Vol. 64, No. 16 (2015)
SPECIAL ISSUE — Quantum metrology and control
Nowadays, the nonlinear optical process of spontaneous parametric down-conversion is considered as the canonical approach for creating entangled-photon pairs. We consider three pairs of entangled photons emitted by the parametric down-conversion source, and introduce a setup for evolving these photons based on linear optics, which is composed of several polarizing beam splitters, beam splitters, and half wave plates. By using the parametric down-conversion source and the setup, we carefully design an efficient scheme for preparing six-photon hyperentangled states in both the polarization and the spatial degrees of freedom. Because we use almost all possible behaviors of the three pairs of entangled photons, the present scheme is efficient for creating six-photon hyperentangled states. Next, in the regime of weak nonlinearity we design a quantum nondemolition detection to distinguish the two cases of photons in two special spatial modes. It is worth pointing out that our scheme is much easier to realize, since the strength of the nonlinearities in the process of quantum nondemolition detection can be restricted to the scalable orders of magnitude in practicality.
2015, 64 (16): 160303. doi: 10.7498/aps.64.160303
Photon system is a promising candidate for quantum information processing, and it can be used to achieve some important tasks with the interaction between a photon and an atom (or a artificial atom), such as the transmission of secret information, the storage of quantum states, and parallel quantum computing. Several degrees of freedom (DOFs) of a photon system can be used to carry information in the realization of quantum information processing, such as the polarization, spatial-mode, orbit-angular-momentum, time-bin, and frequency DOFs. A hyperparallel quantum computer can implement the quantum operations on several DOFs of a quantum system simultaneously, which reduces the operation time and the resources consumed in quantum information processing. The hyperparallel quantum operations are more robust against the photonic dissipation noise than the quantum computing in one DOF of a photon system. Hyperentanglement, defined as the entanglement in several DOFs of a quantum system, can improve the channel capacity and the security of long-distance quantum communication, and it can also be conductive to completing some important tasks in quantum communication. Hyperentangled Bell-state analysis is used to completely distinguish the 16 hyperentangled Bell states, which is very useful in high-capacity quantum communication protocols and quantum repeaters. In order to depress the effect of noises in quantum channel, hyperentanglement concentration and hyperentanglement purification are required to improve the entanglement of the quantum systems in long-distance quantum communication, which is also very useful in high-capacity quantum repeaters. Hyperentanglement concentration is used to distill several nonlocal photon systems in a maximally hyperentangled state from those in a partially hyperentangled pure state, and hyperentanglement purification is used to distill several nonlocal photon systems in a high-fidelity hyperentangled state from those in a mixed hyperentangled state with less entanglement. In this reviewing article, we review some new applications of photon systems with multiple DOFs in quantum information processing, including hyperparallel photonic quantum computation, hyperentangled-Bell-state analysis, hyperentanglement concentration, and hyperentanglement purification.
Reduction of quantum noise in one spin component is a significant tool for enhancing the sensitivities of interferometers and atomic clocks. It has been recently implemented for ultra-cold atomic Bose-Einstein condensate (BEC) interferometer. This type of quantum noise reduction reduces the measurement noise near some predetermined phase. However, if the phase is completely unknown prior to measurement, then it is not known which phase quadrature should be in a squeezed state. We introduce a novel planar squeezing uncertainty relation for spin variance in a plane, and analyze how to obtain such a planar quantum squeezed (PQS) state by using a double-well single component BEC, through the use of local nonlinear S-wave scattering interaction between trapped atoms. Here, we consider the PQS that is generated by using two hyperfine states in a two components BEC system, which is useful for quantum metrology. By comparison with the case of two spatial wells, the Hamiltonian parameters can be controlled in a more efficient way. The spin component can be measured by detecting the occupation number difference between the two internal modes, while one needs to observe a spatial interference pattern in the double well BEC case. This is the major difference between the internal and external cases. Another difference is that one can use the Rabi frequency Ω instead of the Josephson parameters to switch the Hamiltonian parameters through using a diabatic technique. Therefore the coupling could be switched off or on to study the different evolutions. PQS simultaneously reduces the quantum noises of two orthogonal spin projections below the standard quantum limit, while increases the noise in the third dimension. This allows the improvement in phase measurement at any phase-angle. PQS states that reductions of fluctuations everywhere in a plane have potential utility in "one-shot" phase measurement, where iterative or repeated measurement strategies cannot be utilized. The improved interferometric phase measurements and planar uncertainty relations are useful for detecting the entanglement in mesoscopic system between two distinguished modes regardless of the third component.
2015, 64 (16): 160306. doi: 10.7498/aps.64.160306
The direct communication protocol of quantum network over noisy channel is proposed and investigated in this study. In communication process, all quantum nodes share multiparticle Greenberger-Horne-Zeilinger (GHZ)-states. The sending node takes the GHZ-state particle in the hand as the control qubit and the particle for sending secret information as the target qubit, which carries out the CNOT gate operation for the control and target qubit. Each receiving node takes the GHZ-state particle in the hand as the control qubit and the particle of the received secret information as the target qubit, in which the CNOT gate operation is repeated to obtain the secret information that contains the bit error. Each receiving node uses the extracted part of qubits as the checking qubits, and then corrects the bit-flip errors using parity check matrix together with the rest part of qubits. As a result, all receiving nodes obtain rectified secret information. In addition to the high security analysis, this study also presents the detailed analyses of the throughput efficiency and the communication performance.
Quantum secure direct communication (QSDC) is one of the most important branches of quantum communication. In contrast to the quantum key distribution (QKD) which distributes a secure key between distant parties, QSDC directly transmits secret message instead of sharing key in advance. To establish a secure QSDC protocol, on the one hand, the security of the quantum channel should be confirmed before the exchange of the secret message. On the other hand, the quantum state should be transmitted in a quantum data block since the security of QSDC is based on the error rate analysis in the theories on statistics. Compared with the deterministic quantum key distribution (DQKD) which can also be used to transmit deterministic information, QSDC schemes do not need extra classical bits to read the secret message except for public discussion. In this article, we introduce the basic principles of QSDC and review the development in this field by introducing typical QSDC protocols chronologically. The first QSDC protocol was proposed by Long and Liu, which can be used to establish a common key between distant parties. In their scheme, the method for transmitting quantum states in a block by block way and in multiple steps was proposed and the information leakage before eavesdropping detection was solved. Subsequently, Deng et al. presented two pioneering QSDC schemes, an entangled-state-based two-step QSDC scheme and a single-photon-state-based quantum one-time pad scheme, in which the basic principle and criteria for QSDC were pointed out. From then on, many interesting QSDC schemes have been proposed, including the high-dimension QSDC scheme based on quantum superdense coding, multi-step QSDC scheme based on Greenberger-Horne-Zeilinger states, QSDC scheme based on quantum encryption with practical non-maximally entangled quantum channel, and so on. We also introduce the anti-noise QSDC schemes which were designed for coping with the collective-dephasing noise and the collective-rotation noise, respectively. In 2011, Wang et al. presented the first QSDC which exploited the hyperentangled state as the information carrier and several QSDC schemes based on the spatial degree of freedom (DOF) of photon, single-photon multi-DOF state and hyperentanglement were proposed subsequently. In addition to the point-to-point QSDC schemes, we also review the QSDC networks. Finally, a perspective of QSDC research is given in the last section.
The cesium fountain clock as primary frequency standard is widely used in the areas, such as time-keeping system, satellite navigation, fundamental physics research, etc. The principle of operation of cesium fountain clock is introduced. The noise source and frequency shift term are ananlyzed. The major noise source influencing frequency stability are cold atom loading time, microwave phase noise related to Dick effect, and detection laser frequency noise. The major frequency bias influencing frequency uncertainty is blackbody radiation frequency shift,cold atom collision frequency shift,distributed cavity phase frequency shift and microwave leakage frequency shift.The key technique to achieve highperformance cesium fountain clock is sumerized. The application of cesium fountain clock is presented. The status of space cesium clock and future primary frequency standard of optical clock are shown.
Atom in Rydberg state has large polarizability, large electric dipole and low ionization threshold field. It is very sensitive to electric field, therefore it can be used to measure the amplitude of electric field, especially the microwave electric field. The new developed scheme is based on quantum interference effects (electromagnetically induced transparency and Autler-Townes splitting) in Rydberg atoms. Instead of the direct amplitude measurement, this method tests the Rabi frequency value of the transmission spectrum which is determined by the microwave electric field strength and the corresponding atom nature. The minimum measured strengths of microwave electric fields are far below the standard values obtained by traditional antenna methods. Compared with the traditional methods, this new scheme has several advantages, such as self-calibration, non-perturbation to the measured field and independence of the probe length. Besides, this scheme can also be used to measure the polarization direction of microwave electric field and realize sub-wavelength imaging. Through adjusting the wavelength of coupling laser, a broadband 1-500 GHz microwave electric field measurement can be achieved. This new scheme is benefitial to conducting the continue electric field measurement and the miniaturization of the test equipment. In this paper, the researches about using Rydberg atom to measure electric field with high precision are reviewed. The basic theory and experimental techniques are introduced. Finally, we discuss a promising method of using Rydberg atom interferometer to detect the accumulated phase in the process of interaction between electric field and Rydberg atoms. This method converts amplitude measurement into phase test, which may improve the precision and sensitivity.
Research progress on preparation, manipulation, and remote sensing applications of high-order orbital angular momentum of photons
2015, 64 (16): 164210. doi: 10.7498/aps.64.164210
Photons are an ideal candidate for encoding both classical and quantum information. Besides spin angular momentum associated with circular polarization, single photon can also carry other fundamentally new degree of freedom of orbital angular momentum related to the spiral phase structure of light. The key significance of orbital angular momentum lies in its potential in realizing a high-dimensional Hilbert space and in encoding a high-dimensional quantum information. Since Allen et al. [Allen L, Beijersbergen M W, Spreeuw R J C, Woerdman J P 1992 Phys. Rev. A 45 8185] recognized the physical reality of photon orbital angular momentum in 1992, rapidly growing interest has been aroused in orbital angular momentum (OAM) from both classical and quantum points of view. Here we present an overall review on the high-order orbital angular momentum of photon, including its preparation and manipulation based on some specific techniques and also its applications. The spatial light modulator is a commercial device that has been widely employed to generate the OAM beams. We make and identify the optical OAM superposition with very high quantum numbers up to l=360. Recently, the metallic spiral phase mirrors were also developed to produce high-order OAM beams up to l=5050. In addition, the Q-plates made of anisotropic and inhomogeneous liquid crystals were invented to generate high-order OAM beams in a polarization-controllable manner, and the OAM superposition of l=± 50 were achieved. Owing to high rotational symmetry, these high OAM beams have been found to have more and more important applications in the fields of high-sensitivity sensing and high-precision measurements. Two fascinating examples are discussed in detail. The first example is that the research group led by Prof. Zeilinger has prepared and observed the quantum entanglement of high orbital angular momenta up to l=±300 by the technique of polarization-OAM entanglement swapping, and they demonstrated that the angular resolution could be significantly improved by a factor of l. Their result was the first step for entangling and twisting even macroscopic, spatially separated objects in two different directions. The second example is that the research group led by Prof. Padgett has demonstrated an elegant experiment of rotational Doppler effects for visible light with l=±20 OAM superposition. They showed that a spinning object with an optically rough surface might induce a Doppler effect in light reflected from the direction parallel to the rotation axis, and the frequency shift was proportional to both the disk's angular speed and the optical OAM. The potential applications in noncontact measurement of angular speed and in significant improvement of angular resolution for remote sensing will be particularly fascinating.
