Vol. 67, No. 6 (2018)
2018, 67 (6): 060201. doi: 10.7498/aps.67.20172235
Boltzmann-Hamel equation using quasi-velocities as variable quantities instead of generalized-velocities,is an extending form of the classical Lagrange equation.It is widely used for establishing the motion equations in constrained mechanical systems because of its unique structure.The classical method to solve Boltzmann-Hamel equation includes two steps.The first step is to substitute the relationship between the quasi-velocities and generalized-velocities into the equation to establish the second order equation relating to generalized-coordinates.The second step is to search for the analytical solutions using the method of separating variables or the method of Lie groups.However this method is not very effective in practice.In fact,the majority of studies only focus on the similarity between the quasi-coordinate form and the linear non-holonomic constraint form,without considering the effects of the selection of quasi-coordinates on the Boltzmann-Hamel equation.Because the quasi-coordinates in Boltzmann-Hamel equation can be selected freely,the problem of simplifying the Boltzmann-Hamel equation in holonomic system by choosing the appropriate quasi-coordinates is studied in this paper.Using the method of geometrodynamic analysis,the relationship between quasi-coordinates in the time-invariant configuration space and frame field is indicated based on the frame field theory of manifolds.The Boltzmann-Hamel equation in holonomic system is then derived from the tangle of geometric invariance.It differs from the ordinary methods,such as the action principle or d'Alembert's method.It is demonstrated that Boltzmann-Hamel equation can be simplified into an integrable form in homogenous configuration space with zero generalized-force or zero curvature configuration space with non-zero generalized-force.The process of simplifying the equation is provided in detail and the feasibility of this method is verified through two examples.The result in this paper reveals the close link between the intrinsic curvature of the time-invariant configuration space and the structure of Boltzmann-Hamel equation.The simplest form of Boltzmann-Hamel equation under the generalized-coordinate bases field (Lagrange equation) corresponds to the configuration space of zero curvature,and the simplest form of Boltzmann-Hamel equation under the frame field corresponds to the homogenous configuration space (more often,constant curvature space).For the complex motion equations,it should be transformed first into Boltzmann-Hamel equation,then the intrinsic curvature of the time-invariant configuration space will be calculated.If the conditions mentioned in this paper are satisfied, the Boltzmann-Hamel equation can be simplified into the simplest form by choosing appropriate quasi-coordinates,from which,the analytical solutions can be obtained,furthermore,this frame field derived by the appropriate quasi-coordinates can be used as a tool to study the symmetry and the conserved quantity of this holonomic mechanical system.The results in this paper provide a new way to search for the analytical solution of motion equations.
Influence of parameter prior information on effect of colored noise in Bayesian frequency estimation
2018, 67 (6): 060301. doi: 10.7498/aps.67.20171911
Parameter estimation, which undertakes one of the vital missions in quantum metrology, has attracted a lot of attention in recent years. A large number of investigations on the frequency estimation have been carried out. Most of them are based on Cramér-Rao bound estimation approach in which almost perfect knowledge of the parameter to be estimated is given. In reality, however, one has inadequate prior knowledge about the parameter to be estimated. Then the Bayesian estimation approach in which we can perform the estimation even if we only have partial prior information about the parameter would be an ideal choice. Prior information about the parameter can play a significant role in Bayesian statistical inference. So it is interesting to know how the prior knowledge affects the estimation accuracy in the estimation process. In the solid-state realization of probe system, material-specific fluctuations typically lead to the major contribution to the intrinsic noise. Then it is interesting to study the effects of colored noise on the quantum parameter estimation. In this work, we study the inhibitory effects of prior probability distribution of the parameter to be estimated on the effects of colored noise under the framework of Bayesian parameter estimation theory. In particular, we estimate the intensity of a magnetic field by adopting a spin-1/2 system which is influenced by the colored noise with 1/fα spectrum. To evaluate the accuracy of estimation, we obtain the Bayes cost analytically which can be applied to the noisy channels. We mainly focus on the inhibitory effect of prior probability distribution of measured parameter on the non-Gaussianity of noise. We find that for the case of broad prior frequency distribution, the influence of non-Gaussianity on the estimation is very weak. While for the case of narrow prior frequency distribution, the influence of non-Gaussianity on the estimation is strong. That means that in the Bayesian approach, when we have enough prior information about the frequency, the non-Gaussianity can conduce to the improvement of the accuracy of the estimation of the frequency. When we lose the prior information, we also lose the improvement of the accuracy from the non-Gaussianity. The uncertainty of the prior information tends to eliminate the effects of the non-Gaussianity of the noise.
Generation of four-photon hyperentangled state using spontaneous parametric down-conversion source with the second-order term
2018, 67 (6): 060302. doi: 10.7498/aps.67.20172230
Nowadays,the generation of multiphoton entangled states is almost realized by combining the coupled entangled photons emitted from spontaneous parametric down-conversion (SPDC) with the first-order term.In this case,one may focus mainly on the first-order term,and then avoid multipair emission events by restricting experimental parameters.On the other hand,for the higher-order terms in SPDC source,these emitted entangled photons have interesting features.For example,they are entangled maximally not only in photon number for the spatial modes,but also in polarization degree of freedom.In general,two photons,which are entangled in two or more degrees of freedom,are called hyperentangled pair of photons or hyperentangled state.We present a scheme to generate the four-photon hyperentangled state based on four indistinguishable photons emitted from SPDC source with the second-order term.Consider two SPDC sources with equal probability of emission of photons in respective spatial modes.With the passive linear optical devices,i.e., beam splitters,half wave plates,polarizing beam splitters,etc.,under the condition of registering a specified four-photon coincidence,we can obtain the four-photon hyperentangled state in which the photons are entangled in both polarization and spatial-mode degrees of freedom.Here,of course,for an arbitrary fourfold coincidence detection,one obtains a canonical four-photon Greenberger-Horne-Zeilinger (GHZ) state.Then we show the results of fourfold coincidence detections and the corresponding probabilities for the four-photon GHZ states,where the generation of the four-photon hyperentangled state is included as long as we are not to distinguish the two detectors located at the same locations. As a result,our scheme has two notable features.When we only consider the second-order emission,since it is not needed for us to distinguish between the two SPDC sources,the present scheme is simple and feasible.Also,based on the postselection with fourfold coincidence detection,our scheme is suitable for the normal first-order emission where we restrict the four photons emitted from the same source.In this sense,our scheme is efficient.In a word,we describe a method to generate the four-photon hyperentangled state with the second-order emission in SPDC source,which may contribute to the exploration of multipair entanglement with higher-order emissions from the SPDC source.
2018, 67 (6): 060701. doi: 10.7498/aps.67.20172108
As is well known a linearly polarized resonant laser will cause atoms to generate a magnetic tensor moment (MTM) by polarizing them. When there exists an external magnetic field, it is possible that the moment will precess around the field. In the presence of a radio frequency (RF) exciting source, we investigate theoretically the dependence of time-independent (direct current, DC), the first and second harmonic signal of the MTM precession on magnetic vector field, and obtain its analytical solution by solving the Liouville equation. The results show that the interference of both harmonic components will result in the precession spectrum evidently varying. A detailed explanation is described in the following. For the DC signal, Rabi frequency Ω of 1/(2√2) is a spectral splitting threshold. When it is greater than the threshold, the interference will cause single resonant absorption dip characterized usually to split into two dips, which has not been reported before to the best of our knowledge, and the separation between both the dips may be expressed as √3√Ω2+Ω4 -Ω2-1. For the first harmonic signal including symmetric and antisymmetric component, an interference fringe will appear near the center of antisymmetric part when Ω >1/(2√2), simultaneously its symmetric part behaves like the above dc component, such as splitting threshold and separation between both dips. With regard to the second harmonic signal, it is found that the interference can also lead to the width of the second harmonic decreasing to 38% compared with the case of the first harmonic signal. At the optimum RF Rabi frequency, on the assumption that noise spectral density is constant, it is theoretically shown that the most sensitive magnetometer, realized by the DC component or the first or second harmonic signal of the precession, depends only on the angle between the light polarization and the measured magnetic field.In fact, we are able to obtain the modules of the measured magnetic vector by RF resonant frequency. The angle between the magnetic field and the laser polarization is determined just by the ratio of the intensity of the DC component to the intensity of the second harmonic signal and the ratio between the intensities of the symmetric parts of two harmonic signals in resonance, and another orientation angle between the measured field projection at the plane perpendicular to the light polarization and the direction of RF source depends on the phase difference between the antisymmetric components of both harmonic signals. Consequently, we demonstrate a vectorial atomic magnetometer that is realized by using the RF source and the linearly polarized resonant laser without rotating laser polarization. This kind of atomic magnetometer with simple sensor structure is easy to integrate vector magnetometer array which will be suitable for solving the inverse problem and geomagnetic navigation.
