Magnetic resonance image reconstruction using trained geometric directions in 2D redundant wavelets domain and non-convex optimization

Reducing scanning time is significantly important for MRI. Compressed sensing has shown promising results by undersampling the k-space data to speed up imaging. Sparsity of an image plays an important role in compressed sensing MRI to reduce the image artifacts. Recently, the method of patch-based directional wavelets (PBDW) which trains geometric directions from undersampled data has been proposed. It has better performance in preserving image edges than conventional sparsifying transforms. However, obvious artifacts are presented in the smooth region when the data are highly undersampled. In addition, the original PBDW-based method does not hold obvious improvement for radial and fully 2D random sampling patterns. In this paper, the PBDW-based MRI reconstruction is improved from two aspects: 1) An efficient non-convex minimization algorithm is modified to enhance image quality; 2) PBDW are extended into shift-invariant discrete wavelet domain to enhance the ability of transform on sparsifying piecewise smooth image features. Numerical simulation results on vivo magnetic resonance images demonstrate that the proposed method outperforms the original PBDW in terms of removing artifacts and preserving edges.     

基于宽边耦合螺旋结构的低频小型化极化不敏感超材料吸波体

设计、仿真并实验验证了基于宽边耦合螺旋结构的低频小型化超材料吸波体. 实验测试结果表明, 该超材料吸波体在1.39 GHz吸收率达到最大为98%, 其单元尺寸和总厚度均为6.8 mm, 约为1/32工作波长, 实现小型化窄带吸波. 由于吸波体中螺旋结构是旋转对称排列的, 因而其对垂直入射电磁波的极化方向是不敏感的. 此外, 该超材料吸波体对斜入射横电和横磁极化电磁波在60°时, 仍具有强吸收. 关键词： 超材料 吸波体 小型化     

Structures and stabilities of the donor–acceptor complexes HXPY (X=Al,B; Y=H,F, OH)

The optimized geometries, complexation energies, etc. of HXPY (X?=?Al, B; Y?=?H, F, OH) donor–acceptor complexes have been investigated at the B3LYP/6-311+G(d,p), MP2/6-311+G(d,p) and/or CCSD(T)/6-311+G(d,p) levels. The results show that HBPY (Y?=?H, F, OH) is more stable than the corresponding HAlPY (Y?=?H, F, OH), F (or OH) substitution on phosphorus results in decreasing complex stability, and the stronger the electron-attracting nature of the substitution atom, the more stable the complex. Moreover, the thermodynamic and kinetic properties of the formation reaction of these donor–acceptor complexes were also examined within the temperature range 200–800?K using the general statistical thermodynamics and Eyring transition state theory with Wigner correction. It is concluded that the formation of HBPY is thermodynamically favoured over that of the corresponding HAlPY, especially at low temperature, and is kinetically favoured over that of the relevant HAlPY (Y?=?H, F, OH), especially at high temperature.     

Relativistic R-matrix studies of photoionization processes of Ar^5＋

The 3s–np photoionization processes of the ground state 2P1/2 and the metastable state 2P3/2 of Ar5＋ are investigated using our recently developed relativistic R-matrix code, where the interactions between the bound states and the continuum states are included. Both resonance positions and the oscillator strengths are in much better agreement with the absolute experimental measurements by Wang et al.[Wang J C, Lu M, Esteves D, Habibi M, Alna’washi G and Phaneuf R A 2007 Phys. Rev. A 75 062712] with a resolution of 80 meV than their theoretical results. The contributions of the two experimental unresolved transitions are distinguished in our calculations, which show that the transitions from the ground state also make significant contributions to some resonances. Our theoretical results are also in good agreement with the measurements for the first resonance with a higher resolution of 20 meV.     

碳纳米管锁模双包层光纤激光器的实验研究

本文采用双包层掺镱光纤作为增益介质,用单壁碳纳米管作为饱和吸收体,获得最高输出功率为336 mW的锁模脉冲激光.用飞秒激光诱导水击穿法直接在单模光纤上制备出D形区,通过在D形光纤上滴涂单壁碳纳米管溶液,成功制备出碳纳米管饱和吸收体,并对其饱和吸收特性进行测试,发现其调制深度为27％.利用该饱和吸收体作为锁模器件,制备出具有环形腔结构的锁模光纤激光器.当抽运功率为4W时,获得了脉宽为93.8 fs,中心波长为1083.8 nm,3 dB谱宽为8.6 nm,重复频率为5.59 MHz,平均功率为336 mW的飞秒脉冲激光输出.     

HTS双饼线圈的加工工艺及实验研究

详细介绍了高温超导双饼线圈的加工工艺和流程,针对绕制时带材弯曲形变使临界电流退化的技术难点,重点阐述了其解决方法。在液氮温度、自场环境下对采用该加工工艺制作的YBCO双饼线圈及Bi2223/Ag双饼线圈进行低温实验,测试其V-I特性曲线。实验结果表明:该工艺制作的双饼线圈性能良好,工艺安全可靠,是合适的。同时,实验测量数据也为线圈在磁体中的进一步应用提供了可靠依据。     

Subluminal to superluminal light propagation in a V-type three-level atomic system interacting with squeezed vacuum

We study the light propagation properties in a V-type three-level atomic system interacting with two independent broadband squeezed baths. Using the graphic method, the dependences of the absorption and the dispersion of the system on the coupling field strength, the incoherent pumping field strength and the intensity of the squeezed vacuum are analysed. The result shows that the probe pulse propagation can be changed from subluminal to superluminal speed.     

光锥与CCD耦合器件调制传递函数的测试方法

采用直接对倾斜刀口成像的方法测量光锥与CCD耦合器件的调制传递函数.这种方法对静止刀口成像,利用刀口与光锥纤维列间的一定倾斜角度,在垂直于刀口的不同行之间形成不同的值,代替了传统刀口扫描中的扫描装置,并无需保证刀口与阵列的平行,相应地减小了测量误差. 在数据处理中采用了二次平均法,即对同一幅图像得出的不同位置值进行平均及多次测量结果平均,在一定程度上消除了由于离散性产生的空间非平移不变性的影响.     

Characteristics of an add-drop filter composed of a Mach-Zehnder interferometer and double ring resonators

A planar lightwave circuit (PLC) add-drop filter is proposed and analyzed, which consists of a symmetric Mach-Zehnder interferometer (MZI) combined with double microring resonators. A critical coupling condition is derived for a better box-like drop spectrum. Comparisons of its characteristics with other schemes,such as a MZI with a single ring resonator, are presented, and some of the issues about device design and fabrication are also discussed.