2015, 64 (16): 164211. doi: 10.7498/aps.64.164211
Cavity optomechanics originated from the research of interferometric detection of gravitational waves, and later became a fast-growing area of techniques and approaches ranging from the fields of atomic, molecular, and optical physics to nano-science and condensed matter physics as well. Recently, it focused on the exploration of operating mechanical oscillators deep in the quantum regime, with an interest ranging from quantum-classical interface tests to high-precision quantum metrology. In this paper, recent theoretical work of our group in the field of quantum measurement with cavity optomechanical systems is reviewed. We explore the quantum measurement theory and its applications with several unconventional cavity optomechanical schemes working in the quantum regime. This review covers the basics of quantum noises in the cavity optomechanical setups and the resulting standard quantum limit of precision displacement and force measurement. Three novel quantum measurement proposals based on the hybrid optomechanical system are introduced. First, we describe a quantum back-action insulated weak force sensor. It is realized by forming a quantum-mechanics-free subsystem with two optomechanical oscillators of reversed effective mass. Then we introduce a role-reversed atomic optomechanical system which enables the preparation and the quantum tomography of a variety of non-classical states of atoms. In this system, the cavity field acts as a mechanical oscillator driven by the radiation pressure force from an ultracold atomic field. In the end, we recommend a multimode optomechanical transducer that can detect intensities significantly below the single-photon level via adiabatic transfer of the microwave signal to the optical frequency domain. These proposals demonstrate the possible applications of optomechanical devices in understanding of quantum-classical crossover and in achieving quantum measurement limit.
2015, 64 (16): 164212. doi: 10.7498/aps.64.164212
In this review, the recent development of nano-particle detection using Raman lasers in the whispering gallery mode microcavities is presented. The fabrication of the microcavity, the working principles are given and the recent experimental progress is reviewed. Recent years, the demand for nano-particle sensing techniques was increased, since more and more nano-particles of sizes between 1 nm and 100 nm are employed in areas such as biomedical science and homeland security. In these applications, label-free, rapid and real-time sensing requirements are necessary. Whispering gallery mode (WGM) micro-resonators have high-quality factors and small mode volumes, and have achieved significant progress in the nano-particle sensing field. There are various measurement mechanisms for nano-particle sensing using WGM cavities, including resonance mode broadening, resonance frequency shift, and mode splitting changes. The key point to improve sensing limit is to narrow the resonance mode linewidth, which means reducing the optical cavity losses, or equivalently to enhance quality factor. An important approach to narrowing the mode linewidth is to fabricate active resonators that provide gain and produce laser by doping rare earth irons. According to Schawlow-Townes formula, the linewidth of corresponding laser will be narrower than that of the original optical cavity mode. Active resonators have outstanding performances in particle detection. However, doping process requires complex fabrication steps, and rare earth irons laser demands a certain pumping wavelength band. A new approach is to use low threshold Raman laser in an optical micro-resonator. The binding of nano-particles on WGM micro-resonator induces resonance mode splitting. Raman lasers of the two splitting modes irradiate the same photon detector and generate a beat note signal. By monitoring the jumps of the two split mode differential signals, one can easily recognize the nano-particle binding events, thus achieving real time nanoparticle detection. Using Raman laser in WGM cavities in nano-particle sensing is no longer limited by the stringent requirement of a suitable pump light source, which greatly expands the applicability of this method in different environments. It does not need additional fabrication process as compared with the rare earth doping method. It has also better biological compatibility, which makes it a promising technique in biomedical applications. Recently, two groups, i.e., Li et al. (Proc. Natl. Acad. Sci. 111 14657) from Peking University, and zdemir et al. from University of Washington and Tsinghua University, have successfully completed the demonstration experiments. zdemir et al. (Proc. Natl. Acad. Sci. 111 E3836) have achieved a nano-particle sensing limit down to 10 nm without labelling, and Li et al. (Proc. Natl. Acad. Sci. 111 14657) realized real-time detection of single nano-particles with WGM cavity Raman laser in an aqueous environment. Both experiments have shown the great potential of the new approach. The new technique can also be used in other photonic systems, such as the photonic crystal or metal materials.
2015, 64 (16): 167303. doi: 10.7498/aps.64.167303
Because of the long coherence time and the easy way to achieve the qubit scalability, quantum dot spin qubit has obtained considerable attentions recently. Single spin manipulation is usually achieved using the traditional electron spin resonance technique. This method not only needs a static Zeeman field, but also needs an ac magnetic field which is perpendicular to the static one. However, it is not easy to produce a local ac magnetic field experimentally. Recently, instead of an ac magnetic field, an ac electric field can also be used to manipulate an electron spin, an effect called electric-dipole spin resonance. As is well-known, there is no direct interaction between the spin and the electric field. Thus, the electric-dipole spin resonance must be mediated by some mechanisms. These mediums in the quantum dot can be: the slanting magnetic field, the spin-orbit coupling, and the electron-nucleus hyperfine interaction. This paper summarizes three main mechanisms of the electron-dipole spin resonance in semiconductor quantum dot.
2015, 64 (16): 167601. doi: 10.7498/aps.64.167601
With the development of quantum information science, the active manipulation of quantum systems is becoming an important research frontier. To build realistic quantum information processors, one of the challenges is to implement arbitrary desired operations with high precision on quantum systems. A large number of quantum control methods and relevant numerical techniques have been put forward in recent years, such as quantum optimal control and quantum feedback control. Nuclear magnetic resonance (NMR) spin systems offer an excellent testbed to develop benchmark tools and techniques for controlling quantum systems. In this review paper, we briefly introduce some of the basic control ideas developed for NMR systems in recent years. We first explain, for the liquid spin systems, the physics of various couplings and the causes of relaxation effects. These mechanisms govern the system dynamics, and thus are crucial for constructing rigorous and efficient control models. We also identify three types of available control means: 1) raido-frequency fields as coherent controls; 2) phase cycling, gradient fields and relaxation effects as non-unitary controls; 3) radiation damping effect as feedback control mechanism. Then, we elucidate some important control tasks, which may arise from the conventional NMR spectroscopy (e.g., pulse design and polarization transfer) or from quantum information science (e.g., algorithmic cooling and pseudo-pure state preparation). In the last part, we review some of the most important control methods that are applicable to NMR control tasks. For systems with a relatively small number of spins, it is possible to use analytic optimal control theory to realize the target unitary operations. However, for larger systems, numerical methods are necessary. The gradient ascent pulse engineering algorithm and pulse compiler techniques are the most successful techniques for implementing complicated quantum networks currently. There are some interesting topics of utilizing radiation damping and relaxation effects to achieve more powerful controls. Finally, we give an outline of the possible future work.
2015, 64 (16): 168301. doi: 10.7498/aps.64.168301
Particles in industrial flows can be charged under an action of external electric field, while in the absence of external electric field, tribo-electrostatic charges are almost unavoidable in gas-solid two-phase flows due to the consecutive particle contacts. The particle charging may be beneficial, or detrimental. In the past decade considerable progress has been made in understanding the physics of particles charging. However, the particle charging mechanism, especially in the gas-solid phase flow, is still poorly understood. The purpose of this review is to present a clear understanding of the particle charging and movement of charged particle in two-phase flow, by summarizing the charging mechanisms, physical models of particle charging, and methods of charging/charged particle entrained fluid flow simulations. In this review, charged particles in industry, which would be beneficial (triboelectrostatic separation, electrostatic precipitator) or detrimental (electrification in gas-solid fluidized bed and manufacturing plant) are discussed separately. The particle charging through collisions could be attributed to electron transfer, ion transfer, material transfer, and/or aqueous ion shift on particle surfaces. For conductive particle contacts, the difference in work function is often used to explain the charge transfer. For insulation particle contacts, the charging tendency can be explained by the ion transfer and material transfer. In addition, aqueous ion shift transfer would be an important charge transfer mechanism considering water content in environmental conditions and the influences of temperature and humidity. The charges on particle through collision can be quantitatively predicted by using the particle charging model. According to the differently induced ways of charge transfer, the charging models are related to the external electric field, asymmetry contact, and/or aqueous ion shift on particle surfaces. In fact, the motions of particles in industry are influenced by fluid flow. The effect of fluid on particle dynamics makes the particle charging more complicated. Thus it is more reasonable to study the particle charging from the viewpoint of the gas-solid two-phase flow. The method combining particle charging model with computational fluid dynamics and discrete element method is applicable to the studying of the particle charging/charged processes in gas-solid two phase flow in which the charge behaviors are significantly influenced by the fluid mechanics behavior. By this method, the influence factors of particle charging, such as gas-particle interaction, contact force, contact area, and various velocities, are described systematically. This review presents a clear understanding of the particle charging and provides theoretical references on controlling and utilizing the charging/charged particles in industrial technology.
Operator-ordering identities for mutual transformation of power of coordinate-momentum operators obtained by a new concise method
2015, 64 (16): 160302. doi: 10.7498/aps.64.160302
Since the foundation of quantum mechanics, operator-ordering identities for mutual transformation of power of coordinate-momentum operators have been a fundamental and tough topic. To the best of our knowledge, this topic has not been tackled smoothly because there is no elegant and direct way to investigate it. In this paper we report a very concise and novel method to handle this topic, i.e., we employ the generating function of two-variable Hermite polynomial and the characteristics of ordered operators to derive a series of operator-ordering identities for mutual transformation of power of coordinate-momentum operators: they surly possess potential applications. The essence of our method lies in the fact that coordinate-momentum operators can be permutable within ordered product of operators, just as the scenarios in P-Q ordering, Q-P ordering and Weyl ordering. We also derive the integration transformation formula about two-variable Hermite polynomial in phase space. The correspondence relation between operator ordering and quantization recipe is established. The beauty of theoretical physics is embodied extensively in the paper.
2015, 64 (16): 160305. doi: 10.7498/aps.64.160305
The transfer of quantum states between distant nodes is one of the most fundamental tasks in quantum-information processing. Recent studies show that the antiferromagnetic spin chain initially prepared in a multi-excitation state can provide suitable pathways for quantum state transfer. In this paper, we investigate the quality of state transfer through a uniformly coupled antiferromagnetic spin chain where the initial state of the channel varies with the number of spin excitations. Firstly, by analyzing the dynamics of observables for the output qubit using the information-flux approach, the explicit relation about how the average fidelity of state transfer depends on the initial state of the spin channel is obtained. The results show that the average fidelity of state transfer through a multi-excitation spin channel only relates to the parity of the number of spin excitations in the channel. Then we compare the maximum average fidelity of state transfer through the odd-excitation with those through the even-excitation spin channels, and provide a simple criterion to optimize the quality of state transfer by choosing appropriate channels from the odd-excitation and the even-excitation channels. Compared with the previous studies which initialize the chains into the ground state of the ferromagnetic medium or the Nel state, the maximum average fidelity of state transfer is evidently enhanced by using the optimized channel. Moreover, we analyze the entanglement distribution through the channel having different number of spin excitations via the information-flux approach. It is found that the quality of entanglement distribution not only relates to the number of initial spin excitations present in the channel, but also depends on the initial ordering of these excited spins. The numerical results suggest that the amount of distributed entanglement and duration of distribution in the channel where all spins are down or up are larger than those in other excited channels. Based on these results, we can choose appropriate quantum channels for state transfer and entanglement distribution in practice.
2015, 64 (16): 160501. doi: 10.7498/aps.64.160501
Random number generator plays an important role in many domains, including secret communication, radar waveform generation, etc. However, the existing methods for generating random numbers cannot meet the actual demand for speed. Even worse, the use of analog device will restrict the speed of generator and robustness of system. As a result, researchers start to turn their eyes to digital implementation which is stabler and more efficient than the analog counterpart. Unfortunately, digital methods still have the disadvantages of dynamical degradation because of word length limitation effect. Though some remedies, such as increasing computing precision, cascading multiple chaotic systems, pseudo-randomly perturbing the chaotic system, the switching multiple chaotic systems and error compensation method are proposed, but the limitations are still inevitable. In recent researches, continuous-time chaotic oscillators are used with digital devices to realize random number generator, and a new approach is proposed to solve the dynamical degradation of digital chaotic system by coupling the given digital chaotic map with an analog chaotic system, where the analog chaotic system is applied to anti-control the given digital chaotic map. However, this method also requires a whole continuous-time system realized with analog devices, which confines the system performance. In this paper, a new digital-analog hybrid chaotic map with only one analog capacitor is constructed to produce random numbers. Firstly, the block diagram of digital-analog hybrid system based on the single capacitance feedback is given, and the model of the system is derived from the block diagram. Secondly, the simple logistic map is applied to the model and its nonlinear dynamics behaviors are analyzed and compared to verify the correctness and effectiveness of the proposed method. Then a more complex two-way coupled saw tooth map is used to produce pseudorandom sequences through simulation smoothly. When designing the circuits of the system, a digital-analog hybrid implementation with field programmable logic gate array and a single analog capacitor is used to realize chaotic maps, showing that it can overcome the finite word length effect of digital implementation. NIST, a general statistical test suiting for random and pseudorandom number generator cryptographic applications, is used to test the sequences produced by the new system. The results show that the new hybrid system is insensitive to the evolution of circuit parameters and the randomness of sequence is in accordance with the practical application. The circuit implementation verifies the numerical simulation and theoretical results. The high speed digital devices and a single analog capacitance are applied to the proposed random sequence generator, and therefore it can be integrated easily into the systems of digital encryption, secure communication and radar waveform generation.
