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In this paper, a high efficiency and high reliability 915 nm semiconductor laser is designed and fabricated, which is a key component of the fiber lasers. In order to maximize the electro-optic conversion efficiency of the device, a double asymmetric large-cavity waveguide structure is adopted in the design, and the quantum well structure, waveguide structure, doping, and device structure are systematically optimized. Device simulations show that the device's maximum electro-optical conversion efficiency reaches 67% at an ambient temperature of 25℃. The material is grown by Metal Organic Chemical Vapor Deposition(MOCVD), and a laser chip having a light emitting region width of 95 μm and a cavity length of 4.8 mm is prepared. Tests show that the efficiency of the packaged device and other parameter indices have reached the advanced level of similar devices in the world. In the case that the threshold current is 1 A at room temperature, the slope efficiency is 1.18 W/A, the maximum electro-optic conversion efficiency is 66.5%, the output power is 12 W, and the electro-optical conversion efficiency reaches 64.3%. It can be seen that the test results are in good agreement with that of the device theory simulation. After approximately 6 000 hours of long-life accelerated testing, the device power does not attenuate, indicating that the produced high-power 915 nm laser chip has very high reliability.

The advent of high-power ultra-short ultra-intense laser pulses opens up the new frontiers of relativistic nonlinear optics, high-field physics, laser-driven inertial confined fusion, etc. In recent years, with the construction of high power laser facilities at a multi-petawatt (PW) level and above, the interaction between laser and matter enters into a new realm of high field physics, where extremely rich nonlinear physics is involved. In addition to classical nonlinear physics involving wave-particle interactions, relativistic effects, and ponderomotive force effects, the quantum electrodynamic (QED) effects occur, such as radiation reaction force, electron-positron pair production, strong γ-ray radiation, QED cascades, and vacuum polarization. This paper presents a brief overview of electron-positron pair creation and bright γ-ray emission driven by the extremely intense laser fields.

Strong terahertz (THz) radiation of MV/cm can be generated from air via two-color laser scheme. In this paper, we introduce three recent theoretical and experimental researches conducted by Wang et al., in which they explored the long-standing problem of THz generation mechanism and extended the scheme with uncommon frequency ratio. In the widely-studied two-color laser scheme, the frequency ratio of the two lasers is usually fixed at 2/1=1:2. In 2013 they predicted according to the plasma current model, for the first time, that the two-color scheme can be extended to a new frequency ratio 1:2n, where n is an positive integer. In 2017 they found that the frequency ratio can be further extended to much broader values. In that year, their experiments showed, for the first time, efficient THz generation with new ratios of 2/1=1:4 and 2:3. They observed that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization, but the polarization adjustment becomes inefficient by rotating the other laser polarization, which is inconsistent with the symmetric nature in the susceptibility tensor required by the multi-wave mixing theory; the THz energy shows similar scaling laws with different frequency ratios, which is inconsistent with the scaling predicted according to the multi-wave mixing theory. These experimental results are in agreement with the plasma current model and particle-in-cell simulations. Therefore, their studies not only push the development of the two-color scheme, but also show that the THz generation mechanism should be mainly attributed to the plasma current model.

Al_(12)Si_(3.75)Ge_(0.25)O_(26) mullite powder was prepared by sol-gel method using Al(NO_3)_3,Cl_3GeCH_2CH_2COOH and TEOS(Si(OC_2H_5)_4)as precursors.The mullitization process of precursors was monitored by TG-DSC,XRD and FT- IR.After the reduction of Al_(12)Si_(3.75)Ge_(0.25)O_(26),strong room temperature photoluminescence(PL)can be observed at 565, 613,682,731,and 777 nm,respectively.The PL intensity was related with the reduction temperature,and the sample reduced at 500℃had the highest PL intensity among those samples reduced at different temperatures.The XPS spectra showed that some Ge~(4+) had been reduced to Ge~0 under those temperatures.The average size of Ge nanoparticles was about 1.95 nm estimated by Raman spectra analysis.

A new SA (simulated annealing) method is presented for the design of longitudinal shim coils for NMR (nuclear magnetic resonance) equipment. First, AGL (adaptive Gauss-Lobatto) quadrature used by the traditional SA method is analyzed. Two Kinds of complete elliptic integrals are introduced to integrate the spatial magnetic field, which increase the computing speed by 13.3 times. Then, the System energy calculation is extended to the field of the z-axis. The results illustrate that the coil-set efficiency designed by the new SA method is 15% higher than that of the traditional one, and the field inhomogeneity is reduced frm 1.20% to 0.49%. Finally, a prototype of coil assembly for NMR rock-core analyzer is built, and the experimental results are in good agreement with the theoretically calculated results. * Supported by the National Key Technologies Research and Development Program of China (Grant No. 001BA210A03), and Key Project of 2005 Beijing Science and Technology Programme(Grant No. D0205002...

