Laser emission at 4.5 THz from ¹⁵NH₃ and a mid-infrared quantum-cascade laser as a pump source

We present an optically pumped terahertz gas laser, which is based on a mid-infrared quantum-cascade laser as a pump source, a transversely pumped standing wave resonator, and ¹⁵NH₃ as a gain medium. We observe several laser lines around 4.5 THz, corresponding to rotational transitions in the ν₂ band of ammonia. So far, these are the highest frequencies obtained from a QCL-pumped THz gas laser. The involved molecular transitions are unambiguously identified by high-resolution spectroscopy.