3D photon counting imaging at 1550 nm with 1.5-GHz sine-wave-gated InGaAs/InP GAPD

We demonstrated a 3D laser imaging system at 1550 nm with a 1.5-GHz sine-wave gated Geiger-mode InGaAs/InP avalanche photodiode (APD). An optical fiber bundle with 100 individual fiber outputs was implemented at the focal plane of the telescope, providing a 2.5-mrad imaging view. The system used single-pixel near-infrared single-photon detector to measure photons at fiber outputs instead of a photon counting array. The 1.5-GHz gated Geiger-mode InGaAs/InP APD with a timing jitter of 290 ps was operated in quasi-continuous mode with detection efficiency of ∼4.3%. We achieved higher than 6-cm surface-to-surface resolution at single-photon level, showing a potential of low-energy and eye-safe laser imaging system for long-distance measurements.     

New side-pump scheme for large mode area fiber lasers by wrapping polished fibers conjugate caustic removed

With the purpose of uniform gain coefficient along the axial direction in the core of gain guided and index anti-guided fiber, a new kind of side-pump scheme by wrapping particularly treated fibers around the central GGIAG fiber is reported in this paper. According to leakage mode analysis on wrapped fiber and numerical simulation on this pumping structure having ytterbium doped central fiber and conventional wrapped side fiber, we find this kind of pump scheme can improve the uniformity of gain coefficient along the fiber direction, and also, it can support multi channels of pumping sources by adding more side fibers to improve the total effective incident pumping power.     

Strain rate effects on compressive behavior of covalently bonded CNT networks

In this study, strain rate effects on the compressive mechanical properties of randomly structured carbon nanotube (CNT) networks were examined. For this purpose, three-dimensional atomistic models of CNT networks with covalently-bonded junctions were generated. After that, molecular dynamics (MD) simulations of compressive loading were performed at five different strain rates to investigate the basic deformation characteristic mechanisms of CNT networks and determine the effect of strain rate on stress–strain curves. The simulation results showed that the strain rate of compressive loading increases, so that a higher resistance of specimens to deformation is observed. Furthermore, the local deformation characteristics of CNT segments, which are mainly driven by bending and buckling modes, and their prevalence are strongly affected by the deformation rate. It was also observed that CNT networks have superior features to metal foams such as metal matrix syntactic foams (MMSFs) and porous sintered fiber metals (PSFMs) in terms of energy absorbing capabilities.     

ICP-AES测定高浓度基体下杂质元素的偏最小二乘法研究

建立了ICP-AES测定高浓度基体中微量杂质元素的偏最小二乘方法(PLS)。研究表明,PLS能有效校正高浓度基体干扰引起的测量误差,比多元光谱拟合法(MSF)能承受的基体浓度更高。当基体与杂质的含量比为1 000∶1～20 000∶1时,该方法的加标回收率在95%～105%之间。对于干扰效应与基体浓度呈非线性相关的体系,普通PLS的预测准确度不高,但使用基于样品浓度矩阵变换的偏最小二乘法(LIN-PPLS),则明显改善了预测的准确度。分别用MSF、普通PLS和LIN-PPLS对水系沉积物国家标准物质GBW07312中的Co,Pb和Ga进行测定,结果表明,LIN-PPLS的预测准确度优于普通PLS,而普通PLS的预测准确度优于MSF。     

Analysis of laser micro drilled holes through aluminium for micro-manufacturing applications

Conventional laser machining of aluminium with long wavelength lasers has its inherent problems due to the high reflectivity of aluminium to laser radiation (Handbook of Optics, vol 1, 2nd ed. New York: McGraw-Hill; 1995). Laser processing at shorter wavelengths reduces the reflectivity of the workpiece to the incident laser radiation and can also reduce the dimensions of the obtainable machining geometries. This paper reviews the limiting factors in the micro machining of aluminium using a diode pumped solid state (DPSS) Nd:YAG laser operated at 1064, 532, and 355 nm. The geometries of the laser-machined samples were investigated using interferometric, and optical methods to assess how the processing fluence and wavelength will affect the obtainable precision for successful integration of the laser in a micromachining CAD/CAM system.     

Experimental study of flame stabilization in low Reynolds and Dean number flows in curved mesoscale ducts

Flame stabilization during non-premixed combustion in curved ducts with a diameter of the order of magnitude of the premixed flame thickness of the reactants was investigated experimentally, since it has been established that this is a configuration with potential advantages in the context of “micro”-combustion. It was shown that, in such “mesoscale” tubes, a stable flame oscillation including extinction/re-ignition phenomena can be established for steady boundary conditions. These oscillations lead, under appropriate conditions, to sound emission in the 50–350 Hz range. This was a mode of stabilization in addition to the “classical” steady flamelet, stabilized through thermal losses to the duct walls at higher values of the Reynolds number. Curvature of the duct was shown to have minimal effect on reactant mixing, which was diffusion-controlled, but significantly affected flame thickness and stabilization. To probe the fuel-oxidizer mixing in the U-shaped, optically accessible tubes, laser induced fluorescence of acetone fuel dopant was used, and the flame structure was studied using OH PLIF. The various stabilization regimes are discussed in terms of the Reynolds and Dean numbers of the tube flow.     

