Stress mechanism of Y1-xGdxBCO thin film with Gd substitution prepared by F-free metal organic deposition method
The plasma discharge channel in three-dimensional helical shape induced by pulsed direct current (DC) discharge without external stable magnetic field is discovered experimentally. It can be observed by intensified charge-coupled device camera that a luminous plasma structure fast propagates along a helical path in the form of guided streamer (ionization wave). And the propagation of the streamer is stable and repeatable. We take this streamer which propagates along the helical discharge path as the study object, and explain its mechanism by constructing an electromagnetic model. The result shows that the helical shape plasma plumes can exhibit two different chiral characteristics (right-handed and left-handed helical pattern). While the discharge parameters such as pulse frequency, boundary condition, etc. can all affect the propagating characteristics of helical streamers. The electromagnetic radiation driven by pulsed DC power inside the dielectric tube which forms the wave mode is an important source of the poloidal electrical field. The helical steamers form when the poloidal electrical field is close to the axial electrical field. The velocities of the propagation in poloidal and axial direction are estimated respectively, and the hybrid propagation modes involving the interchangeable helical pattern and the straight-line pattern propagating plasmas are explained from the viewpoint of multi-wave interaction. Recently, the second-generation YBa2Cu3O7-δ (YBCO) high temperature superconducting materials have attracted much attention and become a hot research point. The YBCO coated conductors are widely used in transmission cables, motors, generators and magnetic energy storage systems due to their high critical current densities and high irreversible fields. To obtain high critical current, it is necessary to increase the thickness of YBCO film. However, as the thickness increases, the cracking of the film appears and the a-axis grains form, which causes the critical current density to decrease drastically, hence the critical current declines, i.e., the so called "thickness effect" appears. In order to overcome the "thickness effect", a great many of efforts have been devoted to it. It is realized gradually that the growth orientation of the c-axis can be controlled by the stress of film, which can be achieved through the substitution of Y by Gd and Sm each with a larger ionic radius. However, the systematical study of the evolution of the stress mechanism with the substitution ratio is still lacking due to the extreme complexity of the stress manipulation. Therefore, a series of Y1-xGdxBCO thin films with different substitution ratios is deposited on lanthanum aluminate substrates by the fluorine-free metal organic deposition method in order to reveal the evolution of the stress mechanism with Gd substitution. The growth orientations, microstructures and lattice vibration characteristics of the films are analyzed by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The results show that the lattice constant of the film increases and the orientation of the c-axis changes with the Gd substitution ratio for x increasing to a value less than 0.5, and the blue shift of the O(2)/O(3) mode of the Raman spectrum decreases with increasing x. For x=0.5, the blue shift of the O(2)/O(3) mode vanishes, indicating the free standing film with optimal c-axis orientation. However, with the further increase of Gd content, the film structure is deteriorated, and the performance is degraded as well. The red shift of the O(2)/O(3) mode occurs and the frequency decreases with increasing x. Our results indicate that the stress mechanism can be manipulated by controlling the content of various ionic radii in Y1-xGdxBCO films. The free standing film with optimal c-axis orientation can be obtained through adopting an appropriate substitution ratio, i.e., the ratio of m Y:Gd equaling 1:1. These results suggest that manipulation of the stress mechanism is a promising method to overcome the "thickness effect" effectively.
Simulation of spontaneous emission spectrum of degenerate Ge under large injection level
Germanium (Ge) is considered as a promising material for silicon (Si) based light source. Based on tensile strain and n-type heavy doping approaches, the light emitting efficiency of Ge can be improved. Nevertheless, due to the difficulty in introducing large tensile strain into Ge, the photoluminescence or electroluminescence of Ge is demonstrated under degenerated states currently. Traditional spontaneous emission (SE) theory deduced from Boltzmann approximation is inapplicable for this case. To accurately analyze the SE properties of Ge, the influences of strain, temperature and doping on quasi-Fermi level and subsequent SE spectrum of degenerated Ge are theoretically investigated based on Fermi-Dirac distribution model. Owing to large density of states (DOS) in heavy hole (hh) the valance band (VB) and L valley, it is found that compressive strain has a negligible effect on the quasi-Fermi level under carrier concentration of 1019-1020 cm-3, while tensile strain is of benefit to the improvement of carrier occupation levels, leading to dramatic increases of both peak and integrated intensities of SE spectra. Although the peak intensity of SE from Γ-hh transition is larger than that from Γ-1h transition regardless of strain levels in Ge, the integrated intensities of SE from Γ-hh and Γ-1h transitions are almost equal. With the increase of sample temperature, the carriers acquire lager kinetic energy, resulting in more dispersive distribution of electrons (holes) in Γ valley (VB). However, more electrons (holes) are induced into conduction (valence) band at the same time. And according to Varshini's law the energy difference between Γ and L valleys is reduced at higher temperature. Thus, both the peak and integrated intensities of the SE spectra become larger at higher temperature. It is impressive that n-type doping can greatly enhance the SE intensity compared with p-type doping irrespective of strain levels in Ge, demonstrating the significance of n-type doping in the enhancement of Ge SE. Furthermore, it is found that m factors, which can be extracted from linear fitting of log L-log Δn curves, diminish at heavier doping concentration. Under tensile strain condition, the variation of m factors for Ge SE with the sample temperature becomes less sensitive, implying that the tensile strain can effectively enhance the temperature stability of Ge SE. These results provide a significant guidance for analyzing the SE properties of degenerated Ge and other degenerated semiconductors.
Growth and carrier transport properties of highly oriented films of the semiconducting polymers via solution dip-casting
Effective control of molecular orientation and packing as well as the film texture of organic semiconductor plays a crucial role in achieving high performance of the electronic device such as high carrier mobility. Development of facile and scalable solution processing method for film deposition is one of the important routes to such a goal.
In this paper, we report on the successful preparation of the large area, macroscopically aligned film of the semiconducting polymer P(NDI2OD-T2) and PTHBDTP via an improved solution dip-coating process in which a tilted substrate is immersed in the dilute solution. Polarized optical microscopy images reveal the parallel stripe structures of both kinds of the deposited films. The chain backbones of both P(NDI2OD-T2) and PTHBDTP are highly aligned along the descending direction of solution level in the dip-coating process as indicated from polarized UV-vis spectra and X-ray diffraction measurements. Furthermore, the atomic force microscopy images of the oriented films of both kinds of polymers clearly exhibit the highly preferentially oriented nanofibril-like domains, parallel to the alignment direction of chain backbone. We elucidate the dip-coating growth process in our experiment in terms of the surface tension-and solvent evaporation-guided self-assembly of chain backbones at the substrate-solution interface near the solution surface. The influence of film texture on carrier transport property is examined by fabricating field effect transistor (FET) based on the aligned film of semiconducting polymer. The FET device of the aligned P(NDI2OD-T2) exhibits a remarkable enhancement of electron mobility by a factor of four compared with the unaligned devices, as well as a large mobility anisotropy of 19. Such a transport behavior is proposed to be attributed to the characteristic charge conducting pathways induced by chain backbone alignment in the polymeric film. In this case, fast intra-chain transport contributes to the majority of device current when the channel current is parallel to the alignment direction of the film, while charge transport will be limited severely by the inter-chain hopping within the fibrous domain and across the disordered domain boundary when the current is perpendicular to alignment direction. The facile method developed here presents a promising approach to fabricating the low-cost, high-performance organic electronic devices.
Electronic structure and optical absorption properties of β-AgVO3 with vacancy defects
Semiconductor photocatalysts have received much attention due to their applications of wastewater treatment and air purification. The monoclinic β-AgVO3, which has narrow band gap (2.11 eV) and can respond to visible light, has been considered as one of the promising semiconductor photocatalysts. The vacancy defects always exist in β-AgVO3 prepared under the conventional synthesis conditions and have important influences on the structure and properties of β-AgVO3. Systematic theoretical study of the vacancy defects in β-AgVO3 is still lacking. In this paper, using density functional theory plus U (DFT+U) approach, the Ag vacancy, O vacancy and Ag-O bivacancy in β-AgVO3 are studied. The formation energy, band structure, differential charge density and optical absorption spectrum of β-AgVO3 with vacancy defects are carefully investigated. When the U values are chosen as 6 eV and 2.7 eV for the Ag-4d and V-3d electrons respectively, the reasonable lattice parameters and band gap value can be obtained for β-AgVO3. By comparing the formation energies of different Ag and O vacancies, we find that the dominating vacancy defects in β-AgVO3 are Ag3 and O1 vacancies, and the formation of Ag vacancy is much easier than that of O vacancy. The analyses of the total and partial density of states indicate that the conduction band arises mainly from V-3d orbit, and the valence band is mainly composed of Ag-4d and O-2p states for β-AgVO3. With Ag3 vacancy, O1 vacancy or Ag3-O1 bivacancy, the band gaps of β-AgVO3 all decrease in different degrees. The Ag3 vacancy behaves as p-type donor, allowing the Fermi level to shift down to the valence band maximum. However, O1 vacancy and Ag3-O1 bivacancy both act as n-type donors, and the Fermi level shifts to the conduction band minimum. The change of the Fermi level for the vacancy defect systems also means that the charge transfer occurs among the atoms around the vacancy, which is analyzed by calculating the differential charge density. The Ag3 vacancy and O1 vacancy have little effects on the light absorption of β-AgVO3 in the range of visible light, while O1 vacancy and Ag3-O1 bivacancy in β-AgVO3 cause the obvious absorption of light in the near infrared region.
