved thin layer model agree with those from the three-order weakly nonlinear model. In addition, we compare the deformation and evolution of the layer from our model with results of the numerical simulation. In the linear regime, the amplitudes of the bubble and spike obtained from our model agree with those from the numerical simulation. And the evolution of the perturbed interface obtained from the improved thin layer model is consistent with that from the numerical simulation. In the nonlinear regime, the evolution trends of the total amplitude of the bubble and spike for both the improved thin layer model and numerical results are the same. However, the amplitude of the bubble is obviously greater than that of the spike in the later stage of the perturbation. This is because of some shortcomings in the improved thin layer model. The first shortcoming is that ignoring the dynamical pressure in the pressure difference. In fact, the shear velocity of the fluids plays an important role in the nonlinear regime of the perturbation. The second shortcoming is that the surface area of the upper interface equals the lower interface in the whole perturbation process of the present model. Thus, the present model can be used to describe the nonlinear evolution of the perturbed interface before the mushroom structure. Finally, it is worth noting that the improved thin layer model can be used to describe the deformation and nonlinear evolution of a thin layer for arbitrary Atwood number with a perturbation of large initial amplitude and arbitrary distribution. The initial perturbations of the triangular and rectangular waves are also discussed.

2018, 67 (9): 094101.
doi: 10.7498/aps.67.20172608

Abstract +

The resolution of traditional far-field imaging system is generally restricted by half of wavelength of incident light due to the diffraction limit. The reason is that evanescent waves carrying subwavelength information cannot propagate in the far-field and make no contribution to the imaging. To realize the far-field super-resolution imaging, the imaging system should be able to collect both propagation and evanescent waves. Many ideas were presented to provide feasible alternatives but with narrow frequency band. In this paper, a wideband metalens is proposed to realize far-field super-resolution based on stereometamaterials. A typical model of stereometamaterials is studied, which consist of a stack of two identical spiral resonators in each cell, with various twist angles. For each case, there are two observable resonances (-and +), obviously. The phenomenon can be explained as the plasmon hybridization between the two resonators due to their close proximity. The case with a twist angle of 90 is chosen as the basic cell to constitute the stereo-metalens (S-ML). The last S-ML can work in a frequency range from 1.06 to 1.53 GHz, which is much wider than the planar-metalens. Simulations of near-and far-field spectra are conducted to validate the conversion between evanescent waves and propagation waves. Then with the help of antennas in the far-field to receive the information, sub-wavelength image can be reconstructed. The simulations in frequency-and time-domain are performed to verify the super-resolution characteristics of the S-ML. In frequency-domain, an imaging simulation of L-shaped extended target is combined with multiple signal classification imaging method. The resolution defined by full width at half maximum is 19 mm, corresponding to /12. For comparison, a similar simulation without the S-ML is performed, indicating a resolution of 1.5. It shows the ability of the S-ML to enhance the imaging resolution. In time-domain, by using time reversal technique, the spatial super-resolution characteristic of the S-ML is validated. Compared with the planar-metalens, the S-ML has good spatial super-resolution characteristic. All results show that the S-ML has a good potential application in imaging.

2018, 67 (9): 094202.
doi: 10.7498/aps.67.20172528

Abstract +

It is a simple and commonly-used approach to use an inclined plane reference wave to remove zero-order diffraction and conjugated image in digital off-axis holography. However, this method is encountering a difficulty, since an additional carrier frequency is incorporated into the inclined reference wave and it is difficult to accurately obtain this additional carrier frequency via experimental measurement, a certain tilt distortion of the phase image will occur in the hologram reconstruction. In this paper, a numerical reference plane algorithm is proposed to solve this problem. This method innovatively constructs a numerical reference plane which is able to exactly characterize the tilt of the phase image by choosing three different points from a local flat of the reconstructed image, and establishes a mathematical relation between the plane parameters and the carrier frequency of the reference wave, which is used as a criterion of correcting the tilt distortion of the phase image in the subsequent iterative computation. The procedures of the algorithm are as follows. 1) Input the nominal carrier frequencies, (fx', fy') of the plane reference wave and reconstruct the hologram. 2) Unwrap the phase with PUMA algorithm and suppress the noise using bilateral filtering and short time Fourier transform with wavelet shrinkage. 3) Construct the numerical reference plane reflecting the image inclination and establish the mathematical relation between the plane parameters and the carrier frequencies of the reference wave. 4) Perform the iterative computation to correct the nominal carrier frequencies, (fx', fy') by using the differential coefficients, (a, b) of the reference plane equation as the criterion. 5) Output the computation result and the corrected phase image. The algorithm is simple and effective. It is able not only to achieve accurate correction to the tilt phase distortion, but also to exactly obtain the additional carrier frequency of the inclined plane reference wave. Since in the phase unwrapping reconstruction, the proposed approach combines with bi-lateral filtering processing, wavelet shrinking and short time Fourier transform to remove the noise influence while the image details are preserved, the method would still be valid under the influences of environmental and system noise. The experimental result supports the theoretical prediction very well.

