A topology optimization method based on the solid isotropic material with penalization interpolation scheme is utilized for designing gradient coils for use in magnetic resonance microscopy. Unlike the popular stream function method, the proposed method has design variables that are the distribution of conductive material. A voltage-driven transverse gradient coil is proposed to be used as micro-scale magnetic resonance imaging (MRI) gradient coils, thus avoiding introducing a coil-winding pattern and simplifying the coil configuration. The proposed method avoids post-processing errors that occur when the continuous current density is approximated by discrete wires in the stream function approach. The feasibility and accuracy of the method are verified through designing the z-gradient and y-gradient coils on a cylindrical surface. Numerical design results show that the proposed method can provide a new coil layout in a compact design space.

The well-known multi-dimensional reconciliation is an effective method used in the continuous-variable quantum key distribution in the long-distance and the low signal-to-noise-ratio scenarios. The virtual channel employed to exchange data is generally established by using a finite-dimensional rotation in the reconciliation procedure. In this paper, we found that the finite dimension of the multi-dimensional reconciliation inevitably leads to the mismatch of the signal-to-noise-ratio between the quantum channel and the virtual channel, which may be called the finite-dimension effect. Such an effect results in an overestimation on the secret key rate, and subsequently induces vital practical security loopholes.

We study the topological properties of Bogoliubov excitation modes in a Bose-Hubbard model of three-dimensional (3D) hyperhoneycomb lattices. For the non-interacting case, there exist nodal loop excitations in the Bloch bands. As the on-site repulsive interaction increases, the system is first driven into the superfluid phase and then into the Mott-insulator phase. In both phases, the excitation bands exhibit robust nodal-loop structures and bosonic surface states. From a topology point of view, these nodal-loop excitation modes may be viewed as a permanent fingerprint left in the Bloch bands.

Stochastic resonance (SR) is studied in an under-damped bistable system driven by the harmonic mixing signal and Gaussian white noise. Using the linear response theory (LRT), the expressions of the spectral amplification at fundamental and higher-order harmonic are obtained. The effects of damping coefficient, noise intensity, signal amplitude, and frequency on spectral amplifications are explored. Meanwhile, the power spectral density (PSD) and signal-to-noise ratio (SNR) are calculated to quantify SR and verify the theoretical results. The SNRs at the first and second harmonics exhibit a minimum first and a maximum later with increasing noise intensity. That is, both of the noise-induced suppression and resonance can be observed by choosing proper system parameters. Especially, when the ratio of the second harmonic amplitude to the fundamental one takes a large value, the SNR at the fundamental harmonic is a monotonic function of noise intensity and the SR phenomenon disappears.

We design a hybrid integrated chaotic semiconductor laser with short-cavity optical feedback. It can be assembled in a commercial butterfly shell with just three micro-lenses. One of them is coated by a transflective film to provide the optical feedback for chaos generation while insuring regular laser transmission. We prove the feasibility of the chaos generation in this compact structure and provide critical external parameters for the fabrication by theoretical simulations. Rather than the usual changeless internal parameters used in previous simulation research, we extract the real parameters of the chip by experiment. Moreover, the maps of the largest Lyapunov exponent with varying bias current and feedback intensity K_{ap} demonstrate the dynamic characteristics under different external-cavity conditions. Each laser chip has its own optimal external cavity length (L) and feedback intensity (K_{ap}) to generate chaos because of the different internal parameters. We have acquired two ranges of optimal parameters (L=4 mm, 0.12 < K_{ap} < 0.2 and L=5 mm, 0.07 < K_{ap} < 0.12) for two different chips.

A new feedback control method is derived based on the lattice hydrodynamic model in a single lane. A signal based on the double flux difference is designed in the lattice hydrodynamic model to suppress the traffic jam. The stability of the model is analyzed by using the new control method. The advantage of the new model with and without the effect of double flux difference is explored by the numerical simulation. The numerical simulations demonstrate that the traffic jam can be alleviated by the control signal.

The performance of a superconducting quantum interference device (SQUID) gradiometer is always determined by its pick-up coil geometry, such as baseline and radius. In this paper, based on the expressions for the coupled flux threading a magnetometer obtained by Wikswo, we studied how the gradiometer performance parameters, including the current dipole sensitivity, spatial resolution and signal-to-noise ratio (SNR), are affected by its pick-up coil via MatLab simulation. Depending on the simulation results, the optimal pick-up coil design region for a certain gradiometer can be obtained. To verify the simulation results, we designed and fabricated several first-order gradiometers based on the weakly damped SQUID with different pick-up coils by applying superconducting connection. The experimental measurements were conducted on a simple current dipole in a magnetically shielding room. The measurement results are well in coincidence with the simulation ones, indicating that the simulation model is useful in specific pick-up coil design.

SPECIAL TOPIC—Recent advances in thermoelectric materials and devices

In the present work, firstly, a first-principles study of the structural, electronic, and electron transport properties of the Hg_{x} Mg_{(1-x)} Te (HMT) ternary compound is performed using the ABINIT package and the results are compared with Cd_{0.9}Zn_{0.1} Te (CZT) as a current room-temperature photodetector. Next, the response functions of Hg_{0.6}Mg_{0.4} Te and Cd_{0.9}Zn_{0.1} Te under electromagnetic irradiation with 0.05 MeV, 0.2 MeV, 0.661 MeV and 1.33 MeV energies are simulated by using the MCNP code. According to these simulations, the Hg_{0.6}Mg_{0.4} Te ternary compound is suggested as a good semiconductor photodetector for use at room temperature.

We report a clock transition spectrum approach, which is used to calibrate the zero-crossing temperature and frequency drift of an ultralow expansion (ULE) cavity with a Hertz level resolution. With this approach, the linear and nonlinear drifts of the ULE cavity along a variety of controlled temperatures are clearly presented. When the controlled temperature of ULE cavity is tuned away from the zero-crossing temperature of the ULE cavity, the cavity shows larger and larger nonlinear drift. According to our theoretical analysis and experimental results, we investigate more details of the drift property of the ULE cavity around the zero-crossing temperature, which has seldom been explored before. We can definitely conclude that the zero-crossing temperature of our ULE cavity used in an ytterbium (Yb) lattice clock is around 31.7℃.

The frequency-comb structure in the extreme ultraviolet (XUV) and vacuum ultraviolet (VUV) regions can be realized by the high-order harmonic generation (HHG) process driven by frequency-comb fields, providing an alternative approach for the measurement of an unknown frequency in XUV or VUV. We consider the case of two driving frequency-comb fields with the same repetition frequency and the carrier frequencies of fundamental-and third-harmonics, respectively. The many-mode Floquet theorem (MMFT) is employed to provide a nonperturbative and exact treatment of the interaction between a quantum system and the frequency-comb laser fields. Multiphoton transition paths involving both fundamental-and third-harmonic photons are opened due to the coupling of the third-harmonic frequency-comb field. The multiphoton transition paths are superpositioned when the carrier-envelope-phase shifts (CEPs) fulfill the matching condition. And the interference of the multiphoton transition paths can be controlled by tuning the relative envelope delay between the fields. We find that the quasienergy structure, as well as the multiphoton resonant high-order harmonic generation (HHG) spectra, driven by the two frequency-comb fields can be coherently controlled via the interference of multiphoton transition paths. It is also found that the spectral intensities of the generated harmonics can be modulated, and the modulation behavior is harmonic-sensitive.