Magnetic entropy change and electrical transport properties of rare earth Tb doped manganites La4/3Sr5/3Mn2O7
The magnetic transition process in double-layer perovskite manganites is rather different from that in the counterpart compound with standard perovskite structure. In this paper, the magnetic phases below room temperature as well as the order of magnetic phase transition in terbium (Tb) doped La4/3Sr5/3Mn2O7 are studied by analyzing the magnetization curves, including thermal hysteresis, magnetic entropy change and its universal curve. The electrical conductivities with and without applied magnetic field are also discussed.
Both the undoped and the doped samples (La1-xTbx)4/3Sr5/3Mn2O7 (x=0, 0.025) are prepared through the conventional solid-state reaction of mixed La2O3, Tb2O3, MnCO3 and SrCO3 whose purities are all higher than 99.9%. The mixture is calcined twice at 1000℃ for 12 h. Subsequently, the compactly compressed tablet of the calcined mixture is sintered in air at 1350℃ for 24 h.
The data of X-ray diffraction show that the crystallographic structures of both samples are in the Sr3Ti2O7-type tetragonal phase with the space group I4/mmm. The refinement result indicates that the smaller radius of doped Tb3+ reduces all three lattice parameters as well as the c/a ratio, which is attributed to the preferential occupation of Tb3+ on the R site in rocksalt layer instead of the P site in perovskite layer.
The temperature and field dependence of magnetization M(T, H), are recorded using the vibrating sample magnetometer of physical property measurement system (Quantum Design). Upon reducing the temperature, both samples exhibit two magnetic phase transitions from the paramagnetic phase at high temperature to the two-dimensional shortrange-ordered ferromagnetic state at the intermediate temperature, and finally the three-dimensional long-range-ordered antiferromagnetic state at low temperature. The zero-field-cooling and field-cooling curves display the characteristics of spin-glass behavior which may be due to the competition between B-site ferromagnetic and antiferromagnetic interactions associated with the randomly distributed A-site ions.
The magnetic entropy changes of the samples are obtained through analyzing the magnetization data. The maximal magnetic entropy changes under 7 T magnetic field of the two samples are -4.60 J/(kg·K) and -4.18 J/(kg·K), respectively. The doped Tb ions reduce the transition temperatures, Tc2D and Tc3D, as well as the maximal value of magnetic entropy change, and increases the transition temperature range. The re-scaling curves of magnetic entropy change at different magnetic fields do not fall into a universal one, rather disperse in a wide interval, which suggests that the system undergoes a weak first-order transition at Tc3D. This conclusion is supported by the thermal hysteresis observed in the magnetization data.
In addition, the electrical resistivity of the doped sample can be explained by using the small polaron model, which is different from three-dimensional variable-range hopping mechanism of undoped sample. On reducing temperature, the doped sample undergoes metal-insulator transition at temperature TP about 115 K, which is different from the undoped sample that shows the shoulder-shaped MI transition peaks. Under finite fields, the magnetoresistance value of intrinsic nature is about 56% near Tc3D.
Magnetic properties and exchange coupling of Nd-Ce-Fe-B nanocomposite films
In the early 1980 s, the soft and hard magnetic nano-two-phase permanent magnet materials were developed and exchange coupling model was put forward. Moreover, the theoretical maximum magnetic energy product could reach 120 MGOe (1 Oe=79.5775 A/m). However a great many of experimental research results are always disappointing for theoretical calculation, but previous studies have shown that there exists also a strong exchange coupling in hard magnetic phase, which can improve the magnetic property of magnet.
In this paper, nanocomposite Ta(50 nm)/NdFeB(100 nm)/Ta(2 nm)/NdCeFeB(100 nm)/Ta(2 nm)/NdFeB(100 nm)/Ta(40 nm) multilayer films with Ta underlayers and coverlayers are fabricated on Si substrates by direct current sputtering. A 50 nm Ta underlayer and a 40 nm coverlayer are sputtered at room temperature to align the easy axis of the RE2Fe14B grains to the direction perpendicular to the film plane and to prevent the magnetic film from oxidizing, respectively. The 2 nm Ta spacer layer serves as suppressing the diffusion of elements between different magnetic layers. The NdFeB and NdCeFeB magnetic film are deposited at 630℃ and 610℃, respectively, and then they are followed by in situ rapid thermal annealing at 645-705℃ for 30 min. The microstructures and morphologies of the films are characterized by X-ray diffractometry with Cu Kα radiation, atomic force microscope, and magnetic force microscope. The magnetic properties of the films are measured with vibrating sample magnetometer.
The influences of annealing temperature on magnetic property and crystal structure of the film are investigated. The results show that the magnetic property of the film improves gradually with the increase of annealing temperature, but deteriorates sharply when the temperature reaches above 695℃. When the annealing temperature is 675℃, the coercivity Hci of the film reaches 10.1 kOe and the remanence 4πMr⊥ is 5.91 kG (1 G=103/(4π) A/m), with a magnetic field applied to the direction perpendicular to the plane of the Nd-Ce-Fe-B thin film. The X-ray diffraction results show that the grains of the hard magnetic phase (2:14:1 phase) grow almost along the substrate normal (c-axis direction), of course, with a certain misorientation. Through the magnetization reversal process of the Nd-Ce-Fe-B thin film, it is found that the minimum value of Mrev moves in the direction of decreasing Mirr as the applied magnetic field increases, which is similar to the domain wall bowing model. This indicates that there is a strong local domain wall pinning in the film. Moreover, the remanence curve shows that the pinning type mechanism is indeed not dominant in the magnetization reversal process of the Nd-Ce-Fe-B thin film after annealing at 685℃. In addition, Henkel plots are also investigated in the films at different annealing temperatures. It is believed that nonzero δm is due to the interaction between particles in the magnet. It can be stated based on the measuring results that there exists a strong magnetic exchange coupling effect in the Nd-Ce-Fe-B thin film.
Excited-state dynamics of m-dichlorobezene in ultrashort laser pulses
The excited state dynamics of aromatic hydrocarbon has attracted a great deal of attention due to its important role in photophysics and atmosphere chemistry. With the benefit of ultra-short laser pulses, the ultrafast phenomenon can be studied in a time resolved way. In the present work, m-dichlorobenzene, a typical model of aromatic hydrocarbon, is investigated by the femtosecond time resolved time-of-flight mass spectroscopy. In order to reveal its excited state dynamics, m-dichlorobenzene is pumped to the excited state after absorbing one 200/267 nm photon, and then ionized by absorbing 800 nm photons. Time resolved mass spectra are recorded with time of flight. At 200 nm, m-dichlorobenzene is excited to a (π, π*) state. Three decay components are observed in the transient profiles of m-dichlorobenzene ions, which correspond to three competition channels in the excited states. The first channel is an ultrafast dissociation process via a repulsive state with (n, σ*) or (π, σ*) character, and the lifetime is (0.15±0.01) ps. The second channel is an internal conversion process from the populated excited state to the hot ground state, and the lifetime of the redistribution of the internal vibration in the hot ground state is (4.94±0.08) ps. The third channel is an intersystem crossing process to the triplet state, and the lifetime is (110.09±4.33) ps. Moreover, the transient profiles of C6H4Cl+/C6H4+ display similar decay tendencies to the transient profile of parent ion, except that longer lifetime constants ((127.38±29.29) ps for C6H4Cl+, and (123.76±37.12) ps for C6H4+, respectively) are observed. It is likely that the fragment ions result from the dissociative ionization of the parent molecule. At 267 nm, m-dichlorobenzene is excited to the first excited state with (n, σ*) character. Only C6H4Cl2+ and C6H4Cl+ are observed in the two-color mass spectrum. A slow decay component (~(1.06±0.05) ns) is obtained for both the parent ion and the fragment ion. It is attributed to an intersystem crossing process from the first excited state S1 to the triplet state T1. Furthermore, the transient profile of C6H4Cl+ displays other decay components, i.e., (2.48±0.09) ps, in addition to the slow decay component. This fast decay process can be attributed to an internal conversion process from the populated excited states to the hot ground states. The present study provides a more in-depth understanding of the ultrafast excited state dynamics of m-dichlorobenzene.
Particle simulation and analysis of threshold for multicarrier multipactor
The multicarrier multipactor is a phenomenon that can be observed in vacuum environment due to the effect of secondary electron emission. Accurate analysis of the threshold of multicarrier multipactor is crucial for the long-term reliability of high-power spaceborne microwave system, and therefore it has been attracting more and more interests in fields of high-power microwave community, plasma physics and aerospace engineering. Recently, a new mechanism of multicarrier multipactor, termed “long-term” multipactor, induced by sustained accumulation of residual electrons between successive envelope periods of multicarrier signals has received much attention. Comparing with the “single-event” multipactor induced by the electron accumulation inside a single envelop period, researchers tend to believe that the threshold of the long-term discharge should be lower. However, recent experimental results show an opposite conclusion. In this work, in order to investigate the contradiction between the experimental and theoretical studies on the thresholds of multicarrier multipactors, particle simulations are used to simulate the evolution process of the multicarrier multipactor under the same conditions and judgement criterion. The behavioral characteristics and occurrence condition for multicarrier multipactors, especially the single-event ones, are analyzed based on a power scanning analysis, and the conflicting results are effectively explained. Our simulations show that if the evolution process of a multipactor can be divided into three phases, i.e., establishment phase, critical phase and saturation phase, the experimental reflection coefficient can be corresponding to the reflection coefficient simulated in the critical phase. The simulation results indicate that the type of the multipactor discharge would depend on the configuration of multicarrier signals. For multicarrier signals with relatively narrow bandwidths, single-event multicarrier multipactors could occur in the first place at a lower threshold power. Therefore, the threshold of a long-term discharge is not necessarily lower than that of a single-event one. This conclusion is important for estimating and suppressing the multicarrier multipactors in the design of high-power spaceborne microwave components.