2018, 67 (6): 060401. doi: 10.7498/aps.67.20172374
Black hole spectroscopy is an important content in the quantum properties of black holes. In this paper, we use the adiabatic invariants of black holes to investigate the entropy spectrum and area spectrum of the Kerr black hole in gravity's rainbow. Firstly, by considering the particles passing through the event horizon, the adiabatic invariance action for the modified Kerr black hole is calculated. Here, the Euclidean coordinate and the period of the Euclidean time of a loop about the event horizon are used. Combined the obtained adiabatic invariants with the Bohr-Sommerfen quantization condition, the equally spaced entropy spectra that are the same as the original Beckenstein spectra are derived. The entropy spectrum of the gravity's rainbow is independent of the test particle energy. Next, using the first law of the black hole thermodynamics and the black hole entropy spectrum, the area spectrum of the modified Kerr black hole is studied. Due to the quantum gravity effect of the gravity's rainbow, the obtained area spectrum is different from the original Beckenstein spectrum. The present area spectrum is non-equidistant and dependent on the horizon area of the black hole. With the decrease of black hole area, the area space gradually turns smaller. When the black hole reaches the minimum area on a Planck scale, the area quantum is zero. Thus the black hole area no longer decrease and a remnant of the black hole radiation appears. In the case of a large black hole, the correction of the area spectrum to the equally spaced spectra can be ignored, and the area spectrum of the Kerr black hole in gravity's rainbow can return to the original Beckenstein spectrum. It is also shown that like the entropy spectrum, the area spectrum of the gravity's rainbow does not depend on the energy of the test particles either. In addition, the entropy of the modified Kerr black hole in gravity's rainbow is discussed by using the first law of the black hole thermodynamics. The black hole entropy with quantum correction items as the area reciprocal to the Beckenstein-Hawking entropy is derived and the relation between the quantum correction items and the area is discussed. In addition, the consistency between the entropy correction and the area correction for the modified black hole is analyzed. The current research supports that in different spacetimes including quantum corrected spacetimes, the black hole entropy spectrum has the universality, but the black hole area spectrum is dependent on the area due to the spacetime quantum properties.
2018, 67 (6): 060501. doi: 10.7498/aps.67.20172470
Chaos is a seemingly random and irregular movement, happening in a deterministic system without random factors. Chaotic theory has promising applications in various areas (e.g., communication, image encryption, geophysics, weak signal detection). However, observed chaotic signals are often contaminated by noise. The presence of noise hinders the chaos theory from being applied to related fields. Therefore, it is important to develop a new method of suppressing the noise of the chaotic signals. Recently, the denoising algorithm for chaotic signals based on collaborative filtering was proposed. Its denoising performance is better than those of the existing denoising algorithms for chaotic signals. The denoising algorithm for chaotic signals based on collaborative filtering makes full use of the self-similar structural feature of chaotic signals. However, in the parameter optimization issue of the denoising algorithm, the selection of the filter parameters is affected by signal characteristic, sampling frequency and noise level. In order to improve the adaptivity of the denoising algorithm, a criterion for selecting the optimal filter parameters is proposed based on permutation entropy in this paper. The permutation entropy can effectively measure the complexity of time series. It has been widely applied to physical, medical, engineering, and economic sciences. According to the difference among the permutation entropies of chaotic signals at different noise levels, first, different filter parameters are used for denoising noisy chaotic signals. Then, the permutation entropy of the reconstructed chaotic signal corresponding to each of filter parameters is computed. Finally, the permutation entropies of the reconstructed chaotic signals are compared with each other, and the filter parameter corresponding to the minimum permutation entropy is selected as an optimal filter parameter. The selections of the filter parameters are analyzed in the cases of different signal characteristics, different sampling frequencies and different noise levels. Simulation results show that this criterion can automatically optimize the filter parameter efficiently in different conditions, which improves the adaptivity of the denoising algorithm for chaotic signals based on collaborative filtering.
2018, 67 (6): 060702. doi: 10.7498/aps.67.20172599
This paper presents a spatial modulation Fourier transform micro-spectrometer based on micro-optical elements. The infrared microstructure diffractive optical elements, multi-step micro-mirrors and microlens array are introduced to realize the miniaturization of the instrument. In addition, the structure and basic principle of Fourier transform infrared micro-spectrometer are introduced. The design theory of micro-collimation system is analyzed based on the negative dispersion, the abberation correction and the arbitrary phase modulation characteristics of diffractive optical element. Combined with the characteristics of micro-static interference system, the micro-focusing coupled optical system is analyzed and designed. Based on the wave aberration theory and the Sellmeier dispersion formula, the influence of residual aberration on spectral recovery and the diffraction efficiency of diffraction surface in single-chip hybrid diffractive-refractive collimating lens are studied. The effects of diffraction of multi-stage micro-mirrors and the aperture diffraction of microlens array on spectral recovery are studied by using the scalar diffraction theory. Furthermore, the influence of axial assembly error of relay system on the whole system performance is studied. The results show that the diffraction efficiency of the diffraction surface, the diffraction of the multistage micro-mirror and the microlens array have no effect on the recovery spectrum when the working band of the system is 3.7-4.8 μm. Finally, in order to verify the accuracy of the system design results, an optical simulation software is used to simulate the infrared micro-Fourier transform spectrum. The accuracy of the system model is verified by the simulation that the reconstructed spectrum is in agreement with the ideal spectral curve and the actual spectral recovery error is 2.89%. The medium-wave infrared micro-static Fourier transform spectrometer has no movable parts and adopts micro-optics element to replace the traditional infrared lens. Therefore, it has the advantages of not only good stability, but also small size and light weight so that it is helpful in on-line monitoring applications and provides a new design idea about the micro-Fourier transform spectrometer.
ATOMIC AND MOLECULAR PHYSICS
Effect of transition metal element X (X=Mn, Fe, Co, and Ni) doping on performance of ZnO resistive memory
2018, 67 (6): 063101. doi: 10.7498/aps.67.20172459
Resistance random access memory (RRAM) based on resistive switching in metal oxides has attracted considerable attention as a promising candidate for next-generation nonvolatile memory due to its high operating speed, superior scalability, and low power consumption. However, some operating parameters of RRAM cannot meet the practical requirement, which impedes its commercialization. A lot of experimental results show that doping is an effective method of improving the performance of RRAM, while the study on the physical mechanism of doping is rare. It is generally believed that the formation and rupture of conducting filaments, caused by the migration of oxygen vacancies under electric field play a major role in resistive switching of metal oxide materials. In this work, the first principle calculation based on density functional theory is performed to study the effects of transition metal element X (X=Mn, Fe, Co, and Ni) doping on the migration barriers and formation energy of oxygen vacancy in ZnO. The calculation results show that the migration barriers of both the monovalent and divalent oxygen vacancy are reduced significantly by Ni doping. This result indicates that the movement of oxygen vacancies in Ni doped ZnO is easier than in undoped ZnO RRAM device, thus Ni doping is beneficial to the formation and rupture of oxygen vacancy conducting filaments. Furthermore, the calculation results show that the formation energy of the oxygen vacancy in ZnO system can be reduced by X doping, especially by Ni doping. The formation energy of the oxygen vacancy decreases from 0.854 for undoped ZnO to 0.307 eV for Ni doped ZnO. Based on the above calculated results, Ni doped and undoped ZnO RRAM device are prepared by using pulsed laser deposition method under an oxygen pressure of 2 Pa. The Ni doped ZnO RRAM device shows the optimized forming process, low operating voltage (0.24 V and 0.34 V for Set and Reset voltage), and long retention time (>104 s). Set and Reset voltage in Ni doped ZnO device decrease by 80% and 38% respectively compared with those in undoped ZnO device. It is known that the density of oxygen vacancies in the device is dependent on the oxygen pressure during preparation. The Ni doped ZnO RRAM device under a higher oxygen pressure (5 Pa) is also prepared. The Ni doped ZnO RRAM device prepared under 5 Pa oxygen pressure shows a little higher Set and Reset voltage than the device prepared under 2 Pa oxygen pressure, while the operating voltages are still lower than those of undoped ZnO RRAM. Thus, the doping effect in the ZnO system is affected by the density of oxygen vacancies in the device. Our work provides a guidance for optimizing the performance of the metal oxide based RRAM device through element doping.