The effect of delay time on photoelectron spectra and state populations of a four-level ladder K_{2} molecule is investigated by a pump1-pump2-probe pulse via the time-dependent wave packet approach. The periodical motion of the wave packet leads to the periodical change of the photoelectron spectra. The Autler-Townes triple splitting appears at zero delay time, double splitting appears at nonzero delay time between pump1 and pump2 pulses, and no splitting appears at nonzero delay time between pump2 and probe pulses. The periodical change of the state populations with the delay time may be due to the coupling effect between the two pulses. It is found that the selectivity of the state populations may be attained by regulating the delay time. The results can provide an important basis for realizing the optical control of molecules experimentally.

We study the fractal rhythm in the ionization of Rydberg helium and lithium atoms in an electric field by using the semiclassical method. The fractal structures present a nested relationship layer by layer in the initial launch angles of the ionized electrons versus the escape time, which is defined as the rhythm attractor, and exhibit similar rhythm endings. The gradually enhanced chaotic regions of the escape time plots tend to broaden as the scaled energy increases. In addition, the fractal rhythm changes synchronously with the oscillations of the kinetic energy spectrum. We note that the intrinsic quality of the fractal rhythm is closely related to the kinetic energy distribution, that is, the inherent dynamic properties of the Hamiltonian equations have an impact on the fractal regularities. In addition, different ionizing closed trajectories exhibit iterate properties and the inherent beauty of symmetry. Our results and analysis can not only reveal new laws in the ionization of Rydberg atoms, but also promote the establishment of the dynamic mechanism of fractals.

Electron-impact single ionization cross sections for W^{q+} (q=4-5) were calculated using the flexible atomic code (FAC) in the level-to-level distorted-wave method, considering the explicit branching ratio. The calculated cross sections are compared with the available theoretical and experiment results in detail. In the case of the contribution from the same channles as the available theoretical results, all of the calculated ionization cross sections agree with the experimental measured cross sections. But the present calculated results are larger than the experimental measurement when all channels contributions are included. Some important channels excitation autoionization (EA) contributions, such as the excitation to higher higher nl' subshell from 4f and 5[s,p], were not included into the available theoretical calculation. In general, the distorted-wave (DW) results are overestimated.

Laser-induced electron diffraction (LIED), in which elastic scattering of the returning electron with the parent ion takes place, has been used to extract atomic potential and image molecular structures with sub-Å precision and exposure time of a few femtoseconds. So far, the polarization and exchange effects have not been taken into account in the theoretical calculation of differential cross section (DCS) for the laser-induced rescattering processes. However, the validity of this theoretical treatment has never been verified. In this work, we investigate the polarization and exchange effects on electron impact elastic scattering with rare gas atoms and ions. It is found that, while the exchange effect generally plays a more important role than the polarization effect in the elastic scattering process, the exchange effect is less important on electron-ion collisions than on electron-atom collisions, especially for scattering in backward direction. In addition, our calculations show that, for electron-atom collisions at incident energies above 50 eV, both the polarization and exchange effects can be safely neglected, while for electron-ion collisions, both the polarization and exchange potentials do not make substantial contributions to the DCS at incident energies above 20 eV and scattering angles larger than 90°. Our investigation confirms the validity of the current LIED method.

Cold molecules have great scientific significance in high-resolution spectroscopy, precision measurement of physical constants, cold collision, and cold chemistry. Supersonic expansion is a conventional and versatile method to produce cold molecules with high kinetic energies. We theoretically show here that fast-moving molecules from supersonic expansion can be effectively decelerated to any desired velocity with a rotating laser beam. The orbiting focus spot of the red-detuned laser serves as a two-dimensional potential well for the molecules. We analyze the dynamics of the molecules inside the decelerating potential well and investigate the dependence of their phase acceptance by the potential well on the tilting angle of the laser beam. ND_{3} molecules are used in the test of the scheme and their trajectories under the impact of the decelerating potential well are numerically simulated using the Monte Carlo method. For instance, with a laser beam of 20 kW in power focused into a pot of 40 μ in waist radius, ND_{3} molecules of 250 m/s can be brought to a standstill by the decelerating potential well within a time interval of about 0.73 ms. The total angle covered by the rotating laser beam is about 5.24° with the distance travelled by the potential well being about 9.13 cm. In fact, the molecules can be decelerated to any desired velocity depending on the parameters adopted. This scheme is simple in structure and easy to be realized in experiment. In addition, it is applicable to decelerating both molecules and atoms.

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

Femtosecond laser filamentation is a method of generating terahertz, which has wide application in terahertz sub-wavelength resolution imaging. In this paper, the plasma filament formed by femtosecond laser focusing was terminated with an alumina ceramics at different positions and the influence of the cutting off position of the plasma filament on the terahertz wave was studied. The results showed that the terahertz amplitude increases as the position approaches the end of the filament gradually. The stability of amplitude and peak frequency of the terahertz generated by the filament formed by two-color femtosecond laser via a lens with a longer focal length is lower than that through a lens with a shorter focal length, especially the terahertz amplitude at the end of the filament. The study will be helpful for future researchers in the field of THz sub-wavelength imaging utilizing femtosecond laser filament.

Bloch surface waves (BSWs) are excited in one-dimensional photonic crystals (PhCs) terminated by a graphene monolayer under the Kretschmann configuration. The field distribution and reflectance spectra are numerically calculated by the transverse magnetic method under transfer-matrix polarization, while the sensitivity is analyzed and compared with those of the surface plasmon resonance sensing method. It is found that the intensity of magnetic field is considerably enhanced in the region of the terminated layer of the multilayer stacks, and that BSW resonance appears only in the interface of the graphene and solution. Influences of the graphene layers and the thickness of a unit cell in PhCs on the reflectance are studied as well. In particular, by analyzing the performance of BSW sensors with the graphene monolayer, the wavelength sensitivity of the proposed sensor is 1040 nm/RIU whereas the angular sensitivity is 25.1°/RIU. In addition, the maximum of figure of merit can reach as high as 3000 RIU^{-1}. Thus, by integrating graphene in a simple Kretschmann structure, one can obtain an enhancement of the light-graphene interaction, which is prospective for creating label-free, low-cost and high-sensitivity optical biosensors.

The propagation dynamics of a chirped Airy vortex (CAiV) beam with x-polarization in uniaxial crystals orthogonal to the optical axis is studied analytically and numerically. The effect of the ratio of extraordinary and ordinary refractive indices, the chirp parameter, as well as the propagation distance is analyzed, which shows that the focused position of the CAiV beams can be controlled through changing the ratio of the extraordinary and ordinary refractive indices. In addition, with the propagation distance increasing, the asymmetry of the intensity and the angular momentum of the CAiV beam during propagation becomes much more visible. The variation of the chirp parameters can change the attenuation velocity of the vortex as well.

The exceptional point (EP) is a significant and attractive phenomenon in an open quantum system. The scattering properties of light are similar to those in the open quantum system, which makes it possible to achieve EP in the optic system. Here we investigate the EP in a Fabry-Pérot (F-P) resonant coupling structure. The coupling between different types of F-P resonances leads to a near zero reflection, which results in a degeneration of eigenstates and thus the appearing of EP. Furthermore, the multi-wavelength EPs and unidirectional invisibility can be achieved which may be used in integrated photonics systems.