In order to get better glass-forming abilities (GFAs), Ni atoms are partially replaced by Cu and Co atoms in Al84Ni12Zr4 alloys. Thermal analysis shows that the reduced crystallization temperature Trx has no direct correlation with the GFA of the alloys. However, it is notable that prepeaks have been found in the total structure factors of the amorphous Al84Ni(12−x)Zr4Cux and Al84Ni(12−x)Zr4Cox alloys. In addition, the results prove that the intensity of the prepeaks influences the GFA powerfully. The amorphous alloys with larger intensity of the prepeak show better GFA. The influence of prepeaks on the GFA can be explained by the atomic configuration difference among the liquid, crystal and glass states.

The nanocrystallization behaviour of Zr70Cu20Ni10 metallic glass during isothermal annealing is studied by employing a Monte Carlo simulation incorporating with a modified Ising model and a Q-state Potts model. Based on the simulated microstructure and differential scanning calorimetry curves, we find that the low crystal-amorphous interface energy of Ni plays an important role in the nanocrystallization of primary Zr2Ni. It is found that when T

The dynamic evolution of the lamellar eutectic of binary alloys in directional solidification is studied in detail using the Monte Carlo technique. The simulated results can be summarized into two aspects: (1) the lamellar spacing λ is found to be inversely proportional to the chemical potential difference Δμ, predicting a linear relationship between the kinetic supercooling ΔTk and total supercooling at the solid/liquid (S/L) interface; (2) as the solidifying velocity R is low, the dynamic product λ2R shows a considerable dependence on temperature gradient GT in the liquid in front of the S/L interface, although this dependence becomes much weaker at a high R.

The source terms of a tensile dislocation are presented in terms of the wavenumber and circular frequency. Expressions for displacements on the surface of a half-space due to an arbitrary oriented dislocation are also obtained by using the generalized ray method. The result shows that the far-field waveforms contributed by tensile source are different from other sources. Considering that not all large earthquakes can be explained with the shear dislocation model, these expressions may be efficient in analysing some earthquakes.

Magnetic resonance imaging (MRI) has been a primary diagnostic technique due to its high imaging quality, noninvasion and non-radiation capacity. However, the application of conventional whole body MRI is restricted by its massive size, high installation and management cost. Dedicated MRI overcomes the shortcomings of whole body MRI and has great importance in medical diagnosis. The challenge is that the design of superconducting magnet for extremity MRI is largely constrained by physiological structure of human body. As a result, a limited longitudinal length with high field homogeneity in a 160 mm diameter sphere volume (DSV) is required for superconducting magnet of extremity MRI. In this article, a non-shielded 1.5 T extremity dedicated superconducting magnet is designed by using both 0-1 integer programming and genetic algorithm and fabricated with a comprehensive consideration of superconductivity wire consumption, central magnetic field intensity and imaging region homogeneity. The NbTi superconducting wire is chosen for coil winding, and copper-to-superconducting ratio of the wire is 1.3. The sizes of cross-section of the bare wire and the insulated wire are 0.75 mm×1.20 mm and 0.83 mm×1.28 mm respectively, and the critical currents at 4.2 K and 5 T are both about 935 A.According to the size constraint of the magnet, we first calculate the current carrying zone of the superconducting coils and divide it into grid elements with parallel current. The size of each grid element is equal to that of the superconducting wire, and the distribution of non-rectangular coils is obtained by using 0-1 integer programming. In order to obtain a higher homogeneity of magnetic field, a reverse current zone is manually created in the wide blank area of the feasible current carrying zone. Using the results above, we then optimize the distribution of coils and build a rectangular model which facilitates the fabrication by using genetic algorithm. The inductance of the magnet is 1.8094 H, the operating current is 402.09 A, the stored energy is 146.27 kJ and the peak magnetic field of current carrying zone is 5.48 T. The calculated peak-to-peak homogeneity in 160 mm DSV is about 22 ppm. Taking into consideration the factors such as mechanical error and cold shrinkage, the estimated homogeneity would reach 60 ppm (peak-to-peak) with passive shimming.The 1.5 T extremity dedicated superconducting magnet is successfully fabricated through a series of processes such as winding, curing, assembly and welding. The prototype magnet has a room temperature bore of 280 mm in diameter and a total length of 520 mm, and the volume of liquid helium vessel is about 30 liters. To reduce the evaporation of liquid helium, a 1.5 Watt two-stage Gifford-McMahon refrigerator is employed to cool the system and maintain the evaporation rate of Helium at zero level. The range of 5 Gauss line of the magnet is 3.2 m in the radial direction and 2.6 m in the axial direction. Moreover, the magnet is magnetized to 1.5 T after being conditioned three times and the measured homogeneity in 160 mm DSV achieves 55 ppm (peak-to-peak) and 3.4 ppm (Vrms) after passive shimming using silicon steel pieces.