2018, 67 (6): 063102. doi: 10.7498/aps.67.20172022
The C24H38O4 (dioctyl phthalate, DOP) is a main component of the plasticizer. In order to study the influence of external electrical field on molecular structure and spectrum of DOP, the method B3LYP of the density functional theory at B3LYP/6-311G(d,p) level is employed to calculate geometrical parameters of the ground state of DOP molecule under different external electric fields (from 0 to 0.0125 a.u.) in this article. On this basis, the ultraviolet-visible absorption spectrum of DOP is calculated by using the time-dependent density functional theory in the same fundamental group and compared with the ultraviolet absorption peak of the molecules, measured by UNICO ultraviolet and visible spectrophotometer. Finally, by using the time-dependent density functional theory in the same fundamental group, we study wavelengths and oscillator strengths of the first twenty-six excited states of DOP molecule in external electric field. The obtained results are as follows. The strongest absorption of ultraviolet-visible absorption spectrum appears in the end absorption band from n to σ* transition. The stronger absorption occurs in the E band of benzene electronic transition from π to π*. The molecular geometry parameters are strongly dependent on the external field intensity. The dipole moment of DOP molecule is proved to first decrease and then increase with the sharp increase of external field, but the total energy first increases and then decreases with the increase of the external field intensity. The ultraviolet absorption peaks of excited states of DOP are proved to have observably red shift, and the oscillator strength sharply decreases with the increasing of the field intensity.
icMRCI+Q study on spectroscopic properties and predissociation mechanisms of electronic states of BF+ cation
2018, 67 (6): 063301. doi: 10.7498/aps.67.20172114
In this paper, we study the spectroscopic properties and predissociation mechanisms of 14 states, which come from the first two dissociation channels of the BF+ cation. The potential energy curves of 14 Λ-S (X2Σ+, 12Π, 22Π, 22Σ+, 14Σ+, 14Δ, 14Σ1, 12Δ, 12Σ1, 32Σ+, 14Π, 24Π, 24Σ+, and 32Π) and corresponding 30 Ω states are calculated using the complete active space self-consistent field method, which is followed by the valence internally contracted multireference configuration interaction approach with the Davidson modification. To improve the reliability and accuracy of the potential energy curves, the core-valence correlation and scalar relativistic corrections, as well as the extrapolation of potential energy to the complete basis set limit are taken into account. The spin-orbit coupling is computed using the state interaction approach with the Breit-Pauli Hamiltonian. Based on these potential energy curves, the spectroscopic parameters and vibrational levels are determined for all the bound and quasi-bound Λ-S and Ω states. The present ground-state spectroscopic constants match well with the available experimental data. In addition, the vertical and adiabatic ionization potentials from the X1Σ+ state of BF molecule to the X2Σ+, 12Π, and 22Σ+ states of BF+ cation are calculated. The results of BF+(X2Σ+) ← BF(X1Σ+) ionization are in good agreement with the measurements. Various curve crossings of Λ-S states are revealed. We calculate the spin-orbit matrix elements between two interacting electronic states in the curve crossing region. With the help of present spin-orbit coupling matrix elements, we analyze the predissociation mechanisms of X2Σ+ and 32Π states along with the perturbations of the nearby states to 22Π, 14Σ+ and 32Σ+ states for the first time. The predissociation of X2Σ+ and 32Π states have a chance to occur around the vibrational levels υ"=30 and υ'=0 due to spin-orbit coupling, respectively. The present results also indicate that the υ' ≥ 9 vibrational levels of 22Π state are perturbed by the crossing states 22Σ+, 14Σ+, 14Δ, 14Σ1, 12Δ, 12Σ1, 32Σ+, and 14Π, that the υ' ≥ 4 vibrational levels of 14Σ+ state are perturbed via the interacting states 14Σ1 and 12Σ1, and the great perturbations between υ' ≥ 4 vibrational levels of 32Σ+ state and υ' ≥ 0 vibrational levels of 14Π state. For the 30 Ω state, we also calculate the relative energies of dissociation limits compared with the lowest one matching well with the experimental ones. Finally, the Franck-Condon factors, Einstein coefficients, and radiative lifetimes are evaluated for the 22Π (υ'=0-9)-X2Σ+, 22Σ+ (υ'=0-2)-X2Σ+, (3)1/2-(1)1/21st well, and (2)3/2 (υ'=0-9)-(1)1/21st well transitions.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2018, 67 (6): 064201. doi: 10.7498/aps.67.20171824
Photonic spin Hall effect (SHE) is an interesting transport phenomenon, and has attracted growing attention. The spin-dependent splitting of photonic SHE as a weak effect is just tens of nanometers so that it can usually be detected indirectly with the weak measurement techniques. To detect it directly and use it properly, many efforts have been devoted to enhancing the photonic SHE. Recently, the surface plasmon resonance (SPR) excited by a pure nanometal structure is used to enhance the photonic SHE. However, the pure metal permittivities are limited, therefore the regulation of the photonic SHE is also restricted. It is worth mentioning that the alloy made from the pure metal with different composition proportions can achieve the artificial control of permittivity. More importantly, the alloy can also be used to manipulate the SPR. In this paper, we systematically investigate the photonic SHE in a nanoalloy structure composed of BK7 glass, alloy film and air in order to realize the enhancement of photonic SHE. First of all, the resonant angle of SPR varying with the permittivity of alloy is studied by using the angular spectrum theory of beam. It is found that the resonant angle of the SPR is mainly influenced by the real part of the permittivity of alloy, while the imaginary part has little influence on it. The resonant angle of SPR will increase with the increase of the real part of the permittivity. Secondly, the spin-dependent splitting is studied by changing the alloy permittivity when the incident angle is set to be a resonant angle. We find that the distribution of the larger spin-dependent splitting at the resonant angle is zonal. The optimal permittivity of alloy film is ε2=-2.8 + 1.6i, and the alloy can be composed of Ag and Ni according to the Bruggerman theory. Under the condition of the optimal permittivity, the spin-dependent splitting reaches about 1.2×105 nm at a resonant angle of 51.5°, which is about 40 times larger than the previous result in a pure nanometal structure. Finally, when the incident angle is fixed at 44.1°, it is revealed that the spin-dependent splitting varying with the permittivity is axially symmetric and spherical radiation is centered at a maximum value. The farther away from the center, the smaller the corresponding beam shift is. The alloy permittivity in the spherical radiation center is ε2=-10.6 + 1.2i, which can be composed of Au and Ag. The value of spin-dependent splitting reaches about 8000 nm, which is greatly improved when compared with the previous maximum value 3000 nm in a pure nanometal structure. These findings can effectively enhance the photonic SHE and provide theoretical basis for the research and development of nanophotonic devices such as the SPR-based sensor.