A novel algorithm of fifth-degree cubature Kalman filter for orbit determination at the lower bound approaching to the number of cubature points
With more satellites launched into orbits during recent years, monitoring and cataloging of satellites play an important role in improving the utilization rate of space resource and alleviating the pressure of orbit resource. Groundbased radar, a kind of sensor in space surveillance system, does not consider the influences of the weather and other special circumstances. And it is a key technology in space target tracking by using the measurement data for real-time orbit determination. Due to the influence of orbital perturbation, the satellite orbital dynamic model is a nonlinear system. The optimal estimation of the orbital state can be achieved by means of nonlinear filtering based on the measured ranging, velocity and angle data with measurement noise, which is the essence of real time orbit determination and has important research value. The extended Kalman filter (EKF) and unscented Kalman filter (UKF) are most widely used nonlinear Kalman filters. However, the first-order Taylor expansion of nonlinear function in EKF degrades the filtering accuracy. And the weight value in UKF might be negative for the high-dimensional system, which may directly affect the filtering stability. As an important method in nonlinear filtering, cubature Kalman filter (CKF) has better accuracy and stability than UKF. However, CKF only has third-degree filtering accuracy. In order to improve the filtering accuracy further, some fifth-degree cubature Kalman filters are proposed, mainly including the fifth-degree cubature Kalman filter and the fifth-degree spherical simplex-radial cubature Kalman filter. The optimality of the radial integral cannot be guaranteed by using the moment matching method in these fifth-degree filters, so a high-degree cubature quadrature Kalman filter (HDCQKF) is proposed. The radial integral is calculated using the high-degree Gauss-Laguerre formula in HDCQKF. However, the aforementioned filtering algorithm leads to an increase in the number of cubature points, thereby improving the accuracy, and the number of cubature points increases polynomially with the increase of system dimension. Once the algorithm is applied to a high-dimensional system, or the processor has a relatively poor performance, it may impose a heavier computing burden, thus the real-time performance decreases. Therefore, it is necessary to study how to reduce the computational complexity of the fifth-degree filtering algorithm. In order to improve the real-time performance of orbit determination on condition that the accuracy of orbit determination is kept, a novel fifth-degree cubature Kalman filter for orbit determination is proposed at the lower bound approaching to the number of cubature points. The key problem in the nonlinear Kalman filter is to calculate the multidimensional integral in the form of “nonlinear function×Gaussian probability density function”, and the integral is approximated using a fifth-degree numerical cubature rule, in which the number of cubature points required is only one more than the theoretical lower bound. The abovementioned cubature rule is embedded into the nonlinear Kalman filtering framework, from which the update steps of the novel cubature Kalman filter are derived. Then, the equations of state and measurement for real-time orbit determination are obtained. The J2 perturbation and atmospheric drag perturbation are taken into account in the state equation, and the coordinate transformation is used to derive the nonlinear relationship between the orbital state and measurement element. The simulation results show that the proposed fifth-degree cubature Kalman filter can achieve a higher filtering accuracy than the CKF and the same accuracy as the existing fifth-degree filters, but has the fewest cubature points and the best real-time performance, which proves the effectiveness of the proposed algorithm.
Identifying the influence of GaN/InxGa1-xN type last quantum barrier on internal quantum efficiency for III-nitride based light-emitting diode
GaN/InxGa1-xN-type last quantum barrier (LQB) proves to be useful for Ⅲ-nitride based light-emitting diode (LED) in enhancing the internal quantum efficiency (IQE) and suppressing the efficiency droop level that often takes place especially when the injection current is high. In this work, GaN/InxGa1-xN-type LQB reported by the scientific community to enhance the IQE is first reviewed and summarized. Then, the influences of indium composition and thickness of the InxGa1-xN layer on the performance of LED incorporated with the GaN/InxGa1-xN-type LQB are studied. Through analyzing energy band diagrams calculated with APSYS, we find that the oriented LQB features an electron depletion due to the polarization induced negative charges at the GaN/InxGa1-xN interface. The electron depletion enhances the electron blocking effect and reduces the electron accumulation at the InxGa1-xN/AlGaN interface, leading to an improved IQE for the LED. In addition, increasing the indium composition of the InxGa1-xN layer will generate more negative interface charges, which result in further increased conduction band barrier height for the electrons and reduced electron leakage. On the other hand, for the GaN/InxGa1-xN-type LQB with a fixed indium composition, there exists an optimum thickness for the InxGa1-xN layer in maximizing the improvement of IQE for the LED, mainly because the interaction between two mechanisms co-exists when varying the thickness of the InxGa1-xN layer, i.e., the initial increase in the InxGa1-xN layer thickness will lead to an increased conduction band barrier height, which prevents electrons from leaking into the InxGa1-xN layer. However, further increasing the InxGa1-xN layer thickness to a certain value, tunneling effect will kick in as a result of the simultaneously reduced GaN thickness-the electrons will tunnel through the thin GaN layer in the LQB from the quantum wells to the InxGa1-xN layer. This will cause electrons to increase in the InxGa1-xN layer. Therefore, as a result of the interaction between the above-mentioned two mechanisms, there is an optimum thickness for the InxGa1-xN layer such that the electrons in the InxGa1-xN layer will reach a minimal value, which in turn will lead to a maximized conduction band barrier height for the AlGaN electron blocking layer and facilitate the performance of LEDs.
Butt-joint design in a uni-traveling carrier photodiode array monolithic with an arrayed waveguide grating by the selective area growth technique
Monolithic integration of an InP-based O-band 4-channel arrayed waveguide grating (AWG) to a uni-traveling carrier photodiode (UTC-PD) array is realized by the selective area growth (SAG) technique. The passive-active buttjoint design is introduced and experimentally proved to ensure both good compatibility between the PD fabrication process and the SAG technique, and high photodiode quantum efficiency under the complex butt-joint geometry. An extended coupling layer is adopted between the AWG output waveguides and the PD mesa. The extended coupling layer length, the regrowth boundary edge position and the AWG etching edge position relative to the heterogeneous butt-joint boundary, and the refractive indices of the PD collector and coupling layer are optically simulated and optimized by a finite-difference time-domain method.
It is found that the extended coupling layer, compared with the un-extended situation, ensures a good matched optical field from AWG to PD and could reduce nearly 30% quantum efficiency loss when connecting seamlessly to the regrown InP AWG top cladding layer. A stable high efficiency around 80% is maintained within an extended layer length from 7.5 μm to 15.0 μm. The regrowth boundary edge into the coupling region will cause a drastic efficiency oscillation up to 20% period with the increase of distance. The efficiency drop is also attributed to the light scattering at the regrowth boundary edge, caused by the optical field mismatch, while the oscillation comes from the alternative light power concentration between the coupling layer and the core layer, for the light scattering is only obvious when the light power is well concentrated in the coupling layer. The AWG etching edge position deviation from the butt-joint boundary, however, exerts little influence on the PD quantum efficiency, which is believed not to bring obvious coupling loss during device fabrication.
The higher UTC-PD collector refractive index is proved to be crucial for further better optical coupling from the coupling layer to the PD, with quantum efficiency rapidly increasing from around 0.1 to 0.8 when the index is increased from 3.20 to 3.42. By comparison, the efficiency is little affected by the coupling layer refractive index from 3.34 to 3.42.All things considered, we select a 10 μm extended coupling layer, the refractive indices of both PD collector and the coupling layer to be 3.42, and align both the regrowth boundary edge and the AWG etching edge to the heterogeneous butt-joint boundary, and a PD quantum efficiency of 80% is expected.
Owing to the extended coupling layer at the butt-joint, the SAG technique facilitates the PD fabrication process. The overgrown AWG top cladding layer ridge stretches out 4.67 μm toward the PD, but not over the mesa yet, hence has little influence on the PD fabrication accuracy. The monolithic chip presents a uniform photodiode quantum efficiency of 76%, which accords well with theoretical value and confirms the butt-joint design. Central wavelengths for the four channels are 1347.0 nm, 1325.0 nm, 1308.0 nm, and 1286.5 nm, respectively. The low crosstalk level (below -22 dB) also indicates a good de-multiplexer performance.