A new achromatic phase retarder based on a metal-multilayer dielectric grating structure is designed using the rigorous coupled wave analysis method and the genetic algorithm. The optimized phase retarder can maintain phase retardation around 90° from 900 nm to 1200 nm, and the maximum deviation is less than 4.5% while the diffraction efficiencies of TE and TM waves are both higher than 95%. Numerical analysis shows the designed phase retarder has a high fabrication tolerance of groove depth, duty cycle and incident angle. This achromatic phase retarder is simple in design and stable in performance, and can be widely used in optical systems.

The scintillation index (SI) of a Gaussian-Schell model (GSM) beam in a moderate-to-strong anisotropic non-Kolmogorov turbulent atmosphere is developed based on the extended Rytov theory. The on-axis SI in a marine atmosphere is higher than that in a terrestrial atmosphere, but the off-axis SI exhibits the opposite trend. The on-axis SI first increases and then begins to decrease and saturate as the turbulence strength increases. Turbulence inner and outer scales have different effects on the on-axis SI in different turbulent fluctuation regions. The anisotropy characteristic of atmospheric turbulence leads to the decline in the on-axis SI, and the rise in the off-axis SI. The on-axis SI can be lowered by increasing the anisotropy of turbulence, wavelength, and source partial coherence before entering the saturation attenuation region. The developed model may be useful for evaluating ship-to-ship/shore free-space optical communication system performance.

We propose a metasurface which consists of three conductive layers separated by two dielectric layers. Each conductive layer consists of a square array of square loop apertures, however, a pair of corners of each square metal patch surrounded by the square loop apertures have been truncated, so it becomes an orthotropic structure with a pair of mutually perpendicular symmetric axes u and v. The simulated results show that the metasurface can be used as a wideband transmission-type polarization converter to realize linear-to-circular polarization conversion in the frequency range from 12.21 GHz to 18.39 GHz, which is corresponding to a 40.4% fractional bandwidth. Moreover, its transmission coefficients at x-and y-polarized incidences are completely equal. We have analyzed the cause of the polarization conversion, and derived several formulas which can be used to calculate the magnitudes of cross-and co-polarization transmission coefficients at y-polarized incidence, together with the phase difference between them, based on the two independent transmission coefficients at u-and v-polarized incidences. Finally, one experiment was carried out, and the experiment and simulated results are in good agreement with each other.

A single-image passive ranging and three-dimensional (3D) imaging system with chiral phase encoding was proposed in 2011[Opt. Lett.36, 115 (2011)]. A new theoretical analysis of the system in space domain is presented in this paper. We deduce the analytic relationships between the object distance and the point spread function, and between the object distance and the encoded image, respectively. Both the point spread function and the processed spectrum of the encoded image have two spots, which will rotate with the variation of the object distance. Then the depth map is extracted from the encoded image and it can be used to set up 3D images. The theoretical analysis is verified by a wavefront coding system with a chiral phase which is generated by a phase-only liquid-crystal spatial light modulator. The phase generated by the liquid-crystal spatial light modulator is more flexible than the fixed phase mask and can be adjusted in real time. It is especially suitable for observing the object with a large depth of field.

We analyze the transport property of a single photon in a one-dimensional coupled resonator waveguide coupled with a Λ-type emitter assisted by an additional cavity. The reflection and transmission coefficients of the inserted photon are obtained by the stationary theory. It is shown that the polarization state of the inserted photon can be converted with high efficiency. This study may inspire single-photon devices for scalable quantum memory.

Two dimethylamino-carbaldehyde derivatives with different π-bridge lengths were prepared, and their transient optical properties and photophysical mechanisms were investigated by transient absorption spectroscopy and Z-scan measurements. Owing to the difference in molecular structures, the two compounds exhibit different populations of locally excited states and, therefore, they also produce different transient absorption spectra. After photoexcitation, both molecular materials exhibit a wide excited state absorption band from 450 nm to 1000 nm. Meanwhile, the excited state lifetimes are dramatically different, 2 ns and 100 ps, for the two molecules. A figure of merit greater than 2 at the wavelength of 1000 nm is obtained. The results show that modulating the population of the locally excited states in this type of molecule can be a promising approach for obtaining optical switching and solar cell materials.

Water purification is required for environmental protection. In this paper, we propose and demonstrate a rapid, effective and low-cost approach to collect numerous impurities (microparticles) in water on the basis of laser-induced thermal convection. We introduce a heat source by using a fiber tip, which is fabricated into a non-adiabatic-tapered shape. In order to improve the laser power absorption efficiency, we coat a gold film with a thickness of 300 nm on the fiber tip. Due to absorption, the laser power transferred from the fiber to the water results in thermal convection. The forces generated from the thermal convection drive the microparticles to move towards the fiber tip, thereby performing microparticle collection and achieving water purification. Laser-induced thermal convection provides a simple, high-efficiency and low-cost method of collecting microparticles, which is a suitable and convenient for local water purification.

A comprehensive investigation is carried out to compare the spectroscopic properties, absorption saturation behaviors and lasing properties of a Yb ion in tetragonal LuPO_{4} and LuVO_{4} isomorphic crystals. Significant distinctions are revealed in many aspects of the lasing behavior for the Yb ion doped in the two crystal hosts, and Yb:LuPO_{4} proves to be superior to Yb:LuVO_{4} since it enables efficient laser action to be much more easily achieved. With a 0.6 mm thick crystal plate of Yb:LuPO_{4}, an output power of 3.30 W can be generated with an optical-optical conversion efficiency of 50.8%; whereas with a 2 mm long miniature crystal rod, the output power produced can reach 8.35 W with an optical-optical conversion efficiency of 46.7%.

We report on the experimental investigation and theoretical analysis of a nanosecond pulse high power ultraviolet (UV) 278 nm laser by fourth-harmonic generation (FHG) of a 1112-nm Nd:YAG amplifier in LiB_{3}O_{5} (LBO) and CsB_{3}O_{5} (CBO) crystals. The UV laser delivers a maximum average power of 10.3 W at 278 nm with peak power of 36.8 kW under input pump power of 41 W at 556 nm. This is, to the best of our knowledge, the highest output power at the specific UV wavelength of 278 nm. We also performed the theoretical investigation on the FHG with a model in the Gaussian approximation of both spatial and temporal profiles, especially accounting for the two-photon absorption effect in CBO crystal for the first time. The average output power, pulse width, and beam spatial distribution of the UV laser were simulated. The theoretical calculations are in close agreement with the experimental results.

An injection-seeded single-frequency Q-switched Nd:YAG laser is accomplished by using a phase modulated ramp-fire technique. A RbTiOPO_{4} (RTP) electro-optic crystal is selected for effective optical path length modulation of the slave self-filtering unstable resonator. This single-frequency laser is capable of producing 50 mJ pulse energy at 1 Hz repetition rate with a pulse width of 16 ns. The standard deviation of laser pulse intensity for consecutive 100 shots from the mean pulse intensity is less than 1.05%. A spectral linewidth of less than 0.5 pm with a frequency jitter of about 14 fm over 30 min is obtained.

