The pressure wave emitted by a pulsating bubble affects the motions of other bubbles, so in an acoustic field bubbles are in a state of coupled oscillation. In this paper, a cluster with cavitation bubbles inside is considered, and a mathematical model is developed to describe the dynamics of the bubbles of the same radius inside a spherical cluster when the effects of coupled oscillation are included. Based on this new model, the nonlinear acoustic response of cavitation bubbles is analyzed numerically. Comparison of our model with those in the literature, shows that bubbles are suppressed heavily. Because of the coupled oscillations of bubbles, the motions of a bubble are affected by more constraints in the system, which cause the decrease of natural frequency of the bubbles. The nonlinear acoustical response of bubbles is improved by the coupled oscillation in a bubble cluster. With the rise in number density of the cluster, the suppression of bubble oscillation is enhanced. For a cluster of 1 mm radius, when the bubble number is below 500, the change of bubble number may cause a sharp decrease of maximum radial displacement of the bubbles. In cavitation region, there are bubble clusters and large-sized bubble, and the moving large bubble can absorb small bubbles from the surface of bubble cluster, so the bubble numbers inside a cluster varies with time, which may change the acoustic response of coupled oscillating bubbles. The increase of the liquid static pressure can suppress the oscillation of bubbles too, and there is a sensitive region (1-2 atm) that affects remarkably the acoustical response of bubbles. Driving ultrasound can affect the motion of bubble greatly. The range of cavitation bubble size is narrowed when the wave frequency increases. The bubbles whose initial radii are close to 5 μm are easy to be activated by ultrasound under given acoustic conditions, i.e. sizes of bubble cluster, surrounding liquid and inner gas. The cluster oscillation of bubbles may suppress the motion of individual bubbles, and weaken the cavition effects caused by individual bubbles. However, the collapse time of the bubbles may be delayed, and the cavitation region may become larger than that for a single bubble. As a result, cavitation effects are amplified in the cluster region.

In granular materials, particles constitute a complex force chains network through contact with each other, and elastic energies are stored due to deformation of particles. This elastic behavior is macroscopic manifestation of inter-particle contacts. Elastic constants or elastic moduli are of fundamental importance for granular material. Due to the hyper-static property of inter-particle forces, the bulk elastic energy stored in the contacts is metastable in the viewpoint of energy landscape, i.e. a high energy state may approaches a more stable state (i.e. relatively lower state) under the action of external perturbations or internal stress, resulting in the elastic modulus reduction. This process is the so-called elasticity relaxation. It may be more obvious in granular materials.
The time-dependent behavior of granular materials, especially the creep, has been studied in experiments and numerical simulations, while the stress relaxation has few reported investigations. Stress relaxation is defined as the process in vohich the initial strain is maintained and the stress decays with the time. From energetic viewpoint, elastic energy is stored in the deformation of particles. The granular system is in a metastable state when confined in a state easy to break the balance. Generally speaking, the shape and grading of particles, volume fraction, surface friction properties, initial structure features, ageing time, loading strain rate will all play important roles in stress relaxation.
In this work, it is believed that the elastic relaxation is the only mechanism to describe the stress relaxation, and the mechanism of it is analyzed from the viewpoint of the potential energy surface. Stress relaxation is calculated by means of the so-called two-granular temperature theory (TGT) we developed previously (Sun Q et al. 2015 Sci. Rep. 5 9652). The stress decays fast at the beginning, then decreases gradually slowly to a stable value. The logarithmic fit is first proposed to describe the stress decay in the compressed system. Calculated results of stress relaxation match well with the measured results in a recently published paper (Miksic A, Alava M J 2013 Phys. Rev. E 88 032207). Both elastic energy and granular temperature may be reduced with increasing time. It is found that the initial value of the granular temperature has a great influence on the stress relaxation, and at present its effect is input by trial and error. It would be a major problem how to determine the initial value of the granular temperature. Moreover, the relaxation coefficient of elastic stress is basically chosen as a function of granular temperature which is described by the Arrhenius equation that need be further investigated.