2018, 67 (6): 064205. doi: 10.7498/aps.67.20172228
Single-photon laser ranging is a new generation of lidar which represents the future lidar development trend.It uses the single photon detector as the receiving device.Due to the fact that single-photon detector possesses the ultra-high sensitivity,the single-photon laser ranging is much easier to achieve the high density as well as the high coverage target sampling.However,the existence of the range work error in single-photon laser ranging,resulting from the fluctuation in the number of signal photoelectrons restricts the improvement of the ranging accuracy.In this paper,the range walk error model based on the lidar equation and the statistical property of single-photon detector is established.Then the relation between the range walk error and the number of signal photoelectrons is also derived.The range walk error of single-photon laser ranging is predicted and the corresponding compensation for the original result is obtained,with the derived function and the detection probability model of single-photon laser ranging.The experiment for its proof is also carried out.In the experiment,the number of signal photoelectrons is changed by the different attenuators for the same target and at the same distance.When the attenuator is changed,the pulse width of echo signal changes very little (about 3.2 ns).However,the average number of signal photoelectrons varies between 0.03 counts and 4.3 counts.So the range walk error,resulting from the fluctuation in the number of signal photoelectrons cannot be ignored.For example, when using an attenuation of 1/10 pass rate,the average number of signal photoelectrons is about 4.3 counts and the range walk error is almost 46 cm,which is the main factor of the range error.The reduction of the range walk error is achieved by applying the correction of the range walk error in this paper.After correction,the standard deviation of the range walk error decreases significantly from 15.17 cm to 1.16 cm.The mean absolute error is also reduced from 11.56 cm to 0.99 cm.Generally,the range walk error has an unnegligible influence on the ranging accuracy.The experimental result confirms that the theoretical model is accurate.It also shows that the bigger the number of the received signal photoelectrons,the greater the range walk error is,and the accuracy of single-photon laser ranging is improved by applying the technique proposed in this paper.Briefly,this paper presents the technical method of optimizing the design and evaluating the performance of single-photon laser ranging.
2018, 67 (6): 064101. doi: 10.7498/aps.67.20172502
Materials can be experimentally characterized up to terapascal pressures by sending a laser-induced shock wave through a sample that is pre-compressed inside a diamond-anvil cell. Pre-compression expands the ability to control the initial condition, allowing access to thermodynamic states from the principal Hugoniot and enter into the 10 TPa to 100 TPa (0.1-1 Gbar) pressure range that is relevant to planetary science. We demonstrate here a laser-driven shock wave in a water sample that is pre-compressed in a diamond anvil cell. The compression factors of the dynamic and static techniques are multiplied. This approach allows access to a family of Hugoniot curves which span the P-T phase diagram of fluid water to high density. According to the loading characteristics of the SG-Ⅱ high-power laser, the traditional diamond anvil cell is improved and optimized, and a new diamond anvil cell target adapting to high power laser loading is developed. In order to adapt to laser shock, the diamond window should be thin (100 μm) enough so that the shock can propagate to the sample before the side rarefaction erodes too much the shock planarity. With a thickness of 100 mm over an aperture of 600 μm diameter, a pre-compressed water sample at 0.5 GPa can be obtained. The water is pre-compressed to 0.5 GPa by using the diamond anvil cell. Hugoniot curve is partially followed starting from pre-compression at a pressure of 0.5 GPa. Pressure, density, and temperature data for pre-compressed water are obtained in a pressure range from 150 GPa to 350 GPa by using the laser-driven shock compression technique. Our P-ρ-T data totally agree with the results from the model based on quantum molecular dynamics calculations. These facts indicate that this water model can be used as the standard for modeling interior structures of Neptune, Uranus, and exoplanets in the liquid phase in the multi-Mbar range and should improve our understanding of these types of planets.
2018, 67 (6): 064202. doi: 10.7498/aps.67.20172127
Fresnel incoherent correlation holography (FINCH) is a relatively innovative technology, which can achieve incoherent holograms by using the correlation between the object information and a Fresnel zone plate. In this method, the optical wave front scattered from an object propagates and is incident on a spatial light modulator which a phase mask is mounted on, and then the optical beam is split and phase shifted. The biggest advantage of the FINCH is that it can be matched with any standard optical imaging technology, which can realize microscopic imaging, telescopic imaging, spectroscopic imaging, etc. based on incoherent digital holography, and has important application prospect in remote sensing, astronomy, microscopy, and material analysis. In this paper, based on phase modulation characteristic of spatial light modulator, two types of masks are used. The first mask has an optical axis. And the results show that when the distribution intervals of the three phases on the spatial light modulator (SLM) are larger, the reconstruction image is clearer. On this basis, a new method of mode mounting on the SLM is put forward. The second mask has dual-lens array mode with three phases of 0°, 120°, and 240°, and the three phases respectively correspond to their corresponding optical axis, which means that the mask has three optical axes. Both of the two masks can achieve the single-shot of FINCH. By comparing the two mask forms, we find that the field-of-view of the first mask is larger, which can image the entire resolution board; however, because the sub-phase shift holograms are mixed together and cannot be extracted, the quality of the reconstructed image is worse. The second one can extract three sub-holograms, and the reconstructed image has better quality; but because of smaller imaging field of view, it is suitable for the real-time imaging of micro-organisms and objects. Experiments show that a compound digital hologram including three phase-shifting elements is recorded in charge-coupled device in this way. Three sub-holograms with different phase shift angles are extracted from the compound hologram, and there is no overlapping among the three phase shift holograms. Therefore, the three-phase-shifting technique is usually employed. The sample is reconstructed by numerical reconstruction algorithm. The proposed method may be useful in dynamic process real-time measurement and three-dimensional analysis of the object, and thus providing a new way to promote the development of incoherent digital holography.
2018, 67 (6): 064204. doi: 10.7498/aps.67.20172049
Sinusoidal phase-modulated signal light through the Fabry-Perot interferometer can produce a beat signal. Moreover, its amplitude monotonically changes with the signal light frequency. So the beat signal amplitude can be used to measure laser-Doppler-shift. In addition to the beat signal, the phase-modulated signal also contains a direct current (DC) signal, and it still contains a large amount of Doppler-shift information, but the information is not utilized, resulting in the waste of Doppler information. In this paper, this kind of phase-modulated laser-Doppler-shift measurement method is improved to simultaneously utilize the useful information in the DC and beat signal for the Doppler-shift measurement. The specific method is to use the ratio of beat signal amplitude to DC signal amplitude to define a new parameter used in Doppler-shift measurement. The signal light intensity terms in DC and beat signal can be eliminated, so the improved phase-modulated laser-Doppler-shift measurement method does not need to measure the signal light intensity, which makes its structure further simplified and a noise channel eliminated. By comparing the frequency change curves between the newly defined parameter and the beat signal amplitude theoretically, we find that they have the same distribution rule. This theoretical result shows that the improved phase-modulated laser-Doppler-shift measurement method will keep the same working mode as un-improved one, and can inherit its advantages. In theory, by comparing the measurement sensitivity curves, it is proved that the improved phase-modulated laser-Doppler-shift measurement method has higher measurement sensitivity and dynamic range than the un-improved one. The useful information included in the DC signal is the modulated signal light intensity transmittance of Fabry-Perot interferometer. So the improvement is essential to introduce the advantages of edge-technique laser-Doppler-shift measurement method based on the Fabry-Perot interferometer into the phase-modulated method for achieving higher performance. Two phase-modulated laser-Doppler-shift measurement methods before and after improvement are separately used to measure the frequency-shifted controllable signal light reflected by a hard object. The experimental results are in accordance with the theoretical analysis results very well. The comparison of experimental result between the two methods shows that the improved phase-modulated laser-Doppler-shift measurement method can approximately double the measurement dynamic range and reduce about 35% measurement standard deviation compared with the un-improved one.
Highly precise and real-time measurements of 13CO2/12CO2 isotopic ratio in breath using a 2 μm diode laser
2018, 67 (6): 064206. doi: 10.7498/aps.67.20171861
Real-time breath gas analysis with high accuracy, precision and time resolution, as a promising, non-invasive, fast and reliable tool, is important in medical diagnostics. Especially stable isotopologues of carbon dioxide is applied to multiple research areas including the diagnosis of gastrointestinal diseases. Helicobacter pylori (H. pylori) is one of the most frequent bacterial infectious diseases in human beings and is now recognized as one of the key risk factors for chronic gastritis, peptic ulcers, stomach cancer and lymphoma. In contrast to traditional invasive tests, the most reliable non-invasive method in the diagnosis of the H. pylori infection is considered to be 13C-urea breath test which is implemented by measuring the 13CO2/12CO2 isotope ratio in human breath. Tunable diode laser absorption spectroscopy (TDLAS) has the advantages of fast response, low drift, good gas selectivity and high detection sensitivity, and it is very convenient to develop a high precision, real-time and online measurement system. A precision laser spectrometer for the measurement of CO2 isotope abundance in human breath (with CO2 concentration of 4%-5%) or high concentration gas is designed and evaluated based on TDLAS technology. The spectrometer contains a novel compact dense-pattern multipass cell with a small volume of 280 cm3 and an effective optical path length of 26. 4 m. The cell is in conjunction with a fiber-coupled distributed feedback diode laser operating at 2.008 μm. Wavelength modulation spectroscopy approach is used. The mass flow, pressure and temperature of the cell are actively controlled, and able to keep long-term stability. The influence of laser power fluctuation is eliminated by fitting the baseline with cubic polynomial to normalize the raw spectrum. Moving window regression is used to remove the influence of frequency drift on measuring isotope abundance. The system measurement precision is improved by wavelet denosing and Kalman filtering. The experimental results demonstrate that moving window regression method not only extends the stability time of the system but also improves the measurement precision of isotope abundance well, the wavelet denoising improves the signal-to-noise ratio by 2 times that by the method of multi-spectral average, the stability time of the system is 100 s given by Allan variance, and the measurement precision of CO2 isotope ratio is 0. 067‰ after Kalman filtering. The use of small multi-pass cell and the default of denoising devices make the system more portable and improve the real-time and online measurement performance of the system. In addition to the measurement of 13CO2/12CO2 isotope ratio in human breath, by replacing different lasers, the spectrometer can also be used to measure trace gas concentration and the stable isotope abundance of many gas molecules in atmosphere. Therefore, the spectrometer will have broad applications in the areas of medical diagnosis, carbon cycle study and environmental monitoring.
