极性分子量子气体

超冷极性分子的研究是当前原子分子和光物理研究的前沿之一.由于分子是组成物质材料的基本单元,所以这一前沿研究会很自然地和化学、量子信息、凝聚态物理和天体物理密切地交叉,并且有可能带来新的技术突破.操控分子间的相互作用也因此成为几代物理学家长期探索的重要科学问题.在过去的30年间,研究人员已经成功地将原子冷却     

Effect of tunneling ionization on dynamic alignment and post-ionization alignment of nitrogen molecules irradiated by different laser intensities

The dynamic alignment and post-ionization alignment of nitrogen molecules are investigated while considering the effect of tunneling ionization. The effects of tunneling ionization on the angular distribution are calculated when the molecules are irradiated by different laser intensities. The results show that laser intensity directly affects the time and extent of dynamic alignment. Furthermore, the extent of post-ionization alignment is not only determined by laser intensity but also affected by the final extent of dynamic alignment. The post-ionization alignment will dominate during the process of molecular (or molecular ion) rotational alignment for femtosecond laser pulse. The time of tunneling ionization is a significant factor to the final ensemble angular distribution of molecular ions when laser intensity is low.     

Tuning the structural properties of femtosecond-laser-induced nanogratings

We present the results of our investigations on the formation process of nanogratings in fused silica and the influence of fabrication parameters, thereby identifying ways to systematically control the grating properties. Nanogratings, self-organized nanostructures with subwavelength periodicity, are formed in certain parameter ranges during femtosecond-laser processing of transparent materials, resulting in characteristic birefringent modifications. They provide the opportunity for the fabrication of arbitrary three-dimensional birefringent elements with position-dependent retardation. Based on our findings, we were able to fabricate birefringent elements with various precise retardations in otherwise isotropic fused silica.     

Diffusion in the transient dynamic response regime

An ensemble Monte Carlo approach is used to calculate the diffusion of a pulse of carriers in GaAs. On short-time scales, comparable to the relaxation times, the non-stationary diffusion process is non-markovian in space and appears not to obey a stationary-state definition of the Einstein relation.     

Field-free three-dimensional alignment of polyatomic molecules

We experimentally demonstrate field-free, three-dimensional alignment (FF3DA) of polyatomic asymmetric top molecules. We achieve FF3DA in sulfur dioxide gas using two time-delayed, orthogonally polarized, nonresonant, femtosecond laser pulses. Our method avoids the use of rotational revivals and is therefore more robust to temperature. The alignment is probed using time-delayed coincidence Coulomb explosion imaging. FF3DA will be important for all molecular imaging, dynamics, or spectroscopy experiments for which random alignment leads to a loss of information.     

Nonadiabatic alignment of asymmetric top molecules: field-free alignment of iodobenzene

Nonadiabatic laser alignment of an asymmetric top molecule is studied using the combination of a quantum dynamical theory and time-resolved photofragment imaging experiments. In particular, the degree of alignment of iodobenzene, induced by an intense, linearly polarized picosecond laser pulse, is calculated and measured. Pronounced alignment is obtained under field-free conditions.     

Intense-field alignment of molecules confined in octahedral fields

The combined effect of static octahedral potential and anisotropic interactions with intense linearly polarized light on the rotational motion of linear molecules is investigated. Avoided crossings of quantized energy levels corresponding to pendular states with different degrees of alignment are found by varying the strength parameters for the light and static fields. High alignment is achieved for both cooperative and competitive choices of the relative directionality of the two fields, thus presenting means for controlling the dynamics of impurity centers with respect to the surrounding media.     

Evolutionary algorithms in the optimization of dynamic molecular alignment

We numerically study the optimization of dynamic molecular alignment by shaped femtosecond pulses. It is found that an accurate synchronization of the applied field to the molecular response is needed to obtain maximum alignment. Simple approaches based on low-dimensional parametrizations of the electric field are shown to be clearly inferior, especially in the strongly nonperturbative regime.     

Numerical simulation of atomic diffraction in the Raman-Nath regime

The theory of atomic diffraction from a classical standing wave light field in the presence of spontaneous emission in the Raman-Nath regime was developed by Tanguy et al. [6]. We describe the basis of computationally efficient methods for performing calculations in this regime and show their agreement with recent experimental results of Gould et al. [4].     