We study high-order harmonic generation (HHG) from multi-center asymmetric linear molecules numerically and analytically. Our simulations show that odd and even HHG spectra of the asymmetric multi-center system respond differently to the change of the molecular structure. Specifically, when the internuclear distances between these nuclei of the molecule have a small change, the odd spectra usually do not change basically, but the even spectra differ remarkably. Based on this phenomenon, a simple procedure is proposed to probe the positions of these nuclei with odd-even HHG. Our results shed light on attosecond probing of the structure of multi-center molecules using HHG.

The underlying mechanism of the spectral cleaning effect of the cross-polarized wave (XPW) generation process was theoretically investigated. This study shows that the spectral noise of an input spectrum can be removed in the XPW generation process and that the spectral cleaning effect depends on the characteristics of the input pulses, such as the chirp and Fourier-transform-limited duration of the initial pulse, and the modulation amplitude and frequency of the spectral noise. Though these factors codetermine the output spectrum of the XPW generation process, the spectral cleaning effect is mainly affected by the initial pulse chirp. The smoothing of the spectrum in the XPW generation process leads to a significant enhancement of the coherent contrast.

To improve the modeling accuracy of radiative transfer, the scattering properties of aerosol particles with irregular shapes and inhomogeneous compositions should be simulated accurately. To this end, a light-scattering model for nonspherical particles is established based on the pseudo-spectral time domain (PSTD) technique. In this model, the perfectly matched layer with auxiliary differential equation (ADE-PML), an excellent absorption boundary condition (ABC) in the finite difference time domain generalized for the PSTD, and the weighted total field/scattered field (TF/SF) technique is employed to introduce the incident light into 3D computational domain. To improve computational efficiency, the model is further parallelized using the OpenMP technique. The modeling accuracy of the PSTD scheme is validated against Lorenz-Mie, Aden-Kerker, T-matrix theory and DDA for spheres, inhomogeneous particles and nonspherical particles, and the influence of the spatial resolution and thickness of ADE-PML on the modeling accuracy is discussed as well. Finally, the parallel computational efficiency of the model is also analyzed. The results show that an excellent agreement is achieved between the results of PSTD and well-tested scattering models, where the simulation errors of extinction efficiencies are generally smaller than 1%, indicating the high accuracy of our model. Despite its low spatial resolution, reliable modeling precision can still be achieved by using the PSTD technique, especially for large particles. To suppress the electromagnetic wave reflected by the absorption layers, a six-layer ADE-PML should be set in the computational domain at least.

In this work, uranine-dyed zinc (tris) thiourea sulfate (ZTS) monocrystals, 26 mm×15 mm×10 mm in size, were synthesized by the solution method at ambient temperature. Their purity, crystallinity, lattice parameters, and functional modes were studied by x-ray diffraction, Fourier transform-infrared spectroscopy (FT-IR), and FT-Raman spectroscopy analyses. The sodium ion content of the crystals from the dye was confirmed by elemental analysis. The diffused reflectance spectral analysis of the dyed crystal revealed a characteristic absorption band at 490 nm attributed to the presence of the dye. The calculated band gaps of the non-dyed and dyed crystals were 4.53 and 4.57 eV, respectively. A green emission peak at ~(512±4) nm was observed in the photoluminescence spectrum of the uranine-dyed crystals. A differential scanning calorimetry study confirmed that the thermal stability improved owing to the addition of the dye. Dielectric and microhardness studies were conducted to examine the significant improvements in the corresponding properties of dyed crystals. The results demonstrated the competency of the dyed ZTS crystals for applications in optoelectronic devices.

The design, fabrication and performance of narrow-band rugate minus filters are investigated in this paper. A method of fabricating graded-index coatings by rapidly alternating deposition of low (SiO_{2}) and high (Al_{2}O_{3}) refractive index materials is presented to fabricate a rugate structure. The narrow-band rugate minus filter design and fabrication approaches are discussed in detail. The experimental results, including transmittance spectrum, surface damage test and damage morphology investigated with a scanning electron microscope, demonstrate the high performance of the as-fabricated spatial filter and confirm the feasibility of the fabrication method for narrow-band rugate minus filters.

We report on the fabrication and properties of an optical waveguide in Nd^{3+}-doped phosphate glass. The planar waveguide was obtained by 550-keV proton implantation with a dose of 8.0×10^{16} ions/cm^{2}. The proton-glass interaction was simulated by the stopping and range of ions in matter (SRIM software). The characteristics of the waveguide including the refractive index profile and the near-field intensity distribution were studied by the reflectivity calculation method and the end-face coupling technique. The optical waveguide demonstrated multi-mode behavior at the wavelength of 632.8 nm. The propagation features of the proton-implanted Nd^{3+}-doped phosphate glass waveguide shows its potential to operate as an integrated photonic device.

An analytical variational method for the ground state of the biased quantum Rabi model in the ultra-strong coupling regime is presented. This analytical variational method can be obtained by a unitary transformation or alternatively by assuming the form of the ground state wave function. The key of the method is to introduce a variational parameter λ, which can be determined by minimizing the energy functional. Using this method, we calculate the physical observables with high accuracy in comparison with the numerical exact ones. Our method evidently improves over the widely used general rotating-wave approximation (GRWA) in both qualitative and quantitative aspects.

An optical method of generating narrowband Lamb waves is presented. It is carried out with a laser line array in a thermoelastic regime implemented by the Michelson interference technique, where the formed array element spacing can be flexibly and conveniently changed to achieve selective mode excitation. In order to simulate the displacement response generated by this array, its intensity distribution function is presented to build a theoretical analysis model and to derive the integral representation of the displacement response. The experimental device and measuring system are built to generate and detect the Lamb waves on a steel plate. Numerical calculation results of narrowband Lamb wave displacement signals based on the theoretical model show good agreement with experimental results.

This paper presents a numerical study on the simultaneous reconstruction of temperature and volume fraction fields of soot and metal-oxide nanoparticles in an axisymmetric nanofluid fuel sooting flame based on the radiative energy images captured by a charge-coupled device (CCD) camera. The least squares QR decomposition method was introduced to deal with the reconstruction inverse problem. The effects of ray numbers and measurement errors on the reconstruction accuracy were investigated. It was found that the reconstruction accuracies for volume fraction fields of soot and metal-oxide nanoparticles were easily affected by the measurement errors for radiation intensity, whereas only the metal-oxide volume fraction field reconstruction was more sensitive to the measurement error for the volume fraction ratio of metal-oxide nanoparticles to soot. The results show that the temperature, soot volume fraction, and metal-oxide nanoparticles volume fraction fields can be simultaneously and accurately retrieved for exact and noisy data using a single CCD camera.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Hydration water can even decide the physicochemical properties of hydrated organic molecules. However, by far the most important hydration number for organic molecules, in particular polyethylene glycol which we are concerned with here, usually suffers from a large discrepancy. Here, we provide a scheme for accurate and unambiguous quantification of the hydration number based on the universal water-content dependence of glass transition temperature for aqueous solutions, testified by experimental results for polyethylene glycol molecules of a molar weight ranging from 200 to 20000. Moreover, we also clarify the fundamental misunderstanding lying in the definition and quantification of hydration water for PEG molecules in the literature, therein the hydration number for PEG in water-rich solutions has been determined at a critical concentration, across which the properties of the solution display obviously distinct water-content dependence.