2015, 64 (16): 160502. doi: 10.7498/aps.64.160502
Nonlinear time series denoising is the premise for extracting useful information from an observable, for the applications in analyzing natural chaotic signals or achieving chaotic signal synchronizations. A good chaotic signal denoising algorithm processes not only a high signal-to-noise ratio (SNR), but also a good unpredictability of a signal. Starting from the compressed sensing perspective, in this work we provide a novel filtering algorithm for chaotic flows. The first step is to estimate the strength of the noise variance, which is not explicitly provided by any blind algorithm. Then the second step is to construct a deterministic projection matrix, whose columns are polynomials of different orders, which are sampled from the Maclaurin series. Since the noise variance is provided from the first step, then a sparsity level with regard to this signal can be fully constructed, and this sparsity value in conjunction with the orthogonal matching pursuit algorithm is used to recover the original signal. Our method can be regarded as an extension to the local curve fitting algorithm, where the extension lies in allowing the algorithm to choose a wider range of polynomial orders, not just those of low orders. In the analysis of our algorithm, the correlation coefficient of the proposed projection matrix is given, and the reason for shrinking the sparsity when the noise variance increases is also presented, which emphasizes that there is a larger probability of error column selection with larger noise variance. In the simulation, we compare the denoising performance of our algorithm with those of the wavelet shrinking algorithm and the local curve fitting algorithm. In terms of SNR improvement for the Lorenz signal, the proposed algorithm outperforms the local curve fitting method in an input SNR range from 0 dB to 20 dB. And this superiority also exists if the input SNR is larger than 9 dB when compared with the wavelet methods. A similar performance also exists concerning the Rössler chaotic system. The last simulation shows that the chaotic properties of the originals are largely recovered by using our algorithm, where the quantity for "chaotic degree" is described by using the proliferation exponent.
2015, 64 (16): 160701. doi: 10.7498/aps.64.160701
Currently, there is no standard method of evaluating the performance of the gas leak infrared imaging detection system. The evaluating criterions vary greatly and are deficient in aspects of completeness and accuracy, such as noise equivalent temperature difference, noise equivalent concentration path length, and minimum detectable leak rates. Minimum resolvable gas concentration (MRGC) is a latest proposed parameter for evaluating the performance of a passive gas leak infrared imaging detection system, which takes full advantage of the comprehensive evaluation capability of the temperature resolution and spatial resolution of the minimum resolvable temperature difference (MRTD) model. The MRGC takes into account the environmental and gas state parameters, the size of the gas plume and other factors which influence the MRGC measurement. However, the MRGC measurement system is complicated and many state parameters need to be controlled, especially the wide range and dedicated gas concentration meters are required. Therefore, the mathematical model of MRGC is derived and established. By comparing the principles and measurement methods of the performance parameters, MRGC and MRTD, a novel MRGC equivalent measurement evaluation method is proposed, on condition that the minimum resolvable radiation differences are equal. Using ethylene and ammonia as the target, the equivalently measured results of MRGC are obtained. The results show that the MRGC increases with the spatial frequency increasing and the smaller the temperature difference is between the gas and the background blackbody, the faster the MRGC increases. What is more, when the spatial frequency is fixed, MRGC increases with the gas temperature approaching to the background temperature. The background temperature varies asymptotically, which means that if the gas temperature equals the background temperature, the system cannot detect the gas four-bar pattern, no matter what the gas concentration is (here, the maximum gas concentration is 1 million ppm under normal pressure.). The directly measured and equivalently measured results of ethylene are in good agreement within errors of less than ±20%, and the maximum error is 18.26% at a spatial frequency of 0.214f0, which demonstrates the feasibility and effectiveness of the method. Because the equivalent measurement method only needs the traditional MRTD measurement results and the gas infrared spectrum database, it is simple and reliable, which is very significant for the study and application of the gas leak infrared imaging detection systems.
ATOMIC AND MOLECULAR PHYSICS
2015, 64 (16): 163201. doi: 10.7498/aps.64.163201
Rydberg atoms, with large principal quantum number, exhibit certain properties, such as long lifetimes and strong interactions with fields and other atoms, which have been extensively investigated recently. One of the properties is the electromagnetically induced transparency (EIT) of Rydberg ladder system, which can be used to measure the radio frequency (RF) field with high sensitivity. In this paper, we investigate the quantum coherent effect of cesium Rydberg atom in a three-level ladder system involving the ground state (6S1/2), the excited state (6P3/2) and 49S1/2 Rydberg state in room temperature vapor cell. The probe laser (852 nm) drives the transition of 6S1/2(F=4)→6P3/2(F'=5), while the coupling laser (510 nm) couples the Rydberg transition of 6P3/2 (F'=5)→nS1/2. A typical electromagnetically induced transparency spectrum is obtained when the weak probe laser is scanned through the transition of 6S1/2(F=4)→6P3/2(F'=5) and the coupling laser tuning to Rydberg transition. The two-photon RF spectra are observed when the RF field with a frequency of ～16.9 GHz couples the Rydberg transition of 49S1/2→47D3/2, where the EIT signal is split into two EIT peaks due to the interaction between the RF field and Rydberg atoms. The dependences of EIT splitting on the power of RF field are investigated. The results show that the EIT splitting increases with the power of RF field, which can inversely be used to measure the RF field with a higher spatial resolution in the future.
2015, 64 (16): 163202. doi: 10.7498/aps.64.163202
In this paper, the photodetachment cross section of negative hydrogen ion inside a tube cavity with an equilateral triangle cross section is investigated by the traditional quantum approach. Then the analytic formulas each as a function of photon energy having been derived, some interesting oscillations in the photodetachment cross section are shown from the numerical illustrations. The formulas indicate that the oscillations are related to the positions of the ion and the photon polarization. The polarization of photons being perpendicular to the normal direction of the triangle, the cross sections apparently display large amplitude sawtooth-shaped oscillations, while being parallel to the normal direction of the triangle, oscillations are still present and observable from the quantum calculations, although the amplitudes of the oscillations are rather small. The subtle effect is also observed in the quantum theory for photodetachment in an electric field. The formulas also reveal threshold behaviors in the photodetachment cross sections. The threshold is expressed as Eth=(8π2/9l2)(m2+n2-mn), where l is the length of the triangle side, n and m are for all integers with m≥2n. When the polarization of photons is perpendicular to the normal direction of the triangle and the energy of the detached electron is above each threshold, the threshold behavior is Δσ∝(E-Eth)-1/2. When the polarization of photons is parallel to the normal direction of the triangle and the energy of the detached electron is above each threshold, the threshold behavior is Δσ∝(E-Eth)1/2. Furtherly, if the negative hydrogen ion is placed near one corner of the equilateral triangle, the quantum results show agreement with those from the closed-orbit theory when the negative hydrogen ion is in a wedge with an opening angle of 60 degrees. If that occurs, the five sinusoidal oscillations, each of which will correspond to one closed orbit, can be extracted from the photodetachment cross sections. These five closed orbits are definitely the orbits when the negative hydrogen ion is in a wedge with an opening angle of 60 degrees.
2015, 64 (16): 163601. doi: 10.7498/aps.64.163601
The molecular dynamics model is adopted to investigate the dynamical behavior of hydrogen cluster irradiated by an intense femtosecond laser. Being contrary to the predictions from the Coulomb explosion model, this paper points out that the explosion of hydrogen cluster is anisotropic. The component of proton energy along the laser polarization direction is much larger than the component perpendicular to the polarization direction. This paper discusses the mechanism responsible for the anisotropy explosion. In the process of the interaction of femtosecond laser with cluster, the electrons undergo the inner ionization and then oscillate along the direction of laser polarization. During the oscillation of electrons, part of them will escape from cluster. The escaping of the electrons would lead to two correlation effects. First, the anisotropic distribution of the electric field caused by the oscillation of electrons would not be neutralized. For one thing, during the oscillating of electrons, they will be pulled to one pole of cluster so the electric field of the opposite pole would be larger, the electrons in this region will experience larger Coulomb repulsive force and gain lager acceleration. For another thing, the electron number contained in the cluster will decline during each laser cycle. So the proton in this region will gain a pure acceleration. Second, during the oscillation of electrons, part of electrons will escape from cluster. During their escaping they will pull the protons at the pole and the protons move toward the direction of electron escaping direction. These two correlation effects cause the anisotropic explosion of hydrogen cluster. This paper also discusses the influences of cluster and laser parameters on the degree of anisotropy.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
A broad stopband common-mode suppression defected ground structure filter with complementary structure
2015, 64 (16): 164101. doi: 10.7498/aps.64.164101
A low-cost defected ground structure (DGS) wideband stopband filter adopting complementary structure is proposed, which is designed for common-mode noise suppression in high-speed differential signals. The filter is etched below the low cost FR4 printed circuit board. To avoid stimulating the common-mode noise, the DGS cells on ground planes are kept symmetrical to the central line of the two differential signal lines. Both sides of the filter adopt a symmetric cup-shape DGS structure and the middle of the filter adopts a symmetric umbrella-type structure. All of the DGS structures are complementary, which makes the filter compact and miniaturized. What is more, because the spaces among the three DGS are closer, there exist the mutual inductances among them, which are utilized to achieve a wide stopband filter. The simulated result demonstrates the proposed filter has a wideband bandwidth of 6.8 GHz over 20 dB. In order to analyze the effect of compact structure of the filter, a filter having the same DGS patterns but large spaces among them is compared with it. The simulated result demonstrates that the stopband bandwidth of the compared filter has a wideband bandwidth of 4.4 GHz over 20 dB, of which the bandwidth is about 2.4 GHz less than that of the proposed filter. It is obvious that there exists a mutual inductance in the compact DGS structure common-mode filter, which plays an important role in broadening the bandwidth of the proposed filter. In order to facilitate analysis, an equivalent model of LC circuit is also given. The equivalent parameters of LC can be deduced from the definition of 3 dB cut-off frequency and resonant frequency, of which the values can be obtained by the HFSS simulation. The simulated and measured results show that the differential signal under the DGS filter is nearly intact, and the common-mode noise can be reduced over 20 dB from 4.6 GHz to 11.4 GHz and over 15 dB from 4.3 GHz to 12 GHz, while the area of the filter is only 10 mm by 10 mm. Compared with the periodic DGS at the same suppression depth of common-mode noise over 20 dB, the method has the advantages that surface area is reduced to no more than 30%, and the stopband width is increased by over 50%.
Design and radar cross section reduction experimental verification of phase gradient meta-surface based on cruciform structure
2015, 64 (16): 164102. doi: 10.7498/aps.64.164102
A two-dimensional phase gradient meta-surface based on cross structure insensitive to polarization is designed and verified by simulation and experiment. Several periodic metal cross structures are integrated into a superstructure, and an additional component of the wave vector on the meta-surface is formed and the direction of refelction wave can be regulated. Thus the backward radar cross section (RCS) reduction can be realized by the mechanism of anomalous reflection. Experimental results indicate that in a frequency range from 3.2 to 3.4 GHz, the reduction of backward RCS of meta-surface reaches a highest value of 18.19 dB in the normal direction of meta-surface and 8 dB on the average in an angular range between -30° and +30°.