Exchange coupling and helical spin order in the triangular lattice antiferromagnet CuCrO₂ using first principles

The magnetic and electronic properties of the geometrically frustrated triangular antiferromagnet CuCrO2 are investigated by first principles through density functional theory calculations within the generalized gradient approximations (GGA)+U scheme. The spin exchange interactions up to the third nearest neighbours in the ab plane as well as the coupling between adjacent layers are calculated to examine the magnetism and spin frustration. It is found that CuCrO2 has a natural two-dimensional characteristic of the magnetic interaction. Using Monte-Carlo simulation, we obtain the Neel temperature to be 29.9 K, which accords well with the experimental value of 24 K. Based on non-collinear magnetic structure calculations, we verify that the incommensurate spiral-spin structure with (110) spiral plane is believable for the magnetic ground state, which is consistent with the experimental observations. Due to intra-layer geometric spin frustration, parallel helical-spin chains arise along the a, b, or a + b directions, each with a screw-rotation angle of about 120°. Our calculations of the density of states show that the spin frustration plays an important role in the change of d-p hybridization, while the spin-orbit coupling has a very limited influence on the electronic structure.     

Patterning on single crystalline silicon by laser scanning and alkaline etching

The applicability of laser processing for small-lot micro-electromechanical system devices is discussed in this paper. This simple process could replace conventional complex processes designed with mass production in mind. Ablation, protrusions or surface modification is revealed to occur by argon ion laser scanning into silicon. Which of them occurs depends on the laser power. It is found that the protrusions are covered by a thin layer of oxide; however, oxidation of the modified surface is not established even though some results suggest it. Surface modification is more applicable to surface patterning than coarse protrusion is because the laser-modified surface can be used as a mask in KOH etching to make sharp patterns. The applicability of this method is indicated by demonstrating pattern width control, patterning over a large area and the fabrication of a 16-bit linear scale.     

Shape-dependent local internal stress of α-Cr2O3 nanocrystal fabricated by pulsed laser ablation

The α-Cr₂O₃ single-crystal nanocondensates were fabricated by pulsed laser ablation in air and characterized by analytical electron microscopy regarding shape-dependent local internal stress of the anisotropic crystal. The nanocondensates formed predominantly as rhombohedra with well-developed surfaces and occasionally hexagonal plate with thin edges and blunt corners. Such nanocondensates showed Raman shift for the CrO₆ polyhedra, indicating a local compressive stress up to ca. 4 GPa on the average. Careful analysis of the lattice fringes revealed a local compressive stress (0.5% strain) at the thin edge of the hexagonal plates and a local tensile stress (0.3–1.0% strain) near the relaxed , , and (0 0 0 1) surfaces of truncated rhombohedra. The combined effects of nanosize, capillarity force at sharp edge, and specific surface relaxation account for the retention of a local internal compressive stress built up in an anisotropic crystal during a very rapid heating–cooling process.     

Advantages of using multiple-echo image combination and asymmetric triangular phase masking in magnetic resonance venography at 3 T

The present work explores the possibility of localizing veins with magnetic resonance venography using susceptibility weighted imaging. It also seeks new approaches, directed by the spatial specificity of activated brain regions, that have sufficient precision for practical use in functional MRI studies. A 3D flow compensated multiple gradient echo sequence, featuring optimized T2* weighting within a reasonable time of acquisition (11 min) and a small voxel size (0.5x0.5x1 mm3), was used to acquire MR images at 3 T. Post-processing consisted of homodyne filtering, linear phase scaling and magnitude masking prior to minimum intensity projection (mIP). The multiple echo approach provided a satisfactory (48+/-7%) increase in signal-to-noise ratio with respect to conventional methods. Specific features of the blood oxygenation level-dependent phase effect were simulated and used for designing and exploring different phase masking methods in relation to vessel morphology and MRI voxel geometry. As with simulations, the best results were obtained with an asymmetric triangular phase masking, featuring an improved venographic contrast without any increase in the full-width at half-maximum. The multiple echo approach provided satisfactory vessel localization capacity by using asymmetric triangular phase masking and a 4-mm-thick mIP. The venographic contrast obtained enabled the detection of vessels with diameter down to approximately 500 microm, suggesting the applicability of the proposed method as an additional technique in fMRI studies.