Plasma normal modes in ion-beam-plasma systems were experimentally investigated previously only for the waves propagating in the downstream (along the beam) direction. In this paper, the ion wave excitation and propagation in the upstream (against the beam) direction in an ion-beam-plasma system were experimentally studied in a double plasma device. The waves were launched by applying a ramp voltage to a negatively biased excitation grid. Two kinds of wave signals were detected, one is a particle signal composed of burst ions and the other is an ion-acoustic signal arising from the background plasma. These signals were identified by the dependence of the signal velocities on the characteristics of the ramp voltage. The velocity of the burst ion signal increases with the decrease of the rise time and the increase of the peak-to-peak amplitude of the applied ramp voltage while that of the ion-acoustic signal is independent of these parameters. By adjusting these parameters such that the burst ion velocity approaches to the ion-acoustic velocity, the wave-particle interaction can be observed.

Different discharge morphologies in atmospheric Ar and He plasmas are excited by using a pulsed microwave hairpin resonator. Ar plasmas form an arched plasma plume at the opened end of the hairpin, whereas He plumes generate only a contracted plasmas in between both tips of metal electrodes. Despite this different point, their discharge processes have three similar characteristics:(i) the ionization occurs at the main electrode firstly and then develops to the slave electrode, (ii) during the shrinking stage the middle domain of the discharge channels disappears at last, and (iii) even at zero power input (in between pulses) a weak light region always exists in the discharge channels. Both experimental results and electromagnetic simulations suggest that the discharge is resonantly excited by the local enhanced electric fields. In addition, Ar ionization and excitation energies are lower than those of He, the effect of Ar gas flow is far greater than that of He gas, and the contribution of accelerated electrons only locates at the domain with the strongest electric fields. These reasons could be used to interpret the different characteristic plume morphologies of the proposed atmospheric Ar and He plasmas.

The Landau damping behavior in a cylindrical inhomogeneous warm magnetized plasma waveguide has been studied. The radial inhomogeneity for different characteristic lengths (L_{0}) with strong spatial dispersion has been taken into account. The analyses have been considered for two limits ω_{ce} < ω_{pe} and ω_{ce} > ω_{pe}. Due to the radial inhomogeneity of the plasma, all essential equations for studying the Landau damping are calculated in the Bessel-Furrier and differential Bessel-Furrier expansions. The dependence of Landau damping on the inhomogeneity, temperature and external magnetic field for electrostatic modes is scrutinized and described in detail through numerical calculations. The associated numerical results are presented and discussed.

A miniaturized 2.45 GHz permanent magnet electron cyclotron resonance (PMECR) ion source, which has the ability of producing a tens-mA H^{+} beam, has been built and tested at Peking University (PKU). Its plasma chamber dimension is Φ 30 mm×40 mm and the whole size of the ion source is Φ 180 mm×130 mm. This source has a unique structure with the whole source body embedded into the extraction system. It can be operated in both continuous wave (CW) mode and pulse mode. In the CW mode, more than 20 mA hydrogen ion beam at 40 kV can be obtained with the microwave power of 180 W and about 1 mA hydrogen ion beam is produced with a microwave power of 10 W. In the pulse mode, more than 50 mA hydrogen ion beam with a duty factor of 10% can be extracted when the peak microwave power is 1800 W.

Rayleigh-Taylor instability (RTI) in cylindrical geometry initiated by velocity and interface perturbations is investigated analytically through a third-order weakly nonlinear (WN) model. When the initial velocity perturbation is comparable to the interface perturbation, the coupling between them plays a significant role. The difference between the RTI growth initiated only by a velocity perturbation and that only by an interface perturbation in the WN stage is negligibly small. The effects of the mode number on the first three harmonics are discussed respectively. The low-mode number perturbation leads to large amplitudes of RTI growth. The Atwood number and initial perturbation dependencies of the nonlinear saturation amplitude of the fundamental mode are analyzed clearly. When the mode number of the perturbation is large enough, the WN results in planar geometry are recovered.

A detailed analysis of the synchrotron radiation intensity and energy of runaway electrons is presented for the Experimental Advanced Superconducting Tokamak (EAST). In order to make the energy of the calculated runaway electrons more accurate, we take the Shafranov shift into account. The results of the analysis show that the synchrotron radiation intensity and energy of runaway electrons did not reach the maximum at the same time. The energy of the runaway electrons reached the maximum first, and then the synchrotron radiation intensity of the runaway electrons reached the maximum. We also analyze the runaway electrons density, and find that the density of runaway electrons continuously increased. For this reason, although the energy of the runaway electrons dropped but the synchrotron radiation intensity of the runaway electrons would continue rising for a while.

The sterilization of the simulated unearthed silk fabrics using an atmospheric pressure plasma jet (APPJ) system employing Ar/O_{2} or He/O_{2} plasma to inactivate the mycete attached on the silk fabrics is reported. The effects of the APPJ characteristics (particularly the gas type and discharge power) on the fabric strength, physical-chemical structures, and sterilizing efficiency were investigated. Experimental results showed that the Ar/O_{2} APPJ plasma can inactivate the mycete completely within 4.0 min under a discharge power of 50.0 W. Such an APPJ treatment had negligible impact on the mechanical strength of the fabric and the surface chemical characteristics. Moreover, the Ar ions, O and OH radicals were shown to play important roles on the sterilization of the mycete attached on the unearthed silk fabrics.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

The compressibility and pressure-induced phase transition of β-Si_{3}N_{4} were investigated by using an angle dispersive x-ray diffraction technique in a diamond anvil cell at room temperature. Rietveld refinements of the x-ray powder diffraction data verified that the hexagonal structure (with space group P63/m, Z=2 formulas per unit cell) β-Si_{3}N_{4} remained stable under high pressure up to 37 GPa. Upon increasing pressure, β-Si_{3}N_{4} transformed to δ-Si_{3}N_{4} at about 41 GPa. The initial β-Si_{3}N_{4} was recovered as the pressure was released to ambient pressure, implying that the observed pressure-induced phase transformation was reversible. The pressure-volume data of β-Si_{3}N_{4} was fitted by the third-order Birch-Murnaghan equation of state, which yielded a bulk modulus K_{0}=273(2) GPa with its pressure derivative K'_{0}=4 (fixed) and K_{0}=278(2) GPa with K'_{0}=5. Furthermore, the compressibility of the unit cell axes (a and c-axes) for the β-Si_{3}N_{4} demonstrated an anisotropic property with increasing pressure.

Ab initio molecular dynamics simulations were performed to investigate the effect of similar elements on the short-to medium-range atomic packing features in Ce_{70}Al_{30} and La_{70}Al_{30} glass-forming alloys. 4f electrons of Ce element in Ce_{70}Al_{30} alloy were properly treated in electronic calculations. The local atomic structures in both alloys are qualitatively similar. However, the local environments of Al atoms in Ce_{70}Al_{30} alloy show fluctuation with temperature in the cooling process, which could result from 4f electrons of Ce elements. Surprisingly, the medium-range atomic packing features of Al atoms in both MGs are quite different, although Ce and La elements are similar. These findings are useful for understanding the enhanced glass-forming ability by similar element substitution in RE-based MGs from a medium-range structure perspective.

