Development of novel schemes of cavity-enhanced Raman spectroscopy for the sensitive detection of gas-phase species in the environment

光谱学是环境科学的核心。光谱技术在分析应用中是不可或缺的，用于识别和量化与环境相关的物种。拉曼光谱是这方面的一项重要技术，因为它可以分析用其他方法难以检测的物质。在凝聚相中，拉曼光谱已成为主要的分析技术。然而，由于拉曼跃迁固有的弱点，拉曼光谱在痕量气体分析中的应用迄今尚未得到广泛的应用。在这个建议中，一种替代的方法，敏感的拉曼光谱的建议，腔增强拉曼光谱与二极管激光器。该方案采用半导体激光器作为拉曼泵浦源，在光腔中进行放大。与标准拉曼实验相比，激光功率的这种增加将使自发拉曼信号强得多。为了进一步提高灵敏度，将使用具有特定反射镜的特殊腔，其还放大拉曼信号本身（受激拉曼，被动放大）。在最后的实验中，将探索通过用相应的光接种腔来选择性地放大目标物质的跃迁的可能性（受激拉曼，主动放大）。预期腔增强将使拉曼信号增加几个数量级;自发的拉曼信号与激光泵浦功率成线性比例，并且在良好的光学腔中，可以容易地实现10000倍的功率积累。此外，受激拉曼方案具有比自发拉曼方案高几个数量级的灵敏度。光腔中的功率积累和受激拉曼激发都将导致比常规拉曼技术高得多的灵敏度。所提出的拉曼方案的预期增加的灵敏度将被要求用于痕量气体检测和环境中污染物的监测。这些方案将允许使用便携式仪器进行新的和改进的分析测量，该便携式仪器适用于对环境中的污染物进行真实的实时现场测量，该仪器可以同时测量空气的所有成分，具有很大的选择性和灵敏度，混合比的动态范围很大。此外，这些新仪器有可能促成与环境科学有关的更多新应用，包括分子氢的痕量探测、氮和氧的同位素选择性测量以及同位素示踪实验。这一发展拉曼光谱新技术的提议有可能导致拉曼光谱作为气相中主要分析工具的出现，与拉曼光谱目前在液体和固体化学分析中的重要作用相当。

Spectroscopy is at the heart of environmental sciences. Spectroscopic techniques are indispensable in analytical applications to identify and quantify species which are relevant for the environment. Raman spectroscopy is an important technique in this context since it can analyse substances which are difficult to detect with other methods. In the condensed phase, Raman spectroscopy has become a major analytical technique. Applications of Raman spectroscopy for trace gas analysis, however, has not found wide-spread use so far due to the inherent weakness of Raman transitions. In this proposal, an alternative approach to sensitive Raman spectroscopy is suggested, cavity-enhanced Raman spectroscopy with diode lasers. In this proposal, a diode laser as Raman pump source is amplified in an optical cavity. This increase in laser power will make spontaneous Raman signals much stronger compared to a standard Raman experiment. To increase sensitivity even further, a special cavity with specific mirrors will be used which also amplifies the Raman signals themselves (stimulated Raman, passively amplified). In a final experiment, the possibility to selectively amplify the transitions of a target species by seeding the cavity with the corresponding light will be explored (stimulated Raman, actively amplified). It is expected that cavity enhancement will increase Raman signals by orders of magnitude; spontaneous Raman signals are linearly proportional to the laser pump power, and in a good optical cavity, power build up by a factor of 10000 can easily be achieved. In addition, stimulated Raman schemes have orders of magnitude higher sensitivity than spontaneous Raman schemes. Both power build up in an optical cavity and stimulated Raman excitation will result in a much greater sensitivity than conventional Raman techniques. The expected increased sensitivities of the proposed Raman schemes will be required for the trace gas detection and monitoring of pollutants in the environment. These schemes would allow new and improved analytical measurements with a portable instrument suitable for real time, in situ field measurements of pollutants in the environment with an instrument that can measure all components of air simultaneously with great selectivity and sensitivity with a large dynamic range for mixing ratios. In addition, these new instruments have the potential to enable further new applications relevant to environmental sciences, including trace detection of molecular hydrogen, and isotope-selective measurements of nitrogen and oxygen and isotope tracer experiments. This proposal to develop new and innovative technologies for Raman spectroscopy has the potential to lead to the emergence of Raman spectroscopy as a major analytical tool in the gas phase, comparable to the important role Raman spectroscopy currently has for chemical analysis of liquids and solids.