Hafnium carbides (Hf-C system), known as ultra-high temperature ceramics, have attracted growing attention because of their unique features. In this paper, we carry out researches on the stable crystal structures in the Hf-C system at high pressures, using a variable-composition ab initio evolutionary algorithm implemented in the USPEX code. In addition to the ambient-pressure structures HfC (Fm3m), there are two new compounds Hf₃C₂ and Hf₆C₅ and two high-pressure structures of HfC. When pressures are lower than 100 GPa, no new structures are found other than those at ambient pressure, and Hf₃C₂ and Hf₆C₅ become metastable at 20 GPa and 100 GPa, respectively. At 200 GPa, a new compound Hf₂C is found, and the stable structure HfC has changed from Fm3m to C2/m. At 300 GPa, another new compound HfC₂ is found. At 400 GPa, the stable structure of HfC has changed again to the space group Pnma. And at 500 GPa, the stable structures are Hf₂C, HfC₂ and HfC (Pnma), no new structures are found except those at 400 GPa. The composition-pressure phase diagram that shows the pressure range of stable structures in Hf-C system is simulated by calculation of their enthalpies. When the pressures are lower than 15.5 GPa and 37.7 GPa, Hf₃C₂ and Hf₆C₅ are stable, respectively, and their space groups are both of C2/m. And Hf₂C and HfC₂, with space group I4/m and Immm, respectively become stable structures when the pressure is higher than 102.5 GPa and 215.5 GPa, respectively. The phase-transition route of HfC is Fm3m→C2/m→Pnma, and the two phase-transition pressures are 185.5 GPa and 322 GPa, respectively, which are different from the conclusion of Zhao. Then we will show and discuss the newly predicted high-pressure structures and their crystallographic data, such as volume, lattice constants and atom positions. The crystal structures of HfC are described in the literature. The structure of Hf₂C contains 12 atoms in the conventional cell, and carbon atoms lie at the center of decahedron consisting of 8 hafnium atoms. In the structure of HfC₂, carbon atoms form the quasi-graphite sheets and hafnium atoms lie betweent the two sheets. The dynamical and mechanical stabilities of the high-pressure structures have been verified by calculations of their phonon dispersion curves and elastic constants. And the bulk modulus and shear modulus of HfC₂ are larger than those of the other three high-pressure structures. Finally we will study their electronic properties, band structures, density of states (DOS), electron localization functions (ELFs), and the Bader charge analyses of these structures are simulated based on the first-principle. The band structure and density of states show that these four high-pressure structures have weak metallic and strong Hf-C covalent bond. The Bader charge analysis further proves the strong Hf-C covalent bond and weak ionic bond. And ELF shows the existence of C—C covalent bond. In summary, the Hf—C bond shows strong covalence, weak metallicity and ionicity, and the C—C bond is covalent.

The study of physical properties of silicon nano-materials is very important for its application in semiconductor technology. Doping is beneficial to improving the physical properties of silicon nano-materials, it can improve the application value as well. Young's modulus of the crystal in the direction of [100] of the doped silicon nano-film is studied by an analytical model, which is based on the semi-continuum approach. In the model, the strain energy is obtained from the Keating strain energy model. The relationship between the Young's modulus and film thickness are also discussed. Results show that the Young's modulus decreases with the increase of the thickness of the silicon film, especially with the small size; the variation tendency of the Young's modulus of doped silicon films is similar to the pure silicon film. And the Young's modulus decreases as the doping concentration decreases for different doping position. Neither the doping concentration nor the doping position, it is the thickness that shows the most important effect on the Young's modulus of the doped silicon nano-film. Findings in this paper may serve as a reference for similar study, and can offer a totally new idea of the doped monocrystalline silicon materials as well.

As is well known, a solid oxide fuel cell(SOFC) is a device that can convert chemical energy directly into electrical energy. The possibility to grow thin films of oxide materials is of great importance in the development of SOFC. Recently, it is expected that oxide heterostructures, with almost ideal interfaces, should lead to the interesting artificial materials with some novel properties. So we have prepared superlattice electrolyte thin films.
On the MgO single-crystal substrates, multilayer epitaxial thin-film heterostructures of 20 mol% samarium-oxide-doped ceria (Ce_0.8Sm_0.2O_2-δ, SDC) and 8 mol% yttria-stabilized zirconia (Y₂O₃: ZrO₂YSZ) have been deposited in turn, using the pulsed laser deposition (PLD) technique. Five different superlattices (SDC/YSZ)_N (N=3, 5, 10, 20, 30) films are fabricated, keeping the total thickness constant (300 nm), but with a different number of hetero-interfaces. The choice of coupling SDC and YSZ aims to have both layers in the superlattices made of oxygen-ion conductors with compatible crystallographic features. On one hand, we have to remove any potential contribution of the deposition substrate to the total conductivity, and the superlattices may be grown on <110>-oriented MgO single-crystalline wafers. On the other hand, the YSZ electrolyte film requires that the temperature should be higher, and SDC as the electrolyte leakage; both should be combined with each other, so as to have complementary advantages. In this way, not only the temperature of SOFCs is reduced, but also the leakage avoided. Because YSZ and SDC both belong to cubic fluorite structure and have similar lattice parameters, the lattice constant of YSZ is 5.14 Å, and the lattice constant of SDC is 5.44 Å. Compatible crystal properties, oxygen ion conductivity, and great matching defects can lead to a semi-coherent interface between the ZrO₂ and CeO₂ layers. The interface effect is obvious. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and AC impedance mearurements are used to characterize the surface morphology, phase structure and electrical properties of the electrolyte film. Results show that the superlattice (SDC/YSZ)_N electrolyte films will form obviously interface and better superlattice structure. This kind of thin films do not show crack, with no element diffusion at the interfaces, growing uniformly, densely and smoothly. Electrochemical measurements show a sizable increase in conductivity with increasing number of SDC/YSZ interfaces. So it is an ideal low-temperature fuel cell electrolyte materials.

