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

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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.