Shape transformations of opening-up vesicles with one hole
So far two kinds of solutions to the problem of opening-up vesicles with one hole have been found. One is cup-like shape found by Umeda and Suezaki (2005 Phys. Rev. E 71 011913), the other is dumbbell shape with one hole, found by our group. As seen in the context of the bilayer coupling (BC) model, the former corresponds to relatively small reduced area difference Δa, and the latter corresponds to relatively large value of Δa. The relationship between these two kinds of shapes is not clear. Viewing from the angle of the cup-like shape, whether one can obtain the dumbbell shape by increasing Δa is not known. In this paper, we try to clarify this problem by solving the shape equations for free vesicles and adhesive vesicles based on the BC model. Firstly, we solve the set of Euler-Lagrange shape equations that satisfy certain boundary conditions for free vesicles. A branch of solution with an inward hole is found with the reduced area difference Δa slightly greater than 1. It is verified that the solution named cuplike vesicles, which was found by Umeda and Suezaki, belongs to another solution branch (Δa < 1) with an outward hole near Δa=1. According to this result, we make a detailed study of these two solution branches for free vesicles and vesicles with adhesion energy. We find that there is a gap near Δa=1 between the two solution branches. For Δa in this gap, there is no opening-up solution. For adhesive vesicles, the gap will move towards the right side slowly with increasing adhesive radius. In order to check whether the two solution branches can evolve into closed shapes, we also make a calculation for closed vesicles. For free closed vesicles, we find that there is only the sphere solution when Δa is exactly equal to 1 for Δp=0 (in order to comply with the opening-up vesicle, no volume constraint is imposed on it), while for adhesive vesicles there exist closed solutions in a region of Δa without volume constraint. Both studies for free vesicles and adhesive vesicles show that these two kinds of opening-up vesicles belong to different solution branches. They cannot evolve from one to the other with continuous parameter changing. And strictly speaking, they cannot evolve into the closed vesicles. With increasing Δa, the opening-up branch on the right side of the gap can evolve into an opening-up dumbbell shape with one hole via the self-intersection intermediate shapes. Another interesting result is that for adhesive opening-up vesicles, in the Δa parametric space, the solutions are folded for a solution branch, which means that there exist several shapes corresponding to the same Δa value in the folding domain. This phenomenon has never occurred in previous study of the closed vesicles under the BC model. The influences of Δa on the shape and energy of the free vesicles and adhesive vesicles are also studied.
Carrier selective contacts:a selection of high efficiency silicon solar cells
Solar cell has two basic units:the photon absorption layer and the contact layer. The contact layer is a region between the highly recombination-active metal interface and the photon absorption layer. It is vital to reduce the recombination loss between the photon absorption layer and the contact layer in pursuit of the higher conversion efficiency of silicon solar cell. In recent years, carrier selective contact is arousing research interest in photovoltaic industry because it is deemed as one of the last remaining obstacles in approaching to the theoretical efficiency limit of silicon solar cell. In this paper, three different types of carrier selective contacts are analyzed, which includes:1) sandwiching a heavily doped thin layer between the photon absorption layer and the metal interface, which is the so-called emitter or back surface field; 2) aligning the conduction bands or the valence bands of two materials; 3) inducing the band bending through a high work function metal oxide contacting crystalline silicon. Based on one-dimensional solar cell simulation software wxAMPS, three different silicon solar cell structures are numerically simulated, which includes:1) diffused homojunction silicon solar cell[(p+)c-Si/(n)c-Si/(n+)c-Si]; 2) silicon heterojunction solar cell with amorphous silicon thin films[(p+)a-Si/(i)a-Si/(n)c-Si/(i)a-Si/(n+)a-Si]; 3) silicon heterojunction solar cell with metal oxide thin films[(n)MoOx/(n)c-Si/(n)TiOx], then the energy band structures and the spatial distributions of carrier concentrations of solar cells in the dark are discussed. The simulation results show that the key factor of carrier selective contacts is the asymmetric spatial distribution of the carrier concentrations, i.e. the asymmetric conductivities of electrons and holes. This leads to the formation of high resistance to electrons and low resistance to holes, or high resistance to holes and low resistance to electrons, so the holes will go through the contact easily and the electrons will be blocked simultaneously, or the electrons will go through the contact easily and the holes will be blocked simultaneously. Therefore a hole selective contact or a electron selective contact is formed, respectively.
Recent progress of high-coherence ultrafast electron sources
Microscopic dynamic process of material structure which determines the inherent property of substance takes place on a molecular and atomic scale. Understanding the underlying mechanisms of the various fundamental processes has always been the goal of chemistry, physics, biology and materials science. With Ahmed Zewail's pioneering work in the field of femtoscience, the time-resolved electron diffraction, combining the pump-probe and electron diffraction technique, has become an excellent tool with sufficient temporal precision to directly deliver insights into ultrafast phenomena on an atomic level. Central to this method is the ultrashort electron pulses generated from a metal photocathode.
However, up to now, owing to the initial size, effective temperature, energy dispersion and inherent coulomb repulsion of electron source, the state-of-the-art transverse coherence of conventional planar cathode photoemission source is still insufficient to resolve the complex chemical and biological organic molecules. Hence, in recent years, many efforts have focused on developing high-coherence ultrashort electron sources. The main methods include minimizing the initial beam size, weakening the space charge, reducing the effective temperature, and matching the photon energy of laser with the work function of cathode material.
In this review, we firstly summarize the history and advantages of the electron probe, secondly sketch out the figure of merit of the electron source. And then taking coherence as the main line, we review recent progress in common planar photoemission sources, and discuss the latest development of tip-based electron sources and cold atom electron sources in terms of their generation mechanisms, unique properties and research progress. Finally, the development and future applications of the diffraction technique are prospected. In general, the high-coherence length of photoelectric surface source is often at the expense of the current. The needle source can obtain the highest coherence length, but it is similar to femtosecond single-electron pulse, which must be less than one electron per pulse to eliminate the electron-electron coulomb interaction. Thus, a diffraction pattern can only be formed by accumulating millions of shots. The cold atom electron source, which has a transverse coherence greater than 15 nm and a peak brightness similar to conventional electron source's, is sufficient for some molecular systems in biochemistry. In short, with the improvement of coherence and the emergence of new electron sources, it is possible to reveal complex organic and inorganic structures, especially the dynamic behaviors of protein, and promote the understanding of nanoscale energy transport, solid-liquid and solid-gas interfacial dynamics and chemical reaction and so on. High-coherence electron sources not only serve in the diffraction experiments, but also play a key role in developing ultrafast electron microscopy, coherent diffraction imaging and ptychography.
Modelling and simulation of DNA hydrogel with a coarse-grained model
Recently supramolecular hydrogels have become a hot research point in the field of hydrogels. As promising building block for supramolecular hydrogel, DNA has received considerable attention for its designability and excellent mechanical strength, and DNA hydrogel has shown great potential applications in biological and medical areas. To better understand the structure and property of DNA hydrogel, computational simulation is a very powerful tool to complement experimental study. However, owing to the large size of DNA hydrogel system and long time scale of self-assembly process, it is practically unachievable to simulate the system directly at an all-atom level. Coarse-grained simulations should be developed. In this article, we propose a highly coarse-grained model to investigate the mesoscopic structure of well-designed pure DNA hydrogel constructed by Y-shape DNA blocks and linear DNA linkers with sticky ends. In this model, we ignore almost all the atomic details of the building blocks and only give a coarse-grained description of their shapes, and carefully design the Lennard-Jones (LJ) interaction between coarse-grained particles in order to take into account the fact that any of the three arms of a Y block can only interact with a single linker (i.e., the bond is saturated). To design a suitable interaction, here we use a combination of LJ repulsive potential between like particles and LJ attracting potential between unlike particles. Our simulation results show that the hydrogel has two states, namely, homogeneous liquid-like state at high temperature and spongy gel-like state at low temperature. State of this system is related to the degree of cross-linking which is described by average cross-linking pair number per Y-scaffold here. We find that the pair number per Y-scaffold is positively correlated with the concentration of hydrogel blocks, which is consistent with experimental results. We also investigate the distribution of local structure by using voronoi cells, then predict the hole size of the hydrogel network. By the micro-rheology method, we then determine more precisely the value of the transition temperature to be 0.06ε/kB-0.10ε/kB, which is also consistent with experimental result. The quantitative relation between transition temperature and binding energy of sticky ends can hopefully provide guidance for the optimal design of DNA hydrogels. The qualitative and even semi-quantitative agreement between our simulation results and experimental results indicates that our coarse-grained model is a suitable and effective one for this pure DNA hydrogel system. The basic ideas of our model can be generalized to more complicated DNA hydrogel systems.
