Sub-Doppler Double-Resonance Spectroscopy of Methane using a Frequency Comb Probe

We use a 3.3 μm continuous wave optical parametric oscillator as a pump and a 1.67 μm frequency comb as a probe to record 36 sub-Doppler double-resonance transitions in the 3v3 band of methane (including 26 previously unreported) with ∼1.5 MHz center frequency accuracy.