Based on Landau-de Gennes theory and two-dimensional finite-difference iterative method, the spontaneous distortion in hybrid alignment nematic cells with M=±1/2 disclination lines is investigated by establishing two models. The fine structures of defect cores are described in the order space S^{2}/Z_{2}. The joint action of elastic anisotropy (L_{2}/L_{1}) and biaxiality of defects induces the spontaneous twist distortion, accompanied by the movement of the defect center to the upper or lower plate. For each model, four mixed defect structures appear with the same energy, which are defined as energetically degenerated quadruple states.

Molybdenum disulfide quantum dots (MoS_{2} QDs) were synthesized via a hydrothermal method using sodium molybdate and cysteine as molybdenum and sulfur sources, respectively. The optimal hydrothermal time was studied. Furthermore, the as synthesized water-soluble MoS_{2} QDs were used as a fluorescence probe for the sensitive and selective detection of copper ions. The fluorescence of the MoS_{2} QDs was quenched after the addition of copper ions; the reason may be that the transfer of the excited electron from QDs to copper ions leads to the reduction of the radiative recombination. The fluorescence quenching of MoS_{2} QDs is linearly dependent on the copper ions concentration ranging from 0.1 μM to 600 μM, the limit of detection is 0.098 μM, which is much lower than that of existing methods. Moreover, the MoS_{2} QDs show highly selectivity towards the detection of copper ions.

Polycrystalline cubic boron nitride (PcBN) compacts, using the mixture of submicron cubic boron nitride (cBN) powder and hexagonal BN (hBN) powder as starting materials, were sintered at pressures of 6.5-10.0 GPa and temperature of 1750℃ without additives. In this paper, the sintering behavior and mechanical properties of samples were investigated. The XRD patterns of samples reveal that single cubic phase was observed when the sintering pressure exceeded 7.5 GPa and hBN contents ranged from 20 vol.% to 24 vol.%, which is ascribed to like-internal pressure generated at grain-to-grain contact under high pressure. Transmission electron microscopy (TEM) analysis shows that after high pressure and high temperature (HPHT) treatments, the submicron cBN grains abounded with high-density nanotwins and stacking faults, and this contributed to the outstanding mechanical properties of PcBN. The pure bulk PcBN that was obtained at 7.7 GPa/1750℃ possessed the outstanding properties, including a high Vickers hardness (~61.5 GPa), thermal stability (~1290℃ in air), and high density (~3.46 g/cm^{3}).

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

The hydrostatic-pressure-dependent mechanical stability and optoelectronic behavior of Zn_{4}B_{6}O_{13} (ZBO) are calculated using the exchange-correlation functional Perdew-Burke-Ernzerhof generalized gradient approximation and the hybrid functional PBE0 based on density functional theory. The calculated and experimental unit cell volumes and Vickers hardness of ZBO at zero pressure agree well. ZBO is mechanically stable under the critical pressure of 52.98 GPa according to the generalized stability criteria. Furthermore, Young's modulus and Vickers hardness decrease with increasing hydrostatic pressure. The strength and type of ZBO bonds are investigated by population and electron density difference. The electronic structure at zero pressure reveals that ZBO is an indirect band gap semiconductor, and the calculated 5.62-eV bandgap coincides well with the 5.73-eV experimental value, highlighting the success of the hybrid functional PBE0 calculations of electronic properties. The band gap almost increases as a second-order polynomial of pressure, and the indirect nature does not change with the applied external pressure. The optical reflectivity and absorption coefficient show that ZBO is an excellent ultraviolet photodetector. Our calculation results suggest that the elastic and optical properties of ZBO are highly stable over a wide pressure range.

The tuning of electron and phonon by ion doping is an effective method of improving the performances of thermoelectric materials. A series of lower concentration K-doped Ca_{3-x}K_{x}Co_{4}O_{9} (x=0, 0.05, 0.10, 0.15) polycrystalline ceramic samples are prepared by combining citrate acid sol-gel method with cold-pressing sintering method. The single-phase compositions and plate-like grain morphologies of all samples are confirmed by x-ray diffraction and field emission scanning electron microscope. The effects of lower concentration K doping on the thermoelectric properties of the material are evaluated systematically at high temperatures (300-1026 K). Low concentration K doping causes electrical conductivity to increase up to 23% with little effect on the Seebeck coefficient. Simultaneously, the thermal conductivity of K-doped sample is lower than that of the undoped sample, and the total thermal conductivity reaches a minimum value of approximately 1.30 W·m^{-1}·K^{-1}, which may be suppressed mainly by the phonon thermal conduction confinement. The dimensionless figure-of-merit ZT of Ca_{2.95}K_{0.05}Co_{4}O_{9} is close to 0.22 at 1026 K, representing an improvement of about 36% compared with that of Ca_{3}Co_{4}O_{9}, suggesting that lower concentration K-doped Ca_{3}Co_{4}O_{9} series materials are promising thermoelectric oxides for high-temperature applications.

In this work, the spin dynamics of a centrosymmetric WSe_{2} bilayer has been investigated by the two-color time-resolved Kerr rotation together with helicity-resolved transient reflectance techniques. Two depolarization processes associated with the direct transition are discovered at a low temperature of 10 K, with the characteristic decaying time of ~3.8 ps and ~20 ps, respectively. The short decay time of ~3.8 ps is suggested to be the exciton spin lifetime of the WSe_{2} bilayer, which is limited by the short exciton lifetime of the WSe_{2} bilayer and the rapid intervalley electron-hole exchange interaction between K^{+} and K^{-} valley in the same layer as that of monolayer. The long decay time of ~20 ps is suggested to be the spin lifetime of photo-excited electrons, whose spin relaxation is governed by the rapid intervalley scattering from the K valley to the global minimum Σ valley and the subsequent interlayer charge transfer in WSe_{2} bilayer. Our experimental results prove the existence of the spin-polarized excitons and carriers even in centrosymmetric transition metal dichalcogenides (TMDCs) bilayers, suggesting their potential valleytronic and spintronic device applications.

Electromagnetic screening is studied from the perspective of fluid mechanics by generalizing the Drude theory, which unifies three known results:Thomas-Fermi screening of the longitudinal field in both metals and superconductors, the skin effect of the transverse field in metals, and the Meissner effect in superconductors. In the special case of superfluid electrons, we slightly generalize the London equations to incorporate the longitudinal electric fields. Moreover, regarding the experiments, our study points out that the dynamical measurement may overestimate the superfluid density.

We study the stability of vortices pinning and dynamics in a superconducting thin strip containing a square array of antidot triplets by using the nonlinear Ginzburg-Landau (GL) theory. Compared with the regular square array of circular holes, the vortices are no longer pinned inside the circular holes, but instead stabilized at the center of the antidot triplets depending on the geometry parameters. Moreover, the influences of the geometry parameters and the polarity of the applied current on the current-voltage (I-V) characteristics are also studied. The critical current for the sample turning into a normal state becomes smaller when the hole diameter D is smaller and the spacing B between the holes is larger. Due to the asymmetric pinning sites, our numerical simulations demonstrate that the positive and negative rectified voltages appear alternately in the resistive state of the sample under an ac current of square pulses.