2015, 64 (16): 164103. doi: 10.7498/aps.64.164103
With the development of high power microwave technology, the demands for electron beam repetition frequency, current density, response time and emission uniformity are higher and higher. Carbon nanotube (CNt) cathode has been widely investigated, because of its special structure and excellent field emission characteristics. CNt cathode is regarded as a thin film high current cathode, and the interface bonding will affect vacuum performance, stability, lifetime and repeat ability. The direct growth of CNt is a simple and effective means for preparing cathode. When electron energy reaches 1 MeV and the pulse upward gradient attains approximately 60 kV/ns, for CNt cathode, its the emission beam intensity reaches 15 kA and the peak bundle density attains about 1 kA/cm2, the response between beam voltage and current is fast. With the increase of repetition frequency, the emission stability decreases gradually. When the emission power is 15 GW and the emission stability repetitive frequency is 50 Hz, the cathode emission is stable. However with the increase of frequency, the stability becomes weak. When the repetition frequency reaches 100 Hz, voltage and current are almost split into two sections, and the delay time is obviously different. The relation between the voltage and the current meet the exponent law, which is different from the field emission characteristic. After a 1000 shot emission, the morphology of CNt cathode is intact, desorption from the interface of CNt does not happen. So the emission mechanism is flashover plasma emission. Through analyzing the experimental data and considering the plasma expansion effect on diode gap, the plasma speed can be estimated to be about to 3.9 cm/μs.
2015, 64 (16): 164104. doi: 10.7498/aps.64.164104
Nowadays, X-ray nanoprobe plays an important role in many research fields, ranging from materials science to geophysics and environmental science, to biophysics and protein crystallography. Refractive lenses, mirrors, and Laue lenses, can all focus X-rays into a spot with a size of less than 50 nm. To design a refractive lens at fixed wavelengths, absorption in the lens material can be significantly reduced by removing 2πup phase-shifting regions. This permits short focal length devices to be fabricated with small radii of curvatures at the lens apex. This feature allows one to obtain a high efficiency X-ray focusing. The reduced absorption loss also enables optics with a larger aperture, and hence improving the resolution for focusing. Since the single Kinoform lens can focus hard X-ray into a spot on a nanoscale efficiently, it has very important application prospect in X-ray nano-microscopy and nano-spectroscopy. We present a theoretical analysis of optical properties of the single Kinoform lens. Using Fermat's principle of least time, an exact solution of the single Kinoform lens figure is derived. The X-ray diffraction theory is reviewed. The complex amplitude transmittance function of the X-ray single Kinoform lens is derived. According to Fourier optics and optical diffraction theory, we set up the physical model of X-ray single Kinoform lens focusing. Employing this physical model, we study how the focusing performance of hard X-ray single Kinoform lens is influenced by the material, the photon energy, the number of steps and the vertex radius of curvature. We find that diamond single Kinoform lens can achieve a smaller focusing beam size with higher intensity gain than Al and Si single Kinoform lens. The single Kinoform lens designed at a certain photon energy can also focus other photon energies with different lateral beam sizes, axial beam sizes, intensity gains and focusing distances. The numbers of steps of a single Kinoform lens can be lessened with the thickness of step increasing, while the single Kinoform lens keeps good focusing performance. To improve the focusing performance further, reducing the vertex radius of curvature is proposed. Following these rules, a single Kinoform lens is optimally designed to focus 30 keV hard X-ray down to a lateral size of 14 nm (full-width at half-maximum, FWHM) and an axial size of 62 μm (FWHM) with an intensity gain of four orders of magnitude and transmittance of 30%.
2015, 64 (16): 164201. doi: 10.7498/aps.64.164201
White light emitting diode (LED) is expected to replace the incandescent lamp and becomes the next generation of lighting source because of its long life expectancy, high tolerance to humidity, and low power consumption. It is proposed that for the indoor visible light communication system white LED should be used as both lighting source and base station for its properties of high brightness and high speed modulation. Indoor visible light communication has a very wide area of applications, but there is a lack of research on receiver optical antenna that functions as an energy concentrator so as to increase received power. In order to meet the needs of high gain and meanwhile large field of view of receiver optical antenna for indoor visible light communication, two-cascade optical antenna is designed. It is shown that the field of view from 40 to 60 degrees can meet the requirements for high speed communication by analyzing the relationship between signal-to-noise ratio and data rate of different fields of view. The performances of the traditional optical antenna of Fresnel lens and the compound parabolic concentrator are simulated and analyzed by TacrePro. The gains of the designed two-cascade optical antenna are discussed at different incident angles. The results show that two-cascade optical antenna has better performance than traditional receiver optical antenna. The distribution of received power of two-cascade optical antenna is analyzed by using Matlab. The received average power by using two-cascade optical antenna is about 7 dBm larger than that without any optical antenna. The designed optical antenna provides a field of view of 40 degrees and enough gain for indoor visible light communication system.
2015, 64 (16): 164202. doi: 10.7498/aps.64.164202
A special lattice resonance can be observed when the array period of a metal nanoparticle array matches the resonant wavelength of the localized plasmon resonance of an isolated particle. The lattice resonance is sharper and its linewidth is narrower than the localized plasmonics resonance of a single particle. According to the modified long wavelength approximation approach, we discuss the extinction cross-section of the rectangular array in terms of the array factor and the particle polarizability. In this paper we emphasize the polarization characteristics of the regular array when the laser is incident vertically under different polarizations, and we also discuss in detail the variation of the array factor with the direction of electric dipole, and its influence on extinction cross section of the particle array. The square lattice with big size is polarization independent, while the rectangular lattice is polarization dependent. The coupling between the neighboring particle dipoles along the two lattice vectors of the regular array gives rise to a maximum value of its array factor, which determines a minimum value of the extinction cross section. When the incident light is polarized along one of the lattice vectors, the dipole coupling along that direction can be ignored since the particles are located in the far field of its neighboring particles, and the relevant peak in the array factor disappears.
Multi-transverse-mode and wavelength split characteristics of vertical cavity surface emitting laser
2015, 64 (16): 164203. doi: 10.7498/aps.64.164203
We study the multi-transverse mode distributions and the wavelength splittings with different designed oxide apertures of the oxide-confined VCSEL. By developing the effective index model and BPM algorithm theory, the characteristics of transverse optical field distribution are calculated with circular aperture and ellipsoid aperture, which are compared with our experimental results of multi-wavelength spectra of high-order transverse modes. The results show that the orthogonality of the different crystal orientation modes will be broken by the oxidation-induced ellipsoid aperture, and the maximum wavelength spltting of the degenerated high-order mode is 0.037 nm, which can be reduced as the diameter of aperture increases. The results in this paper will provide a useful reference for multi-transverse mode locking of oxide-confined VCSELs.
2015, 64 (16): 164204. doi: 10.7498/aps.64.164204
In this paper we present an all-fiber directly pumped fiber laser in master oscillator power amplifier configuration based on domestically manufactured fibers. In the amplifier stage of the laser, the gain fibers adopt the 20/400 μm Yb-doped double cladding fibers manufactured separately by Wuhan Fiber Home Technologies Group and China Electronics Technology Group Corporation No. 46 Research Institute in two individual experiments. Via this homemade amplifier stage, the system achieves a 1080 nm fiber laser with output powers of 3050 W and 3092 W respectively with two types of fibers. When the gain fiber of the amplifier adopts the YDF manufactured by Wuhan Fiber Home Technologies Group, the corresponding extraction efficiency and the optical-to-optical efficiency reach 67.3% and 63.0% respectively. No residual pump laser is found in the spectrum of output laser, and the beam quality is measured to be M2<2. Similarly, when the gain fiber of the amplifier adopts the YDF manufactured by China Electronics Technology Group Corporation No. 46 Research Institute, the corresponding extraction efficiency and the optical-to-optical efficiency reach 68.2% and 63.9% respectively. To the best of our knowledge, this is the best result ever reported for directly pumped all-fiber laser. Meanwhile, as we use the domestically manufactured fiber as the gain fiber in the amplifier stage, the result verifies the usage of homemade active fibers in 3-kilowatt level fiber laser. By combining the results of the high power fiber laser, the low efficiency of domestic fiber laser in our experiment might be explained to be due to defects in fiber manufacturing process, the inhomogeneous refractive index of the core, structural flaw of the homemade fiber observed by the microscopic images of the cross section and the splicing fuse of homemade fibers. The main difficulty of these two experiments lies in the heat dissipation of the gain fiber in the amplifier stage. Also, due to the restriction of experimental condition, photodarkening test is unable to run for a longer period of time, which is the focus of our further work. Therefore, measures such as refining fiber manufacturing techniques, increasing pump power and optimizing the length of fiber are suggested to be taken in order to obtain a higher output power from homemade fiber laser.
2015, 64 (16): 164205. doi: 10.7498/aps.64.164205
Tunable coherent deep ultraviolet (DUV) light sources, especially ultrashort pulse DUV lasers have great applications in the fields of time-resolved, material processing, spectroscopy, laser spectroscopy and laser fusion. In the UV region, the best choice of generating the laser pulses in the femtosecond or picosecond regime is the frequency up-conversation technique based on second order nonlinearities. Over the past three decades, quite a lot of nonlinear crystals, such as LiB3O5, βup-BaB2O4, KBe2BO3F2 and Ba1-xB2-y- zO4SixAlyGaz have been developed and employed for generating the femtosecond pulses in the blue, ultraviolet, and even the deep-ultraviolet region. A tunable deep ultraviolet femtosecond laser is experimentally studied based on the new nonlinear crystal Ba1-xB2-y-zO4SixAlyGaz It is a kind of low-temperature phase barium metaborate single crystal belonging to a trigonal system, doped with one or more elements selected from Si, Al and Ga. As an optimized β-BaB2O4 crystal, Ba1-xB2-y-zO4SixAlyGaz completely overcomes the shortcomings of deliquescence compared with β-BaB2O4, and its nonlinear efficiency and optical damage threshold have also been greatly improved. Using two crystals as second harmonic generation is to compensate for the spatial walk-off effect and the light path walk-off due to refraction effect The optical axis of the second Ba1-xB2-y-zO4SixAlyGaz is twice the phase matching angle with respect to the first one. In a femtosecond regime, short pulse provides high efficient frequency conversation due to their high peak powers, but the group velocity mismatch is a cognitive factor to limit conversion efficiency. It is obvious that after the frequency doubling, the second harmonic pulse and fundamental pulse separate from each other. The second harmonic pulse lags behind the fundamental pulse as they propagate through the crystal and the second harmonic pulse is broadened into a longer pulse duration than the fundamental pulse The method to compensate for the group velocity mismatch is to adjust the path length between the fundamental and second harmonic pulse by means of time delay line. It consists of beam splitters and mirrors. Tunable deep ultraviolet pulse within a wavelength range from 192.5 to 210 nm is produced, with a maximum average power of 5.8 mW, under a 2.78 W fundamental power. The average power of second harmonic, third harmonic and fourth harmonic are 1.28 W, 194 mW and 5.8 mW at the fundamental wavelength of 800 nm, corresponding to conversion efficiencies of 46.14%, 15.16% and 3% from the previous stage, respectively. The duration of the third harmonic pulse is 640.4 fs at 266.7 nm as measured by the cross-correlation technique.