We investigate theoretically the electromagnetic propagation properties of graphene plasmons in a comb-like dielectric-graphene-dielectric (DGD) waveguide. The effective index of surface plasmon mode supported by the waveguide is analysed numerically, and it is found that the effective refractive index increases with the refractive index of the dielectric and decreases with Fermi energy of the graphene sheet. For a comb-like DGD waveguide with a finite branch length, a subwavelength plasmon filter can be formed by Fabry-Perot resonance caused by the reflection of the guided mode at the branch. The central frequencies of the gaps can be changed by varying the length of the branch, Fermi energy, the refractive index of the dielectric and the layer number of graphene sheets. The analytic and simulated result reveals that a novel nanometric plasmonic filter in such a comb-shaped waveguide can be realized with ultracompact size in a length of a few hundred nanometers in the mid-infrared range. We find that the frequencies of the stopband increase with Fermi energy and the layer number of graphene sheets, while will they decrease nonlinearly with the length of the branch and the refractive index of the dielectric. In addition, the width of the gap can be increased with the number of comb branches. Such electromagnetic properties could be utilized to develop ultracompact photonic filters for high integration.

In this paper, experimental results are reported about the new Al_0.25Ga_0.75N/GaN high electron mobility transistor (HEMT) with a step AlGaN layer. The rule of 2DEG concentration variation with the thickness of AlGaN epitaxial layer has been applied to the new AlGaN/GaN HEMTs: The step AlGaN layer is formed at the gate edge by inductively coupled plasma etching, the 2DEG concentration in the etched region is much lower than the other parts of the device. A new electric field peak appears at the corner of the step AlGaN layer. The high electric field at the gate edge is decreased effectively due to the emergence of the new electric field peak, and this optimizes the surface electric field of the new AlGaN/GaN HEMTs. The new devices have the same threshold voltage and transconductance as the conventional structure, -1.5 V and 150 mS/mm. That means, the step AlGaN layer does not affect the forward characteristics of the AlGaN/GaN HEMTs. As the more uniform surface electric field distribution usually leads to a higher breakdown voltage (BV), with the same gate to drain length L_GD=4 μm, the BV can be improved by 58% for the proposed Al_0.25Ga_0.75N/GaN HEMTs as compared with the conventional structure. At V_GS=1 V, the saturation currents (I_sat) is 230 mA/mm for the conventional Al_0.25Ga_0.75N/GaN HEMT and 220 mA/mm for the partially etched Al_0.25Ga_0.75N/GaN HEMT (L_Etch=4 μm, L_GD=4 μm). The decrease of I_sat is at most 10 mA/mm. However, as the BV has a significant enhancement of almost 40 V, these drawbacks are small enough to be acceptable. During the pulse I-V test, the current collapse quantity of the conventional structure is almost 40% of the maximum I_DS(DC), but this quantity in the new devices is only about 10%, thus the current collapse effect in Al_0.25Ga_0.75N/GaN HEMTs has a significant remission for a step AlGaN layer. And as the high electric field peak at the gate edge is decreased, the effect of the gate electrode on electron injection caused by this electric field peak is also included. The injected electrons may increase the leakage current during the off-state, and these injected electrons would form the surface trapped charge as to decrease the 2DEG density at the gate. As a result, the output current and the transconductance would decrease due to the decreased electron density during the on-state. That means, with the region partially etched, the electron injection effect of the gate electrode would be remissed and the stability of Schottky gate electrode would be improved. In addition, due to the decrease of the high electric field at the gate edge, the degradation of the device, which is caused by the high electric field converse piezoelectric effect, will be restrained. The stability of the partially etched AlGaN/GaN HEMT will become better.

Memristor is a nanoscale element with low power consumption and high integration, having great potential in applications. A single memristor has rich electrical properties, and its series-parallel circuit exhibits more abundant dynamic behaviors. However, memristors' coupled effects cannot be ignored in high-density integrated environment. Therefore, this paper first deduces the mathematical model of coupled memristor in detail based on the coupled flux controlled memristors. Second, considering the different polarity connection and coupling strength, we discuss the coupled condition of two flux-controlled memristors in series and parallel connections. Then the detailed theoretical analysis is illustrated, and the variation of memristance in terms of voltage, time and flux as well as the relations between voltage and currents are examined via numerical simulations to further explore the influence of coupled effects on the memristive system. At the same time, a graphical user interface of series-parallel coupled circuit based on Matlab is designed. Through this interface, we can adjust the initial value of memristor and coupling coefficient, select different connection modes, obtain corresponding connection diagram and output waveform which intuitively show the dynamic behavior of different parameters directly and provide experimental reference for further study of the circuit design. Furthermore, this paper shows the influence of initial value on the normal working range of memristors in the presence of coupling. From the table 1 it can be easily obtained that when the memristors are connected in the same direction, the range of memristance without coupling is greater than that with coupling. And the situation is opposite when the memristors are connected in different directions. Finally, the hysteresis curve with different coupling coefficients and the change of memristance with time are shown via building the Pspice simulator of coupled memristors, so the coupling effects of memristor is confirmed by circuit simulations. Experimental results reflect that the coupling with the same polarity enhances the change of resistance, and the coupling with different polarity with slow down it. Such dynamical properties can be well utilized in memristive networks and provide a strong theoretical basis for the comprehensive consideration of the design of memristive system.

