Doppler-Free Two-Photon Cavity Ring-Down Spectroscopy of a Nitrous Oxide (NO) Vibrational Overtone Transition.

We report Doppler-free two-photon absorption of NO at = 4.53 μm, measured by cavity ring-down spectroscopy. High power was achieved by optical self-locking of a quantum cascade laser to a linear resonator of finesse , and accurate laser detuning over a 400 MHz range was measured relative to an optical frequency comb. At a sample pressure of = 0.13 kPa, we report a large two-photon cross-section per molecule of cm s for the (18) rovibrational transition at a resonant frequency of = 66179400.8 MHz.