具有电与磁非线性效应超常介质中光束的聚焦特性

基于Drude模型,采用解析法与数值模拟法研究了具有电与磁非线性效应超常介质中光束的聚焦特性。结果表明超常材料中光束聚焦现象不但可以发生在自聚焦非线性介质中(正折射区)而且可以发生在自散焦非线性介质中(负折射区),且光束中心频率离介质的等离子频率越近,其聚焦效应越明显。特别是当电与磁非线性极化率为异号时,超常介质的非线性符号可以通过调控电与磁等离子频率的相对大小控制,这为主动操控光束的传输提供一种新的自由度。     

抽指加权宽带扇形叉指声表面波滤波器的等效电路分析

扇形换能器可以看成若干个沿孔径方向划分的均匀孔径换能器电端并联而成，根据梅森(Mason)等效电路模型以及由其延伸的混合场等效电路模型，推导出整个扇形换能器的导纳矩阵和频率响应，其中采用复阻抗模型考虑了叉指电极与自由表面的阻抗不连续的因素。本方法能够模拟指间反射、三次行程、指条寄生阻抗以及外围电路等对器件性能的影响；理论模拟与实验结果吻合。     

自辐射场下UN2分子的光谱研究

对铀原子和氮原子分别使用相对论有效原子实势和6-311+G(d)基组, 采用优选的密度泛函B3P86方法, 研究了铀本身产生自辐射场(-0.005–0.005 a.u.)作用下UN₂基态分子的能隙E_g和谐振频率ν. 结果表明: UN₂分子在自辐射场中反对称伸缩振动频率ν₃(σ_g)和对称伸缩振动频率σ₁(σ_g)与实验值1051.1 cm^-1和1008.3 cm^-1 基本符合; E_g随自辐射场场强的增大而趋于减少, 占据轨道的电子容易被激发至空轨道而形成激发态; UN₂分子在自辐射场中趋于不稳定, N₂, O₂等更容易扩散到表面内层而腐蚀铀表面, 加剧了铀在自辐射场中的腐蚀.     

虚拟RLC电参量测试仪

虚拟仪器在测试测量领域、自动控制和工业控制领域有着广泛的应用.为提高本科生普通物理实验训练的起点,北京大学基础物理实验中心将虚拟仪器引入到物理实验教学中.本文以虚拟RLC电参量的测量为例,介绍了虚拟仪器在物理实验中的具体应用.     

论FeCr合金辐照损伤的多尺度模拟

铁素体/马氏体钢具有优异的抗辐照肿胀和抗腐蚀能力,是聚变堆、ADS和四代堆结构材料重要候选者之一.多尺度模拟在核武器自辐照损伤、压力容器辐照脆化、医学材料辐照损伤等方面有着广泛的应用.国际上围绕F/M FeCr合金辐照损伤开展了大量的多尺度模拟工作.论文概述了F/M FeCr合金辐照损伤多尺度模拟研究现状及进展,并针对目前存在的某些问题提出了解决建议. 关键词： 铁素体/马氏体FeCr合金 辐照损伤 多尺度模拟     

腔内倍频调Q激光器中脉冲光场的时间不稳定性研究

以腔内倍频调Q激光器工作原理为基础,研究了多种形式抽运光分布的情况下倍频光的脉冲形状以及脉冲持续过程中光场分布的时间不稳定性,分析了倍频光光束质量随时间的变化规律.发现腔内倍频调Q激光器输出的倍频光光场分布在脉冲的持续过程中并不稳定,呈持续变化.对高斯型抽运的情况研究发现,减小腔长会使脉冲光场分布的不稳定性减小;而增加...     

Enhanced NIR emission in Ce^3＋/Er^3＋-doped YAG induced by Bi^3＋ doping

Ce^3＋/Er^3＋/Bi^3＋ triply-doped yttrium aluminum garnet （YAG） is synthesized using co-precipitation method. The Bi^3＋ concentration-dependent near-infrared （NIR） emission behavior is systemically in- vestigated. The NIR emission of Er^3＋ ions at 1531 nm is enhanced threefold by the addition of 7 mol% Bi^3＋. Bi^3＋doping results in the formation of exciton in YAG and the variation in the local environment of the doped rare-earth ions. The enhancement in NIR luminescence is ascribed to the combined effects of the sensitization of exciton→Ce^3＋ →Er^3＋ and the Bi^3＋ doping-induced adjustment of the local environment for Ce^3＋ and Er^3＋ ions.     

Multiple Parallel Round Leg Design for Quadrature Birdcage Coil in Ultrahigh-Field MRI

Copper foil has been widely employed in conventional radio frequency (RF) birdcage coils for magnetic resonance imaging (MRI). However, for ultrahigh-field (UHF) MRI, current density distribution on the copper foil is concentrated on the surface and the edge due to proximity effect. This increases the effective resistance and distorts the circumferential sinusoidal current distribution on the birdcage coils, resulting in low signal-to-noise ratio (SNR) and inhomogeneous distribution of RF magnetic (B₁) field. In this context, multiple parallel round wires were proposed as legs of a birdcage coil to optimize current density distribution and to improve the SNR and the B₁ field homogeneity. The design was compared with three conventional birdcage coils with different width flat strip surface legs for a 9.4 T (T) MRI system, e.g., narrow-leg birdcage coil (NL), medium-leg birdcage coil (ML), broad-leg birdcage coil (BL) and the multiple parallel round wire-leg birdcage coil (WL). Studies were carried out in in vitro saline phantom as well as in vivo mouse brain. WL showed higher coil quality factor Q and more homogeneous B₁ field distribution compared to the other three conventional birdcage coils. Furthermore, WL showed 12, 10 and 13% SNR increase, respectively, compared to NL, ML and BL. It was proposed that conductor’s shape optimization could be an effective approach to improve RF coil performance for UHF MRI.     

中心波长为28.5nm多层膜镜反射率测量

在“星光-Ⅱ”装置上以类Ne铬x射线激光作为标定源，以平场光栅谱仪为分光元件进行了285nm的Mo/Si多层膜反射镜效率测量.介绍了实验方法，给出了实验结果，本次研制的两块多层膜镜反射率分别为31%和9.6%. 关键词： x射线多层膜反射镜 反射率测量 x射线激光     

ROOM TEMPERATURE LIGHT-THERMO-DETECTOR MADE OF HIGH-Tc SUPERCONDUCTOR

Based on the anisotropic Seebeck tensor coefficients, a light-thermo-radiation detector made of high-T_c superconductor (YBa₂Cu₃O_7-δ) was fabricated, which can function at room temperature. The induced voltaic signals at various modulation frequencies for radiation from He-Ne laser and from a 500 K black body were measured. The noise ratio for detecting He-Ne laser radiation and the D^* for black body were evaluated. Compared with bolometer made of high-T_c superconductor which functions at liquid nitrogen, and with pyroelectric detector, it was found that the advantages of this new device are that the device can work at room temperature with very low noise and very fast response. Although having a lower D^* value than that of bolometer for the present device construction, we point out the possibility and the direction for improving the D^* value, hence the promising prospect of this kind of device.