Many modern electronic devices are operated on a frequency above 1 GHz. Frequencies of electromagnetic noises coming from these devices are usually larger than 10 GHz. High-frequency magnetic losses in the natural resonance mechanism can be used to dissipate the energy of electromagnetic noises. Ferromagnetic nanostructural materials (nano flakes or nanowires) in strong shape anisotropy fields are one of the promising anti electromagnetic interference (EMI) materials due to their large high-frequency magnetic losses. Application of EMI requires that the electromagnetic wave absorbing materials should be lightweight and have a wide absorbing bandwidth. However, most electromagnetic wave absorbing materials reported do not have these features. To meet these demands, the microwave magnetic properties of porous α-Fe nano flakes (length × width × thickness: 300 nm × 100 nm × 10 nm) have been simulated based on micromagnetics theory. Compared to the nano flakes without nano pores, simulation results reveal that the demagnetization fields will be altered if a nano flake contains several pores. Effect of nano pores (diameter =15 nm) in different arrangements (rows × columns: 2×10; 2×5; 2×2; 4×5) on the high-frequency magnetic properties is investigated in this paper. It is found that nano flakes can alter the configurations of magnetic domains. More domains in small sizes in an inhomogeneous localized magnetic anisotropic field have been achieved. Consequently, more high-frequency magnetic loss peaks can be found. Overlapping of magnetic loss peaks implies that it potentially enables to widen the bandwidth of electromagnetic absorption within 10–30 GHz. Furthermore, simulations reveal that the quantity, magnitude and resonance frequencies of the loss peaks are strongly dependent on the quantity and the arrangement of nano pores. Besides, the existence of multi magnetic loss peaks has been studied for ellipsoid objects from the perspective of inhomogeneously localized effective magnetic fields. Results reveal that the frequently observed wide magnetic loss peaks in experimental data may be due to the inhomogeneously localized effective magnetic fields of an absorber containing a plentiful of randomly oriented particles. Clearly, compared to the nano flakes without pores, the nano flakes with pores can significantly reduce the volume density. Therefore, our simulation results show that porous nano flakes can be a good lightweight electromagnetic wave absorber candidate with wide absorbing bandwidths.

In order to achieve the requirement of rapid growth of the magnetic storage density, the slider-disk spacing needs to be reduced to less than 2 nm. However, the slider-disk contact can easily occur within such a narrow spacing, and eventually result in the loss of the stored data in the magnetic recording film, i.e., demagnetization of the magnetic disk. Therefore, research into the magnetomechanical relationship related to the slider-disk contact demagnetization is significantly important to identify the demagnetization mechanism and further improve the anti-demagnetization performance of the magnetic disk. In this study, the nanoscratch experiment and the magnetic force microscope technology are used to investigate the magnetomechanical behavior induced by the slider-disk contact. And according to the phase imaging principle of the magnetic force microscope, the relationship between the information intensity of the magnetic recording layer and the magnetic contrast measured by the magnetic force microscope is found. Thus, a quantitative analysis method is proposed, which is different from the previous qualitative observation of the magnetic domain change. Experimental results show that the critical demagnetization load during the slider-disk contact is 120 μup N. When the slider-disk contact force exceeds the critical demagnetization load, the increase of slider-disk contact force can lead to the decrease of the information intensity of the magnetic recording layer. And the decay rate of the information intensity will be rapidly enhanced after the slider-disk contact force reaches 380 μup N. Moreover, the variation trend of the information intensity with the depth of the residual scratch is the same as that of the information intensity with the slider-disk contact force. Specially, before the slider penetrates the hard carbon layer of the magnetic disk, the slider-disk contact demagnetization still may occur, corresponding to the load cases from 120 μup N to 200 μup N. In addition, for any slider-disk contact force, the area of the surface damage of the hard carbon layer is always greater than that of the demagnetization of the magnetic recording layer. This phenomenon is related to the elasto-plastic force fields in the hard carbon layer and the magnetic recording layer. Moreover, when the slider repeatedly scratches the same location on the surface of the magnetic disk, the information intensity of the magnetic recording layer will decrease with the increase of scratching number. After the scratching number is beyond 20, the elastic shakedown status may occur in the magnetic recording layer, and correspondingly, the information intensity of the magnetic recording layer can be close to a constant value. This result is derived from the work hardening process during the slider-disk repeatedly scratching.

