MRI与MRS在肥胖症研究中的应用进展

肥胖症已经成为严重威胁人类健康的主要慢性疾病之一,磁共振成像(MRI)和磁共振波谱(MRS)技术的结合运用,对于评价脂肪组织分布和蓄积程度具有极大的优势.该文总结了近年来MRI与MRS技术在肥胖症研究中的应用进展,并讨论了MRI与MRS技术在肥胖症临床应用及科学研究中的价值.     

结构光测量技术中的投影仪标定算法

详细介绍了投影机模型,并提出一种简单、高精度的投影仪参数标定算法,该算法将投影仪当作一个逆向的相机,使用一块带有圆形标志点的平面标定板对投影仪进行标定.标定过程中,使用两组不同方向的光栅图像建立投影仪图像和相机图像的对应关系,从而得到投影仪标定所需的图像数据,将投影仪标定转化为成熟的相机标定,然后使用相机标定算法对投影仪进行高精度标定.实验结果表明,所提议的投影仪标定算法操作过程简单,标定精度可达0.312 pixel.     

Low Loss Plastic Terahertz Photonic Band-Gap Fibres

We report a numerical investigation on terahertz wave propagation in plastic photonic band-gap fibres which are characterized by a 19-unit-cell air core and hexagonal Mr holes with rounded corners in cladding. Using the finite element method, the leakage loss and absorption loss are calculated and the transmission properties are analysed. The lowest loss of 0.268 dB/m is obtained. Numerical results show that the fibres could liberate the constraints of background materials beyond the transparency region in terahertz wave band, and efficiently minimize the effect of absorption by background materials, which present great advantage of plastic photonic band-gap fibres in long distance terahertz delivery.     

可切换频段的太赫兹编码超表面

依据Pancharatnam-Berry(PB)相位原理提出了一种可切换频段的太赫兹编码超表面,其顶层是金属-二氧化钒（VO2）复合结构层,中间是聚酰亚胺介质层,底部为纯金属反射层。编码超表面单元按不同的编码序列排列构成的编码超表面,可以产生多种太赫兹波束形式,并且随着VO2的相变,可以在不改变原波束形式的情况下实现从单频段到双频段的切换功能。通过设计不同的编码序列,令编码超表面分别产生了涡旋波和散射波,对于垂直入射的右旋圆极化波（左旋圆极化波类似）：当VO2处于绝缘态时,在频段1.17～1.37 THz处可以产生与编码序列所对应的波束形式;当VO2相变为金属态时,在0.87～0.92 THz和1.4～1.6 THz两个不同频段处分别获得与VO2处于绝缘态时相同的波束形式,进一步拓宽波束产生的频段。所提出的编码超表面利用了VO2的相变特性增加了太赫兹工作的频段,为实现频段可切换的太赫兹编码超表面提供了思路,在太赫兹波束调控中的调频方面有一定参考意义。     

Green-Beam Generation of 40W by Use of an Intracavity Frequency-Doubled Diode Side-Pumped Nd:YAG Laser

A cw diode side-pumped Nd：YAO laser is frequency doubled to 532nm with an intracavity KTP crystal in a Vshaped arrangement, achieving an output power of 40 W corresponding to an optical-optical conversion efficiency of 9.7%. The instabilities and the M2-parameters of the laser are measured at different output powers after the beam is filtered.     

多肽自组装纳米管的研究进展

综述了通过多肽自组装形成纳米管道的各种方法:线形的D,L-α-多肽可以通过分子内的氢键以及主链的空间排斥作用而组装成纳米管,游离寡肽和环肽则可以通过分子间的氢键组装形成纳米管,而两亲多肽则通过疏水相互作用组装成纳米管。     

THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves

We propose a novel scheme for THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves (HSWs). The repeated frequency conversions are accomplished by oscillations of Stoke waves in resonant cavity (RC) where low-order Stokes waves (LSWs) are converted to high-order Stokes waves again and again. The continuous frequency conversions are accomplished by optimized cascaded difference frequency generation (OCDFG) where the poling periods of the optical crystal are aperiodic leading to the frequency conversions from low-order Stokes waves to high-order Stokes waves uninterruptedly and unidirectionally. Combined with the repeated and continuous frequency conversions, the optical-to-THz energy conversion efficiency (OTECE) exceeds 26% at 300 K and 43% at 100 K with pump intensities of 300 MW/cm².     

High-power terahertz radiation from surface-emitted THz-wave parametric oscillator

We report a pulsed surface-emitted THz-wave parametric oscillator based on two MgO:LiNbO3 crystals pumped by a multi-longitudinal mode Q-switched Nd:YAG laser.Through varying the phase matching angle,the tunable THzwave output from 0.79 THz to 2.84 THz is realized.The maximum THz-wave output was 193.2 nJ/pulse at 1.84 THz as the pump power density was 212.5 MW/cm 2,corresponding to the energy conversion efficiency of 2.42×10-6 and the photon conversion efficiency of about 0.037%.When the pump power density changed from 123 MW/cm 2 to 148 MW/cm 2 and 164 MW/cm 2,the maximum output of the THz-wave moved to the high frequency band.We give a reasonable explanation for this phenomenon.     

甲醇缩合生成二甲醚的质谱和量子化学研究

利用质谱分析结果结合量子化学abinitio方法,研究甲醇缩合脱水的反应过程.研究表明,在气相中,甲醇分子主要以二聚体和多聚体的形式存在,据此提出甲醇脱水反应的原始反应物是甲醇二聚体,而不是单个甲醇分子.研究了气相甲醇二聚体、过渡态的能量和结构、甲醇脱水生成二甲醚反应的可能反应途径和反应势垒,对甲醇缩合脱水生成二甲醚的反应性及其影响因素作了讨论.