A new improved two-step method in fabricating Tl_{2}Ba_{2}CaCu_{2}O_{8} (Tl-2212) thin films is presented in this paper. In the first process of dc magnetron sputtering, the thallium content in the precursor film is largely increased by adjusting the ratio of thallium in the sputtering targets. After the second annealing process in the absence of additional thallium pellets or powder source, high-quality Tl-2212 thin films can be obtained. The proper content of thallium in the precursor film provides a relatively stable atmosphere to guarantee the growth of Tl-2212 film. This method avoids the repeated production of the thallium pellets in the post-annealing process, the repeatability and controllability of the experiment are greatly improved. X-ray diffraction (XRD) scans show that all of the sharp peaks of the sample films can be assigned to the (00l) peaks of Tl-2212 phase. The highest superconducting critical temperature (T_{c}) of the films is 105 K and the critical current density (J_{c}) can achieve 1.93 MA/cm^{2} in zero magnetic field at 77 K for a 600 nm film.

For static magnetic properties of the Co/Ni bilayers, macroscopic hysteresis loops and microscopic magnetic moment distributions have been determined by the object oriented micromagnetic framework (OOMMF). It is found that when the bilayer systems are fully decoupled, the magnetizations of the two phases reverse separately. The coercivity of the bilayers decreases to a valley value sharply with increasing interfacial exchange coupling and then rises slowly to a platform. On the other hand, we have carried out an atomistic simulation for the laser-induced ultrafast demagnetization of the Co/Ni bilayer. A larger damping constant leads to a faster demagnetization as well as a larger degree of demagnetization, which is consistent with the first-principle theoretical results. For the magnetization recovery process, the damping constant has different influences on the recovery time with various peak electron temperatures, which is ignored in previous atomistic simulations as well as the Landau-Liftshit-Bloch (LLB) micromagnetic calculations. Furthermore, as the interfacial exchange coupling increases, the ultrafast demagnetization curves for Co and Ni become coincident, which is a demonstration for the transition from two-phase phenomenon to single-phase phenomenon.

We suggest an experimental scheme that a single nitrogen-vacancy (NV) center coupled to a nearest neighbor ^{13}C nucleus as a sensor in diamond can be used to detect a static vector magnetic field. By means of optical detection magnetic resonance (ODMR) technique, both the strength and the direction of the vector field could be determined by relevant resonance frequencies of continuous wave (CW) and Ramsey spectrums. In addition, we give a method that determines the unique one of eight possible hyperfine tensors for an (NV-^{13}C) system. Finally, we propose an unambiguous method to exclude the symmetrical solution from eight possible vector fields, which correspond to nearly identical resonance frequencies due to their mirror symmetry about ^{14}N-Vacancy-^{13}C (^{14}N-V-^{13}C) plane.

We perform a proof-of-principle experiment that uses a single negatively charged nitrogen-vacancy (NV) color center with a nearest neighbor ^{13}C nuclear spin in diamond to detect the strength and direction (including both polar and azimuth angles) of a static vector magnetic field by optical detection magnetic resonance (ODMR) technique. With the known hyperfine coupling tensor between an NV center and a nearest neighbor ^{13}C nuclear spin, we show that the information of static vector magnetic field could be extracted by observing the pulsed continuous wave (CW) spectrum.

Based on density functional theory, first-principles calculation is applied to study the electronic properties of undoped and Ag-doped ZnO-Σ7 (1230) twin grain boundaries (GBs). The calculated result indicates that the twin GBs can facilitate the formation and aggregation of Ag substitution at Zn sites (Ag_{Zn}) due to the strain release. Meanwhile, some twin GBs can also lower the ionization energy of Ag_{Zn}. The density of state shows that the O-O bonds in GBs play a key role in the formation of a shallow acceptor energy level. When Ag_{Zn} bonds with one O atom in the O-O bond, the antibonding state of the O-O bond becomes partially occupied. As a result, a weak spin splitting occurs in the antibonding state, which causes a shallow empty energy level above the valence band maximum. Further, the model can be applied to explain the origin of p-type conductivity in Ag-doped ZnO.

The ceramics La_{0.85}Cr_{0.15}TiO_{x} and La_{0.7}Cr_{0.3}TiO_{x} are prepared by conventional solid-state reaction method. The dielectric properties of Cr-doped LaTiO_{x} as a function of frequency (0.1 kHz ≤ f ≤ 1 MHz) and temperature (77 K ≤ T ≤ 360 K) are studied. The blocks are then annealed in a flowing O_{2} or Ar/H_{2} to convert their oxygen content and the tests mentioned above are performed. The highly oxygenated samples exhibit extremely high low-frequency dielectric constants at room temperature (~10^{6}). The results show that the oxygen stoichiometry could significantly influence the dielectric properties of Cr-doped LaTiO_{x}.

We report a theoretical study of pumped spin currents in a silicene-based pump device, where two time-dependent staggered potentials are introduced through the perpendicular electric fields and a magnetic insulator is considered in between the two pumping potentials to magnetize the Dirac electrons. It is shown that giant spin currents can be generated in the pump device because the pumping can be optimal for each transport mode, the pumping current is quantized. By controlling the relevant parameters of the device, both pure spin currents and fully spin-polarized currents can be obtained. Our results may shed a new light on the generation of pumped spin currents in Dirac-electron systems.

This paper reports the plasmonic lasing of a split ring filled with gain material in water. The lasing mode (1500 nm) is far from the pump mode (980 nm), which can depress the detection noise from the pump light. The laser intensities of the two modes simultaneously increase by more than 10^{3} in amplitude, which can intensify the absorption efficiency of the pumping light and enhance the plasmonic lasing. The plasmonic lasing is a sensitive sensor. When a single protein nanoparticle (n=1.5, r=1.25 nm) is trapped in the gap of the split ring, the lasing spectrum moves by 0.031 nm, which is much larger than the detection limit of 10^{-5} nm. Moreover, the lasing intensity is also very sensitive to the trapped nanoparticle. It reduces to less than 1/600 when a protein nanoparticle (n=1.5, r=1.25 nm) is trapped in the gap.

In this study, we design periodic grille structures on a single homogenous thin plate to achieve anisotropic acoustic metamaterials that can control flexural waves. The metamaterials can achieve the bending control of flexural waves in a thin plate at will by designing only one dimension in the thickness direction, which makes it easier to use this metamaterial to design transformation acoustic devices. The numerical simulation results show that the metamaterials can accurately control the bending waves over a wide frequency range. The experimental results verify the validity of the theoretical analysis. This research provides a more practical theoretical method of controlling flexural waves in thin-plate structures.

INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Tailoring the electronic states of the AlN/diamond interface is critical to the development of the next-generation semiconductor devices such as the deep-ultraviolet light-emitting diode, photodetector, and high-power high-frequency field-effect transistor. In this work, we investigate the electronic properties of the semipolar plane AlN(1101)/diamond heterointerfaces by using the first-principles method with regard to different terminated planes of AlN and surface structures of diamond (100) plane. A large number of gap states exist at semi-polar plane AlN(1101)/diamond heterointerface, which results from the N 2p and C 2s2p orbital states. Besides, the charge transfer at the interface strongly depends on the surface termination of diamond, on which hydrogen suppresses the charge exchange at the interface. The band alignments of semi-polar plane AlN(1101)/diamond show a typical electronic character of the type-Ⅱ staggered band configuration. The hydrogen-termination of diamond markedly increases the band offset with a maximum valence band offset of 2.0 eV and a conduction band offset of 1.3 eV for the semi-polar plane N-AlN(1101)/hydrogenated diamond surface. The unique band alignment of this Type-Ⅱ staggered system with the higher CBO and VBO of the semi-polar AlN/HC(100) heterostructure provides an avenue to the development of robust high-power high-frequency power devices.