It is of great significance to research on methods for obtaining the initial magnetization curve, the important magnetic property of ferromagnetic materials. In the existing methods, a time-varying magnetic field is adopted as the excitation field. To obtain the initial magnetization curve, magnetic field and induced magnetic flux density in the specimen have to be measured step-by-step as the excitation field changes, and this is inefficient. Thus, a calculation method for initial magnetization curve based on time-space transformation is proposed in this paper. In this method, an elongated rod or a circular ring is used as the specimen. A spatially varying magnetic field generated by constant magnetization is utilized as the excitation field. The strength of the excitation field changes with the spatial positions of the specimen. Under the action of the excitation field, the magnetic field strength within the specimen is calculated by means of the responding magnetic field strength on the surface of the specimen according to the continuity of the tangential magnetic field strength. While, based on the Gauss' law for magnetism, the law of approach to saturation and the basic equation of magnetization curve in Rayleigh region, the induced magnetic flux density within the specimen can be calculated from the responding magnetic flux density on the surface of the specimen. After obtaining the magnetic field strength and magnetic flux density in the specimen, the initial magnetization curve can be obtained. To verify theoretically the correctness of the method, simulations are carried out with an elongated rod and a circular ring. In experiments, a spatially varying magnetic field generated by DC coils is applied on the specimen as the excitation field. The initial magnetization curve calculated from the magnetic field strength and magnetic flux density on the surface of the specimen is similar to the known initial magnetization curve. Experimental results also show that when adopting an elongated rod rather than a circular ring as the specimen, this calculation method for initial magnetization curve is simpler and the error in the results is smaller, which are different from those obtained by existing measurement methods for initial magnetization curve. In addition, in order to study the influence of the limiting factors in practical applications of the calculated results, further research is conducted based on the simulation data. Results show that when choosing a proper elongated rod as the specimen, the initial magnetization curve can be calculated from the magnetic field strength and magnetic flux density on the surface of the specimen under the constant magnetization, also the induced magnetic field flux in the specimen does not have to be measured by the encircling detecting coil which makes this method easy to operate. Namely, this method is feasible in practice. This paper may be a theoretical guidance for exploring new measurement methods for initial magnetization curve.

Electret has caught wide attention because it can produce a lasting and stable electrostatic field in the application of MEMS devices such as miniwatt electret generator, electret motor, electret sensor, electret transducer, and so on. Of all the above applications, a remarkable feature is that the electrostatic field distribution on electret surface is patterned in millimeter size or even smaller. However, the charge storage performance of electret in miniature size will dramatically get worse in contrast with the macro-electret. Therefore, it is very important to develop an applicable preparing method to maintain the stability of electrostatic field distribution in micro-patterned electret. In this paper, it is reported that a fluorinated ethylene propylene copolymer (FEP) with evaporated aluminum grid electrode a 25 μm thickness topped with at a step of 2 or 3 millimeter are successfully prepared to form the electret with well grid distribution of electric field(abbreviated as grid electret) by means of corona charging and thermal charging technology. Effect of grid width and charging temperature on the charge storage performance is studied. After stored for 150 days, the grid distribution of electric field on the FEP surface becomes clear and organized. The potentials of the area covered by aluminum electrode are close to zero, while that of uncovered area still remain high. The potential differences between the covered and uncovered by aluminum electrode area are identical in different charging methods, it is 110 V (electric field 44 kV/cm) for the sample with an electrode width of 2 mm, and 130 V (electric field 52 kV/cm) for the sample with an electrode width of 3 mm. Results also show that the initial surface potentials of the grid electrets prepared by corona charging is higher than that by thermal charging, but the former decays more rapidly. For the same charging method, the narrower the aluminum electrode area can lead to the lower initial surface potential, and the higher charging temperature causes the larger initial surface potential. According to the principle of corona charging and thermal charging technology it is concluded that the difference of charge storage capability between FEP and aluminum can account for the grid distribution of electric field on the FEP surface.

Electret attracts increasing attention nowadays because of its lasting and stable electrostatic field. To achieve the widespread use of electret material, higher density and better stability of the electret charge storage as well as well-distributed electrostatic field must be ensured at the same time. Based on the mechanism of interface polarization on double-layer media, a novel charging technology for electret is reported in this paper; and PP film is successfully charged through an auxiliary layer to form electret by this proposed method. Effect of charging temperature and charging voltage on the charge storage performance of the as-prepared PP film electret is investigated by means of surface potential measurement. Also its charge storage performance at high temperatures is explored by thermally stimulated discharge technique. Furthermore, its electrostatic field distribution in the directions of X and Y is measured. Results show that the interface polarization charging is more excellent than the corona charging. At a certain temperature, the surface potential of PP film electret increases with increasing charging voltage and both are in a good linear relationship. This is in good agreement with the theoretical analysis in terms of the equation of electret charge accumulation during the charging process. It is shown that in the case of constant charging voltage within the range of 0.5–3.0 kV, the effect of charging temperature is not obvious when the temperature is below 75 ℃; however, when the temperature is higher than 75 ℃, the surface potential of PP film electret increases with increasing temperature. In addition, its surface potential may change a little with time so it has an excellent charge storage stability. The distribution of its surface potential shows that it exhibits an homogeneous electrostatic field due to interface polarization charging.