2018, 67 (9): 094203.
doi: 10.7498/aps.67.20172464

Abstract +

Computer-generated hologram (CGH) makes possible the three-dimensional (3D) display of true stereo. It has characteristics of strong flexibility, small noise, easy replication, and computable virtual object. However, there are still some difficulties with the CGH 3D display presently, such as slow computation speed of complex object hologram, small size and small field angle of 3D scene, much noise of reconstruction image, and true color display. In this paper, the problem of reconstruction image noise and true color display of the CGH are studied, and the hologram of true color 3D object with complex morphologies is calculated. First of all, the angular-spectrum layer-oriented method can avoid error caused by the paraxial approximation and be used to accurately generate and calculate 3D object hologram. And it also has advantages of efficient computation, reduced complexity, and less storage memory. We achieve the true color display of a 3D object by using the angular-spectrum method based on intensity and depth maps. We also analyze the problem of multi-wavelength sampling, and mitigate the phenomenon of frequency mixing effectively. Then, we propose to use the Gerchberg-Saxton (GS) algorithm along with the angular-spectrum layer oriented method to reduce the speckle noise in the reconstruction image. The root mean-square error (RMSE) and peak signal-to-noise ratio (PSNR) of the reconstruction image by angular-spectrum layer-oriented method with the GS algorithm are compared with those obtained in the case without using the GS algorithm. The RMSE and PSNR are the main methods of evaluating the image quality. Smaller RMSE and bigger PSNR correspond to higher quality of the image. The hologram and reconstruction image of the true color locomotive with complex morphologies are calculated using the method proposed in this paper and the locomotive is divided into three parts:head, middle and tail. The RMSE and the PSNR of reconstruction image of the head are approximately 0.77 and 65.7, respectively. The RMSE and the PSNR of reconstruction image of the middle are approximately 0.68 and 70.0, respectively, and so are those of the tail. Comparing with the traditional angular-spectrum layer-oriented method, the RMSE of the reconstruction images of the head, middle and tail are reduced approximately by 0.11, 0.40, 0.41, and the PSNR are increased approximately by 1.15, 5.70, 4.13, respectively. The simulation results show that the speckle noise is suppressed effectively and the quality of the reconstruction image is improved when the GS algorithm along with the angular-spectrum layer oriented method is used. The proposed method is more suitable for the calculation of complex 3D objects with true color.

2018, 67 (9): 094206.
doi: 10.7498/aps.67.20172345

Abstract +

In recent years, dual repetition-rate mode-locked lasers with slightly different pulse repetition rates, as newly developed ultrafast lasers, have attracted great interest and shown their applications in ultrafast dual-comb spectroscopy, asynchronous optical sampling without mechanical movement, etc. The traditional dual-comb system composed of a pair of independent optical frequency combs with slightly detuned comb spacing is still considered expensive, complex and fragile. It is imperative to develop practical and compact dual-comb devices. Dual repetition-rate ultrafast lasers generating asynchronous ultrafast pulses directly from a single cavity can be a promising alternative to the current dual-laser-based comb source. A dual-comb setup based on single laser has the advantages of compact structure, low cost and intrinsic mutual coherence. This technique paves the way for developing the compact, robust and environmental-immune dual-comb systems. In this paper we develop an alternative dual repetition-rate mode-locked Yb:YAG ceramic laser that emits a pair of pulses with spatially separated beams from a single cavity by using a semiconductor saturable absorber mirror and a dual-path pump configuration. In our experiment, a high quality transparent Yb:YAG ceramic prepared by non-aqueous taper-casting method is selected as the gain medium, which is pumped by a 940 nm laser diode. A dual-path pump configuration consisting of a pair of polarization beam splitters and a pair of half-wave plates is designed, in which total pump power from a laser diode is divided equally for pumping the two separate laser beams. When the total absorbed pump power is 5.6 W, dual repetition-rate continuous mode-locked laser operation is achieved under the gain-loss balanced cavity condition. The pulse repetition rates of Pulse1 and Pulse2 are 448.918 MHz and 448.923 MHz, respectively. The difference between repetition rates is 5 kHz mainly caused by the different optical path lengths in the cavity. Under an absorbed pump power of 7 W, the maximum total output power extracted from this laser reaches 170 mW, i.e., 89 mW for Pulse1 and 81 mW for Pulse2. The two mode-locked pulses have nearly identical spectral shapes centered at 1029.6 nm and 1029.8 nm, respectively. The spectral bandwidths for Pulse1 and Pulse2 are 1 nm and 1.16 nm, respectively. The corresponding pulse durations are 2.8 ps and 2.6 ps for the Pulse1 and Pulse2 respectively. Our scheme integrates the advantages of self-starting operation, high repetition-rate, suppression of gain competition. These results indicate that dual-path pump configuration is feasible for dual-repetition-rate mode-locked lasers. These co-generated, dual repetition-rate pulses from one laser cavity possess similar laser characteristics and can be operated independently by dual-path pump configuration. This laser has potential advantages of compact, cost-effective and high-stability for single-cavity-based dual-comb applications in dual-comb spectroscopy, distance ranging, etc.

