Sub-Doppler optical-optical double-resonance spectroscopy using a cavity-enhanced frequency comb probe

Experimental setup and procedures

To demonstrate cavity-enhanced frequency comb OODR spectroscopy we use a high-power 3.3 µm (3000 cm⁻¹) CW pump and a 1.68 µm (5950 cm⁻¹)-centered frequency comb probe, as shown in Fig. 1a. The pump frequency is Lamb-dip locked to a CH₄ transition from a vibrational ground state and populates a selected assigned state in the ν₃ band, while the comb simultaneously probes sub-Doppler hot-band transitions from the pumped ν₃ state and Doppler-broadened transitions from the ground state, as shown in Fig. 1b. The final states reached by the…