Graphene exhibits excellent ultrafast optical properties due to its unique electronic structure. In this paper we investigate theoretically the ultrafast dynamic optical properties of graphene based on the Bloch-equations, and introduce the theoretical model of graphene. First, we give the energy which has a linear relationship with the wave vector k. The behavior of electrons in the vicinity of the two Dirac points can be described by the massless Dirac-equation, thus we have the Dirac equation of graphene. Second, we discuss the interaction between graphene and light field. The Bloch-equations of graphene are obtained through the Heisenberg equation and then we discuss the photon carriers,electric polarization and optical current change over time by analyzing the Bloch-equations. It is found that the nonequilibrium carriers in graphene induced by a terahertz field can be built in 20-200 fs due to the Pauli blocking and the conservation of energy principle. The photon carrier density will increase with the frequency of enhanced light field. Thus an optical current can be created rapidly within 1 ps. A graphene system responds linearly to the external optical field for √2ev_FE₀t<<ħω, while the graphene systems respond nonlinearly to the external optical field, where E₀ and ω are respectively the intensity and the frequency of the light, t is the time and v_F the Dirac velocity in graphene. The electric polarization and optical current increase with increasing photon energies. These theoretical results are in agreement with recent experimental findings and indicate that graphene exhibits important features and has practical applications in the ultrafast optic filed, especially in terahertz field.

Polarization-insensitive metasurfaces are of great value in practical applications. In this paper, we present a polarization-insensitive reflective phase-gradient metasurface operating in optical communication band which has almost the same electromagnetic (EM) responses for both x-and y-polarized incident waves with high-efficiency anomalous reflection.
The reflective metasurface employs a typical metal (Au)-insulator (SiO₂)-metal (Au) structure, in which the top metal layer consists of periodic arrays of isotropic cross-shaped gold antennas, i.e. unit cells. The supercell of the metasurface is composed of five unit cells with their dimensions different from each other. The normally incident waves are reflected by the metal-grounded plane, but the reflection phases of both x-and y-polarized waves are controlled by changing the dimensions of their unit cells. Based on the finite-difference time-domain simulations, we investigate the polarization-dependent EM responses of this metasurface under the illumination of linearly polarized incident plane waves. Selecting carefully five cross-shaped gold antennas in different dimensions, we obtain polarization-insensitive metasurface with high-performance anomalous reflection in optical communication band.
First, in order to investigate the polarization sensitivity of the proposed metasurface, we study the EM responses for x-and y-polarized incident waves, since arbitrary linearly-polarized EM waves can be separated into two orthogonally-polarized components. We find that the two orthogonally-polarized incident EM waves have almost the same phase and amplitude response with the phase nearly linearly changing from 0 to 2πup within a supercell, hence a constant gradient of phase discontinuity is introduced and anomalous reflection will occur. We further analyze the reflected electric-field patterns and the far-field intensity distributions, from which we find that the reflected beams exhibit low-distortion wavefronts and the scattered light is predominantly reflected into the anomalous mode. As a consequence, high-efficiency anomalous reflection is realized, with a 70% reflectivity at the operating wavelength of 1480 nm. Furthermore, we look into the incident-angle dependence of the proposed metasurface, and find that the designed metasurface can exhibit polarization insensitivity within a broad incident angle ranging from -30° to 0°.
In conclusion, we propose a broad-angle polarization-insensitive reflective gradient metasurface with high-efficiency anomalous reflection, which has potential applications in optical communications, signal processing, displaying, imaging and other fields.

A large amount of tin oxide (SnO₂) nanowire arrays were synthesized on the flexible conductive carbon fiber substrate by thermal evaporation of tin powders in a tube furnace. The temperature, as well as the flow rate of the carrier N₂ gas and the reaction O₂ gas, plays an important role in defining the morphology of the SnO₂ nanowires. Morphology and structure of the as-grown SnO₂ samples are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Results show that all the samples possess a typical rutile structure, and no other impurity phases are observed. The morphology changes from rod to wire with the increase of reaction temperature. Ratio of length to diameter of the nanowires increases first and then decreases with the flow ratio of N₂/O₂ gas. The optimum synthesis conditions of SnO₂ nanowire are: reaction temperature 780 ℃, N₂ and O₂ flow rates being 300 sccm and 3 sccm respectively. In our growth process, the nanowire grows mainly due to the vapor-liquid-solid (VLS) growth process, but both the VLS process and surface diffusion combined with a preferential growth mechanism play the important role in morphology evolution of the SnO₂.
Field emission measurements for Samples 1-6 are carried out in a vacuum chamber and a diode plate configuration is used. Relationship between the growth orientation, aspect ratio, density and uniformity of the arrays and field emission performances will be investigated first. Results reveal that the field emission performance of SnO₂ nanostructures depends on their morphologies and array density. The turn-on electric field (at the current density of 10 μupA/cm²) decreases and the emission site density increases with tin oxide array density, and the turn-on electric field of Sample 5 (synthesized at 780 ℃, nitrogen and oxygen flow rates being 300 sccm and 3 sccm respectively) is about 1.03 V/μm at a working distance of 500 μm. By comparison, for the turn-on electric fields of the not well-aligned SnO₂ nanowire arrays we have 1.58, 2.13, 2.42, 1.82, and 1.97 V/μm at 500 μm. These behaviors indicate that such an ultralow turn-on field emission and marked enhancement in β (～ 4670) can be attributed to the better orientation, the good electric contact with the conducting fiber substrate where they grow, and the weaker field-screening effect. Our results demonstrate that well-aligned nanowire arrays, with excellent field-emission performance, grown on fiber substrate can provide the possibility of application in flexible vacuum electron sources.

