Connection between vibrational instabilities of molecules in surface-enhanced Raman spectroscopy and Raman lasing

To observe a vibrational instability in a molecule or Raman lasing, the molecules should be placed in a resonator and illuminated by a laser. Both these phenomena are self-oscillations of either photons in a resonator or nuclei in molecules. We show that, thanks to the coupling of the forced oscillations of electrons in a molecule with its nucleus vibrations, these two effects are manifestations of the same phenomenon. When the ratio of damping rates of the molecule and the resonator is large, the number of coherent photons is also large, causing Raman lasing. In this case, the number of quanta of the coherent molecular vibrations is negligible. In the opposite case, the number of coherent vibration quanta is large, causing vibrational instability.