Cavity-enhanced phase-matched coherent Stokes Raman spectroscopy

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 10⁹ 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.