Geometrical optimization of Cu-Au-Pd clusters based on the construction of inner cores
The trimetallic cluster has become a hot topic in the field of basic scientific research due to its special catalytic, magnetic and chemical activities. It is very important to determine the stable structures of clusters. In order to optimize the stable structure of large size Cu-Au-Pd cluster, a modification algorithm of adaptive immune optimization algorithm based on the construction of inner cores, called AIOA-IC algorithm, is proposed. The only difference between AIOA and AIOA-IC lies in their starting structures. Instead of generating the starting structure randomly in AIOA, an inner core in the AIOA-IC method is used for generating the starting structure. Several motifs, such as decahedron, icosahedron, face centered cubic, six-fold pancake structure, and Leary tetrahedron, are randomly selected as the inner cores. The size of the inner core is determined according to the cluster size. The Gupta potential based on the second moment approximation of tight binding potential is used to describe the interatomic interaction between Cu-Au-Pd clusters, and the corresponding potential parameters, such as the cohesive energy, lattice constants, and elastic constants are obtained by fitting the experimental values. To test the efficiency of the proposed algorithm, the stable structure of Ag-Pd-Pt cluster with 60 atoms is optimized. The results show that the new structure has lower energy than the cluster reported in the literature. It can be seen that the AIOA-IC algorithm has a stronger ability to search for the potential energy surface of the Gupta potential. Furthermore, the proposed algorithm is used to optimize the stable structures of 38-atom and 55-atom Cu-Au-Pd clusters. The structures of the investigated Cu6AunPd32-n, CunAu6Pd32-n and CunAu32-nPd6 (n=1-31) clusters can be categorized into three types:five-fold, six-fold, and truncated octahedron. Moreover, it is found that the compositions of Cu, Au and Pd atoms in the trimetallic clusters affect the structural type of the cluster. However, the Cu13AunPd42-n, CunAu13Pd42-n, and CunAu42-nPd13 (n=1-41) clusters each have a structure of complete Mackay icosahedron. Furthermore, the order parameter results show that Cu, Au and Pd atoms each have a significant segregation phenomenon. For the 147-atom Cu12Au93Pd42 cluster, the structure is also of an icosahedron. The central atom is Au, and the inner shell and sub-outer shell are occupied by 12 Cu and 42 Pd atoms, respectively. The outer shell is filled with 92 Au atoms. The results show that the Cu, Pd and Au atoms tend to be distributed in the inner shell, sub-outer shell, and outer shell, respectively. This can be further explained by the results of the atomic radius and the surface energy.
Influence of atmosphere attenuation on quantum interferometric radar
There has been aroused much interest in quantum metrology such as quantum radar, due to its applications in sub-Raleigh ranging and remote sensing. For quantum radar, the atmospheric absorption and diffraction rapidly degrade any actively transmitted quantum states of light, such as N00N and M&M' states. Thus for the high-loss condition, the optimal strategy is to transmit coherent state of light, which can only provide sensitivity at the shot-noise limit but suffer no worse loss than the linear Beer's law for classical radar attenuation.
In this paper, the target detection theory of quantum interferometric radar in the presence of photon loss is thoroughly investigated with the model of Mach-Zehnder interferometer, and the dynamic evolution of the quantum light field in the detecting process is also investigated. We utilize the parity operator to detect the return signal of quantum interferometric radar with coherent-state source. Then we compare the detection result of quantum radar with that of classical radar, which proves that the quantum radar scheme that employs coherent radiation sources and parity operator detection can provide an N-fold super-resolution, which is much below the Rayleigh diffraction limit; besides, the sensitivity of this scheme can also achieve the shot-noise-limit.
Also, we analyze the effect of atmospheric attenuation on the performance of quantum radar, and find that the sensitivity is seriously influenced by atmospheric attenuation:only when the reference beam and the detection beam have the same transmissivity, will the sensitivity increase monotonically with increasing the photon number per pulse N, otherwise it first increases and then decreases with increasing N. Further, the sensivity is directly proportional to 1/√N for the first case.
In conclusion, we investigate the effects of atmospheric absorption on the resolution and sensitivity of quantum radar, and find that one can overcome the harmful effects of atmospheric attenuation by adjusting the transmissivity of reference beam to the atmospheric transmittance.
Experimental technique for multi-qubit nuclear magnetic resonance system
With the development of quantum information and quantum computation science, quantum information processor has been widely used in different areas such as quantum simulation, quantum computation and quantum metrology and so on. To make quantum computer come true, we need to increase the number of controllable qubits of the system and improve the controllability to perform complex quantum manipulation. As a good experimental testbed for quantum information processing, nuclear magnetic resonance (NMR) spin system provides rich and sophisticated quantum control methods. In recent years a lot of multi-qubit experiments have been performed on the platform and a series of experimental technologies have been developed. In this paper, we firstly explain the difficulties of multi-qubit NMR experiments. Then by focusing on the experiment of 7-qubit labelled pseudo-pure state preparation and other relevant experiments, we review the technologies in multi-qubit experiments. Using the radio frequency selective method, the inhomogeneities of the radio frequency pulses are reduced and the spectral resolution is improved. After performing 1/2 spin selective sequence, we can regard the three methyl protons in the sample of crotonic acid as a single 1/2 spin nucleus and treat the whole molecule as a 7-qubit quantum information processor. We utilize Gauss pulses, Hermite pulses, composite pulses and gradient ascent pulse engineering (GRAPE) pulses to implement basic π/2 and π rotation operations. The GRAPE pulses are calculated by subspace GRAPE program to speed up the computation greatly. The errors of the basic pulses caused by chemical shift and J-coupling evolution can be estimated by the program of pulse compilation. It divides the errors of the pulses into a series of post-errors and pre-errors. A program of sequence compilation is used to eliminate the accumulated error of the whole pulse sequence, reduce the number of pulses and optimize the experimental duration. A variety of methods of quantum state tomography have been proposed to improve the efficiency of reading out information about quantum state. As an experimental example, we combine the above experimental technologies and perform the experiment of 7-qubit labelled pseudo-pure state preparation by using the method of cat state preparation. The sequence of cat state preparation consists of three steps:encoding procedure, phase cycling and decoding procedure. We use 14 experiments to realize the phase cycling and acquire the final 7-qubit labelled pseudo-pure state. The total duration of experimental sequence is about 132 ms. All the readout spectra have the similar shapes to the theoretical expectations. Finally we give an outlook for further research in this direction.
High efficiency cross-polarized wave filter for non-vacuum transmission
Development of high-peak power laser system encounters difficulties in producing the pulses with high temporal contrast. To increase the pulse temporal contrast ratio, a nonlinear filter based on crossed-polarized wave (XPW) generation is proposed. The XPW generation relies on a third-order nonlinear process occurring in a nonlinear medium, such as barium fluorite (BaF2) crystal. The XPW process is quite straightforward:a linearly polarized laser pulse is focused on BaF2 crystal positioned between two orthogonally polarizers, high power main pulses due to nonlinear polarization rotation can pass through the second polarizer, while low power unconverted pre-and post-pulses are filtered by the second polarizer. With the XPW technique, pulse contrast can be enhanced by several orders of magnitude. Furthermore, XPW spectrum can be broaden by a factor with respect to the initial spectrum. This efficient pulse cleaner presents many advantages and has proved to be a simple and reliable pulse filter operating in a double chirped pulse amplification system.
Most of previous XPW experiments utilize short focal systems or work off focus due to an intensity limit in the crystal (BaF2). These drawbacks result in a lower conversion efficiency (lower than 10%) when using a single crystal. Dual crystal setup is capable of achieving efficiency more than 20%, yet the configuration restricts the crystal separation to a millimeter level. The use of long focus lens in the XPW device is capable of reaching higher efficiency, with BaF2 crystal positioned in the focal plane. Hence for milljoule pulses, the setup distance increases to tens of meters, resulting in a complicated system and cumbersome configuration.
Considering these limitations, a compact, highly efficient and stable XPW generation using dual-lens system suitable for non-vacuum transmission is presented. The measured nonlinear accumulated phase shows little deterioration of pulse quality. With a compact dual lens system, we realize an excellent XPW conversion of above 22% (internal efficiency of 30%) with using double BaF2 crystals, while a femtosecond laser pulse can experience a spectrum broadening up to a factor of 1.78. The dual-lens configuration overcomes the crystal separation limit, and conversion efficiency exceeds 20% for a crystal separation from 13 cm to 22 cm, which is conducible to flexibility and robustness. The stability for the setup to generate shorter pulses with very high contrast or compensate for spectral gain narrowing in the preamplifier is ensured due to the dual-lens focusing system.
Design of varying f/number of cooled infrared detectors based on spherical reflecting warm shield
As is well known, the f/number of a cooled infrared detector is determined by the aperture and position of the internal cold shield. Moreover, the f/number can be changed by inserting a warm shield in front of the detector. In order to reduce the stray radiation introduced by an ordinary planar warm shield, we propose a method of varying f/number of the infrared detector based on a well-designed spherical reflecting warm shield in this paper. First, an infrared radiation model is established in order to analyze the influence of the stray radiation introduced by the ordinary planar warm shield. Then the design principle of the spherical reflecting warm shield is put forward. By changing the surface shape and emission characteristics, the stray radiation introduced by the ordinary planar warm shield can be obviously reduced. Hence it is beneficial to maintain the performance of the detector effectively while the f/number is changed. To validate the proposed method, a spherical reflecting warm shield and an ordinary planar warm shield are designed to vary the f/number of a cooled infrared detector respectively. To compare the influences of the two warm shields on the cooled infrared detector, radiometric calibration experiments are conducted in a high-low-temperature test chamber. The analyses and experimental results show that the stray radiation of spherical reflecting warm shield is far less than that of the ordinary planar warm shield. Moreover, the noise equivalent temperature difference introduced by the designed spherical reflecting warm shield is lower. Therefore it is indeed better than an ordinary planar warm shield in ensuring the performance of an infrared imaging system.