2018, 67 (9): 094501.
doi: 10.7498/aps.67.20172549

Abstract +

Granular system commonly encountered in industry or nature is comprised of non-spherical grains. Comparing with spherical particles, high discretization and interlocking among non-spherical particles can effectively dissipate the system energy and improve the buffer capacity. The superquadric element based on continuous function envelop can form the geometric shape of irregular particles accurately, and then contact collision action between particles can be calculated easily. In this paper, we provide a comprehensive introduction to particle-particle and particle-boundary contact collision. In addition, considering different shapes and surface curvatures under various contact patterns between super-quadric particles, the linear contact force model cannot be applied to the accurate calculation of the contact force, and a corresponding non-linear viscoelastic force model is developed. In this model, the equivalent radius of curvature at a local contact point is adopted to calculate the normal contact force, and the tangential contact force is simplified based on the contact model of spherical elements. To examine the validity of the algorithm and this model, we compare the discrete element analytical results with the analytical results for a single cylinder impacting a flat wall and the previous experimental results for spherical granular material under impact load, and this method is verified by good agreement between the simulated results and the previous experimental results.
According to the aforementioned method, we study the buffer capacity of non-spherical particles under impact load by the discrete element method, and the influences of granular thickness and particle shapes on the buffer capacity are discussed. The results show that a critical thickness Hc is obtained for different particle shapes. The buffer capacity is improved with increasing the granular thickness when H Hc, but is independent of the granular thickness and particle shapes when H Hc. Moreover, the impact peak and initial packing fraction increase significantly with increasing the blockiness. Rectangular particles account for the highest packing fraction, and the packing fraction of cylindrical particles is higher than the packing fraction of spherical particles. Therefore, Rectangular particles are more likely to form dense face-face contacts and ordered packing structures with high packing fraction. These denser packings prevent the particles from their relatively moving, and thus reducing the buffering capacity of the particles. Furthermore, the impact peak and initial packing fraction decrease with increasing or reducing the aspect ratio of cylindrical particles and the aspect ratio of rectangular particles. The aspect ratio of particle can be used to adjust the dense packing structure and reduce the stability of the system. It means that the particles have more effective buffer capacity for the non-spherical particle system.

###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

2018, 67 (9): 096101.
doi: 10.7498/aps.67.20180027

Abstract +

The linear bipolar devices and integrated circuits (ICs) which are subjected to ionizing radiation exhibit parametric degradations due to current-gain decrease, and the amount of degradation on various types of bipolar devices is much more significant at low-dose-rate than at high-dose-rate. Such an enhanced low-dose-rate sensitivity (ELDRS) is considered to be one of the major challenges for radiation-tolerance testing intended for space systems. Therefore, it is of great significance to explore an efficient and practical test for the ELDRS in the linear bipolar devices and ICs. The different experiments have been implemented on four types of bipolar ICs for evaluating their responses to low-dose-rate irradiation. The experiments involve the dose rate switching approach performed under high to low-dose-rate irradiation and temperature switching approach performed under high to low temperature irradiation. Good agreement is observed between predictive curves obtained at dose rate switching irradiation and the low-dose-rate results, and the irradiation time for the dose rate switching approach is reduced from 4 months to a week. Further, the results also suggest that the device degradation rate can affect the prediction of the total dose. This is because the curves examined at different doses have a lot of overlap when the devices with fast degradation rates are performed. In addition to temperature switching irradiation, the radiation response of the same type of device is much more significant than that obtained in low-dose rate irradiation, and this method will shorten the irradiation time to 12 h. Based on the analysis of mechanisms behind the switched dose rate and temperature irradiation, switching temperature irradiation can accelerate the release of protons and buildup of interface traps, which is the key physical mechanism for ELDRS. Firstly, a higher irradiation temperature can enhance the transport of holes and release of protons to form interface traps, resulting in the enhanced degradation occurring at first dose examined. Further, the reducing temperature sequence suppresses the hydrogen dimerization process during the irradiation that follows, which is strongly temperature dependent and contributes to interface trap annealing. Moreover, further decrease in temperature can restrict the interface trap annealing because the barrier for this process is higher and it has less opportunity to take place at lower temperature. Additionally, the hydrogen molecules converted from hydrogen dimerization may extend the liberation of protons, by the hydrogen molecules cracking mechanisms, leading to the additional degradation. Therefore, the temperature switching irradiation is shown to be a conservative and efficient method for ELDRS in bipolar devices, and this provides an insight into hardness assurance testing.

###### CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

2018, 67 (9): 097301.
doi: 10.7498/aps.67.20172221

Abstract +

Molecular device is the ultimate electronic devices in the view point sense of scale size.Electron transport in molecular device shows obvious quantum effect,and the transport property of molecular device will be strongly affected by the chemical and structural details,including the contact position and method between the molecule and electrodes,the angle between two electrodes connecting to the molecule.However,we notice that in the existing reports on device simulations from first principles the two electrodes are always in a collinear case.Even for multi-electrode simulations,one usually used to adopt orthogonal electrodes,namely,each pair of the electrodes is in a collinear case.As the electrode configuration will clearly affect the transport property of a device on a nanometer scale,the first principles quantum transport studies with non-collinear electrodes are of great importance,but have not been reported yet.In this paper,we demonstrate the calculations of a transport system with non-collinear electrodes based on the state-of-the-art theoretical approach where the density functional theory (DFT) is combined with the Keldysh non-equilibrium Green's function (NEGF) formalism. Technically,to model a quantum transport system with non-collinear electrodes,the center scattering region of the transport system is placed into an orthogonal simulation box in all the other quantum transport simulations,while one or two electrodes are simulated within a non-orthogonal box.This small change in the shape of the simulation box of the electrode provides flexibility to calculate transport system with non-collinear electrodes,but also increases the complexity of the background coding.To date,the simulation of transport system with non-collinear electrodes has been realized only in the Nanodcal software package.
Here,we take the Au-benzene (mercaptan)-Au molecular devices for example,and systematically calculate the quantum transport properties of the molecular devices with various contact positions and methods,and specifically,we first demonstrate the effect of the angle between the two electrodes on the transport property of molecular device from first principles.In our NEGF-DFT calculations performed by Nanodcal software package,the double- polarized atomic orbital basis is used to expand the physical quantities,and the exchange-correlation is treated in the local density approximation,and atomic core is determined by the standard norm conserving nonlocal pseudo-potential.Simulation results show that the chemical and structural details not only quantitatively affect the current value of the molecular device,but also bring new transport features to a device,such as negative differential resistance.From these results,we can conclude that the physics of a transport system having been investigated in more detail and a larger parameter space such as the effect of the contact model having been assessed by a comparison with ideal contacts,further understanding of the transport system can be made and more interesting physical property of the device can be obtained,which will be useful in designing of emerging electronics.

2018, 67 (9): 097501.
doi: 10.7498/aps.67.20180212

Abstract +

In order to tune the crystalline texture evolution and magnetic properties of the Sm-Fe film, molecular beam vapor deposition method is used to fabricate the Sm-Fe films. Sm content, thickness, and high magnetic field are used to affect the crystalline texture and magnetic properties. X-ray diffraction is used to analyze the texture evolution. Atomic force microscope is used to observe the surface morphology and roughness. Energy-dispersive X-ray spectroscopy is used to measure the compositions of the film. Vibrating sample magnetometer is used to test the magnetic properties. The results show that the crystalline textures are tuned through the Sm content. The crystalline texture evolution and high magnetic field have significant effect on the magnetic properties of the Sm-Fe film. The Sm-Fe film with 5.8% atomic content is of bcc crystal structure and is of amorphous structure with 33.0% Sm. Neither the thickness nor the high magnetic field has an influence on the crystalline texture. The surface roughness and particle size on the surface of the amorphous film are smaller than those of the crystal film. A 6 T high magnetic field increases the surface particle size and reduces the surface roughness. Saturation magnetization Ms of the amorphous film is 47.6% lower than that of the crystal film (1466 emu/cm3, 1 emu/cm3=410-10 T). The 6 T high magnetic field reduces the Ms of crystal and amorphous film by about 50%. The coercivity Hc values of the Sm-Fe films are in a range of 6-130 Oe (1 Oe=103/(4) A/m). The Hc of the amorphous film is higher than that of the crystal film. The 6 T high magnetic field increases the Hc of the crystal film and reduces the Hc of the amorphous film. The highest reduction is 95%. The anisotropy of the crystal film transforms to isotropy of the amorphous film. High magnetic field increases the anisotropy of the crystal film. The squareness of the crystal film is much higher than that of the amorphous film. High magnetic field has a significant effect on the measured magnetic field to obtain saturation magnetization in the film. This measured saturation magnetic field increases in the amorphous film and decreases in the crystal film after the high magnetic field has been exerted during the film growth. These results indicate that the Sm content and high magnetic field can be used to tune the crystal textures and magnetic properties of the Sm-Fe films.

2018, 67 (9): 097801.
doi: 10.7498/aps.67.20180114

Abstract +

Metamaterials, composed of subwavelength resonators, have extraordinary electromagnetic properties which rely on the sizes and shapes of the resonance structures rather than their compositions. Recently, achieving electromagnetically induced transparency (EIT) in metamaterial system, also called electromagnetically-induced-transparency-like (EIT-like) analogue, has attracted intense attention. Many studies of EIT-like metamaterials have been reported at microwave, terahertz, and optical frequencies numerically and experimentally. However, most of the EIT-like metamaterials can only control the transmission window by changing the structure size of the metamaterial which restricts the practical applications of the EIT-like metamaterial. Therefore, a broadband tunable EIT-like metamaterials based on graphene in terahertz band is presented in this paper, which consists of a cut-wire as the bright resonator and two couples of H-shaped resonators in mirror symmetry as the dark resonators. The transmissivity of the metamaterial structure is simulated by the software CST Microwave Studio. And the simulation results show that the transmission window of this structure is in a frequency range from 1.05 THz to 1.46 THz, which is attributed to the interference between the plasmon resonance of wire resonators and the LC resonance of H-shaped resonators. In addition, increasing the number of dark mode resonators leads to an increase in transmission window bandwidth. Furthermore, a broadband tunable property of transmission amplitude is realized by changing the Fermi level of graphene. When the graphene Fermi level gradually increases from 0 eV to 1.5 eV, the transmission amplitude of the transmission window gradually decreases from 87% to 20%, which realizes the broadband tunability of transmission window. At the same time, the distribution of the electric field at a central frequency of 1.26 THz is simulated to analyse the transmission mechanism. Finally, the EIT metamaterial samples are prepared and the transmission curves of the samples are tested by terahertz time-domain spectroscopy. Such an EIT-like metamaterial not only realizes the broadband EIT property but also realizes the characteristic of the tunable amplitude of the transmission window, which has potential applications in designing the active slow-light devices, terahertz active filtering and terahertz modulator.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