2015, 64 (16): 164206. doi: 10.7498/aps.64.164206
Based on the phenomenon of the s-polarization extraordinary optical transmission through subwavelength metallic grating on a dielectric film, the same phenomenon in bilayer metallic nano-grating has been found. In order to analyze the s-polarization transmission in this specific structure, the rigorous coupled-wave analysis and finite-different time-domain method is applied: the former is used for analyzing the transmission of the structure exactly and the latter is used for acquiring the optical field distribution of the structure. Using the equivalent refractive method, the equivalent mechanical model of the bilayer metallic grating is founded, which is as much of extraordinary optical transmission as the original model, to discover the relationship between the polymer and the s-polarization transmission. The comparison of distribution of field-intensity for two bilayer structures, with or without the polymer, illustrates that the existence of the polymer is the main reason to the s-polarization transmission peak appearance. Because the existence of the polymer can be treated as a waveguide and the s-polarization is coupled by metal grating and then turns to a surface wave, there is a resonant phenomenon occurred in the polymer area under the incident light with particular wavelength. In addition, the effect of geometrical parameters of the polymer, such as the refractive index and the thickness of the polymer, the effect of the thickness of the metal film on s-polarization transmittance are discussed. Increasing the refractive index of the polymer leads to the red shift of transmission peak both in the original bilayer model and the equivalent model, which indicates that the two models have the same property. The transmission peak can be explained by the Fabry-Perot-like resonance, and the red shift of transmission peak is result from the change of the resonance condition due to the refractive index increase. The polymer thickness increase results in the addition of the resonance modes and the corresponding transmission peaks. The cycle of the peak is calculated and the result is similar to the length of the Fabry-Perot-like cavity. However, the thickness of metal layer does not impact the position of the s-polarization transmission peak. In conclusion, the polymer which sustains a waveguide elecromagnetic mode is necessary for the extraordinary optical transmission, and the existence of Fabry-Perot-like resonance in the polymer film is the main reason of the resonant peak appearing.
2015, 64 (16): 164207. doi: 10.7498/aps.64.164207
We demonstrate a femtosecond fiber master oscillator power amplifier system of high-stability and high-quality pulse compression, which is applied to corneal refractive surgery. The nonlinear-polarization-evolution mode-locking in a hybrid-cavity Yb-fiber oscillator consisting of both polarization-maintaining (PM) and non-PM fibers is demonstrated. In this paper, the PM-fibers introduced into the mode-locked fiber oscillator partially replace non-PM fibers. This alternative approach is conducive to reducing the adverse effect of uncontrolled fiber birefringence, which originates from the on-PM fiber suffering environmental temperature fluctuation and mechanical vibration. Once the length of non-PM fiber is comparable to the fiber beat length, the uncontrolled fiber birefringence caused by environment starts to abate the laser robustness, repeatability, and reliability of mode-locking. The stability becomes notoriously worse for long-cavity Yb-fiber oscillators. The PM-fibers adopted in the mode-locked fiber oscillator could improve the mode-locked stability of nonlinear-polarization-evolution self-started with long-cavity Yb-fiber oscillators. We study the dependence of the compressed pulse quality on the parameters of input pulse pre-chirp injected into the Yb-doped fiber amplifier. Due to the nonlinear-chirp and third order dispersion, the mode-locked pulse shape in time-domain will produce distortion during power being amplified in the Yb-doped fiber amplifier. A diffraction grating to adjust the pre-chirp of the input pulse from positive value to negative value launched into the Yb-doped fiber amplifier is placed between mode-locked Yb-fiber oscillator and Yb-doped fiber amplifier. We vary the pre-chirp by changing the distance between the diffraction grating and triangular prism and then adjust the second grating pair to compress the amplified pulses into its shortest pulse duration of full-width at half maximum measured by an autocorrelator. The experimental results show that the best compression quality of mode-locked pulse occurs at the negative pre-chirp with a measured pulse width of 183 fs. Deviation from this optimum pre-chirp degrades the compressed-pulse quality and features an increased temporal pedestal. The fiber laser produces self-started mode-locking at a repetition rate of 19.4 MHz, an average power of 1.2 W, a pulse width of 183 fs. The homemade femtosecond fiber laser is used to perform surgery on ex vivo pig corneas. The surgey shows that the light burst of femtosecond laser in corneal tissues could cut pig corneas, which proves that the femtosecond fiber laser satisfies the surgical operation requirements of animal corneas.
2015, 64 (16): 164208. doi: 10.7498/aps.64.164208
The quasi-phase matching optical parametric oscillator tuning methods, i.e. grating period tuning, temperature tuning, pumping wavelength tuning, and angle tuning are more simple and convenient than the traditional mechanical tuning which requires inserting the frequency selective element into the cavity. However, itneed to be improved for wavelength tuning in quick response and high-accuracy control. In this paper, the tunability of multiple optical parametric oscillator (MOPO) based on MgO:QPLN has been studied under an applied electric field. Based on the linear electro-optic effect of LiNbO3, we theoretically analyze the feasibility of achieving parametric light wavelengths tuning by applying electric field of particular direction on the z-direction of LiNbO3. We study the relationships between the ability of electric field tuning and the polarization structure parameters, and analyze the feasibility of MgO:QPLN with high positive and negative superlattice domain ratio for electric field tuning. The relationship of output wavelength and loading voltage is achieved by simulation. Simulation results show that the tuning rates of 1.57 μm signal light and 3.84 μm idler light are about 0.27 and 0.93 nm/kV, respectively. By reasonably controlling the temperature of MgO:QPLN crystal, tuning sections of the parametric light electric field could be linked orderly, and the output wavelength of parametric lights could be turned continuously in a wide range, which greatly expands the spectral bandwidth of the electric field tuning. In the experiment, a high-repetition-rate acousto-optic Q-switched Nd:YVO4 laser at 1064 nm is applied as the pumping source. The laser works at 200 kHz with a pulse width of 9.756 ns and its maximumouput power is 22.8 W on average. When the temperature of MgO:QPLN is stable at 20℃, the average output power of 1.57 μm signal light and 3.84 μm idler light are 1.7 and 0.72 W, respectively, and the corresponding pulse width of the two parametric lights is 9.132 and 8.463 ns. By loading proper electric field on MgO:QPLN (on-load voltage：-3000 V-+3000 V), we achieve the electric tuning of the parametric light at 3.84 μm range, and the bandwidth of spectral tuning is about 6 nm with the tuning rate approaching 1 nm/kV. Combining the temperature tuning with the electric field tuning, we further achieved high-precision continuous tuning of the parametric light in a wide spectrum range. All the final experimental results agree basically with the theoretical analysis one. In addition, results show that the electric field tuning performs better than the temperature tuning in accuracy control and quick response. For MgO:QPLN-MOPO, its irregular domain configuration could work adequately by introducing the electric tuning.
Two-dimensional observation of atmospheric trace gases based on the differential optical absorption spectroscopy technique
2015, 64 (16): 164209. doi: 10.7498/aps.64.164209
A two-dimensional observation method of atmospheric trace gases based on differential optical absorption spectroscopy technique is reported in this paper. The conventional multi-axis differential absorption spectrum system is improved to make the telescope point to different directions. Thereby, the trace gas information of different azimuthal angles and consequently the distribution and variation of pollution gases around the measurement point can be obtained simultaneously. Using this method, NO2 concentration and distribution as well as O4 slant column densities are obtained. High degree of similarity is shown between O4 slant column density simulated by radiation transfer model and the measured data. Based on the measured O4 data, light path information can also be extracted. By combining with radiation transfer model, the light path differences caused by different profile modifications are corrected. The corrected NO2 slant column density is further converted into the volume mixing ratio. By comparing the calculated NO2 mixing ratio with the long-path-differential optical absorption spectroscopy data, the results show good consistency with each other.
2015, 64 (16): 164301. doi: 10.7498/aps.64.164301
The space charge in air is closely related to the mechanism of corona discharge. In order to study the onset and sustainability of corona discharge, acquiring the distribution of space charge is necessary but there still exists a puzzle which has not been settled. According to the sound pulse method, in this paper we present a kind of signal processing algorithm to analyze the electric field which is generated by modulating the space charge in the sound field. The electric filed is dependent on the form of sound emission and space charge density. The waveform of electric field is related to space charge density. Through the proposed algorithm, the space charge density can be obtained by analyzing electric field signal. The area in which the space charges need to be measured, is divided into elements. Each element is small enough so that the space charge quantity in each element is assumed to be the same. The following assumption is accepted during numerical simulation: space charge densities in the wave fronts are the same. The curve of electric field produced, received by electric field antenna, is the vector sum of electric filed produced by each element, and then calculated by numerical simulation. In order to satisfy the assumption in each measurement case, the requirements for sound emission system under different cases are discussed. In different cases, different sound emission systems are required. For space charges which are distributed uniformly, plane wave or spherical wave is suitable; for one-dimensional space charge distribution, plane wave is necessary; for space charge two-dimensional or three-dimensional space charge distribution, plane wave array is availed. What is more, a corresponding measuring system is developed which can be used for measuring the space charge density. This system mainly contains the producing of sound pulse, producing of space charges and the receiving of electric field signal. The producing of sound pulse is designed according to the measurement requirement for multi-needle-to-plate geometry which is assumed that space charge is distributed uniformly in the gap. With the experimental model, the space charge density in multi-needle-to-plate geometry is calculated according to the algorithm proposed in this paper. The result is compared with the calculated one by the method of corona currents, verifying the proposed method.
Ordered detection of layered space-time signals based on the propagation delays of underwater acoustic channels
2015, 64 (16): 164302. doi: 10.7498/aps.64.164302
The multiple-input multiple-output (MIMO) architecture with the layered space-time codes is a very promising solution for the high data rate underwater acoustic communications. The realization of this potential advantage, however, needs the essential layered space-time signal processing methods for canceling the interference resulting from the multipath propagation and the asynchronous arrivals of the sub-streams due to the different propagation delays, and the interference between the transmitted streams superposed in each receiving hydrophone. In this paper, the low-complex layered space-time signal detection scheme for the underwater acoustic communications is investigated. A propagation delay-based ordered successive interference cancellation (OSIC) algorithm is proposed at first. Sub-streams are sorted at the receiver according to the arrival orders resulting from the relative propagation delays inherent in the underwater acoustic channels from the transmitting transducers to the receiving hydrophones. The sub-stream with the first arrival is detected first. The proposed OSIC algorithm based on the "first-come first-go" principle has an advantage in the reduction of the interference from yet-to-be-detected sub-streams, therefore improving the detection performance at each step. The analysis manifests that the delay-based ordering is an optimal detection ordering to minimize the probability of overall block error for the asynchronous space multiplexing architectures. Then the ordering procedure is given which is performed by estimating the relative delays between the MIMO channels and requires only one ordering before the signal detection. This channel estimation-based method simplifies dramatically the ordering procedure and the calculations, therefore reducing substantially the calculation complexity of the layered signal detection. Finally, the single-carrier frequency domain equalization is employed to compensate for the multipath interference and the asynchronous arrival interference from the underwater acoustic propagation. Numerical results show that the performance gain can be obtained with the delay-based OSIC detection algorithm relative to the detection without ordering; moreover the gain increases substantially with the data rate. The investigation results demonstrates, on the other hand, that the inherent relative propagation delay in the underwater acoustic channels leading to asynchronous interference to the signal detection can be turned into an advantage to improve the performance with the efficient space-time signal processing algorithms.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Measurement of jet evolution and electron energy spectrum during the process of laser-driven magnetic reconnection
2015, 64 (16): 165201. doi: 10.7498/aps.64.165201
Magnetic reconnection (MR) is a universal physical process in plasma, in which the stored magnetic energy is converted into high-velocity flows and energetic particles. It is believed that MR plays an important role in many plasma phenomena such as solar fare, gamma-ray burst, fusion plasma instabilities, etc.. The process of MR has been studied in detail by dedicated magnetic-driven experiments. Here, we report the measurements of magnetic reconnection driven by Shenguang II lasers and Gekko XVII lasers. A collimated plasma jet is observed along the direction perpendicular to the reconnection plane with the optical probing. The present jet is very different from traditional magnetic reconnection outflows as known in the two-dimensional reconnection plane. In our experiment, by changing the delay of optical probing beam, we measure the temporal evolution of jet from 0.5 ns to 2.5 ns and its velocity around 400 km/s is deduced. Highcollimated jet is also confirmed by its strong X-ray radiation recorded by an X-ray pinhole camera. With the help of optical interferograms we calculate the jet configuration and its density distribution by using Abel inverting technique. A magnetic spectrometer with an energy range from hundred eV up to one MeV is installed in front of the jet, in the direction perpendicular to the reconnection plane, to measure the accelerated electrons. Two cases are considered for checking the acceleration of electrons. The results show that more accelerated electrons can be found in the reconnection case than in the case without reconnection. We propose that the formation and collimation of the plasma jet, and the electron energy spectrum may be possible directly influenced by the reconnection electric field, which is very important for understanding the energy conversion in the process of MR and establishment of the theoretical model. Finally the electron energy spectra of three different materials Al, Ta and Au are also shown in our work. The results indicate that the higher atomic number material can obtain a better signal-noise ratio, which provides some helpful references for our future work.