超辐射机制相对论返波管的数值计算

运用相对论返波管非线性理论,推导出描述相对论返波管束波互作用的非线性自洽方程组;数值求解了该非线性自洽方程组,揭示了相对论返波管中由非稳态束波互作用产生的超辐射现象的基本规律：输出超辐射波峰值功率大约与参与互作用电子总电荷量的平方成正比。并将超辐射和由稳态束波互作用所产生的受激辐射作了相关比较,结果表明：超辐射是短电子束脉冲产生非稳态束波互作用的结果,而受激辐射是长电子束脉冲产生稳态束波互作用的结果;不断增加电子束脉冲宽度,辐射机制由超辐射转变为受激辐射。     

超辐射机制相对论返波管的数值计算

 运用相对论返波管非线性理论,推导出描述相对论返波管束波互作用的非线性自洽方程组;数值求解了该非线性自洽方程组,揭示了相对论返波管中由非稳态束波互作用产生的超辐射现象的基本规律：输出超辐射波峰值功率大约与参与互作用电子总电荷量的平方成正比。并将超辐射和由稳态束波互作用所产生的受激辐射作了相关比较,结果表明：超辐射是短电子束脉冲产生非稳态束波互作用的结果,而受激辐射是长电子束脉冲产生稳态束波互作用的结果;不断增加电子束脉冲宽度,辐射机制由超辐射转变为受激辐射。     

Simulations of resonant X-ray emission spectra of molecules

ab initio calculations. We let each molecule, or group of molecules, represent one particular aspect: calculations on molecular oxygen, benzene, carbon dioxide, aniline, fullerenes, chlorofluoromethanes, carbon monoxide, and polyenes are used to illustrate, in order, the effects of parity selection rules and core hole localization, vibronic coupling and symmetry breaking, frequency dependence of symmetry breaking, the role of chemical shifts, the character of band gap excited spectra, polarization anisotropies, the role of screening, and the role of excitons. Accepted: 6 March 1997     

Metamaterials电磁特性的数值仿真研究

利用HFSS软件,对电磁波在左手材料中的后向波传输、平板成像特性和近零折射率媒质的定向辐射特性进行了全波仿真研究,分别验证了左手材料与近零折射率媒质的上述特性。利用AnsoftDesigner软件,对去掉并联电感和去掉串联电容的、基于微带线的两种简化的左右手复合传输线构成的具有磁单负和电单负特性介质的电磁特性进行了仿真研究,验证了电磁波在匹配的MNG-ENG介质对结构的隧穿特性。     

Simulations of dynamic resistive evaporation in a vacuum

The model of dynamic resistive evaporation in vacuum has been considered, and the model takes into account the structural peculiarities of the corresponding evaporator. In the model, the dependences to determine the time of material heating up to evaporation temperature as well as dynamic characteristics of the evaporation have been obtained. It has been shown that the obtained characteristics are nonharmonic and periodically repeated. The adequacy of the developed model to the physical model has been corroborated. It has been found that the discrepancy between the experimental and calculated time characteristics of shutter movement is not higher than 5%. The recommendations for using the suggested model to fabricate of thin films of multicomponent materials via thermal evaporation have been considered.     

Functional MR imaging in the awake monkey: effects of motion on dynamic off-resonance and processing strategies

Functional MR imaging of the alert, behaving monkey is being used more and more often to detect activation patterns and guide electrophysiological research investigating the neural basis of behavior. Several labs have reported fMRI data from the awake monkey, but none of them has studied and systematically corrected the effects of monkeys' motion on fMRI time series. In this study, a significant refinement of acquisition and correction strategies is reported that can be used to minimize magnetic susceptibility artifacts induced by respiration and by jaw and body movement. Real-time acquisition of sensor signals (e.g., signals induced by jaw and body movement) and MR navigator data were combined to optimize fMRI signal-correction strategies. Within trials, the artifact-induced off-resonance changes were small and mainly reflected the effects of respiration; between trials, movements caused major changes of global frequency and shim (>20 Hz/cm). Several methods were used to assess the stability of the fMRI series: k-space analysis ('dynamic intensity and off-resonance changes in k-space', dubbed DICK and DORK) and image analysis using a Laplace operator and a center-of-mass metric. The variability between trials made it essential to correct for inter-trial variations. On the other hand, images were sufficiently stable with our approach to perform fMRI evaluations on single trials before averaging of trials. Different motion correction strategies were compared: DORK, McFLIRT (rigid body model with three translations and three rotations) and 2D image alignment based on a center-of-mass detection (in-plane translation). The latter yielded the best results and proved to be fast and robust for intra- and inter-trial alignment. Finally, fMRI in the behaving monkey was tested for spatial and temporal reproducibility on a trial-to-trial basis. Highly activated voxels also displayed good reproducibility between trials. On average, the BOLD amplitude response to a short 3-s visual stimulus was close to 2%.