Hydrogen is considered as a potentially ideal substitution for fossil fuels in the future sustainable energy system because it is an abundant, clean and renewable energy carrier. A safe, efficient and economic storage method is the crucial prerequistite and the biggest challenge for the wide scale use of hydrogen. The nanomaterial is one of the most promising hydrogen storage materials because of its high surface to volume ratio, unique electronic structure and novel chemical and physical properties. It has been demonstrated that pristine nanostructures are not suitable for hydrogen storage, since they interact weakly with hydrogen molecule and their hydrogen storage density is very low. However, the hydrogen storage capacity of the nanostructures can be significantly enhanced through substitutional doping or decoration by metal atoms. Using density functional theory, we investigate the properties of hydrogen adsorption on Li-decorated C₂₄clusters. Results show that the preferred binding site for Li atom is the pentagonal rings. The interaction of Li atoms with the clusters is stronger than that among Li atoms, thus hindering effectively aggregation of Li atoms on the surface of the cluster. The decorated Li atoms are positively charged due to electron transfer from Li to C atoms. When H₂ molecules approach Li atoms, they are moderately polarized under the electric field, and adsorbed around the Li atoms in molecular form. Each Li atom in the Li-decorated C₂₄ complexes can adsorb two to three H₂ molecules. The H-H bond lengths of the adsorbed H₂ molecules are slightly stretched. The average adsorption energies are in the range of 0.08 to 0.13 eV/H₂, which are intermediate between physisorption and chemisorption. C₂₄Li₆ can hold up to 12 H₂ molecules, corresponding to a hydrogen uptake density of 6.8 wt%. This value exceeds the 2020 hydrogen storage target of 5.5 wt% proposed by the U. S. Department of Energy.

Microstructures and electronic structures of Cu₂ZnSnS₄ (CZTS) grain-boundaries (GB) are studied by the first-principles electronic structure method. Some special twist grain-boundaries have low grain-boundary energies and exhibit similar electronic structure as that in a perfect crystal. The twist grain-boundaries such as ∑ 3[221] and ∑ 6[221] have grain-boundary planes parallel to (112) plane, the easiest cleavage plane, so that they have small damages to the crystal structure and small influence on the properties of the materials. Grain-boundary plays two roles in CZTS thin-films: (1) capturing and trapping holes from p-n junctions, and (2) providing fast channels for transportation of majority carriers. As the majority of carriers, the positively charged holes need override a barrier before being trapped by a potential-well in the grain-boundary region. For the minority of carriers, the grain boundary is a high barrier to prevent electrons from transporting across it. The intrinsic nature of the potential barrier is not very clear. By calculating the distributions of static potentials across different grain boundaries of CZTS and also by comparing them with those across different surfaces, we find that the potential barriers at grain boundaries are the remnants of the potential barriers of surfaces, which trap the electrons in the bulk and prevent the electrons from escaping from the bulk to vacuum. When two surfaces get contact to form a grain boundary the corresponding surface barriers will be merged together as one potential barrier of the grain boundary. It is obvious that if a grain boundary intersects with the surface, the escaping work function near the grain boundary is lower than that near the prefect crystal surface. Experiment shows the coexistence of Sn⁴⁺ and Sn²⁺ions. The Sn⁴⁺ ions are located in the bulk by bonding 4 S atoms as neighbors. Our results show that Sn²⁺ ions can appear in the grain-boundary regions, on the surfaces or in the bulk with lattice defects so that Sn²⁺ ions have the lower coordination number by bonding 3 S atoms. The Sn atom is favored to be at the center of S octahedron with six neighboring S (or O) atoms in most sulfides (oxides) of tin. In CZTS, Sn atom is at the center of tetrahedron with 4 neighboring S atoms so that Sn atom is very active to move by structural relaxations. Most importantly the conduction-bands in CZTS are formed by the hybridizations between the s electrons of Sn and p electrons of S so that the conduction-bands of CZTS are sensitively dependent on the distributions and properties of Sn atoms. The appearing of Sn²⁺ ions and the strong structural relaxations of Sn atoms in grain-boundary regions and on surfaces induce extra in-gap states as a new source for the recombination of electron-hole pairs that are un-favored to the photo-voltage effects. Generally, the grain boundary plays a negative role in brittle photo-voltage materials such as Si and GaAs, and the positive role in ductile photo-voltage materials such as CdTe and CIGS (Cu(InGa)Se₂). It means that the growth of the hard and brittle films is very difficult, the micro-cracks and micro-pores are easily created. Our calculations show that CdTe, CIGS and CZTS are all ductile with Poisson-ratio greater than 0.33. This means that CZTS can be used as the absorber of flexible solar cell. By comparing the optical absorption-coefficients of crystals, grain-boundaries, surfaces and nano-particles, we find that the internal surfaces in thin-films with high pore-ratio can create new energy-levels in band-gap, which enhances the recombination between electrons and holes and decreases the optical absorption-coefficients (>1.3 eV). As a result, the high dense CZTS thin-film is required for high-efficient CZTS solar-cell. The positive role of grain boundary is more important if the CZTS film has the large, unique oriented grains and the uniform distribution of grain sizes. The simple and regular grain-boundary network is more beneficial to the coherent transport of majority carriers.