2015, 64 (16): 165202. doi: 10.7498/aps.64.165202
Taking theories and stimulation as the starting points, this paper gives the optimal design for dichroic plate in the advanced technology of imaging diagnostic. Dichroic plate is firstly analyzed theoretically, whose result shows that it is a wavelength-sensitive device, different from the traditional understanding of dichroic plate. In fact, a dichroic plate is a wavelength selective, and its outward manifestation is a frequency selective plate. There are two ways of putting medium into the dichroic plate. It can be seen that with the increase of the medium's semidiameter, the passband bandwidth increases obviously and the rising slope is sharper. When the medium is fully populated, the passband bandwidth reaches its maximum, the freguency increasing upto 12 GHz, and in the meanwhile the rising slope possessing the top speed. The other is to adopt the method of coating. If a hollow dielectric cylinder is filled to become a waveguide, it can also be seen that with the thickening of the wall of the dielectric cylinder, the passband bandwidth also increases obviously and the rising slope is sharper. When the medium is fully filled in the circular waveguide, the result is the same as that of the first method.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2015, 64 (16): 166401. doi: 10.7498/aps.64.166401
An experimental research platform is built on Shenguang Ⅱ high power laser facility for obtaining the equation of state of liquid deuterium which has ability to control the temperature in a range of 12-300 K with an accuracy of ±0.03 K in 80 min. By optimizing the coating processing and cleaning the target, we solve the problems that the residual reflection is too high and serious frosting takes place on the window of the target at low temperature, then we obtain the experimental image with a good signal-to-noise ratio. By using the impedance matching method and velocity interferometer system for any reflector, experimental Hugoniot data of liquid deuterium are obtained at a pressure of about 60 GPa under the output condition of 3ω, 3 ns, 1200 J on Shenguang Ⅱ high power laser, which agrees well with the other published data in the same pressure regime and provides a good foundation for the next experimental study of liquid deuterium equation in 100 GPa pressure regime.
Influence of powdered state on crystallization during laser solid forming Zr55Cu30Al10Ni5 bulk metallic glasses
2015, 64 (16): 166402. doi: 10.7498/aps.64.166402
Laser solid forming (LSF) is a viable and promising manufacturing technique for preparing bulk metallic glasses (BMGs) without size limitation. Owing to the structural heredity of alloy melts, the crystallization characteristic of the powder has an important influence on that of the deposit during LSF process. In this work, the as-prepared Zr55Cu30Al10Ni5 (Zr55) alloy powder and the Zr55 alloy powder annealed at 1000 K are used for LSF of Zr55 BMGs. The influence of the crystallization characteristic of Zr55 alloy powder on the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) in the deposit are investigated. It is found that the as-prepared Zr55 powder prepared by plasma rotating electrode process (PREP) is composed of the amorphous phase and Al5Ni3Zr2 phase. When the heat input of laser is low, there exist some Al5Ni3Zr2 residual phases in the amorphous matrix in the RZ, and there appear some Cu10Zr7, CuZr2 and NiZr2 phases besides the Al5Ni3Zr2 phase in the HAZ for the deposit fabricated by as-prepared Zr55 powders. With the increase of the heat input of laser, the RZ remains the amorphous state since the Al5Ni3Zr2 phase is completely remelted, while there are a large quantity of Al5Ni3Zr2 phases and some other crystallization phases precipitated in the HAZ because the heating and cooling rate decrease in the HAZ during LSF. Fabricated by the fully crystallized annealed powder, the deposit is mainly of the amorphous phase, and almost no Al5Ni3Zr2 phase is found even if the incident laser power is low. It is shown that the crystallization of the deposit fabricated by the annealed powder at the low heat input does not change remarkably with the increase of the deposited layers. The Zr55 deposit with five deposited layers could still keep large volume fraction of amorphous phase. This is mainly because the powder experiences the structure relaxation entirely during the annealing treatment, and the volume fraction of the short/medium-range ordered structure associated with the Al5Ni3Zr2 phase in the powder is reduced. Therefore, the volume fraction of the Al5Ni3Zr2 clusters in re-solidified amorphous RZ in the deposited layer decreases during LSF, which is conducible to the increase of the thermal stability of the already-deposited layer. In result, the area of the HAZ in the subsequent deposition decreases and the precipitation of Al5Ni3Zr2 phase is suppressed. In conclusion, increasing the heat input of laser aggravates the crystallization of the deposited layers, and the Al5Ni3Zr2 cluster in the powder has an important influence on the crystallization behavior of the Zr55 deposited layers.
Effect of thermomigration on the growth kinetics of Cu6Sn5 at liquid-solid interfaces in Cu/Sn/Cu solder joints
2015, 64 (16): 166601. doi: 10.7498/aps.64.166601
With the continuous miniaturization of electronic packaging, micro bumps for chip interconnects are smaller in size, and thus the reliability of interconnects becomes more and more sensitive to the formation and growth of intermetallic compounds (IMCs) at liquid-solid interface during soldering. Thermomigration (TM) is one of the simultaneous heat and mass transfer phenomena, and occurs in a mixture under certain external temperature gradient. In the process of interconnection, micro bumps usually undergo multiple reflows during which nonuniform temperature distribution may occur, resulting in TM of metal atoms. Since the interdiffusion of atoms between solders and under bump metallization (UBM) dominates the formation of interfacial IMCs, TM which enhances the directional diffusion of metal atoms and induces the redistribution of elements, will markedly influence the growth behaviors of interfacial IMCs and consequently the reliability of solder joints. The diffusivity of atoms in liquid solder is significantly larger than that in solid solder and in consequence a small temperature gradient may induce the mass migration of atoms. As a result, the growth of interfacial IMCs becomes more sensitive to temperature difference between solder joints in soldering process. So far, however, few studies have focused on liquid state TM in solder joints, and the growth kinetics of interfacial IMCs under TM during soldering is still unknown to us. In this study, Cu/Sn/Cu solder joints are used to investigate the migration behavior of Cu atoms and its effect on the growth kinetics of interfacial Cu6Sn5 under temperature gradients of 35.33℃/cm at 250℃ and 40.0℃/cm at 280℃, respectively. TM experiments are carried out by reflowing the Cu/Sn/Cu interconnects on a hot plate at 250℃ and 280℃ for different durations. For comparison, isothermal aging experiments are conducted in a high temperature chamber under the same temperatures and reaction durations. During isothermal aging, the growth of interfacial Cu6Sn5 follows a parabolic law and is controlled by bulk diffusion. Under the temperature gradient, asymmetrical growth of interfacial Cu6Sn5 is observed between cold and hot ends. At the cold end, the growth of the interfacial Cu6Sn5 is significantly enhanced and follows a linear law, indicating a reaction-controlled growth mechanism; while at the hot end, the growth of the interfacial Cu6Sn5 is inhibited and follows a parabolic law, indicating a diffusion-controlled growth mechanism. The dissolved Cu atoms from the Cu substrate at the hot end are driven to migration toward the cold end by temperature gradient, providing the Cu atomic flux for the fast growth of the interfacial Cu6Sn5 at the cold end. With the variation of the measured thickness of Cu6Sn5 IMC at the cold end and the simulated temperature gradients, the molar heat of transport Q^* of Cu atoms in molten Sn is calculated to +14.11 kJ/mol at 250℃ and +14.44 kJ/mol at 280℃. Accordingly, the driving forces of thermomigration in molten solder FL are estimated to be 1.62×10-19 N and 1.70×10-19 N, respectively.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Effects of Cu doped ZnO diluted magnetic semiconductors on magnetic and electrical performance from simulation and calculation
2015, 64 (16): 167201. doi: 10.7498/aps.64.167201
At present, the effects on the magnetic and electrical properties of Cu heavily doped ZnO with the mole amount of Cu being in a range of 0.02778-0.16667 are rarely studied by first-principles. Therefore two models for Zn1-xCuxO supercells (x=0.02778, 0.03125) are set up to calculate the band structures and density of states by using the plane-wave ultrasoft pseudopotential based on the spin-polarized density functional theory. The calculation results indicate that the doped systems are degenerate semiconductors, and they are semimetal diluted magnetic semiconductors. As the doping amount of Cu increases, the relative concentration of free holes increases, the effective mass of holes decreases, the electron mobility decreases and the electronic conductivity increases. These results are validated again by the analysis of ionization energy and Bohr radius, and they are consistent with the experimental data. As the doping amount of single-Cu increases from 0.02778 to 0.0625, the volume of doping system decreases, the total energy increases, the stability decreases, the formation energy increases and doping is more difficult. As the same concentration and the different doping modes for double-Cu doped, the magnetic moment of doping system first increases and then decreases with the increasing of spacing of Cu-Cu; while the bonds of nearest Cu–O–Cu lie along the a-axis or b-axis, the magnetic moment of doping system disappears; while the bonds of nearest Cu–O–Cu lie along the c-axis, the Curie temperature reaches a temperature above room temperature. As the doping amount of double-Cu increases from 0.0625 to 0.16667, the total magnetic moment of doping system first increases and then decreases, while the bonds of nearest Cu–O–Cu lie along the c-axis. The calculation results are consistent with the experimental data.
Enhancement and quenching of photoluminescence from Au nanoparticles and CdTe quantum dot composite system
2015, 64 (16): 167301. doi: 10.7498/aps.64.167301
New composite systems consisting of Au nanoparticles (NPs) and CdTe quantum dots (QDs) are fabricated by spin coating chemically synthesizing CdTe QDs on silica substrates which have already been implanted by Ag ions through using a metal vapor vacuum arc (MEVVA) ion source implanter. By thermally annealing the Au ions implanted silica substrates, the growth and redistribution of Au NPs can be controlled, the influence of localized surface plasmon (LSP) of Au NPs on the photoluminescence (PL) of CdTe QDs is well studied. The optical properties, surface morphologies, microstructures, and light emission properties of the Au-ion implanted samples are investigated by using optical absorption spectroscopy, atomic force microscopy, transmission electron microscopy and PL spectra measurements. PL spectra show that the PL intensities from Au NPs and CdTe QDs composite systems can be enhanced or quenched compared with those of CdTe QDs directly spin coated on bare silica substrate. The underlying interaction processes between Au NPs and CdTe QDs are discussed in depth, and the new mechanisms for the PL enhancement and quenching in the Au-CdTe coupled systems are put forward. These results provide a good reference for the future designing of optoelectronic devices with improved luminescence efficiency by LSP of metal NPs.
2015, 64 (16): 167302. doi: 10.7498/aps.64.167302
Majorana fermions are their own antiparticles, which play an important role in fault-tolerant topological quantum computation. Recently, the search for Majorana fermions in condensed matter physics, is attracting a great deal of attention as quasiparticles emerge. In this paper we consider a specific model consisting of double quantum dots and a tunnel-coupled semiconductor nanowire on an s-wave superconductor, since the nanowire may support Majorana fermions under appropriate conditions. We study the electron transport through the double quantum dots by using the particle-number resolved master equation. We pay particular attention to the effects of Majorana's dynamics on the current fluctuation (shot noise). It is shown that the current and the shot noise measurement can be used to distinguish Majorana fermions from the usual resonant-tunneling levels. When there exist Majorana fermions coupling to the double quantum dots, a difference between the steady-state source and drain currents depends on the asymmetry of electron tunneling rates. The asymmetric behaviors of the currents can reveal the essential features of the Majorana fermion. Moreover, the dynamics of Majorana coherent oscillations between the semiconductor nanowire and the double quantum dots is revealed in the shot noise, via spectral dips together with a pronounced zero-frequency noise enhancement effect. We find, on the one hand, that the peak of the zero-frequency noise becomes a dip in the case of weak coupling between double quantum dots and the nanowire; on the other hand, for the strong coupling the dip of the zero-frequency noise becomes even further deep with side dips towards high frequency regimes. Furthermore, the dip of the zero-frequency noise disappears and a zero-frequency noise peak gradually develops when the dot-electrode coupling is tuned by gate voltage. As a result, the combination of the current and the shot noise through double quantum dots allows one to probe the presence of Majorana fermions.