Dissipative variational analysis for bounded dark solitons of electromagnetically induced transparency
2018, 67 (6): 064207. doi: 10.7498/aps.67.20172567
Electromagnetically-controlled precision is one of novel topics in the electromagnetics. To realize the precision controlling of the electromagnetically complicated phenomenon, the systematic characteristics of medium environment needs considering. Based on the cancellation of interference caused by quantum coherence in the systematic environment of material, the electromagnetically-induced transparency (EIT) can be achieved. For this nonlinear phenomenon, due to the advancement of quantum spot and well, the controlling of the bounded sate of quantum in various dimensions of semiconductor can be operated. So the solid system presents a clear superiority of controlling EIT. High power electromagnetic field excites the dynamic characteristics in solid material, which is the result of systematic reaction between field and material. Under the excitation of electromagnetic pulse, because of quantum coherence, the dual-well semiconductor has the ability to induce the dark state of solitons. In the study of the complicated system of multiple physical fields, two aspects need investigating further. Firstly, in the induction process of electromagnetic filed and solid material, the features of high dispersion and nonlinear reaction appear increasingly. Thus, due to the environmental restriction on dispersion and nonlinear reaction, electromagnetic dissipation is a crucial point, which needs considering in the electromagnetically-controlled precision of the EIT. Secondly, compared with the formation of soliton, the coupling reaction of solitons under co-sate is much complicated. The relation among these factors is necessary to be investigated in the formulation of soliton excitation. Therefore, a dual-well semiconductor is employed as solid environment to analyze the dynamic characteristics of dark solitons in the EIT. In order to achieve the controlling of precision and regulating of the effect, the environmental features of solid materials ought to be systematically considered. Accordingly, the variational method is utilized, through which the bounded action of dissipation and nonlinear coherence is effectively studied for the dark solitons under co-sate, and under the condition of exciting dark soliton in the system of EIT. Using the density matrix and electric polarization, the spectrum of dynamic transmission deviation of EIT is calculated in the solid environment. With the assistance of relevant action principle, the bounded relation of dark solitons under co-state is practically investigated in the dissipative environment of solid system. In addition, the space-time trajectory is analyzed in the applicable region of characteristic equations of dark solution. The deduced result indicates that the systematical balance between dissipative weakening and coherent coupling supports the valuable approach to controlling the space-time evolution of dark solitons in precision. The results also show that the special effect has the potential applications in electromagnetically-controlled precision in the quantum information, ray sensor, controllable environment, etc.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Localization of blue phase liquid crystal with ordered crystallographic direction and well-defined micro-patterning
2018, 67 (6): 066101. doi: 10.7498/aps.67.20172517
The construction of uniform orientation of crystallographic direction of blue phase is of great importance for its practical applications and the scientific research of multi-dimensional controllable growth of soft matter. With the consideration of the weak thermal stability of blue phase, the uniform lattice orientation of blue phase is combined with localized polymer-stabilization in this work. So the relatively stable fabrication of micro-patterns for blue phase can be realized, and it is promising for researchers to prepare brand new photonic devices. To the best of our knowledge, the relevant reports are rather rare, and the successful implementation of the above ideas is full of difficulties according to current conditions. In this paper, the uniform, patterned and stable orientation of crystallographic direction of blue phase is achieved by using the aforementioned integrated method. Here in this work, facile rubbing alignment is used as the primary way to realize the uniform lattice orientation. Meanwhile, the polymer-stabilization, as an effective technological way, is used to stabilize the frustrated topological structure of aligned blue phase for a better stability and its application perspective. Furthermore, we construct the well-defined micro-patterned blue phase array including one-dimensional and two-dimensional pattern in virtue of facile and effective localized exposure. Simultaneously, the stability of such a micro-pattern under external field is also investigated to evaluate the validity of stabilized superstructure and characteristic behavior of unstable region. As a result, the micro-patterned blue phase array keeps good state even under the adequate exposure to high voltage. Finally, the potential photonic application is explored based on the above micro-patterns which exhibit good optical diffraction effects in the experiment that follows. In conclusion, it really provides a feasible route for achieving stable control about orientation of soft matter, like liquid crystal, and fabricating field-stable and periodic superstructure. Such a research will speed up the development of blue phase liquid crystal in crystallography, electronics, and photonics.
Lifetime modulation of graphene oxide film by laser direct writing for the fabrication of micropatterns
2018, 67 (6): 066802. doi: 10.7498/aps.67.20172331
The strong, broad and tunable fluorescence emission of graphene oxide (GO) has shown the exciting optical applications in many areas, such as fluorescence imaging in living cell, high sensitive detection of heavy metal ions, and the fabrication of optoelectronic devices. However, the intrinsic heterogeneous fluorescence intensity resulting from the variability in the power density of excitation laser and the non-uniform thickness of GO film, hinders its further applications in the micropatterning, information storage and display technology, which requires homogeneous fluorescence emission. In contrast to the fluorescence intensity, the fluorescence lifetime of GO is determined by the intrinsic nature of chromophores, rather than the film thickness or excitation power density. Here we report that the fluorescence lifetime is homogeneous for GO film, which eliminates the anisotropic optical properties of GO film. By reducing the GO film through the irradiation from a 405 nm continuous-wave laser at a certain power density on a home-built scanning confocal microscope, we find that the lifetime can be precisely modulated by controlling the duration of laser irradiation. It is determined that the lifetime gradually decreases with the increase of duration. As reported in the previous researches, the GO fluorescence originates from the graphene-like confined sp2 clusters and sp3 domains consisting of oxygen-containing functional groups, where the lifetime of sp3 domain is about 1.4 ns, and that of sp2 domain is 0.14 ns. During the photoreduction, the long-lived sp3 domains will decrease or convert into short-lived sp2 domains, resulting in the decrease of lifetime. Hence, by controlling the reduction degree or the ratio of the two domains, the lifetime of GO film can be determined. More importantly, the lifetime distributions of the reduction areas are very narrow, leading to a relatively homogenous background. The precise manipulation of lifetime can be used to fabricate micropatterns with high contrast. Combining with laser direct writing with features of maskless, facile processing ability and high spatial resolution, many versatile micropatterns, such as quick response code, barcode, graphic, alphabet, and numbers can be readily created based on the modulation of fluorescence lifetime. By using three optimized durations of laser irradiation, three distributions with narrow widths are obtained. Based on this processing, the micropatterns with three colors are determined, which indicates that the multimode optical recording can be created on the GO film based on the modulation of fluorescence lifetime. Furthermore, the multilayer micropatterns are also created. The robust and versatile micropatterns with film-thickness and excitation-power-independent features show their promising applications in electronics, photonics, display technology and information storage.
