GaAs体材料及其量子阱的光学极化退相特性

采用飞秒时间分辨瞬态简并四波混频技术，在室温下测量了GaAs体材料及其量子阱材料GaAs/Al_0.3Ga_0.7As的光学极化超快退相时间，当激光中心波长为785nm，受激载流子浓度为10¹¹cm^-2时，它们的退相时间分别为28fs和46fs.量子阱材料的退相时间比体材料的长，这是由于量子阱中的载流子在垂直于GaAs/AlGaAs界面的运动受到限制，运动呈现二维特性，大大减小了载流子的散射概率.实验中观察到瞬态简并四波混 关键词： 时间分辨简并四波混频 飞秒激光脉冲 退相 密度矩阵     

利用半周期激光场调控H_2~+谐波辐射过程

采用数值求解核与电子耦合的薛定谔方程,对H_2~+分子谐波辐射调控进行了研究.计算结果表明,激光场正向时,谐波能量主要来源于负向H核贡献;激光场负向时,谐波能量主要来源于正向H核.随后,通过适当引入半周期激光场,可以有效调节正负H核辐射谐波的强度.最后,适当叠加谐波谱上的谐波,可获得一个46 as的脉冲.该研究对调控分子谐波的辐射过程及阿秒脉冲的输出是有帮助的.     

飞秒脉冲振幅和相位的无干涉条纹重构法测量

对作者所提出的无干涉条纹直接电场重构测量飞秒脉冲的振幅和相位的新方法作出进一步理论分析，并通过实验测量说明该方法的优越性.该方法克服了传统的SPIDER方法的弊病，能得到一组无干涉条纹的图像，排除传统方法必须使用傅里叶变换滤波消除干涉条纹而引进的系统误差，使得该方法能够采用较简便设备且能较准确测量飞秒脉冲强度轮廓和相位.最后给出同一条件下新方法和传统SPIDER方法分别重构的脉冲强度自相关曲线与实验测量结果的比较，以说明新方法的有效性和优越性.     

Spectral compression of femtosecond pulses via frequency intermodulation

A new technique of spectral compression of femtosecond pulses, based on their frequency intermodulation in the frequency mixing process, is demonstrated experimentally. In the exploited phenomenon of frequency intermodulation the phase of the reference pulse is superimposed on the signal instead of phase self- or cross-modulation used in traditional spectral compressors. In the process of frequency mixing a twelve-fold aberration-free spectral compression of 100 fs pulses has been achieved experimentally.     

Propagating beam analysis of fiber wavelength filters with a depressed-index section

A wavelength filter consisting of single-mode and few-mode fibers is investigated numerically. A simple finite-difference beam-propagation method, in which a transparent boundary condition can be imposed, is developed for circularly symmetric waveguides. After confirming the validity of the numerical method by the mode-mismatch loss, we calculate the propagating field in the fiber wavelength filter, in which interference between LP₀₁, and LP₀₂ modes occurs. To improve the filtering operation, a depressed-index fiber is employed for the few-mode fiber. The effects of the radius and refractive index of the depressed section on the transmission power are revealed and discussed. Power is suppressed to less than 0.1% at 1.3 μm, while maintaining power transmission of more than 85% at 1.55 μm. It is also found that the filtering operation shifts to higher wavelengths as the input power is increased when we choose a self-focusing nonlinear material in the depressed section.     

Use of Induced Circularly Polarized Luminescence (CPL) from Racemic D3 Lanthanide Complexes to Determine the Absolute Configuration of Amino Acids

The perturbation of the racemic equilibrium of luminescent D₃ lanthanide(III) complexes by added chiral agents, such as amino acids, may be a useful technique for determining the absolute configuration of the added species. It is shown in this work, however, that simple interpretation of the equilibrium shift of racemic tris-terdentate complexes of Tb(III) with 2,6-pyridine-dicarboxylate by amino acids, as measured by the sign of the resultant circularly polarized luminescence (CPL) in terms of specific structural characteristics is not possible. CPL results for a number of derivatized amino acids are also presented, and some insights into the nature of the chiral discriminatory forces that might be exploited in this kind of study are discussed.     

Construction of universal rotations from point-to-point transformations

For a desired range of offsets, universal rotations of arbitrary flip angle can be constructed based on point-to-point rotations of I(y) with half the flip angle. This approach allows, for example, creation of broadband or bandselective refocusing pulses from broadband or bandselective excitation pulses. Furthermore, universal rotations about any axis can be obtained from point-to-point transformations that can easily be optimized using optimal control algorithms. The construction procedure is demonstrated on the examples of a broadband refocusing pulse, a broadband 120(x) degrees rotation and a z-rotation with offset pattern.     

High resolution heteronuclear correlation NMR spectroscopy between quadrupolar nuclei and protons in the solid state

A high resolution two-dimensional solid state NMR experiment is presented that correlates half-integer quadrupolar spins with protons. In this experiment the quadrupolar nuclei evolve during t1 under a split-t1, FAM-enhanced MQMAS pulse scheme. After each t1 period ending at the MQMAS echo position, single quantum magnetization is transferred, via a cross polarization process in the mixing time, from the quadrupolar nuclei to the protons. High-resolution proton signals are then detected in the t2 time domain during wPMLG5* homonuclear decoupling. The experiment has been demonstrated on a powder sample of sodium citrate and 23Na-1H 2D correlation spectra have been obtained. From the HETCOR spectra and the regular MQMAS spectrum, the three crystallographically inequivalent Na+ sites in the asymmetric unit were assigned. This MQMAS-wPMLG HETCOR pulse sequence can be used for spectral editing of half-integer quadrupolar nuclei coupled to protons.     

Homonuclear decoupled 13C chemical shift anisotropy in 13C doubly labeled peptides by selective-pulse solid-state NMR

We describe a new experiment for measuring homonuclear-decoupled anisotropic chemical shift patterns in doubly 13C-labeled compounds under magic-angle spinning. The experiment combines a pair of selective and non-selective 180 degrees pulses to suppress the 13C-13C scalar and dipolar interactions. This is combined with the recently developed SUPER technique to recouple the chemical shift anisotropy. Demonstrations on 13Calpha and 13CO-labeled amino acids and peptides show that accurate chemical shift powder patterns can be obtained. This permits the use of chemical shift anisotropy for conformational studies of suitably extensively 13C-labeled peptides and proteins.     

Determination of pore space shape and size in porous systems using NMR diffusometry. Beyond the short gradient pulse approximation

The influence of finite length gradient pulses on NMR diffusion experiments on liquids confined to diffuse between two parallel planes is investigated. It is experimentally verified that the pore size decreases when determined using finite gradient pulses if the results are analyzed within the short gradient pulse approximation. The results are analyzed using the matrix formulation. The observed minima in the echo decay profiles are considerably less sharp than theoretical analysis would indicate and we suggest that this is due to the presence of a distribution of pore sizes in the sample. In addition, effects due to the presence of background gradients are discussed. It is argued that effects due to the finite length gradient pulses are relatively minor and in realistic applications the effects due to inhomogeneities in pore sizes and effects due to background gradients will constitute more serious problems in pore size determinations by means of NMR diffusometry.