Compact Isotopic Raman Multigas Trace Detection System

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Spontaneous Raman scattering is a simple and well-studied process that is ideally suited for chemical analysis. However, while routinely implemented in commercial systems to identify molecular species in solids and liquids, few solutions exist for the measurement of gases, due in part to the need for a powerful and spectrally pure laser source. Yet, a compact and inexpensive portable chemical gas analyzer would find a plethora of applications in industrial process control, medical diagnostics and hazards detection. The present project is exploring solutions to bring gaseous spontaneous Raman scattering to a level of maturity that could allow it to be used in consumer-level devices. Implemented with an ordinary laser diode in conjunction with an engineered multi-pass beam geometry that simultaneously defines the laser’s spectral purity via feedback, the technique investigated can detect trace chemicals including isotopologues, i.e., molecules that differ only by their isotopic composition. These efforts are focusing on practically relevant measurements such as atmospheric and breath detection that could pave the way for the realization of versatile and widely deployable “artificial noses”. The project entails training graduate and undergraduate students in an interdisciplinary environment of photonics and quantum physics research. Due to its inherent simplicity and versatility, spontaneous Raman scattering is ideally suited for chemical gas analysis. However, with a small scattering cross-section, the process typically requires an enhancement method, of which the most famous example is probably surface enhanced Raman scattering. Recent years have seen a resurgence of approaches such as cavity, capillary, hollow core fiber, and Purcell-enhanced Raman scattering, some of which have demonstrated gas sensing capabilities in the parts-per-million concentration range. The primary goal of the proposed work is to develop a novel method expected to offer dramatic improvements, in particular, detection limits in the parts-per-billion range, while employing a low-cost compact design with all components operating at room temperature. The novel approach is based on a multi-pass cavity integrated as part of an external cavity diode laser that is multimode and efficient. The project explores methods of producing an engineered electromagnetic environment that optimizes the degree to which Raman scattered photons are collected. It aims at understanding how an optical cavity can be utilized most effectively to enhance the rate of spontaneous Raman scattering for the benefit of trace detection. The project has implications for cavity quantum electrodynamics research and general metrology science as well as a diverse range of applications in isotopologue trace gas sensing.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.