New super junction lateral double-diffused metal-oxide-semiconductor field-effect transistor with the P covered layer
2015, 64 (16): 167304. doi: 10.7498/aps.64.167304
In order to design the lateral double-diffused metal-oxide-semiconductor field-effect transistor (LDMOS) with low loss required for a power integrated circuit, a new super junction LDMOS with the P covered layer which is based on the existing N buffered super junction LDMOS is proposed in this paper for the first time. The key feature of the proposed structure is that the P-type covered layer is partly above the N-type of the super junction layer, which is different from the N buffered super junction LDMOS. In this structure, the specific on-resistance of the device is reduced by using the high doped super junction layer; the problem of the substrate-assisted depletion which is produced due to the P-type substrate of the N-channel super junction LDMOS is eliminated by completely compensating for the charges of the N-type buffered layer and the P-type covered layer, thus improving the breakdown voltage. The charges of the N-type and P-type pillars are depleted completely. A new transmission path at the on-state is formed by N buffered layer to reduce the specific on-resistance, which is similar to the N buffered super junction LDMOS. However, the effect of N-type buffered layer of N buffered super junction LDMOS is not fully used. The drift region of the device is further optimized by the proposed device to reduce the specific on-resistance. The charge concentration of the N-type buffered layer in the proposed device is improved by the effect of charge compensation of the P covered layer. It is clear that high breakdown voltage and low specific on-resistance are realized in the proposed device by introducing the P-type covered layer and the N-type buffered layer. The results of the 3 D-ISE software suggest that when the drift region is on a scale of 10 μm, a specific on-resistance of 4.26 mΩ·cm2 obtained from P covered super junction LDMOS by introducing P covered layer and N buffered layer is reduced by about 59% compared with that of conventional super junction LDMOS which is 10.47 mΩ·cm2, and reduced by about 43% compared with that of N Buffered super junction LDMOS which is 7.46 mΩ·cm2.
Gate length dependence of hot carrier injection degradation in short channel silicon on insulator planar MOSFET
2015, 64 (16): 167305. doi: 10.7498/aps.64.167305
With the continued device scaling and the introduction of new device structures, MOSFET reliability phenomena arising from the hot carrier injection (HCI) stress have received extensive attention from both the academia and the industry community. In this work, the degradations of ultra-scaled silicon on insulator (SOI) MOSFETs under the HCI stress are investigated on devices of different gate lengths (L=30-150 nm). Our experimental data demonstrate that the time evolutions of the threshold voltage change (Vth) under the HCI stress for different gate length devices are the same, and the magnitude of Vth reduces for the shorter devices. The degradation of the device under the HCI stress should be due to both the channel hot carrier (CHC) effect and the bias temperature instability (BTI) effect. The distribution and magnitude of the electric field along the MOSFET's channel are analyzed. It is confirmed that besides the well-known CHC effect in the depletion region close to the drain side, a strong BTI effect co-exists in the channel close to the source side. This degradation mechanism is different from the conventional HCI stress. With the gate length decreasing, the contribution of the aforementioned BTI effect becomes larger, and it dominates in the degradation. One feature of the BTI effects is that the corresponding degradation is small when the gate length is short. This is consistent with our experimental result that the change of Vth is small for the device of short gate length under the accelerated HCI stress. The time evolution of Vth can be described by the equation Vth=A•tn, where A is a constant, t is the stress time, and n is the power law exponent obtained by the curve fitting. In this study, the power law exponent n of pMOSFET is larger than that of nMOSFET. This experimental fact can lead to the point that the BTI effect exists during the HCI stress because the BTI effect in ultra-scaled pMOSFETs is more significant than that in nMOSFETs. The stress-recover experiments of the HCI stress on MOSFTTs show larger recovery in device of shorter gate length. It is found that the ratio of the recovery to the total degradation in the 30 nm gate-length device is almost twice as large as that in the 150 nm device. The degradation from the CHC effect has no recovery, and the larger recovery in the shorter-channel device implies the larger component of the BTI degradation. Another intriguing fact is that our experimental result on SOI MOSFET is inconsistent with the recently reported result on FinFET. We argue that the reported stronger HCI degradation in FinFET may not be ascribed only to the stronger electric field in the shorter channel, but also to the fact that the FinFET' channel is three-dimensionally surrounded by the gate dielectric. This kind of three-dimensional structure significantly increases the chance for electrons or holes to be injected into the dielectric layer. Therefore the HCI reliability of planar SOI MOSFETs may be better than that of FinFETs at the same level of gate length. In conclusion, the BTI effect is an important source of the degradation during the HCI stress in ultra-short-channel device, and it is no more negligible in analyzing the underlying physical mechanism.
2015, 64 (16): 167501. doi: 10.7498/aps.64.167501
The giant magnetoimpedance(GMI) effect of Co-rich microwires makes an opportunity to design sensitive GMI weak magnetic meter sensor. Optimization of magnetic meters needs to improve the GMI response, especially the field sensitivity of microwires. In this study, Co-rich amorphous microwires each with an average diameter of 32 μm are prepared by melt-extracted technique and their GMI characteristics are investigated at frequencies ranging from 0.1 to 10 MHz with and without bias direct voltage applied. Experimental results indicate that the GMI effect of these wires has asymmetric features with the increases of frequency and driving current. It is found that the intrinsic asymmetric GMI (AGMI) response results from the helical anisotropy and magnetization hysteresis of the Co-rich microwires. Furthermore, it is found that there is a pronounced improvement in AGMI response when a bias voltage is applied. In theory, the factor which induces an increase in circular magnetic field causes successive changes in magnetization reversal of the quickly quenched Co-rich microwires with multiple domains and helical anisotropy. As a consequence, the circular magnetization process is enhanced, leading to higher circular permeability and stronger GMI response. Meanwhile, a bias voltage inducing the given circular magnetic field reinforces the magnetization process in a certain direction, which intensifies the asymmetric characteristic of GMI response. For example, the asymmetric ratio between two impedance peaks rises from 1.46% to 12.06% at 1MHz and 3 mA after applying a 1 V bias voltage. Simultaneously, the circular field inclines the magnetization off the axial direction which makes the axially induced magnetization reversal more difficult and occur at a higher switching field. This effect broadens the linear impedance zone; however, it reduces the slope of the impedance with the external field and the field sensitivity increasing to some extent. The balance between these two sides proves that AGMI response is related to the magnetization reversal process which is sensitive to the circular magnetic field. Experimental results indicate that the field sensitivity rises from 616 to 5687 V/T with the impedance linear zone broadening from 0.65 to 1.16 when a 1 V bias voltage is applied, while it decreases to 4525 V/T when the bias voltage futher increases to 2 V at 10 MHz and 5 mA. This reveals that the GMI effect of these amorphous Co-rich microwires with high field sensitivity can be optimized by applying proper bias voltage.
Study of ferroelectric switching and fatigue behaviors in poly(vinylidene fluoride-trifluoroethylene) copolymer nano-films
2015, 64 (16): 167701. doi: 10.7498/aps.64.167701
The nano-films of poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer, with mole ratio of VDFTrFE 70/30, are deposited on titanium-metallized silicon wafer by spin coating technique. Annealing temperature and humidity dependence of polarization switching and fatigue babivors in ferroelectric P(VDF-TrFE) copolymer thin film capacitors have been investigated. Firstly, the effect of different annealing temperature on polarization behavior is revealed. It is found that the polarization of the film is improved by increasing annealing temperatures. When the annealing temperature is higher than 100℃, with increasing switching cycles, the ferroelectric polarization characteristics exhibit a trend of increasing firstly and then decreasing, a top value appears at the number of cycles near 104. A more appropriate heat treatment temperature is 130℃. Further analyses on the crystalline structures with X-ray diffraction show that the degree of crystallinity of the films is strongly dependent on the annealing temperature. It can be seen that the diffraction peak of the ferroelectric phase ( phase) becomes very strong and sharp with increasing annealing temperatre. It is demonstrated that the effect of annealing temperature on ferroelectric properties could be explained by the changes of the degree of crystallinity in these films from the results of X-ray and the polarization behaviors. Meanwhile, the microstructure of the 140 nm film annealed at 130℃ is obtained by using scanning electron microscope, which shows that the film exhibits a worm-like, dense, well-crystallized microstructure. Secondly, for the capacitor P(VDF-TrFE) films with a thickness of 140 nm, the ferroelectric polarization hysteresis loops as functions of electric field for the films at different relative humidities are achieved. It is obvious that the polarization properties depend on the relative humidity during the film preparation process, the polarizaiton fatigue can be further enhanced through a higher relative humidity during the sample preparation. In addition, one of the most important features for ferroelectric material to be used as an alternative FeRAM is the low leakage current density. Therefore, the descriptions of the leakage current density versus different relative humidities are given. It is observed that the voltage behavior of the leakage current has a minor dependence on relative humidity. In a word, these results illustrate that the polarization properties are strongly dependent not only on the annealing temperature, but also the relative humidity in a process for the preparation of the nano-films. Furthermore, according to a re-annealing treatment to improve the crystalline degree of the ferroelectric phase, the influence of the re-annealing process on the fatigue properties of the films is also studied. The polarization fatigue can be improved obviously by a re-annealing process, and the possible origins have been discussed. To further understand the variation of crystallization properties of the samples before and after re-annealing, the crystallinity of the film are studied by the technique of Fourier transform infrared spectroscopy. It is indicated that the crystallinity of the films can partly be recovered through re-annealing treatment. These results are very helpful and provide an available way to improve the ferroelectric polarization and fatigue properties of the ferroelectric nano-films.
Phosphorescent hybrid organic-inorganic light emitting devices with solution-processed small molecule emissive layers
2015, 64 (16): 167801. doi: 10.7498/aps.64.167801
We report efficient phosphorescent hybrid organic-inorganic light emitting devices using poly(ethylenimine) as electron-injection layer and interfacial modifier for metal oxides and solution-processed small molecule emissive layers. As a first step, the hole transport layers with various HOMO levels and the hole mobility values including TAPC, TCTA and mCP are evaluated. The results indicate that devices using a TAPC layer show the best luminance and power efficiencies. Subsequently, the optimum phosphor concentration is determined to be ca. 5%. Enlarged efficiency roll-off with increasing current density is observed in devices using the phosphor concentration above the optimum value. We also investigate the morphologies of films having different phosphor concentrations on the top of bare or PEIE-covered glass substrates, which is closely related to device performance. All the films have the RMS values of ca. 1 nm, indicating high-quality solution processed small molecule films. The blue, yellow and red devices show the maximum external quantum efficiencies of 17.3%, 10.7% and 7.3%, respectively. These efficiencies are 17.0%, 10.6% and 5.8% at 1000 cd·m-2, only showing a small roll-off, which can be attributed to alleviated triplet-triplet and triplet-polaron interactions in a broad carrier recombination zone due to using the mixed hole-transport and electron-transport materials as the co-hosts. In addition, these devices exhibit the respective Commission International Eclairage (CIE) coordinates of (0.16, 0.36), (0.50, 0.49) and (0.66, 0.32), which almost traverse the whole visible light region. Furthermore, the hybrid two-colored white devices show a luminance efficiency of 31 cd·A-1, power efficiency of 14.8 lm·W-1 at 1000 cd·m-2 and the operating current-insensitive CIE coordinates of (0.32, 0.42). The efficiencies represent the significant improvement over the previously reported values, which can be attributed to high-quality small molecule films on PEIE, and in particular to the unique properties of small molecule host materials such as balanced carrier transport and high triplet energy. Further efforts including selection of high-mobility electron transport host material and phosphors with high luminescence quantum yield are made to increase the efficiency and reliability of hybrid organic-inorganic light emitting device.
