基于柱矢量光场的单发五维分辨光谱系统构建及分子取向驰豫超快过程研究

Laser therapy has been considered to be a promising medical technology owning to its advantages of noninvasive,analgesia, safe, convenient and wide indication. Nevertheless, the ambiguity to the mechanism of interaction between laser and biological tissue and the dose-effect relationship determine the contigency and blindness of current laser therapy. Previous investigations have shown that the key to understanding of the interaction mechanism between laser and tissue is to identify the alignment and ultrafast relaxation of the biological molecules under polarized laser radiation. Considering the fact that the biological molecules are fragile to light, we propose to establish single-shot five-dimonsionally resolved spectroscopy system based on four-wave mixing using ultrafast cylindrical vector beam. This new conceptual spectroscopy approach will then be used to study the polarization alignment and transient structure evolution of biological molecules under polarized laser radiation, which would help to uncover the interaction principle between laser and biological tissues. The polarization nonuniformity of cylindrical vector beam and space dependent time delay in the four-wave mixing of BOXCAR configuration enables to obtain polarization- and time-resolved spectra simultaneously from a single shot. Our proposal will not only overcome the drawback that traditional nonlinear spectroscopy can not acquire polarization- and time-resolved measurements simultaneously, but also enables investigation on the irreversible processes which were impossible to be measured by multi-shot five-dimensionally resolved nonlinear spectroscopy. The implementation of this project is potentially to provide a crucial approach for capturing full information of polar molecules in freedoms of polarization, time and space.

激光治疗因无创、无痛、安全、简便、适应性广的优势具有广阔的应用前景。然而，由于激光与生物组织作用的机理和量效关系尚不明确，激光医疗目前还具有一定的偶然性和盲目性。前期研究表明，揭示激光与生物组织作用机理的关键在于明确组织分子在偏振激光作用下的取向和驰豫超快过程。鉴于生物分子光稳定性较差的特点，本项目提出利用超短脉冲柱矢量光构建单发五维分辨四波混频光谱系统，研究生物分子在偏振激光照射下分子极化方向，结构瞬态变化的过程，阐明激光与生物组织作用的机制。利用柱矢量光偏振方向不均匀，以及BOXCAR配置的四波混频过程中时间延迟与空间位置有关的特点，从单帧信号图像即可同时获得偏振分辨和时间分辨光谱。本项目的实施不仅克服了传统时间分辨和偏振光谱无法相互兼顾的缺点，也有效弥补了多发五维分辨非线性光谱技术对不可逆过程无法有效研究的缺陷，有望成为获取极性分子在偏振、时间和空间等各个自由度全部信息的重要手段。

在偏振激光作用下的分子/颗粒物的取向过程是激光与物质相互作用研究中的一个热点领域。实验上，为了获得偏振分辨光谱，通常需要依次测量同一物质在不同偏振激光作用下响应，激光的多次作用会导致物质（尤其是光稳定性较差的物质）的光损伤或降解，导致实验结果的可重复性下降。鉴于此，本项目提出利用柱矢量光偏振方向不均匀，从单帧信号图像同时获得偏振分辨光谱的方法，具体包括以下内容：① 提出了一种基于单束矢量光场的简并多波混频方案。利用柱矢量光场的偏振非均匀但具有轴对称分布的特点，使单束矢量光场通过偏振片之后，获得空间分离的多束相位匹配光束。改变螺旋相位板的拓扑荷数，选择适当的空间滤光片，将空间配置满足相位匹配条件的光束聚焦于Rb原子蒸气池中，得到了简并四波混频、简并六波混频、以及共存的多个简并四波混频信号。由于参与多波混频过程的光束源自同一束矢量光，极大地简化了光学系统，降低了光路调整的复杂程度，提高了抗外界干扰能力，对提高多波混频光谱技术的实用性，以及测量结果的准确性具有重要意义。② 实现了单帧扫描偏振分辨简并四波混频光谱测量，两束泵浦光保持线偏振态，探测光为矢量光场。由于四波混频信号的偏振和强度与探测光的偏振态密切相关，使用偏振态随空间变化的矢量探测光能够产生偏振态和强度随空间变化的简并四波混频信号，因此，从单帧信号图像中即可检索到偏振分辨光谱。③ 使用混合偏振光作为探测光，在 85Rb 原子蒸汽中实现了一种单帧扫描偏振分辨饱和吸收光谱。利用混合偏振探测光的偏振态随空间变化的特性，能够从单帧信号中检索到偏振分辨饱和吸收光谱信息。然后，在实验上对两个共振跃迁处（|5S1/2, F=3> → |5P3/2, F′=4> 和 |5S1/2, F=3> → |5P3/2, F′=3>）的饱和吸收信号的偏振依赖特性进行了探测和比较，发现在“闭合的二能级系统” |5S1/2, F=3> → |5P3/2, F′=4> 处，可以观察到饱和信号具有强烈的偏振依赖特性。然而在|5S1/2, F=3> → |5P3/2, F′=3> 这个“开放的二能级系统”处没有观察到饱和吸收信号的偏振依赖特性。本项目提出并实现了单发偏振分辨光谱，将为研究光稳定性较差的物质提供新思路和新方法。