Lamb-dip spectroscopy of rotational levels with UTC-PD terahertz emitter

Pump-probe saturation spectroscopy in the sub-terahertz region was performed in the rotational transition (J, K) = (16, 0) <- (15, 0) for gas-phase acetonitrile molecules in the counter-propagating configuration. We observed Lamb-dips at much lower excitation powers than previously reported. The linewidth in the zero-pressure limit was 10 kHz, which was estimated from the intensity and pressure dependence. This corresponds to the transit-time broadening.