Purcell-enhanced Raman scattering from atmospheric gases in a high-finesse microcavity

We demonstrate the Purcell spontaneous-emission enhancement effect for Raman scattering from gases in a $\ensuremath{\sim}$10-$\ensuremath{\mu}$m-long Fabry-P\'erot microcavity with a mode volume of $\ensuremath{\sim}$100 $\ensuremath{\mu}$m${}^{3}$ and a peak finesse of 50 000. The cavity increases the pump laser intensity by resonant recirculation while simultaneously enhancing the rate of Raman scattering into a separate cavity mode by modification of the density of states of the intracavity electromagnetic vacuum field. The result is an overall enhancement of $\ensuremath{\sim}$10${}^{7}$ compared to free-space emission into the same solid angle as the cavity, enabling trace gas sensing requiring only minute sample quantities.