Frequency modulated continuous wave (FMCW) laser ranging is one of the most interesting techniques for precision distance metrology. It is a promising candidate for absolute distance measurement at large standoff distances (10 to 100 m) with high precision and accuracy, and no cooperation target is needed during the measuring process. How to improve the measurement resolution in practice has been the research focus of the FMCW laser ranging in recent years.
FMCW laser ranging system uses the method which may convert the measurement of flight time to the frequency measurement, while the ranging resolution can be determined by the tuning range of the optical frequency sweep in theory. The main impact-factor that reduces the resolution is the tuning nonlinearity of the laser source, which may cause an amount of error points within the sampling signal. So a dual-interferometric FMCW laser ranging system is adopted in this paper. Compared to the traditional Michelson scheme, an assistant interferometer is added. The assistant interferometer has an all-fiber optical Mach-Zehnder configuration, and the delay distance is at least 2 times longer than OPD (optical path difference) of the main interferometer. Because it provides the reference length, the length of the fiber must remain unchanged. The interference signal is obtained on the photodetector. At the time points of every peak and bottom of the auxiliary interferometer signal, the beating signal from the main interferometer is re-sampled. The original signal is not the equal time intervals, while the re-sampled signal is the equal optical frequency intervals. Based on the property of the re-sampled signal, a method by splicing the re-sampled signal to optimize the signal processing is proposed, by which the tuning range of the laser source limitation can be broken and high precision can be easily obtained. Also, a simple high-speed measuring method is proposed.
Based on all the above principles, the two-fiber optical frequency-modulated continuous wave laser ranging system is designed. The delay fiber in the FMCW laser ranging system is 40.8 m long, and the tuning speed and tuning range of the laser source are set to 10 nm/s and 40 nm respectively. Experiments show that the optimization method can effectively improve the measurement resolution and measuring efficiency; in the 26 measuring ranges, 50 μm resolution can be easily obtained and the error is less than 100 μm.

Organic materials with strong two-photon absorption response have attracted a great deal of interest in recent years for their many potential applications such as two-photon fluorescence microscopy, optical limiting, photodynamic therapy, and so on. Theoretical study on the relationships between molecular structure and two-photon absorption property has great importance in guiding the experimental design and synthesis of functional materials. Nowadays, quantum chemical calculations become very useful and popular tools in investigating the structure-property relations. At the same computational level, the two-photon absorption properties of different compounds can be compared accurately, and thus provide reasonable structure-property relations. Recently, a series of novel divinyl sulfides/sulfonesbased molecules have been synthesized and it is found that their photophysical properties behave like quadrupolar charge-transfer chromophores. In order to explore their potential two-photon absorption applications, in this paper, the two-photon absorption properties of these new molecules are calculated by using quantum chemical methods. Their molecular geometries are optimized at the hybrid B3LYP level with 6-31+g(d, p) basis set in the Gaussian 09 program. The two-photon absorption cross sections are calculated by response theory using the B3LYP functional with 6-31g(d) and 6-31+g(d) basis sets respectively in the Dalton program. In response theory, the single residue of the quadratic response function is used to identify the two-photon transition matrix element. Using the same methods, the two-photon absorption properties of distyrylbenzene compounds are computed for comparison. The basis set effects on excitation energies and two-photon absorption cross sections have been checked. It is found that the use of large basis sets could probably provide better numerical results, but the overall property trends would not change. Calculations show that the molecule with a triphenylamine group has the largest cross-section due to its strong donor groups. The divinyl sulfones-based dyes have larger cross-sections than the corresponding sulfides-based ones, because divinyl sulfones have stronger capability to accept electrons and at the same time the torsional angles between benzene rings in sulfones-based molecules are smaller than in the sulfides-based molecules. In the applicable wavelength range, these new dyes exhibit large two-photon absorption cross-sections which have the same order of magnitude as the strong two-photon absorption molecules with similar conjugation length. The largest cross section comes to 1613.3 GM calculated by using 6-31g(d) basis set. Molecular orbitals involved in the strongest two-photon absorption excitations are plotted and the charge transfer process is analyzed at length. The divinyl sulfide and sulfone centers behave as electron withdrawing groups and can form effective charge transfer molecules. On the basis of these new molecules, the structure inducing two-photon absorption enhancement is designed by employing isomerism effect. When the benzene rings of carbazole groups are connected with the molecular center, the planarity and charge transfer intensity are increased, and then the two-photon absorption cross-section can be improved dramatically. This study provides theoretical guidelines for the synthesis of new type of active two-photon absorption materials.