Lamb dip spectroscopy with a terahertz quantum-cascade laser

We report on Doppler-free spectroscopy using a free-running terahertz quantum-cascade laser at 3.3 THz in a pumpprobe configuration with a fast Ge:Ga detector. The sensitivity of the setup allows for the observation of a Lamb dip in deuterated water with a resolution of less than 1 MHz and an acquisition time of only 10 ms.