2015, 64 (16): 167802. doi: 10.7498/aps.64.167802
Scintillation glass is an attractive material due to its many advantages including low-cost and easy-manufacturing compared with single crystal. However the low density of glass scintillator restricts its applications. The introduction of heavy components such as PbO and Bi2O3 allows the density of the glass to be easily increased to more than 6.0 g/cm3 which is desirable for most applications. However, it is usually accompanied with a dramatic decrease in the luminescence response of Ce3+ ions. Although Gd2O3 based glass has a relatively high light yield, it is far below the high silica glass. In order to explain why the luminescent efficiency of Ce3+ doped glass with low density is high while that with high density is low, a glass-forming region of SiO2-Al2O3-Gd2O3 ternary system is achieved by high-temperature melt-quenching method. Ce3+doped SiO2-Al2O3-Gd2O3 and SiO2-Al2O3-Gd2O3-Ln2O3 (Ln=Y, La, Lu) scintillation glasses are prepared at reducing atmosphere. Their optical and scintillation properties are investigated. The results show that the content of Gd2O3 can reach as high as 30% mol without phase separation. In addition, the UV cut-off position is red-shifted, PL intensity decreases and decay time reduces from 70 to 37.6 ns with increasing the Gd2O3 concentration. After Lu2O3, La2O3, Y2O3 are added in the glass, the UV cut-off position is red-shifted and PL intensity decreases. Moreover the UV cut-off position is in the order of La>Y>Lu and the decay time is in the order of La2O3 is more than 10% mol, X-ray excited luminescence light emission intensity reduces from 61% of BGO to 13% of BGO. With the UV cut-off position red-shifted, the bandgap of glass becomes narrow, resulting in the 5 d level of Ce3+ ions gradually approaching to the conduction band and the 5 d electrons easily combining with the holes in the glass through the conduction band. Namely, charge transferring quenching occurs. This is the reason why the PL intensity and decay time both decrease. It can also explain why the luminescent efficiency of Ce3+ doped glass with low density is high while that with high density is low.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2015, 64 (16): 168101. doi: 10.7498/aps.64.168101
Friction or lubrication process is a typical process of the energy dissipation. It can be reasonably described and speculated by using the entropy increase and dissipative structure theory of the non-equilibrium thermodynamics. In this paper, we model and analyze the typical thin-film lubrication mechanism based on the theory of thermodynamics, by using the interfacial disjoining pressure to characterize the dominant role of the solid-lubricant interaction on a microscale and establishing the lubrication Stribeck curve based on thermodynamic concepts. The concept of entropy production is adopted to describe the lubrication system, which is defined as the sum of multiplications of the thermodynamic forces and flows. Then the variations and the competing relations between the pairs of thermodynamic forces and flows could be used to reveal the different factors dominated in the lubrication system, such as the solid-liquid interaction, the sliding velocity, and the normal load. In this paper, we assume that all the dissipated energy caused by the viscous resistance of lubricant is converted into heat, then the total entropy increase per surface area at the frictional interface is considered, affected by interfacial disjoining pressure and the one-dimensional heat flow. With the entropy increasing analysis of lubrication process, we find that when the entropy production in the steady state becomes minimum, the total energy dissipation due to friction also becomes minimum, which directly indicates the lowest friction coefficient point at the lubrication Stribeck curve. Moreover, when a lubrication system loses its stability slightly from the equilibrium state, self-organization may occur at the solid-lubricant interface, thus resulting in partially ordering interfacial structures, which may indicate the interfacial structures when tribosystem turns from hydrodynamic lubrication phase into thin-film lubrication phase. In the experimental aspect, the location of the lowest friction coefficient point at the Stribeck curve has a very good correspondence to the minimum entropy point predicted by our thermodynamic model, and the lubrication transition process from hydrodynamic phase to thin-film phase can be explained quite well by the theory of dissipative structures when the system loses its stability. Furthermore, a calculation model of the friction coefficient for thin-film lubrication is obtained when considering the dominant contribution of the solid-lubricant interfacial interaction through an equivalent force method. The calculation data correspond well to the experimental results. In summary, thermodynamic model could effectively characterize the lubrication process in mechanism by revealing the involved multi-scale effect, multi-physical effect and nonlinear coupling effect.
Vesicles exposed to shear flow exhibit a remarkably rich dynamics. With the increase of shear rate, one can observe a tumbling-to-tank-treading transition. Besides, a complex oscillating motion, which has alternatively been called trembling, swinging, or vacillating breathing, has also been predicted theoretically and observed experimentally. While in biological systems, vesicles are always decorated by a large number of macromolecules, rendering the dynamics of vesicles in shear flow much more complex. As a powerful supplement to analytical techniques, the dissipative particle dynamics has been proved to be a useful tool in simulating nonequilibrium behaviors under shear. By replacing the conservative force in dissipative particle dynamics with a repulsive Lennard-Jones potential, the density distortion has been overcome and the no-slip boundary condition is achieved. In this article, a nonequilibrium molecular dynamic method is used to study the dynamics of two-dimensional complex vesicles in shear flow. The dynamical behaviors of the complex vesicles are closely related to shear rate and the size of small grafting vesicle. We first consider a vesicle with two small vesicles symmetrically grafted. At a weak flow, the complex vesicle maintains its equilibrium shape and undergoes an unsteady flipping motion, known as tumbling motion. At a moderate shear rate, the tumbling of the vesicle is accompanied with strong shape oscillation, which is consistent with Yazdani's simulation, in which a breathing-with-tumbling type of motion is observed, and is called trembling in this article. As the shear rate further increases, the vesicle is oriented at a fixed angle with respect to the flow direction, while the vesicle membrane circulates around its surface area, exhibiting a well-known tank-treading motion. For sufficiently large grafted vesicles and at a high enough shear rate, a transition from tank-treading to translating motion is observed, in which the flipping of the vesicle or the circulating of the vesicle membrane is hampered. A crossover regime, namely, the tank-treading/translating mixture motion is also found, where translating motion alternates with tank-treading chaotically. However, when a sufficient number of small vesicles are uniformly grafted to the vesicle, the newly observed translating motion is eliminated. This study can give a deeper insight into the complexity of vesicle motions in shear flow.
High mobility thin-film transistor with solution-processed hafnium-oxide dielectric and zinc-indium-tin-oxide semiconductor
2015, 64 (16): 168501. doi: 10.7498/aps.64.168501
In this paper, bottom-gate-top-contact structured thin-film transistors (TFTs) are fabricated by solution-processing of hafnium oxide (HfO2) dielectrics and zinc-indium-tin-oxide (ZITO) semiconductors. Solution-processed HfO2 films are annealed at different temperatures, and the 500℃ annealed HfO2 dielectrics can exhibit optimizing film properties such as smooth surfaces (the RMS value of HfO2 films is less than 1 nm), low leakage current density (1.25×10-7 A/cm2 at 1 MV/cm), high transmittance (above 80% at the wavelength ranging from 400 to 800 nm) and high relative dielectric constant (about 12). The smooth surface of HfO2 dielectrics is attributed to the decreased charge trapping states at the interface between the HfO2 dielectrics and ZITO semiconductors, and thus improves the device electrical performance and stability. Hence, TFT devices of HfO2 dielectrics annealed at 500℃ show a high saturated field effect mobility of more than 100 cm2·V-1·s-1 a low threshold voltage of -0.5 V, an on-to-off current ratio of 5×106 and a small subthreshold swing of 105 mV/dec. An almost negligible threshold voltage shift is observed under a positive bias stress for 1000 s, indicating the excellent stability of HfO2 TFT devices.
2015, 64 (16): 168701. doi: 10.7498/aps.64.168701
One has proved that the collective structural vibrational modes of proteins are in the terahertz (THz) frequency range. These frequencies relate to the polypeptide backbone and are thought to be essential for conformational dynamics necessary for protein function. Hemagglutinin (HA) is the main surface glycoprotein of the influenza A virus. The H9N2 subtype influenza A virus is recognized as the most possible pandemic strain as it crosses the species barrier, infects swine and humans. In this paper we use principal component analysis (PCA) to study the 7 different concentrations dependent terahertz spectra of hemagglutinin proteins, and detect the binding interaction of HA with the broadly neutralizing monoclonal antibody F10 in liquid phase. Spectrum pretreatment and band selection play a vital role in the THz spectroscopic analysis due to the fact that the original spectrum contains a large amount of interference information. In order to compress variables and extract useful information, we use a variety of pretreatment methods, such as second derivative, multiplicative scatter correction (MSC), least square polynomial fitting derivation, standard normalization, smoothing, moving window median filtering before PCA analysis. We even consider MSC + smoothing + SG second derivative + median filtering as the optimized pretreatment method finally. THz spectrum parameters including refractive index, absorption coefficient, reduced absorption cross-section and dielectric loss angle tangent are calculated in a frequency range of 0.1-1.4 THz for comparison. The results indicate that the reduced absorption cross-section presents the highest correlation response to the concentration variation of HA protein, and the dielectric loss angle tangent appears to be more appropriate for qualitative analysis of HA-antibody binding interaction. PCA method provides a feasible and effective way to find the sensitive parameters for further analyzing the function of protein and the antigen-antibody interaction using terahertz spectrum, whereas an appropriate pretreatment method is required.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Decreasing trend of the middle atmospheric static stability in historical data from rocketsonde network
2015, 64 (16): 169201. doi: 10.7498/aps.64.169201
Brunt-Vaisala frequency squared (N2) measures the static stability of the atmosphere, and reflects the general structure of the atmosphere in term of vertical temperature gradient. For middle atmosphere the response of the middle atmospheric structure to the global warming still lacks investigation currently. The historical data from rocket sounding network in 1962-1991 are used to investigate the long-term trend of N2 in the middle atmosphere. For six stations spanning from the tropical latitudes to the northern mid-latitudes, our estimates show that, in the upper stratosphere and middle mesosphere, i. e., 48-60 km high, the significant decreasing of static stability is observed in an N2 anomalies averaged over 48-60 km range. For two tropical stations, long-term trend in N2 exhibits a similar magnitude, i.e., -0.11×10-4 s-2/decade; it is also observed that the trend increases with latitude, with trend estimates from -0.16×10-4 s-2/decade at 22°N (Barking Sand station) to -0.22×10-4 s-2/decade at 38°N (Wallops Island station).
2015, 64 (16): 169501. doi: 10.7498/aps.64.169501
The space-based surveillance, which would mainly use the space-based visible, has great value for civil and military applications currently and for a fairly long future period. In space-based surveillance, the visible and near-infrared radiation characteristics of the space target are influenced by its attitude variation. This influence is especially prominent in space-based imaging. In some ways, solar radiation cannot arrive at the surface of the space target, or the arriving radiation is not uniformly distributed because of the space target's strong reflection at a particular position. In order to solve these problems, the visible and near-infrared illumination characteristics of the space target surface are analyzed. Moreover, a notion that earth's reflective radiation can be used as illumination for space target imaging is given, and an accurate modeling method is proposed. Firstly, based on diffuse reflectance model, a method of mathematically calculating the illumination at space target's position from earth's reflective radiation is established. And a formula for calculating illumination is derived. Secondly, the coordinates of sun and space target at any time can be obtained by the Satellite tool kit software, in which the complicated multiplying matrix and coordinate transformation algorithm introduced in some references are avoided. Thirdly, the method of estimating earth's reflective radiation region at arbitrary moment is introduced. The grid division method is generated and the uniform sampling is used in each small area. Fourthly, the position of a surface cell is transformed from the sphere reference frame into the J2000.0 inertial frame. The earth's reflective radiation can be calculated through numerical integration. Finally, the illumination from earth's reflective radiation to a sun synchronous orbit satellite in an imaging mission based on space is calculated by the given parameters. The results show that the earth's reflective radiation is luminous enough for space target imaging when the satellite passes through arctic. When the satellite moves on the orbit, we can obtain more detailed information about target satellites' bottom then the ground simulation imaging. The on-orbit imaging results demonstrate the validity of the modeling method, which could support the foundation of our space-based surveillance system theoretically and technically and could be used as a reference of space-based orbit measurement and determination in deep space exploration.