We investigated phase transitions in a diblock copolymer-homopolymer hybrid system blended with nanorods (NRs) by using the time-dependent Ginzburg-Landau theory. We systematically studied the effects of the number, length and infiltration properties of the NRs on the self-assembly of the composites and the phase transitions occurring in the material. An analysis of the phase diagram was carried out to obtain the formation conditions of sea island structure nanorod-based aggregate, sea island structure nanorod-based dispersion, lamellar structure nanorod-based multilayer arrangement and nanowire structure. Further analysis of the evolution of the domain sizes and the distribution of the nanorod angle microphase structure was performed. Our simulation provides theoretical guidance for the preparation of ordered nanowire structures and a reference to improve the function of a polymer nanocomposite material.

Time-dependent density functional theory (TDDFT) method is used to investigate the details of the excited state intramolecular proton transfer (ESIPT) process and the mechanism for temperature effect on the Enol^{*}/Keto^{*} emission ratio for the Me_{2}N-substited flavonoid (MNF) compound. The geometric structures of the S_{0} and S_{1} states are denoted as the Enol, Enol^{*}, and Keto^{*}. In addition, the absorption and fluorescence peaks are also calculated. It is noted that the calculated large Stokes shift is in good agreement with the experimental result. Furthermore, our results confirm that the ESIPT process happens upon photoexcitation, which is distinctly monitored by the formation and disappearance of the characteristic peaks of infrared (IR) spectra involved in the proton transfer and in the potential energy curves. Besides, the calculations of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) reveal that the electronegativity change of proton acceptor due to the intramolecular charge redistribution in the S_{1} state induces the ESIPT. Moreover, the thermodynamic calculation for the MNF shows that the Enol^{*}/Keto^{*} emission ratio decreasing with temperature increasing arises from the barrier lowering of ESIPT.

The mechanism of terahertz (THz) pulse generation with a static magnetic field imposed on a gas plasma is theoretically investigated. The investigation demonstrates that the static magnetic field alters the electron motion during the optical field ionization of gas, leading to a two-dimensional asymmetric acceleration process of the ionized electrons. Simulation results reveal that elliptically or circularly polarized broadband THz radiation can be generated with an external static magnetic field imposed along the propagation direction of the two-color laser. The polarization of the THz radiation can be tuned by the strength of the external static magnetic field.

The conventional magnetic resonance imaging (MRI) equipment cannot measure large volume samples nondestructively in the engineering site for its heavy weight and closed structure. In order to realize the mobile MRI, this study focuses on the design of gradient coil of unilateral magnet. The unilateral MRI system is used to image the local area above the magnet. The current density distribution of the gradient coil cannot be used as a series of superconducting nuclear magnetic resonance gradient coils, because the region of interest (ROI) and the wiring area of the unilateral magnet are both cylindrical side arc surfaces. Therefore, the equivalent magnetic dipole method is used to design the gradient coil, and the algorithm is improved for the special case of the wiring area and the ROI, so the X and Y gradient coils are designed. Finally, a flexible printed circuit board (PCB) is used to fabricate the gradient coil, and the magnetic field distribution of the ROI is measured by a Gauss meter, and the measured results match with the simulation results. The gradient linearities of x and y coils are 2.82% and 3.56%, respectively, less than 5% of the commercial gradient coil requirement.

A unique method is proposed to encapsulate solar cells and improve their power conversion efficiency by using a millimeter-sized cylindrical lens array concentrator. Millimeter-sized epoxy resin polymer (ERP) cylindrical lens array concentrators are fabricated by the soft imprint technique based on polydimethylsiloxane stamps. The photovoltaic measurements show that millimeter-sized ERP cylindrical lens array concentrators can considerably improve the power conversion efficiency of silicon solar cells. The validity of the proposed method is proved by the coupled optical and electrical simulations. The designed solar cell devices with the advantages of high-efficiency and convenient cleaning are very useful in practical applications.

A series of organic solar cells (OSCs) are prepared with PTB7:PC_{71} BM as the host materials and DIB-SQ as the third component. The power conversion efficienty (PCE) of OSCs can be improved from 6.79% to 7.92% by incorporating 6 wt% DIB-SQ into donors, resulting from the enhanced short circuit current density (J_{SC}) and fill factor (FF). The increased J_{SC} of the optimized ternary OSCs should be attributed to the enhanced photon harvesting of teranry active layer by incorporating DIB-SQ. Meanwhile, FF of the optimized ternary OSCs should be due to the optimied phase separation. The open circuit voltage (V_{OC}) of tenray OSCs can be maintained at a constant of 0.75 V, indicating that all photogenerated holes willl be transported along the channels formed by PTB7.

The angular light-scattering measurement (ALSM) method combined with an improved artificial bee colony algorithm is introduced to determine aerosol optical constants and aerosol size distribution (ASD) simultaneously. Meanwhile, an optimized selection principle of the ALSM signals based on the sensitivity analysis and principle component analysis (PCA) is proposed to improve the accuracy of the retrieval results. The sensitivity analysis of the ALSM signals to the optical constants or characteristic parameters in the ASD is studied first to find the optimized selection region of measurement angles. Then, the PCA is adopted to select the optimized measurement angles within the optimized selection region obtained by sensitivity analysis. The investigation reveals that, compared with random selection measurement angles, the optimized selection measurement angles can provide more useful measurement information to ensure the retrieval accuracy. Finally, the aerosol optical constants and the ASDs are reconstructed simultaneously. The results show that the retrieval accuracy of refractive indices is better than that of absorption indices, while the characteristic parameters in ASDs have similar retrieval accuracy. Moreover, the retrieval accuracy in studying L-N distribution is a little better than that in studying Gamma distribution for the difference of corresponding correlation coefficient matrixes of the ALSM signals. All the results confirm that the proposed technique is an effective and reliable technique in estimating the aerosol optical constants and ASD simultaneously.

We used a weather research and forecasting model to simulate a torrential rainstorm that occurred in Xinjiang, China during June 16-17, 2016. The model successfully simulated the rainfall area, precipitation intensity, and changes in precipitation. We identified a clear wave signal using the two-dimensional fast Fourier transform method; the waves propagated westwards, with wavelengths of 45-20 km, periods of 50-120 min, and phase velocities mainly concentrated in the -25 m/s to -10 m/s range. The results of wavelet cross-spectral analysis further confirmed that the waves were gravity waves, peaking at 11:00 UTC, June 17, 2016. The gravity wave signal was identified along 79.17-79.93°E, 81.35-81.45°E and 81.5-81.83°E. The gravity waves detected along 81.5-81.83°E corresponded well with precipitation that accumulated in 1 h, indicating that gravity waves could be considered a rainstorm precursor in future precipitation forecasts.

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