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 microm, 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 ${\sigma }_{13}^{\left(2\right)}=8.0\times {10}^{-41}$ cm⁴ s for the Q(18) rovibrational transition at a resonant frequency of ν ₀ = 66179400.8 MHz.