2018, 67 (9): 098501.
doi: 10.7498/aps.67.20172674

Abstract +

Memristor, memcapacitor and meminductor are novel nonlinear circuit elements with memory, which are also known as the memory elements. Based on the mathematical models of these three circuit elements, from the point of view of mathematical analysis, memristor, memcapacitor and meminductor Simulink based models are established. Simulink models of the memory elements reflect that their values are dependent on their historical states and their state variables, and correctly show their unique memory properties. A series of simulation analyses are done, and the typical characteristics of the three memory elements are obtained, showing the validities of these models. In addition, by studying the circuit characteristics under different parameters and excitations, the changing laws of these equivalent models with frequency and amplitude are obtained, which lay the foundation for research and application based on memristor, memcapacitor and meminductor's Simulink simulator.

2018, 67 (9): 098801.
doi: 10.7498/aps.67.20172779

Abstract +

In the classical planar heterojunction perovskite solar cells (PSCs), the electron conducting TiO2 layer shows lower conductivity than the hole transporting materials such as spiro-OMeTAD, which becomes one of the key problems in improving the power conversion efficiency (PCE) of PSCs. In this study, the surface of compact TiO2 layer is modified by a thin self-assembled dodecanedioic acid (DDDA) molecular layer. The TiO2 substrates are immersed into the DDDA solution for 0.5, 2.5, 4.5, 22 h, respectively. It is found that the PCE of PSCs is improved when using the DDDA modified TiO2, showing optimized PCE of 15.35%0.75% under AM 1.5G illumination at 100 mWcm-2 after 4.5 h modification. The short current density (JSC) of the best device is improved from 20.34 mA cm-2 to 23.28 mA cm-2, with the PCE increasing from 14.17% to 15.92%. And it is found that the hysteresis of the PSC is also reduced remarkably with hysteresis index decreasing from 0.4288 to 0.2430. In the meantime, the device with DDDA modification shows a significant improvement in light stability, keeping 71% of its initial PCE value after 720 min exposure under AM 1.5G illumination at 100 mW cm-2 without encapsulation. As a contrast, the device without DDDA modification keeps 59% of its initial PCE value under the same condition. To reveal the mechanism, we investigate the surface energy level change using ultraviolet photoemission spectroscopy. It is found that after DDDA modification, the valence-band maximum energy (EVBM) of TiO2 decreases from -7.25 eV to -7.32 eV, and the conduction-band minimum energy (ECBM) of TiO2 from -4.05 eV to -4.12 eV. The shifting of energy level optimizes the energy level alignment at the interface between the TiO2 and perovskite. It promotes the transport of electrons from perovskite layer to compact TiO2 layer and obstructs the transport of holes from perovskite layer to compact TiO2 layer more effectively. In addition, the decrease of ECBM implies the increase of conductivity of TiO2. We further design a series of electrical experiments, and confirm that the modification improves the conductivity of TiO2 obviously with both contact resistance and thin-film resistance decreasing. In summary, our results indicate the enormous potential of the compact TiO2 layer with a thin self-assembled DDDA molecular layer modification to construct efficient and stable planar heterojunction PSCs for practical applications.

2018, 67 (9): 098901.
doi: 10.7498/aps.67.20172295

Abstract +

Identifying the most important nodes is significant for investigating the robustness and vulnerability of complex network. A lot of methods based on network structure have been proposed, such as degree, K-shell and betweenness, etc. In order to identify the important nodes in a more reasonable way, both the network topologies and the characteristics of nodes should be taken into account. Even at the same location, the nodes with different characteristics have different importance. The topological structures and the characteristics of the nodes are considered in the complex network dynamics model. However, such methods are rarely explored and their applications are restricted. In order to identify the important nodes in undirected weighted networks, in this paper we propose a method based on dynamics model. Firstly, we introduce a way to construct the corresponding dynamics model for any undirected weighted network, and the constructed model can be flexibly adjusted according to the actual situation. It is proved that the constructed model is globally asymptotic stable. To measure the changes of the dynamic model state, the mean deviation and the variance are presented, which are the criteria to evaluate the importance of the nodes. Finally, disturbance test and destructive test are proposed for identifying the most important nodes. Each node is tested in turn, and then the important nodes are identified. If the tested node can recover from the damaged state, the disturbance test is used. If the tested node is destroyed completely, the destructive test is used. The method proposed in this paper is based on the dynamics model. The node importance is influenced by the network topologies and the characteristics of nodes in these two methods. In addition, the disturbance test and destructive test are used in different situations, forming a complementary advantage. So the method can be used to analyze the node importance in a more comprehensive way. Experiments are performed on the advanced research project agency networks, the undirected networks with symmetric structures, the social network, the Dobbs-Watts-Sabel networks and the Barrat-Barthelemy-Vespignani networks. If the nodes in the network have the same dynamic model, the network is considered to be the homogeneous network; otherwise, the network is heterogeneous network. And experiments can be divided into four categories, namely, the disturbance test, the destructive test on the homogeneous network, the disturbance test and the destructive test on the heterogeneous network. The experimental results show that the methods proposed in this paper are effective and credible.

###### GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

2018, 67 (9): 099701.
doi: 10.7498/aps.67.20172100

Abstract +

In the traditional compressed sensing algorithms, the precision of the time delay estimation is closely related to the number of atoms in the dictionary. The bigger the atom number, the smaller the atomic interval becomes, thus the higher the accuracy of the time delay estimation will be. However, the bigger atom number leads to a higher calculation load. Considering the limited calculation capacity of on-board computer, in order to fast obtain high-accuracy time delay estimation value of the integrated pulsar profile of pulsar in the X-ray pulsar-based navigation, we propose a time delay estimation method based on two-level compression sensing. Compressed sensing mainly includes three parts:the dictionary, the measurement matrix, and the recovery algorithm. Among them, the dictionary size is one of the most important factors that affect the estimation accuracy of the compressed sensing. Aiming to solve the problem of the greater computational load with the increase of the atom number in the dictionary of compressed sensing while improving the accuracy of estimation, we combine the rough estimation with the precision estimation as a two-level dictionary. In the first level, the global phase estimation of the low-dimensional integrated pulsar profile is carried out by making use of the feature of the large atomic interval and the small atomic amount of the rough estimation dictionary. Specifically, first, construct a coarse estimation dictionary according to the low-dimensional standard pulsar profile. Then make dimension reduction sampling on the low-dimensional integrated pulsar profile by the rough estimation measurement matrix based on low-dimensional Hadamard matrix. Finally, use an orthogonal matching pursuit method to obtain the predictive estimation of delay value. In the second level, by taking advantage of the small atomic intervals and numbers of the precise estimation dictionary which are suitable for local estimation, the exact time delay estimation of the high dimensional integrated pulsar profile is performed. Specifically, the original position is first corrected by using the predictive estimation of time delay value, that is, shifting the initial high-dimensional integrated pulsar profile as the input signal of the second level. Then the precise estimation dictionary is constructed according to the partial signal of the length of the high dimension standard pulse profile, using the precise estimation measurement matrix sampling on high-dimensional integrated pulsar profile to obtain measurement value. Finally, the optimal matching position is obtained through the recovery algorithm, which is then combined with the predictive estimation of delay value to calculate the prcis time delay estimation value. Theoretical analysis and experimental results show that the quantity of data in the two level dictionary is two orders of magnitude smaller than in the traditional dictionary. The proposed method reduces the computational complexity greatly compared with traditional compression sensing method in the same time delay estimation accuracy. Therefore, this method has the advantages of high precision and small calculation load.

2018, 67 (9): 099501.
doi: 10.7498/aps.67.20172686

Abstract +

To understand the characteristics of the anisoplanatic error resulting from different return-light experiences between the sodium beacon with a greater angular offset and the science object through atmospheric turbulence, the angular anisoplanatism for sodium beacon is investigated experimentally based on the technique of synchronized range gating. The return-light spot arrays through turbulent atmosphere from the natural star and the sodium beacon with 50 rad angular offsets are synchronously collected by using a single Hartmann wavefront sensor, consequently the synchronous turbulence-induced wavefront distortion sequences are recovered for the on-axis natural star and the off-axis sodium beacon. According to the experimental data, the temporal correlations of the wavefront distributions and decomposed Zernike modes between the on-axis natural star and the off-axis sodium beacon are discussed. By comparing the off-axis sodium beacon with the on-axis natural star, we analyse the statistics of the acquired angular anisoplanatism error and its associated Zernike-modal variances for the off-axis sodium beacon, and derive the Zernike-modal relative anisoplanatic errors as well. Furthermore, the influence of the acquired angular anisoplanatism error on the quality of imaging point spread function (PSF) is studied. The experimental results show that the existence of 50 rad angular deviation between the sodium beacon and the natural star causes that there are a certain correlation between just low-order Zernike modes of these two types of wavefronts (e.g. from the 3rd order to the 9th order), but the correlations between other high-order Zernike modes of these two types of wavefronts are severely degenerated and even these modes are de-correlated, resulting from the improper turbulence probing with off-axis sodium beacon off the ray path from the natural star to the telescope aperture. The angular anisoplanatism error has a great influence on the quality of imaging PSF, which leads to a degradation of Strehl ratio of 0.31-0.22 and beam quality factor of 2.70-3.35. Therefore, the influence may not to be ignored. At the end of this paper, according to the derived experimental turbulence coherence length and the generalized Hufnagel-Valley model, we calculate the theoretical anisoplanatic phase variance for the sodium beacon with 50 rad angular offsets, which is in good accordance with the measured anisoplanatic phase variance. This investigation is useful in promoting our knowledge of sodium beacon angular anisoplanatism effect on turbulence probing.

