Doppler-Free Two-Photon Cavity Ring-Down Spectroscopy of a Nitrous Oxide (N2O) Vibrational Overtone Transition.

We report Doppler-free two-photon absorption of N2O at λ = 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 F = 22730 , and accurate laser detuning over a 400 MHz range was measured relative to an optical frequency comb. At a sample pressure of p = 0.13 kPa, we report a large two-photon cross-section per molecule of σ 13 ( 2 ) = 8.0 × 10 - 41 cm4 s for the Q(18) rovibrational transition at a resonant frequency of ν 0 = 66179400.8 MHz.