First principle calculation and photocatalytic performance of BixWO6 (1.81 ≤ x ≤ 2.01) with oxygen vacancies
2018, 67 (6): 066801. doi: 10.7498/aps.67.20172287
Semiconductor photocatalyst Bi2WO6 has an extensive application prospect in organic contaminant degradation.But its energy band is relatively large and the recombination rate of photon-generated carriers is high,which prohibit its rapid development and applications.Many methods such as ion doping,non-stoichiometry,semiconductor heterojunction have been used to improve the photocatalytic activity of Bi2WO6.But the improvement mechanism is still not very clear.In this paper,by using first principle density functional theory (DFT) calculation,we study the influences of oxygen vacancy on the bond length,charge population,band structure,defect formation energy,and density of states of Bi2WO6.On the basis of DFT calculation results,different non-stoichiometric BixWO6 (x=1.81,1.87,1.89,1.92,2.01) products with oxygen vacancies are synthesized through the solvothermal method.The products are characterized by X-ray diffraction,scanning electron microscopy,X-ray photoelectron spectroscopy,UV-vis diffuse reflectance spectra photoluminescence spectroscopy,and X-ray Fluorescence.The effects of non-stoichiometric Bi element on crystal structure,chemical composition,the number of oxygen vacancies,microstructure,and photocatalytic properties are investigated and the improvement mechanism of the photocatalytic property is explored.The DFT calculation results reveal that the formation energies of Bi16W8O48 are different for the three kinds of oxygen vacancies and the bond lengths of Bi–O and W–O with one oxygen vacancy decrease a little and the bond populations decrease significantly for the Bi and W atoms adjacent to oxygen vacancy.The existence of oxygen vacancies forms O 2p impurity energy level and significantly reduces the band gap of Bi2WO6. The absorption spectra indicate that the absorption intensities in the visible light increase for the Bi16W8O48 cell with oxygen vacancy defects increasing.The DFT calculation results show that oxygen vacancy defects promote the formation of photoelectrons and enhance the photocatalytic performance of Bi2WO6.The experimental results show that non-stoichiometric Bi element makes the crystal structure slightly deformed and significantly affects the number of oxygen vacancies,photoabsorption capacity and the electron-hole recombination of Bi2WO6.The Bi1.89WO6 product has the best photocatalytic performance,and the rhodamine B is degraded by 98% after being irradiated for 180 min by visible light.Therefore,non-stoichiometric semiconductor with oxygen vacancy is testified to be an efficient method of obtaining high activity photocatalyst.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
First-principles study of electronic structures and optical properties of Mn and Cu doped potassium hexatitanate (K2Ti6O13)
2018, 67 (6): 067101. doi: 10.7498/aps.67.20172356
Potassium hexatitanate (K2Ti6O13) is a kind of wide band-gap semiconductor material with potential applications in photocatalysis. Unfortunately, it only responds to the short wavelengths of ultraviolet light, which seriously limits the utilization efficiency of solar energy. To extend its response to visible light, a promising strategy is to partly substitute some other transition metals for the Ti element. In this work, the electronic structures and optical properties of Mn-and Cu-doped K2Ti6O13 are systematically investigated by the first-principles calculations with the aid of the CASTEP module in the Materials Studio software package. The PW91 exchange-correlation functional is used with a plane wave basis set up to a 340 eV cutoff. The computational results show that the Mn-and Cu-doped K2Ti6O13 have impurity bands mainly stemming from the mix of Mn or Cu 3d states with Ti 3d states and O 2p states. Compared with the band gap of pristine K2Ti6O13 (2.834 eV), the band gap of Mn-doped one becomes narrow (2.724 eV), and its impurity energy level in the middle of the band gap can be used as a bridge for electronic transitions to facilitate the absorption of visible light. Although the band gap of Cu-doped K2Ti6O13 slightly increases (2.873 eV), it could be greatly narrowed (1.886 eV) when taking into consideration the impurity energy levels closely connected to the valence band. In addition, the impurity energy levels may form a shallow acceptor and suppress the carrier recombination in the Cu-doped K2Ti6O13. As usual, the calculated imaginary part of dielectric function as a function of photon energy shows that the ε2(ω) value is nearly zero for pure K2Ti6O13 when the photon energy is less than 3.5 eV, whereas there are finite values and also some peaks for the Mn-and Cu-doped ones. These peaks may originate from the impurity energy levels, whose occurrence makes the electron excitation occur readily by low photon energy. Thus, the absorption edges in the doped ones can red-shift to the visible-light region with enhancing absorption intensity. Finally, the simulated absorption spectra of the pristine and doped K2Ti6O13 are consistent with their electronic structures, which further confirms the above analysis. All the results show that the Cu-doped K2Ti6O13 exhibits higher visible-light photocatalytic efficiency than the Mn-doped one. The current work demonstrates that the absorption of visible light can be realized by the Mn or Cu doped potassium hexatitanate, with the effect of the latter being better than that of the former. The obtained conclusions are of great significance for understanding and further developing the potential applications of K2Ti6O13 in the field of photocatalysis.
Effects of Ca2+ doping on dielectric, ferromagnetic properties and magnetic phase transition of SmFeO3 ceramics
2018, 67 (6): 067501. doi: 10.7498/aps.67.20172433
In this paper we deal with the preparation of Sm1-xCaxFeO3(x=0-0.3) ceramics by the solid stat reaction and study the influences of Ca2+ doping on the dielectric,ferromagnetic properties and magnetic phase transition of SmFeO3.The crystalline structures of the Sm1-xCaxFeO3(x=0-0.3) samples are characterized by X-ray diffraction.The dielectric property is measured by a precisive impedance analyzer (HP4294A) in a frequency range from 40 to 110 MHz.The microstructures of Sm1-xCaxFeO3 are imaged with scanning electron microscope under an operating voltage of 20 kV.The coexistence of Fe3+/2+ ions in Sm1-xCaxFeO3 samples is investigated with X-ray photoelectron spectroscopy (XPS).The magnetic properties of Sm1-xCaxFeO3 are measured with the physical property measurement system.The result shows that all the peaks for Sm1-xCaxFeO3 samples can be indexed according to the crystal structure of pure SmFeO3 and their fine crystal structures are obtained by XRD.The lattice parameter a value of SmFeO3 gradually increases,while the values of b and c decrease,and the unit cell volume (V) shrinks slightly with the increase of x.The scan electron microscope images indicate that Ca2+ doping significantly increases the grain size of SmFeO3 ceramic.The average grain sizes of Sm1-xCaxFeO3 samples range from 0.5 to 2μm with Ca2+ doping.The εr values of Sm1-xCaxFeO3(x=0.1,0.2,0.3) measured at 1 kHz are about 5,3 and 2.6 times greater than that of SmFeO3,respectively,and dielectric loss increases by an order of magnitude.The increase of εr is mainly caused by the interaction between the dipole and the space charge orientation polarization.Both the conductance current and the space charge limiting current are the main factors to increase the dielectric loss.The magnetic measurements show that the M-H curves of Sm1-xCaxFeO3(x=0-0.3) samples exhibit saturated magnetic hysteresis loops with the increase of Ca2+,and the Mr values of Sm1-xCaxFeO3(x=0.1,0.2,0.3) are 20,31,and 68 times that of SmFeO3,respectively,indicating the weakly ferromagnetic behavior.The XPS spectrum indicates that the Fe2+ and Fe3+ co-exist in each of Sm1-xCaxFeO3 samples.The ratio of Fe2+/Fe3+ increases with doping Ca2+ increasing,and the magnetic preparation of SmFeO3 is enhanced.It can be attributed to the structural distortion and the formation of Fe2+–O2-–Fe3+ super-exchange.The spin recombination temperature (TSR) and the Neel temperature (TN) are obtained,respectively,to be 438 K and 687 K by measuring the M-T curves.It is noted that both TSR and TN of SmFeO3 samples move toward low temperature with the increase of x,and the spin recombination disappears when x=0.3.This is mainly due to the stability of the magnetic structure of SmFeO3 sample and the interactions of Fe3+–O2-–Fe3+ and Sm3+–O2-–Fe3+ super-exchange.