Quantum trajectory simulation for nonadiabatic molecular dynamics
The mixed quantum-classical (MQC) molecular dynamics (MD) approaches are extremely important in practice since, with the increase of atomic degrees of freedom, a full quantum mechanical evaluation for molecular dynamics would quickly become intractable. Moreover, in some cases, the nonadiabatic effects are of crucial importance in the proximity of conical intersection of potential energy surfaces (PESs), where the energy separation between different PESs becomes comparable to the nonadiabatic coupling. In the past decades, there has been great interest in developing and improving various nonadiabatic MQC-MD protocols. The widely known nonadiabatic MD proposals include the so-called “Ehrenfest” or “time-dependent-Hartree mean-field” approach, the “trajectory surface-hopping” method, and their mixed scheme. Among the trajectory-based surface hopping methods, the most popular one is Tully's fewest switches surface hopping approach. In this approach, the nonadiabatic dynamics is treated by allowing hops from one PES to another, with the hopping probability determined by a certain artificial hopping algorithm.
In our present work, we extend the study of a recent work on the nonadiabatic MQC-MD scheme, which is based on a view that the nonadiabatic MQC-MD actually implies an effective quantum measurement on the electronic states by the classical motion of atoms. The new protocol, say, the quantum trajectory (QT) approach, provides a natural interface between the separate quantum and classical treatments, without invoking artificial surface hopping algorithm. Moreover, it also connects two widely adopted nonadiabatic dynamics methods, the Ehrenfest mean-field theory and the trajectory surface-hopping method. In our present study, we implement further the QT approach to simulate several typical potential-surface models, i.e., including the single avoided crossing, dual avoided crossing, extended coupling, dumbbell and double arch potentials. In particular, we simulate and compare three decoherence rates, which are from different physical considerations, i.e., the frozen Gaussian approximation, energy discrimination and force discrimination. We also design simulation algorithms to properly account for the energy conservation and force direction change associated with the surface hopping. In most cases, we find that the QT results are in good agreement with those from the full quantum dynamics, which is insensitive to the specific form of the decoherence rate. But for the model involving strong quantum interference, like other nonadiabatic MQC-MD schemes, the QT approach cannot give desirable results. Developing better method should be useful for future investigations in this research area.
2000 eV X-ray laser transparent mechanism of neon atom
X-ray transparency occurs during the interaction of X-ray free electron laser with matter. The study of the mechanism of X-ray transparency is of great value for understanding the interaction between X-ray free electron laser and matter. In this paper, the main ionization modes from neutral neon atom till bare nucleus at different flux densities are determined based on the 2000 eV photoionization cross sections and the Auger decay rates of various neon atoms (ions), calculated by the Flexible Atomic Code program. By establishing and solving the rate equations, the formulas of the proportions of various electronic configurations of neon in the main ionization mode are obtained. The proportions of electron configurations in the main ionization modes and the atomic average photoionization cross sections at flux densities of 2000 and 10000 Å-2·fs-1 are calculated by using the formulas. The ratios of the number of hollow atoms to that of complete valence electrons at any time under different flux density laser irradiations are calculated. It is found that both the bare nuclei and the hollow atoms cause X-ray transparency, and a relatively high ratio of the number of hollow atoms to that of complete valence electrons can be achieved by choosing appropriate flux density and pulse duration.
Generation of ultrafast broadband small angle hundreds MeV electron bunches from laser wakefield acceleration
Electrons can be accelerated to a GeV level in centimeters by plasma wakefield driven by laser. With the development of chirped pulse amplification technique, the accelerating field can reach 100 GV/m. The laser driven wakefield acceleration experiments with ionization injection are carried out using 68 TW (1.7 J, 25 fs) laser and a mixture gas of 99% He and 1% N2. In experiment, the output electron beam has broadband spectrum with a maximum cut-off energy of about 290 MeV and a maximum output energy is quite stable in a certain range of laser focal positions. Two-dimensional particle-in-cell simulation is carried out. It is found that the longitudinal phase space is occupied by the continuously injected electrons and the phase space distribution is quite stable after the laser has propagated several millimeters inside plasma. This acceleration process can lead to quite stable maximum output energy of electron beam. These experiments reveal the physical nature of continuous ionization injection, which is very important for improving the performance of ionization injection.
Inversion and reconstruction of the macroscopic photometric characterization model for on-orbit space object
In the field of space object optical situational awareness by space-based optics, the current research focuses on the detecting of long distance point target, the predicting and confirming of target trajectory. It is very important to analyze the on-orbit operation status and basic physical attribution of the space object by remote imaging without any structure or texture information. The analysis method can be used effectively to support the space object status discrimination and the related decision for on-orbit maintenance. In recent years, the number of three-axis stabilization space objects in orbit has increased dramatically. In order to retrieve the optical characteristic parameters of the three-axis stabilization space object surface in a long on-orbit distance, a new method is proposed to reconstruct the macroscopic photometric characterization based on analyzing visible photometric sequence signal. Firstly, based on the principle that solar panel can receive the maximum solar radiation energy, a directing model of solar panel is proposed. Considering the structural characteristics of space object, surface material characteristics, directing characteristics of the solar panels, illumination-observation geometry and the optical system characteristics, the photometric modeling method of the space object oriented to space-based observation is improved. Secondly, the photometric model is equivalent to a two-facet model, then multi-level fusion model of bidirectional reflectance distribution function (BRDF) is used to characterize optical reflection characteristics of complex material surfaces of the space object, and the product of area and reflection corresponding to multi-level BRDF is taken as the parameter of the reconstruction model. Finally, the minimum error between the measured result and reconstruction model of photometric signal is used as the optimization goal, and the linear optimization method is established to realize the inversion of the model parameters. As the space object and the observing satellite are in the same orbit or near orbit condition, the simulating experiment of photometric sequence signal and reconstructing experiment of macroscopic characterization model of optical characteristics are carried out. The simulation result shows that the proposed photometric model can describe the dynamic characteristics of on-orbit space object more comprehensively. And using the on-orbit reconstruction method of the macroscopic photometric model, the photometric signal reconstruction accuracy achieves more than 97% in the near orbit condition. So it is demonstrated that the on-orbit reconstruction method is correct. The method can provide a solution of optical situational awareness for space object on space-based platform, and provides technical support for inversing the attitude and shape of space object.
Nanoscale surface topography imaging using phase-resolved spectral domain optical coherence tomography
Microscopic surface topography plays an important role in studying the functions and properties of materials. Microscopic surface topography measurement has been widely used in many areas, such as machine manufacturing, electronic industry and biotechnology. Optical interferometry is a popular technique for surface topography measurement with an axial resolution up to nanoscale. However, the application of this technique is hampered by phase wrapping, which results in a limited measurement range for this technique. Various digital algorithms for phase unwrapping have been proposed based on the phase continuity between two adjacent points. However, several significant challenges still exist in recovering correct phase with this technique. Optical coherence tomography (OCT) is a non-contact three-dimensional imaging modality with high spatial resolution, and it has been widely used for imaging the biological tissues. In this paper, we demonstrate a method for nanoscale imaging of surface topography by using common-path phase-resolved spectral domain OCT to reduce the influence of phase wrapping. The system includes a superluminescent diode with a central wavelength of 1310 nm and a spectral bandwidth of 62 nm, an optical fiber circulator, a home-made spectrometer, and a reference arm and a sample arm in common-path arrangement. The reference mirror and the sample under investigation are positioned on a same stage in order to further reduce the influence of ambient vibration. The phase difference between two adjacent points is calculated by performing Fourier transform on the measured interferometric spectrum. The phase difference distribution of the surface is obtained first. And then, the surface topography of the sample is constructed by integrating the phase difference distribution. In the traditional methods, phase wrapping occurs if the absolute value of the measured phase is greater than π. However, in the present method, phase wrapping occurs if the absolute value of the phase difference between two adjacent points is greater than π. The maximal detectable absolute value of the phase difference between two adjacent points increases from π for the traditional methods to 2π for the present method. The experimental results indicate that the present system has a high stability and the maximum fluctuation is less than 0.3 nm without averaging. The accuracy of the system is tested with a piezo stage, and the mean absolute deviation of the measured results is 0.62 nm. The performance of the present system is also demonstrated by the surface topography imaging of an optical resolution test target and a roughness comparison specimen. The experimental result shows that the present system is a potential powerful tool for surface topography imaging with an axial resolution better than 1 nm.
Single photon transport properties in the system of coupled cavity array nonlocally coupled to a Λ-type three-level atom
In this paper, we discuss the transport properties of a single photon, which is in a coupled cavity array system where the two nearest cavities nonlocally couple to a Λ-type three-level atom, under the condition of ideal and dissipation, respectively. By employing the quasi-boson picture, the transmission amplitude of the single photon in an open system is investigated analytically. The system where the coupled cavity array nonlocally couples with the three-level atom demonstrates several advantages. Compared with other systems, this system has many parameters to manipulate the single photon transport properties. Moreover, the system of the coupled cavity array that nonlocally couples with the three-level atom may have a wider range of application because the single photon transmission spectrum in this system has three peaks. Furthermore, it has characteristics of its own. At the same value of Rabi frequency Ω, changing the coupling strength between the atom and one cavity of the coupled cavity array shows that there exists an fixed point where the transmission rate is always 1, and the point is corresponding to the frequency of the photon ωc-Ω. In the nonideal case, it is shown that the dissipations of the cavity and the atom affect distinctively the transmission of photons in the coupled cavity arrays. When considering only the dissipation of the atom, the atomic dissipation increases the dips of the single photon transport spectrum, while the peaks have no observable changes. When considering only the dissipation of the cavity, the peaks of the single photon transmission amplitude are diminished deeply, while the cavity dissipation does not have any effect on the dips. In addition, with both the cavity dissipation rate and the number of the cavity increasing, the photon transmission spectrum peaks decrease. A comparison of the dissipative cavity case with the dissipative atom case shows that the incomplete reflect near the peak is mostly caused by the cavity dissipation, and that the incomplete reflect near the dip is mostly caused by the three-level atom dissipation. Specifically, when considering both the atom and the cavity dissipation at the same time, the dips of the single photon transport spectrum are affected by both the atomic and the cavity dissipation. Instead, with the cavity dissipation rate increasing, the photon transmission spectrum dips are reduced. But for the peaks of the single photon transport spectrum, the dips are always determined by the cavity dissipation rate and the number of the cavity, while the atomic dissipation has no significant influence on them.
