We demonstrate phase-matched cavity-enhanced coherent Stokes Raman scattering in hydrogen gas using a high-finesse dispersion-compensated cavity, achieving over 10 9 enhancement compared with spontaneous Raman scattering.
Coherent Raman spectroscopy is widely applied to mitigate the effects of the smallness of scattering cross-sections common in standard Raman spectroscopy. In this work, we use phased-matched coherent “Stokes” Raman scattering in a macroscopic high-finesse optical enhancement cavity pumped using a narrow-band continuous-wave laser to intensify Raman signals derived from a trace of gaseous Raman-active molecules. To demonstrate the superiority of this method, we directly measure the ratio of signals arising from coherent Raman scattering to those arising from spontaneous Raman scattering in a gaseous medium under the same conditions. The measured enhancement ratio exceeds 10 9 and agrees well with the theoretically predicted value, strongly implying that the proposed method is highly promising for the ultrasensitive detection of traces of homonuclear diatomic molecules that are not accessible via absorption spectroscopy.
We report the measurement of degenerate coherent anti-Stokes Raman scattering (CARS) in a high-finesse optical microcavity, with an atmospheric gas as the nonlinear medium. Unlike the well-known bulk process in which index-dispersion compensation is required for phase-matching, efficient microcavity CARS involves a resonant coupling at the Stokes, pump, and anti-Stokes frequencies which can be described using coupled-mode analysis. We show how the interaction is thereby dramatically enhanced in a microscopic sample volume and illustrate the technique for the measurement of CO₂ in air.