2018, 67 (6): 067502. doi: 10.7498/aps.67.20172551
Considerable quantities of Nd-Fe-B magnet wastes are produced every year worldwide. Some Nd-Fe-B magnet wastes in the bulk form, produced during manufacturing, have low coercivity and cannot meet the requirements for applications. Finding an effective way to reuse those wastes by improving the coercivity, without powdering or reproducing process, becomes very important for saving energy and raw materials in manufacture. In this work, the grain boundary diffusion process is carried out on waste Nd-Fe-B sintered magnets by using Pr70Cu30 as a diffusion medium. The effects of diffusion temperature, diffusion time, and annealing time on the magnetic properties of the magnets are investigated. It is found that the coercivity increases when the diffusion temperature increases from 500 to 800℃, the diffusion time increases from 1 to 3 h, or the annealing time increases from 1 to 3 h. By comparing the diffused sample with the simply heat treated sample, we find that the coercivity enhancement by grain boundary diffusion process indeed results from the infiltration of Pr and Cu elements. The coercivity of the magnet increases by 51.9%, from 7.88 kOe (1 Oe=79.5775 A/m) to 11.97 kOe, after 4-hour diffusion at 800℃ followed by 3-hour annealing, with a negligible reduction of remanence Br, achieving a 99.8% recovery of coercivity compared with the commercial N35 magnet. It is noted that 500℃ annealing for 3 h after 800℃ diffusion only slightly increases the coercivity by 4.6%, from 11.44 kOe to 11.97 kOe, which indicates that the annealing process after Pr-Cu grain boundary diffusion may be not indispensable. Based on the microstructure analysis, the diffusion of Pr and Cu is confirmed. However, the distributions of Pr and Cu are inhomogeneous within a range of tens of microns near the surface even though the diffusion has spread throughout the magnet. The structure of main phase grains separated by the continuous grain boundary phase is formed after the grain boundary diffusion process while the core-shell structure is not observed, which suggests that the modification of the grain boundary structure is the main reason for the coercivity improvement. Cu element plays an important role in forming continuous grain boundary phase. In addition, the electrochemical corrosion test shows that higher corrosion current is obtained in the diffused magnet than in the original magnet, though the corrosion potential is improved. The reduced corrosion resistance may be related to the increased RE-rich phase content and the formation of continuous grain boundary phase. The present work is of great importance for increasing the production yield of Nd-Fe-B magnets.
Design of active region for GaAsP/AlGaAs tensile strain quantum well laser diodes near 800 nm wavelength
2018, 67 (6): 067801. doi: 10.7498/aps.67.20171816
As an active region, the tensile strain GaAs1-xPx quantum well plays an important role in the high power semiconductor laser diode with a wavelength of about 800 nm. Accompanied with the improved stability due to the Al-free active region, the GaAs1-xPx quantum well laser also shows a high level of catastrophic optical mirror damage because of the non-absorbing window at the facet, which is formed automatically by the relaxation of the tensile strain GaAs1-xPx material. On the other side, the GaAs1-xPx quantum well laser can provide a transverse magnetic (TM) polarized light source which is important for many solid state laser systems. However, the energy band structure of the tensile strain GaAs1-xPx quantum well is more complicated than that of the compressed or lattice matched quantum well. Although the light hole band is on the top of the heavy hole band for the bulk tensile strain GaAs1-xPx material, the situation may be different from the tensile strain GaAs1-xPx quantum well, in which the first light hole subband lh1 can be either on the top of the first heavy hole subband hh1 or reversed, that will cause the laser to generate either TM or transverse electric (TE) polarized light according to the well structure. So it is meaningful to optimize the tensile strain GaAs1-xPx quantum well structure based on the analysis of the energy band structure. Firstly, according to the 6×6 Luttinger-Kohn theory, the energy band structure of the tensile strain GaAs1-xPx quantum well is calculated by the finite difference method. The relationship between the interband transition energy and the well structure parameters is established. It is found that the well composition x and the well width should increase simultaneously, in order to fix the first subband transition wavelength at about 800 nm. Special attention is paid to the 808 nm quantum well, the valence structures of different well widths are calculated, the detailed analysis of the envelope function shows that the top valence subband is lh1 for wider well width, while it is changed to hh1 for narrower well width. Meanwhile, both the TE and the TM momentum matrix element are calculated as a function of the transverse wave vector for the subband transition from c1 to lh1, lh2, hh1 and hh2, respectively. Further, the threshold optical gains of different well widths are simulated for 808 nm laser diode with the tensile strain GaAs1-xPx quantum well as an active region, the wider well width benefits the TM mode, while the narrower one is favor of TE mode. Finally, according to the threshold carrier density, the relationship between the threshold current density and the well width is analyzed for 808 nm laser diode by considering both the spontaneous and the Auger recombination, an optimum combination of the well width and the well composition exists. For wider well width, the threshold current density will be higher because of the high energy subband carrier filling effect. For narrower well width, the decrease of the optical confinement factor will lead to the increase of threshold current density.
Enhanced performance of inverted polymer solar cell based on Al2O3/MoO3 as composite anode buffer layer
2018, 67 (6): 067201. doi: 10.7498/aps.67.20172311
Inverted polymer solar cell with P3HT:PC61BM as an active layer is fabricated based on Al2O3/MoO3 composite anode buffer layer. Effects of Al2O3/MoO3 composite anode buffer layers with the Al2O3 precursor solutions of different concentrations on the device performance are investigated. It can be found that the Al2O3/MoO3 composite anode buffer layer can effectively enhance the photovoltaic performance and device stability of inverted polymer solar cell. The open-circuit voltage (Voc), short-circuit current (Jsc), filling factor (FF), and photoelectric conversion efficiency (PCE) are 0.64 V, 8.62 mA/cm2, 63.86%, and 3.85% respectively for the control device with MoO3 single buffer layer. In addition, with the increase of the concentration of Al2O3 precursor solution, the photovoltaic performance of the inverted polymer solar cell with Al2O3/MoO3 composite anode buffer layer is gradually improved. For the Al2O3 precursor solution of 0.15%, the photovoltaic performance of the device reaches an optimal value, and the corresponding Voc, Jsc, FF, and PCE are 0.65 V, 11.04 mA/cm2, 64.46%, and 4.64%, respectively. The Jsc and PCE significantly increase by 28% and 20%, respectively, compared with those of the control device with MoO3 single buffer layer. Moreover, after 80 days of measuring the device lifetime, the PCE of the device with the composite anode buffer layer remains at 76% of the original value while the PCE with the single buffer layer is reduced below 50%. The improvement of the device performance should be attributed to the PC61BM receptor near the anode dissolved and washed by isopropyl alcohol solvent from the Al2O3 precursor solution. At the same time, a large number of pits on the surface of the active layer are filled with Al2O3 to make it more smoothly contact the composite anode buffer layer. Therefore, the contact resistance between the active layer and the anode decreases, which enhances hole collection performance of the anode. Simultaneously, the Al2O3 layer can passivate the active layer of the device, thus improving the photovoltaic performance and device stability of inverted polymer solar cell.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Characterization of high-transconductance long-channel hydrogen-terminated polycrystal diamond field effect transistor
2018, 67 (6): 068101. doi: 10.7498/aps.67.20171965
Diamond has a great potential to be used in high-power, high-voltage and high-frequency semiconductor devices due to its wide band gap (5.5 eV), high breakdown field (> 10 MV/cm), high thermal conductivity (22 W/(cm·K)), and good carrier transport property. High-quality polycrystal diamond with large size wafers (up to several inches) is more easily obtained than the expensive monocrystal diamond plate with the size of only several mm2, and the good performance of electronic device on polycrystal diamond has been reported. So we fabricate a normally-on hydrogen-terminated polycrystal diamond field effect transistor with a 4-μm aluminum gate by using a gold mask process. The saturation drain current is 160 mA/mm, and the on-resistance is as low as 37.85 Ω ·mm. The maximum transconductance reaches 32 mS/mm, and the gate voltage range with the transconductance higher than 90% of its maximum value reaches 3 V (-2 V ≤ VGS ≤ -5 V). An Ohmic contact resistance of 5.52 Ω ·mm and a quite low square resistance of 5.71 kΩ/sq for the hydrogen-terminated diamond are extracted from the analysis of transmission line model measurement. On the basis of the analyses of the obtained results, the on-resistance of device dependent on gate voltage, and the capacitance-voltage data measured at the gate-source diode, we find that the hole sheet density under the gate reaches 1.56×1013 cm-2 at a gate voltage of -5 V, and the extracted effective mobility of the holes stays at about 170 cm2/(V·s) in the afore-mentioned gate voltage range with high transconductance. In summary, the high and broad transconductance peak and the low on-resistance are attributed to the relatively low gate-source and gate-drain series resistance, the high-density carriers in the channel, and the high-level mobility achieved over a large gate voltage range. The relevant research of finding proper dielectrics for the gate insulator and the passivation layer is under way to further improve the device performance.