Spectral and laser properties of Nd:YSAG single crystal
In order to solve the problem of low segregation coefficient of Nd3+ ions in YAG crystal, Sc3+ ions are used to replace some Al3+ ions in YAG, so that the YAG cell becomes bigger, thereby the segregation coefficient of Nd3+ ion increases. In this paper, the spectra and laser characteristics of Nd:YSAG single crystal are studied. The (1.5 at.%) Nd3+:Y3Sc2Al3O12 (YSAG) laser crystal is grown by Czochralski method, and X-ray rocking curve shows that the crystal quality is good. The concentration of ions in Nd:YSAG is measured by an electronic probe. The segregation coefficient of Nd3+ ion is calculated to be 0.35, which is approximately twice as much as that of Nd3+ in YAG. The absorption and emission spectra are measured, which indicates that it can be pumped by an 808 nm-laser diode (LD). The strongest emission from the transition 4F3/2→4I11/2 of Nd:YSAG peaks at 1059 nm with an emission cross section of 1.03×10-19 cm2, and the lifetime of 4F3/2 is about 253 μs, which indicates that Nd:YSAG has roughly an efficiency equal to that of Nd:YAG, but the laser upper level lifetime is longer than that of Nd:YAG, which is more favorable for Q-switched laser operation. The 808 nm-LD is used to pump Nd:YSAG rod of 2 mm×2 mm×6 mm, the laser operation with a threshold of 0.85 W and a maximum output power of 1.1 W is realized:the laser slope efficiency is 21.1%, and the optical conversion efficiency is 18.3%. All of the above results show that Nd:YSAG single crystal is a good solid state laser material, which is more favorable for Q-switched laser operation.
Tuning characteristics of fluorescent light source by dye-doped liquid crystal filled hollow fiber
The fluorescent fiber light source has been widely used in many areas, such as optical fiber communication and medical imaging, owing to its low cost and wide optical spectrum. The temperature-sensitive refractive index of liquid crystal makes it a suitable filling material used in the fluorescent light source. The existing work has investigated the filling of liquid crystal into the air holes in cladding of photonic crystal fiber. However, the photonic crystal fiber has the disadvantages of complicated craft and high cost. As is well known, the hollow fiber has the advantages of the easy preparation and low cost, but the filling of liquid crystal into the hollow fiber of fluorescent light source is rarely investigated. In this paper, we investigate that a tunable hollow fiber of fluorescent light source is filled with dye doped liquid crystals. The transmission characteristics of the fluorescent light source are theoretically analyzed. The variation in property of the B4400 fluorescent spectrum is numerically discussed with the dye molecule energy level structure theory. The numerical simulation results show that the relative refractive index is dependent on temperature. It first increases linearly with the increase of temperature and then exponentially increases rapidly till clearing point 61.9 C, finally decreases slowly to a saturated value. In order to find an optimum doping concentration, the doping-concentration-dependent fluorescence output intensity is analyzed by using the super continuum spectrum of YAG pump with a wavelength of 1064 nm. The fluorescence light intensities are amplified at three different selective dye doping concentrations, namely 0.2 wt%, 1 wt% and 2 wt% in the experiment, respectively. The highest output is obtained at the 1 wt% doping concentration, which verifies the selective fluorescence amplification property of the fluorescent source. It is also demonstrated that the transmission characteristics and the tunable range of the liquid crystal filled fluorescent light source can be adjusted by modulating the temperature in experiment. And the temperature-dependence of the fluorescence source is experimentally demonstrated by using the 1 wt% doping concentration dye-doped liquid crystal. Using a pulsed YAG pump with a wavelength of 532 nm, tunable characteristics of the fluorescent light source composed of a dye-doped liquid crystal filled hollow fiber, are studied and show that the central wavelength increases from 590 nm to 605 nm and the spectral width broadens from 228 nm to 236 nm with the increase of the temperature. The proposed fluorescent light source can be controlled by adjusting the temperature within limits. These findings will give a guidance for the practical applications of the dye doped liquid crystal based fluorescent light source, and offer a theoretical foundation for the further study of the liquid crystal filled fluorescent fiber light source.
Method and experiment of path rainfall intensity inversion using a microwave link based on nonspherical rain-induced model
It is important to measure rainfall accurately with high spatial and temporal resolution in meteorology, hydrology, agriculture industry, environment conservation, flood warning and weather forecasting. The use of attenuated information about microwave propagation in rainfall areas to acquire surface precipitation intensity has been shown to be a practical approach to measuring rainfall in recent years. However, the inversion of a single-frequency link is based on the assumption of rainfall attenuation under a certain frequency condition. Further, obtaining parameters that comply with all rainfall events for the rainfall attenuation model is a challenge, often leading to an overestimation of the rainfall intensity. Therefore, based on extended boundary condition method and Gamma raindrop size distribution, an inversion method of the path rainfall intensity by using a microwave link rain-induced attenuation is proposed in order to improve the accuracy of rainfall measurement by microwave rain-induced attenuation. In this paper, we use the characteristics of an atmospheric attenuation model to eliminate the influence of non-rainfall-caused attenuation on the process of rainfall inversion. On the basis of scattering theory and by utilizing the Gamma raindrop size drop, we use the extended boundary condition method to calculate the characteristics of microwave attenuation for Pruppacher-Beard raindrop shape model. The correction model of rainfall effective attenuation and rainfall inversion model of line-of-sight microwave links are proposed, based on the microwave rain attenuation characteristics and raindrop size distribution statistics. In this paper, we propose 15-20 GHz inversion model of path-average rainfall intensity based on nonspherical rain-induced model by using Levenberg-Marquardt optimization algorithm. Meanwhile, we analyze the variations of parameters of rain-induced model under the condition of different temperatures. Besides, we design a line-of-sight microwave experimental system for measuring the rainfall, and the path average rain rate is inversed by rainfall inversion model, which is compared with an OTT disdrometer. The results show that the correlation coefficient of rain rate inversed by microwave link and that of disdrometer are both higher than 0.6 mostly, and the maximum value is 0.96; the error of accumulated rain amount is less than 2.47 mm, the minimum value is 0.28 mm; the relative error of accumulated rain amount is less than 1.84%, the minimum value is 0.44%. The experimental results validate the feasibility and accuracy of rainfall inversion method proposed in this paper. In addition, the experimental result reflects that rainfall intensity retrieved method based on nonspherical raindrop model has advantages over the method based on spherical raindrop model.
Acoustic focusing by thermoacoustic phased array
Broadband acoustic focusing effect based on a thermoacoustic phased array is studied. In this work, according to the relationship between the sound velocity and the temperature, a new type of a thermoacoustic phase control unit is designed by using air with different temperatures surrounded by rigid insulated boundaries and thermal insulation films. The acoustic wave velocity could be adjusted by changing the temperature of air in the unit, and the transmitted and reflected acoustic phase delays can cover the whole 2π interval. On the basis of this thermoacoustic phased array, we design four different types of acoustic focusing lenses. By using eight or two kinds of such units, we realize the transmitted and reflected acoustic focusing effect, respectively. The results show that the thermoacoustic phased array lens has a good focusing performance in a frequency range of 4.0-15.0 kHz. In addition, the center intensity of the focal spot is much greater in the focusing lens with eight phase units, and the design method is simpler and more robust in the focusing lens with two phase units. Compared with other types of focusing lenses, the proposed focusing lens based on the thermoacoustic phased array has the advantages of broad bandwidth, high focusing performance and simple designed method. The results provide a theoretical basis and experimental reference for designing the broadband thermoacoustic phased array devices and new types of acoustic focusing lenses.
