R6G单分子表面增强共振拉曼散射光谱探测研究

以共焦显微系统为平台,研究了不同浓度的R6G银溶胶的表面增强共振拉曼散射(SERRS)光谱, 结果表明不同浓度溶液中的R6G分子表现出了不同的光谱特性。在浓度为10-13mol·L-1的R6G银溶胶中 得到了R6G单分子的表面增强共振拉曼散射光谱,观察到了一些光谱非均匀变化现象,如谱色散、谱线的 强度起伏、拉曼谱的偏振化以及分子的闪烁等,并对这些现象进行了分析,证明得到的是R6G单分子的 SERRS光谱。文章还对单分子检测中的一些关键问题进行了分析与讨论,确定了单分子SERRS光谱检测的 适当条件。     

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.     

最优化“调控”区间分离密度泛函理论的研究进展

发展精确、高效的交换-关联泛函一直是密度泛函理论工作者所追求的神圣目标。传统密度泛函被证实在计算原子或分子体系的某些基态和激发态性能时存在困难,而且预测不具有普适性;另一方面,一些高水平方法如耦合簇（CC）理论和基于格林函数（G）和屏蔽库仑作用（W）近似的多体微扰理论（MBPT）,尽管相对精确但往往需要消耗昂贵的计算成本,因而其研究体系的尺寸和实用性受到了很大的限制。近年来,“最优化”调控区间分离泛函的发展在一定程度上使得上述问题得到改善,尤其是在消耗较少的计算成本前提下能够达到与高水平方法相媲美的预测精度,引起了越来越多的关注。本文首先简要回顾了密度泛函领域的理论背景,在区间分离密度泛函理论的基础上,重点介绍了最优化“调控”的概念;并且结合近期的理论工作对其在实际计算时的表现进行评价;最后,就最优化“调控”方法的前景和应用进行了展望。     

两个双能级原子与辐射场的喇曼相互作用

研究了两个双能级原子与单模辐射腔场的喇曼相互作用。分别计算了两个原子与光场之间具有不同耦合常数和两个原子虽与光场具有相同耦合常数但同时考虑了原子间偶极-偶极相互作用两种情形下的辐射谱。分析了两个原子与辐射场之间耦合常数的相对大小和两原子间偶极作用的计入对辐射谱的影响。 关键词：     

碰撞对钠分子梯形能级系统高位能级布居的影响

以氩气为缓冲气体,在实验上研究了碰撞过程对钠分子梯形能级系统高位能级两种布居过程的影响。对于双光子激发过程,由于消相碰撞,无多普勒加宽的洛伦兹尖峰随缓冲气体压力的增大而很快消失,加入少量缓冲气体时,碰撞激发过程对以多普勒宽度为特征的信号有一定的增强作用,但加入过量缓冲气体会由于消布居作用使信号减弱,对于主要通过碰撞实现高位能级布居的过程,加入缓冲气体对高位能级布居起关键作用,在速率方程近似下解释了上述结果。     

Characteristics of ultrafast K line hard x—ray source from femtosecond terawatt laser—produced plasma

Theoretical studies and analytical scalings were carried out to find the optimized laser parameters and target conditions so that ultrashort hard x-ray pulses and high x-ray power could be achieved.The dependence of laser intensity and wavelength on the yield of K-shell x-ray emission was studied.We propose an optimal design for a foil target for producing high-yield hard x-ray pulses of customizing duration.     

Study on the interaction of intense femtosecond laser pulses with nanometre-sized hydrogen clusters

The interaction of intense femtosecond laser pulses with hydrogen clusters has been experimentally studied. The hydrogen clusters were produced from expansion of high-pressure hydrogen gas (backed up to 8\tm10⁶Pa) into vacuum through a conical nozzle cryogenically cooled by liquid nitrogen. The average size of hydrogen clusters was estimated by Rayleigh scattering measurement and the maximum proton energy of up to 4.2keV has been obtained from the Coulomb explosion of hydrogen clusters under 2×10¹⁶ W/cm² laser irradiation. Dependence of the maximum proton energy on cluster size and laser intensity was investigated, indicating the correlation between the laser intensity and the cluster size. The maximum proton energy is found to be directly proportional to the laser intensity, which is consistent with the theoretical prediction.     

Surface-enhanced resonance Raman scattering spectroscopy of single R6G molecules

Surface-enhanced resonance Raman scattering (SERRS) of Rhodamine 6G (R6G) adsorbed on colloidal silver clusters has been studied. Based on the great enhancement of the Raman signal and the quench of the fluorescence, the SERRS spectra of R6G were recorded for the samples of dye colloidal solution with different concentrations. Spectral inhomogeneity behaviours from single molecules in the dried sample films were observed with complementary evidences, such as spectral polarization, spectral diffusion, intensity fluctuation of vibrational lines and even ``breathing' of the molecules. Sequential spectra observed from a liquid sample with an average of 0.3 dye molecules in the probed volume exhibited the expected Poisson distribution for actually measuring 0, 1 or 2 molecules. Difference between the SERRS spectra of R6G excited by linearly and circularly polarized light were experimentally measured.     

Accurate determination of the absolute phase and temporal-pulse phase of few-cycle laser pulses

A Fourier analysis method is used to accurately determine not only the absolute phase but also the temporal-pulse phase of an isolated few-cycle (chirped) laser pulse. This method is independent of the pulse shape and can fully characterize the light wave even though only a few samples per optical cycle are available. It paves the way for investigating the absolute phase-dependent extreme nonlinear optics, and the evolutions of the absolute phase and the temporal-pulse phase of few-cycle laser pulses.     

Constructive interference via collision—aided radiative excitation

We have studied theoretically constructive interference via collision-aided radiative excitation in an open four-level system using a density matrix approach. The four-level system consists of a Λ-type three-level quantum-beat configuration driven by one laser field and a fourth level coupled by a vacuum mode. It is shown that through the incoherent process (collision), coherence between widely-separated doublets and subsequent constructive interference can be realized. We analyse the effects of the collision-induced coherent and incoherent decay rates, laser intensity, and energy separation of the doublets on the interference. Meanwhile, the constructive interference between the two transition pathways 3P_1/2-4D and 3P_3/2-4D via equal-frequency hybrid excitation and collision-aided radiative excitation in an Na₂-Na system is experimentally observed. A good agreement between the theoretical and experimental results is obtained.