Figure 1: Principle of cavity-enhanced Raman spectroscopy.

(a) Schematic of the cavity formed by the end-facet of an optical fibre (top) and a macroscopic mirror (bottom). CNTs (black lines) are deposited on the macroscopic mirror and interact strongly with the light field when inside the cavity. The large mirror can be positioned laterally by a nanopositioner and the cavity length can be tuned by a piezo actuator (see black arrows). (b) Schematic level diagram for cavity-enhanced Raman scattering showing a ground state (g), a virtual state (v) and a vibrationally excited state (f). The Raman scattering is stimulated by the resonance with the cavity. (c) Calculated cavity finesse from a measurement of the mirror transmission (line) and a direct measurement of the Finesse (blue datapoint) for cavity B (see the ‘Methods’ section). The upward and downward arrows symbolize excitation and stimulated Raman scattering, respectively. Selective enhancement occurs at a cavity resonance, whose wavelength is set by the cavity length. (d) Calculation of the effective Purcell factor as a function of mode volume for different quality factors Q_r of Raman features (solid lines) for Q_r<<Q_c. The dashed line is the ideal Purcell factor, equivalent to the effective Purcell factor for Q_r>>Q_c. It solely depends on the Q-factor of the cavity and is evaluated for cavity B used in the experiments.