Vibro-acoustic stimulating ultrasonic guided waves in long bone
Ultrasonic guided wave is sensitive to waveguide microstructure and material property, which has great potential applications in long cortical bone evaluation. Due to the multimodal dispersion effect, low-frequency guided wave is usually used to avoid multimode overlapping and simplify the signal processing. However, the traditional low-frequency ultrasound transducer is usually designed on a large-scale (around several millimeters), leading to relatively low-spatial resolution. In response to such a technique limit, an ultrasound-stimulated vibro-acoustic method is introduced to excite low-frequency ultrasonic guided waves. There are two excitation ways of the ultrasound-stimulated vibro-acoustic method, i.e., a single amplitude-modulated (AM) beam and confocal beam excitation. In the case of the single beam excitation, a high-frequency signal is modulated by using a low-frequency amplitude. In addition, low-frequency vibration can also be produced by a confocal transducer, where two beams are close to the center frequency and focus on a small region. In this way, the frequency difference between two beams can be selected to generate the arbitrary low-frequency excitation in a given bandwidth on the focus point. In this paper, we first introduce the theory of ultrasonic guided wave in the plate and the basic principle of ultrasound-stimulated acoustic emission. Second, the three-dimensional finite element method is used to simulate the phenomena of the low-frequency ultrasonic guided waves excited by the ultrasound-stimulated vibro-acoustic method. Two Gaussian-function enveloped tone-burst signals close to the center frequencies of 5 MHz are used to excite 150 kHz low-frequency guided wave in a 3 mm-thick bone plate. An ex-vivo bovine bone plate is involved in the experiments to test the feasibility of the proposed method. The axial transmission ultrasonic guided waves are recorded at eight different propagation distances. The time-frequency representation method is used to analyze the dispersive guided waves. The results indicate that both the two confocal beams and the single AM beam are capable of stimulating low-frequency ultrasonic guided waves in the bone plate. The first two fundamental guided wave modes, i.e., symmetrical S0 and asymmetrical A0 are observed in the bone plate. Similar spectrum can be obtained in the two different excitation ways. In the simulation and experiment, two wave packets can be separated in the distance-time diagram of the received signals. Good agreement can be found between the results of time-frequency representation and the theoretical group dispersion curves. This study can enhance the spatial resolution of measuring ultrasonic guided wave in long bone, and improve the flexibility of excitation with arbitrary frequency in a given bandwidth. The study can be helpful for developing the new clinical techniques of using low-frequency guided waves for long cortical bone assessment.
Dynamic modeling method of flexible bodies with contact/impact based on interactive mode
To solve the contact/impact problem of flexible bodies in arbitrary shape, the finite element method is widely used to discretize the contact bodies. In the finite element method, two contact models are mainly used to compute the contact force, i.e., penalty function method and additional constraint method, which are different in constraint imposition strategy. The penalty function method regards the contact effect as a force function of local penetration at the contact point and its rate. This method has gained significant popularity because it does not bring extra dimensions to the dynamic equations and does not need to solve constraint equations either. However, as the non-penetration constraint is not precisely satisfied in the contact process when using the penalty function method, the accuracy of the numerical simulation depends on the choice of the penalty parameter. On the other hand, the additional constraint method can strictly satisfy the contact constraint condition by introducing the Lagrange multipliers into the dynamic equations, but the method poses some numerical difficulties due to the additional effort required to solve the multipliers. Considering the advantages and disadvantages of the two contact methods, the interactive mode method is proposed. This method divides the whole model into local static module and main dynamics module. The static module establishes a local finite element model of the contact region to compute the contact force, and the main dynamics module is used to obtain the kinematic variables of the whole body. In the simulation, the two modules are coupled by exchanging displacements and forces in each time step. In the current integration step, the main dynamics module provides the displacements of the boundaries of the local contact region at first, the values are transferred to the local finite model to compute the contact force next, and then the contact force is fed back to the dynamics module for the calculation of the next step. The proposed method combines the advantages of both the additional constraint method and the penalty function method, in which not only the artificial selection of penalty parameter is avoided, but also the non-penetration constraint of local contact region is satisfied and the numerical solution is convenient. The validity of the proposed method is verified by the comparison between simulation results and experimental results of a rod-plate impact case. Furthermore, a multi-point impact problem of a slider sliding in the gap chute is presented to validate the proposed method of dealing with the general impact problem.
Experimental study on the influence of granular shear deformation on sound propagation
Effective medium theory (EMT) predicts a scaling relation between sound velocity c and pressure P as c∝ (φZ)1/3 (P/E0)1/6, where φ and Z are respectively the packing fraction and the mean coordination number of granular material. In this relation, the granular contact network is represented via two simple parameters φ and Z stemming theoretically from a strong approximation that microscopic and macroscopic granular displacements remain affine. This hypothesis simplifies tremendous computations for sound wave in a granular system, however some experimental results show that the scaling relation is recovered only for the case of very high pressure confinement (larger than 106 Pa for a glass bead system), but for the lower pressure case (less than 106 Pa) the relation does not hold. Owing to the fact that the change of microscopic granular displacement relates to the contact network variation of granular sample, and for better understanding the effect of the variation of contact network on the sound propagation in granular system, we conduct uniaxial shear experiments, in which the granular solid sample, composed of 0.28-0.44 mm glass beads, is cyclically compressed under a series of axial loadings (denoted as Pcomp). After these axial loadings, different contact networks of the sample are formed. Ultrasonic waves are then measured in the granular sample with these different contact networks under a constant axial pressure (denoted as Pobse). It is found that the axial deformation of the granular sample apparently affects the incoherent part of ultrasonic wave, but not the coherent part. A resemblant parameter is introduced to quantitatively discuss the variations of incoherent parts of sound waves in different axial deformations. In this paper, we also compare the frequency and the energy spectra of the sound waves, and find that the tendencies of their varying with the increase of axial deformation are nearly the same. This indicates that during the sound wave propagation in the granular solid sample, the processes of wave scattering and dissipation on particle contacted occur at the same time and the energy dissipation of sound wave in the air among particles can be neglected. In our experiments, compressional wave velocities based on time-of-flight method are also explored. The experimental results show that the velocity increases rapidly at the beginning of the axial deformation, and then tends to a steady value which is predicted by EMT. These illuminate that the variation of contact networks of granular sample may contribute to the deviation of velocity-pressure exponent from the prediction of EMT in low confining pressure.
Characteristics of gain in Ne-like Ar 69.8 nm laser pumped by capillary discharge
In this paper, the theoretical calculation model of the gain coefficient of Ne-like Ar 69.8 nm laser is established. With the collisional-radiative model, the rate equations for the 46.9 nm and 69.8 nm lasers are built by considering the 4 levels of the 2s2p6 1S0, 2p53p 1S0, 2p53p 3P2, and 2p53s 1P1. The gain coefficients per ion density of 46.9 nm and 69.8 nm lasers are calculated on the basis of the rate equations. The results show that the 46.9 nm laser has potential of higher gain than the 69.8 nm laser at an electron temperature of 200 eV. The gain coefficients per ion density at different electron temperatures are also calculated. Under the same electron density, the higher electron temperature is favorable for increasing the gain coefficients per ion density of the 69.8 nm laser. Meanwhile there is also an optimal electron density corresponding to the maximum gain coefficient per ion density of the 69.8 nm laser at a given electron temperature. Then a one-dimensional cylindrical symmetry Lagrangian magneto-hydrodynamics (MHD) code is utilized to simulate the Z-pinch process. The radial distributions of the electron temperatures, the electron densities and the Ne-like Ar ion densities are calculated with the MHD code at the different initial pressures. According to the rate equations for the 69.8 nm laser and the simulation results of the MHD code, the gain coefficient distribution of 69.8 nm laser in the radial direction of the plasma can be determined when the plasma is compressed to a minimum radius. According to the experimental parameters, the maximum gain coefficient of 69.8 nm laser is calculated to be 0.32 cm-1 when the main pulse current is 12 kA. The relationship between the radial distribution of gain coefficient of 69.8 nm laser and the initial pressure is also simulated. The theoretical results show that the optimal initial pressure is in a range of 12-14 Pa, in which the amplitude of gain coefficient is maximum. The experiments about 69.8 nm laser are conducted with Al2O3 capillary which has an inner diameter of 3.2 mm and a length of 35 cm. A main current of 12 kA with a rise time of 32 ns is produced by the main pulse generator, which consists of a Marx generator and a Blumlein line filled with de-ionized water. The Blumlein line is pulse-charged by a ten-stage Marx generator and discharges through the capillary by a self-breakdown main switch pressurized with N2 gas. To reduce the amplitude of main current, we reduce the charging voltage of the Marx generator and increase the conducting inductance of the main switch. Prior to the operation of the main current pulse, the capillary filled with Ar is predischarged by a current of~20 A. The 69.8 nm laser intensity as a function of initial pressure is measured by a 1-m grazing incidence Rowland spectrograph. The experimental results show that the optimum pressure is 16 Pa which is similar to the theoretical result. In addition, the gain coefficient (0.4 cm-1) measured in experiment is slightly higher than that (0.32 cm-1) of the theoretical calculation.
Plasma characteristics of helical streamers induced by pulsed discharges
The plasma discharge channel in three-dimensional helical shape induced by pulsed direct current (DC) discharge without external stable magnetic field is discovered experimentally. It can be observed by intensified charge-coupled device camera that a luminous plasma structure fast propagates along a helical path in the form of guided streamer (ionization wave). And the propagation of the streamer is stable and repeatable. We take this streamer which propagates along the helical discharge path as the study object, and explain its mechanism by constructing an electromagnetic model. The result shows that the helical shape plasma plumes can exhibit two different chiral characteristics (right-handed and left-handed helical pattern). While the discharge parameters such as pulse frequency, boundary condition, etc. can all affect the propagating characteristics of helical streamers. The electromagnetic radiation driven by pulsed DC power inside the dielectric tube which forms the wave mode is an important source of the poloidal electrical field. The helical steamers form when the poloidal electrical field is close to the axial electrical field. The velocities of the propagation in poloidal and axial direction are estimated respectively, and the hybrid propagation modes involving the interchangeable helical pattern and the straight-line pattern propagating plasmas are explained from the viewpoint of multi-wave interaction.