###### REVIEW

## COVER ARTICLE

2018, 67 (9): 090702.
doi: 10.7498/aps.67.20180226

Abstract +

Terahertz (THz) technologies have broad application prospects in ultrafast space communication, heterodyne detection, biological detection, non-destructive testing and national security. Ultrafast THz detectors, which can respond to the THz light with modulation rate larger than 1 GHz, are the key component of fast imaging, space communication, ultrafast spectroscopy and THz heterodyne applications. Theoretically, the traditional THz detectors based on heat effects are difficult to meet the requirements for fast detections, while the semiconductor based THz detectors can work under the condition of ultrafast detection. Photoconductive antennas with ultrafast response time are suitable for room-temperature broad-spectrum THz detections. Schottky barrier diodes, superconductor-insulator-superconductor mixers and hot electron bolometers are promising candidates for high-speed THz spatial heterodyne and direct detections attributable to their high conversion efficiency and low noise. High-mobility field effect transistors based on two-dimensional graphene material have the advantages of high sensitivity and low impedance, which make this kind of device have great potential applications in room-temperature high-speed detections. THz quantum well detectors (THz QWPs) based on inter-subband transitions are very suitable for the applications in high-frequency and high-speed detections because of the advantages of high responsivity, small value and integrated packaging. Recently, we have demonstrated 6.2 GHz bandwidth modulation by using THz QWPs, the fast THz receiving device. On the other hand, low working temperature and low coupling efficiency are the main factors that restrict the applications of THz QWPs. From the Brewster angle, 45 polished facet coupling structure, to one-or two-dimensional metal grating and surface Plasmon polariton coupling configuration, researchers often explore the appropriate coupling mechanism which can not only couple the normal incidence THz light, but also improve the coupling efficiency substantially. The sub-wavelength double-metal micro-cavity array coupling structure has two advantages which make THz QWPs a key candidate for fast imaging and detection in THz band:firstly, the patch antennas on the device surface can effectively increase the light absorption region, and the periodic structure can make the normal incidence THz light fulfill the rule of intersubband transition. Secondly, the sub-wavelength size double metal structure can restrict the light within a very small volume, and the electric current will be enhanced by the resonance effect when the cavity mode is equal to the peak response frequency, which can suppress the dark current and improve the optical coupling efficiency of the device. In this paper, several ultrafast THz detectors are reviewed and the advantages and disadvantages of various detectors are also analyzed.

## EDITOR'S SUGGESTION

2018, 67 (9): 097201.
doi: 10.7498/aps.67.20180244

Abstract +

The pump-orientation-probe technique is a recently-developed novel transient measurement technique, which has unique advantages in probing the ultrafast dynamics of charge separation in colloidal nanostructures. In this technique, the linearly-polarized pump pulse is applied to generating electron-hole pairs, and the circularly-polarized spin-orientation pulse is used to establish the electron spin polarization, whose dynamics is detected by monitoring the polarization change of the linearly-polarized probe pulse. Initially, the wavefunctions of the electron-hole pairs are spatially overlapped, and the lifetime of the electron spin is short because of the strong electron-hole exchange interaction. If the electrons or the holes are trapped by the surfaces of the colloidal nanostructures, the spatial separations between the electrons and the holes weaken the exchange effect, and thus the lifetime of the electron spin is largely lengthened. The evolutions of electrons and holes from their spatial overlap to separation can be revealed by monitoring the change of the electron spin dynamics. Based on the introduction of the conventional two-beam carrier pump-probe and spin pump-probe techniques, the features and optical layout of three-beam pump-orientation-probe technique are described in depth. The application to probing negative or positive photocharging in CdS colloidal quantum dots is taken for example and discussed in depth. Compared with the conventional time-resolved absorption or time-resolved fluorescence spectroscopy, the pump-orientation-probe technique can detect the dynamics of trapping electrons or holes and distinguish the type of charging state easily and directly, which has particular advantages under the high-power excitation condition. Further outlook of the three-beam pump-orientation-probe technique is also presented finally.

2018, 67 (9): 098502.
doi: 10.7498/aps.67.20180074

Abstract +

The influences of hydrogen impurities on the performances of indium-gallium-zinc oxide (IGZO) thin film transistors (TFT) are summarized in this article. Firstly, the sources of hydrogen impurities in the IGZO channels of the TFTs are proposed, which could originate from the residual gas in the deposition chamber, the molecules absorbed on the sputtering target surface, the neighbor films that contain abundant hydrogen elements, doping during annealing processes, etc. The hydrogen impurities in the IGZO films can exist in the forms of hydroxyl groups and metal hydride bonds, respectively. The former originates from the reaction between H atoms and the O2- ions. This reaction releases free electrons, leading to a rise of the Fermi level of IGZO, and thus enhancing the mobilities of IGZO TFTs. The latter incurs negative charges on H atoms, and thus changing the distribution of the subgap density of states, hence improving the negative bias (or illumination) stabilities of IGZO TFTs. Subsequently, various methods are also proposed to characterize hydrogen elements in IGZO, such as secondary ion mass spectroscopy, thermal desorption spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Finally, the effects of hydrogen impurities on the electrical characteristics of the IGZO TFTs, such as the field effect mobilities, subthreshold swings, threshold voltages, on/off current ratios as well as the positive and negative bias stress stabilities, are discussed. The results indicate that hydrogen element concentration and process temperature are two key factors for the device performances. With the increase of hydrogen element concentration in the IGZO channels, the TFTs exhibit higher electron mobilities, lower subthreshold swings and better reliabilities. However, annealing at too high or low temperatures cannot improve the device performance, and the most effective annealing temperature is 200-300℃. It is anticipated that this review could be helpful to the IGZO TFT researchers in improving the device performances and understanding the underlying mechanism.

###### THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS

2018, 67 (9): 091201.
doi: 10.7498/aps.67.20172155

Abstract +

Construction of a valid interaction potential function between quarks is a crucial issue in hadronic physics and also one of the frontier issues. Non-relativistic Breit potential is a common model to describe the interaction between quarks. It is used to successfully calculate the bound states of quarks and quark scatterings. These spur people to improve it. As is well known, the full Breit potential function, which includes the color-Coulomb term, the mass term, the orbit-orbit interaction term, the spin-spin interaction term, the spin-orbit interaction term, the tensor force term, and the constant term, contains singularity factors. How to eliminate the singularity factors is the most urgent task for developing Breit potential model. In this paper, we carry out a replacement method to eliminate the singularity factors in the full Breit quark potential function in coordinate space. Except for the color-Coulomb term and the constant term, remaining terms in the Breit quark potential function are all reconstructed. The replacement of (r) 3 e-r/8 is applied to the mass term and the spin-spin interaction term. The replacement of 1/r (1-e-r)/r is applied to the obit-obit interaction term. The replacement of 1/r3[1-(1+r)e-r]/r3 is applied to the spin-obit interaction term and the tensor force term. We calculate mass splits of heavy mesons and quarkonium species by using the reconstructed potential function and test the validity of the reconstructed potential function. The screening mass used in the calculations is not a simple constant but a variable relating to the quark mass mi and mj. It is found that the simple screening-mass expression cannot give the accurate value of B-meson mass, although it may give the mass splits of light mesons. However, the calculated results of the mass splits of the light mesons -, the heavy mesons, c-J/, b-(1s), c0-c2, etc., are highly consistent with the experimental data only when the screening mass is taken to be the Laurent series, =c-3(a+0.512)-3+ c-2(a+0.512)2 +c-1(a+0.512)-1+c0+c1(a+0.512) with respect to the average quark mass a=(mi+mj)/2. In this case, the mass accuracy of other mesons, especially the six D mesons, is improved significantly. Our calculated results indicate that a valid quark potential model, which gives not only the mass values of light mesons accurately but also the mass splits of heavy quarkonium species, is thus constructed in this paper.

###### ATOMIC AND MOLECULAR PHYSICS

Microwave electromagnetically induced transparency and Aulter-Townes spectrum of cesium Rydberg atom

2018, 67 (9): 093201.
doi: 10.7498/aps.67.20172645

Abstract +

We present an electromagnetically induced transparency and Aulter-Townes (EIT-AT) spectrum of a Rydberg three-level atom that is dressed with a microwave field in a room-temperature cesium cell. The EIT is a quantum coherent effect produced by the interaction of atoms with electromagnetic waves, which leads to the decrease of the absorption for a weak resonant probe laser. AT splitting refers to the phenomenon, that the absorption line splits when an electromagnetic field that is in resonance or near resonance acts on the transition of atoms. Rydberg atoms are extremely sensitive to an external electric field due to their large polarizabilities and microwave transition dipole moments, which can be used to measure the external field. In this work, a Rydberg three-level EIT is used to detect Rydberg atom and AT splitting induced by the microwave field. Cesium levels 6S1/2, 6P3/2 and 50S1/2 constitute a Rydberg three-level system, in which a weak probe laser locking to the transition from 6S1/2 to 6P3/2 couples ground-state transition and the strong coupling laser resonates on the Rydberg transition from 6P2/3 to 50S1/2. The two Rydberg levels 50S1/2 and 50P1/2 are coupled with the microwave field at a frequency of 30.852 GHz, leading to the AT splitting of EIT line and forming an EIT-AT spectrum, which is used to measure the electric field amplitude of microwave. In order to further study the EIT-AT splitting characteristics of the Rydberg levels, we carry out a series of measurements by changing the microwave field. The experimental results show a broadened EIT-AT signal for the weak field range and the four-peak spectrum for the strong field, which is attributed to the inhomogeneity of the microwave field. The microwave in cesium cell, emitted by a function generator, shows inhomogeneous behavior such that the atoms interacting with the laser field experience the different fields, leading to the line broadened and multi-peak EIT-AT spectra. For the microwave transition of nS1/2-nP1/2 in this paper, a pair of EIT-AT lines should be obtained for an electric field value. The broadening of the EIT-AT spectrum and the multi-peak structure here are due to the inhomogeneity of the microwave field measurement. We propose a method to increase the spatial resolution by reducing the length of cesium cell. The result in this work provides a method of measuring the field amplitude and monitoring the distribution of microwave electric field, meanwhile the spatial resolution of the measurements can be improved by reducing the size of the cell.