The structure factor and the coordination numbers of liquid Al–Si alloys with different Si content have been measured by a high temperature X-ray diffractometer. Radial distribution functions (RDFs), the nearest atomic distance and the coordination numbers of eutectic Al–Si alloys before and after being modified with Sr and Sb were studied. The RDFs of the liquid alloy were decomposed by five Gaussian peaks. The results show that a Si–Si covalent bond exists in the liquid of eutectic and hyper-eutectic alloys. Sr in the liquid Al–Si has a capability to weaken the covalent bonds of Si–Si, suppressing the nucleation of the eutectic silicon phase. On the other hand, Sb in the liquid Al–Si increases the order degree of Si atoms, decreasing the supercooling degree of the nucleation and promoting the nucleation of eutectic silicon.

A high-temperature X-ray diffractometer has been used to measure the structure factors and the coordination numbers at different temperatures of liquid Al–12.5% Si alloy with various additions of Sb. Radial distribution functions (RDFs), the nearest neighbor atomic distances, and the coordination numbers of eutectic Al–12.5% Si have been studied before and after being modified by Sb. The results show that a prepeak exists on the structure factor curve. Sb in liquid Al–Si alloys promotes the aggregation and nucleation of Si atoms, decreases the supercooling of solidification, and finally modifies the size and shape of the Si phases in the solid state.

A simple theoretical model proposed recently to evaluate the ability of bulk materials to form single crystals is further tested via vast molecular dynamics simulations of growth for fcc (Ni, Cu, Al, Ar) and hcp (Mg) crystals, especially applied to the growth of bcc (Fe) crystal, showing that the validity of the model is independent of crystal types and the interaction potentials of the constitute atoms.

In this article, a novel designed radio frequency (RF) coil is designed and built for the imaging of puppies in a V-shape permanent magnetic resonance imaging (MRI) system. Two sets of Helmholtz coil pairs with a V-shape structure are used to improve the holding of an animal in the coil. The homogeneity and the sensitivity of the RF field in the coil are analysed by theoretical calculation. The size and the shape of the new coil are optimized and validated by simulation through using the finite element method (FEM). Good magnetic resonance (MR) images are achieved on a shepherd dog.

The vertical-cavity surface-emitting laser（VCSEL） has proved to be a low cost light source with attractive properties such as surface emission, circular and low divergence output beam, and simple integration in two-dimensional array. Many new applications such as in spectroscopy, optical storage, short distance fiber optic interconnects, and in longer distance communication, are continuously arising. Many of these applications require stable and single-mode high output power. Several methods that affect the transverse guiding and/or introduce mode selective loss or gain have been developed. In this study, a method for improving the single mode output power by using metal surface plasmons nanostructure is proposed. Theoretical calculation shows that the outpout power is improved about 50% compared to the result of standard VCSELs.

Transmission of electromagnetic wave in a heavily doped n-type GaAs film is studied theoretically. From the calculations, an extraordinary transmission of p-polarized waves through the film with subwavelength grooves on both surfaces at mid-infrared frequencies is found. This extraordinary transmission is attributed to the coupling of the surface-plasmon polariton modes and waveguide modes. By selecting a set of groove parameters, the transmission is optimized to a maximum. Furthermore, the transmission can be tuned by dopant concentrations. As the dopant concentration increases, the peak position shifts to higher frequency but the peak value decreases.

Based on the analysis of the X-ray diffraction data of liquid Al and Sn, temperature-induced structural transformations are found for liquid Al in the range of 1050—1250℃ and for liquid Sn near 800 and 1200℃. The nearest neighbor distance r1 of Al and Sn decreases with the increase of temperature. The structure of liquid Sn is similar to α-Sn on the short-range order length scale. The physical meaning of the correlation radius rc is discussed.