Development of a 4 T (46 K) 100 mm high-temperature superconducting coil made of homemade MOCVD-YBCO coated conductors
2018, 67 (6): 068401. doi: 10.7498/aps.67.20171491
The second-generation high-temperature superconductor (2G HTS) is a good candidate for high field magnet due to its high critical temperature Tc,high critical current density Jc,and high irreversibility field Hirr.This paper presents the design and development of a 4.08 T (46 K) coil made of homemade 2G HTS.In order to meet the design requirement of HTS coil,the electromagnetic finite element modeling and optimization are carried out on the basis of the research of the properties of YBa2Cu3O7-x(YBCO) tapes.And the design scheme of HTS coil is completed.Then the HTS coil with an inner diameter of 100 mm is successfully constructed according to the scheme.It consists of a stack of 10 double-pancakes with the same outer diameter wound with YBCO tapes.The diameter and height of the HTS coil are 236 and 359 mm,respectively.A total of 1600 meters of YBCO tape are used to wind this HTS coil.We measure the I-V curves of superconducting coil at different cryogenic temperatures.First,liquid nitrogen is used to cool the HTS coil to 77 K,and then the temperature is reduced to 65 K by the decompression cooling method.The cooling coil containing liquid helium is used to exchange heat and cool the solid nitrogen to obtain much lower cryogenic temperature.The maximum operating currents of the HTS magnet at 77,65,and 55 K are 65,147,and 257 A,respectively,corresponding to the center magnetic field of 0.78,1.77,and 3.1 T.At 46 K,the HTS coil with an inner diameter of 100 mm generates a 4.08 T field at the center.And the magnetic field of superconducting coil is basically uniform in the medium plane.The results demonstrate a strong potential of home-made YBCO magnet for direct current high-field applications.
New experimental discovery of channel crystal plane and orientation selection for small-sized uniaxial strained Si PMOS
2018, 67 (6): 068501. doi: 10.7498/aps.67.20172138
The inversion layer mobility of small-sized uniaxial strained Si p-channel metal oxide semiconductor (PMOS) channel is closely related to the crystal plane and crystal orientation. When optimally designing the strained PMOS, the crystal plane and crystal orientation of the channel should be chosen reasonably. At present, there is a theoretical sort model for the inversion layer mobility of Si PMOS channel at 1.5 GPa stress according to the crystal plane and crystal orientation. However, in the actual manufacturing process of device, the process of covering the SiN stress film is fixed, because the channel coefficient of stiffness is aeolotropic. So, the stress intensities of strained PMOS in different crystal planes and orientation channels are different, which causes the theoretical sort model for the inversion layer mobility to be invalid. To solve this problem, the small-sized uniaxial strained Si PMOS and unstrained Si PMOS with different crystal planes and orientations are fabricated by 40 nm technological process of Chinese Academy of Sciences. The result for the inversion layer mobility of Si PMOS channel according to the crystal plane and crystal orientation is obtained by the device transfer characteristic test. Considering the process implementation factors, the relevant conclusion about the inversion layer mobility of small-sized uniaxial strained Si PMOS channel according to the crystal plane and crystal orientation is more suitable to guide the actual device manufacturing than the theoretical sort result predicted in the literature. At the same time, the relevant analysis method can also provide important technical reference for the solution of other strained material MOS.
2018, 67 (6): 064203. doi: 10.7498/aps.67.20172395
Optical microcavities play a key role in both fundamental research on light-matter interaction and also applications such as integrated optics and sensors. Among them, whisper gallery mode (WGM) microcavity outstands itself by low loss, high Q-factor and high sensitivity to their dielectric environment. It can be found to have a variety of applications, including nonlinear optics, quantum electrodynamics, bio-sensors, low-threshold lasers, etc. However, the multi-mode nature of WGM microcavity is inconsistent with the basic requirements for these applications, i.e., a single-mode output and tunable wavelength. Therefore, the modulation of whisper gallery mode towards a unidirectional single-mode output is meaningful for both studying cavity dynamics and developing the above-mentioned applications. Here in this paper a brief review is carried out on the study of coupled dye-doped polymer microcavity processed by femtosecond laser direct-writing (FSLDW). The content covers fabrication, microcavity structure design, lasing and coupling mechanism study. The powerful patterning ability of FSLDW can realize complex three-dimensional microcavity structure design, which follows two schemes. One is to integrate a filter port to a microcavity. The other is to bring two or more microcavities in close proximity to each other for coupling. Based on such schemes, three kinds of microcavity structures, which are stacked microdisks, a microdisk integrated with gratings and stacked spiral-ring and circular-ring microcavity, are developed for the mode modulation. It is shown that all the three kinds of structures support unidirectional single-mode emissions with low lasing threshold. For the case of the stacked microdisks, the coupling can have a vernier effect among their modes and hence the mode selection. For the case of the microdisk cavity integrated with gratings, the gratings work as a filter port to select a certain mode according to their own period. For the case of the stacked spiral-ring and circular-ring microcavities, it is the structure asymmetry of the former that leads to the single-mode output. The mode modulations based on the mentioned microcavity structures have successfully maintained the high Q-factor of WGMs, which makes these cavities promising unidirectional single-mode microlasers. Combining with theoretical simulations, it is confirmed that the mode coupling between the microcavities (or between gratings and a microcavity) is responsible for the mode selection. Moreover, the unique structure design can break the rotational symmetry of the microcavity and hence achieve unidirectional laser emission. By careful designing and processing, successful modulationscan be achieved on a series of polymer microcavities. With both high Q-factor and good lasing directionality, these microcavity lasers could be well explored in integrated optical systems and organic optoelectronic devices.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2018, 67 (6): 065201. doi: 10.7498/aps.67.20172403
Electromagnetic plasma accelerators which can produce plasma jets with hypervelocity and high density have been widely used in the fields of nuclear physics and astrophysics. Parallel-rail accelerator, a type of electromagnetic plasma accelerator, is usually used to generate high density and compact plasma jets. The axial movements of plasma in a parallel-rail accelerator operated at different discharge currents and initial pressures are reported in this paper. Based on current truncation, the momentum of the first plasma jet is measured by a ballistic pendulum. The axial movement characteristics and velocity of the plasma during the acceleration phase are diagnosed by magnetic probes and photodiodes. The accelerator is powered by 14 stage pulse forming networks. The capacitor and inductor in each stage are 1.5 μF and 300 nH respectively, yielding a damped oscillation square wave of current with a pulse width of 20.6 μs. Plasma sheath is formed upon breakdown at the back wall insulator surface and subsequently accelerated by Lorentz force towards the open end of the accelerator. A secondary breakdown generally occurs at the starting end of the rail when the current reverses its direction, and then a secondary axial movement of plasma is formed. We focus on the first plasma jet accelerated by the first half-cycle of current. According to the snowplow model, the plasma velocity is proportional to the current and is inversely proportional to the square root of gas initial density or pressure. The axial velocity of the plasma is in a range from 8 km/s to 25 km/s when the discharge current is varied from 10 kA to 55 kA and the initial pressure is varied from 200 Pa to 1000 Pa. The experimental results show that the experimental velocities of the plasma are about 60%-80% of the theoretical result. It is likely that the viscous resistance of the electrode surface acting on the plasma and the mass increase of plasma caused by the electrode ablation are neglected in the snowplow model. The momentum of the first plasma jet is nearly proportional to the integration of the square of current over time, which is consistent with the predictions of the theoretical model. The maximum momenta of plasma jet at different currents appear at average velocities ranging from 13 km/s to 14 km/s when the plasma just moves to the outlet of the rail in the end of the first current pulse. The measured momentum of plasma jet is actually the total momentum of the truncated current waveform. The ratio of the momentum of the first plasma jet to the total measured momentum is about 87%. The momenta of the first plasma jet are in a range from 1.49 g·m/s to 9.88 g·m/s at discharge currents ranging from 21 kA to 51.6 kA. The experimental plasma momentum is about 75% of the theoretical result. These results show that the viscous resistance of rail electrode surface is about 25% of the Lorentz force, and thus leading to a lower value of plasma momentum.