Precision molecular spectroscopy with a phase-locked terahertz quantum-cascade laser

We demonstrate precision molecular spectroscopy with a phase-locked quantum cascade laser (QCL) over its entire frequency tuning range at 3.4 THz. A Schottky diode harmonic mixer is used to mix the QCL emission with a multiplied microwave signal, referenced to a rubidium frequency standard, to create an intermediate frequency (IF) signal, which is used to phase-lock the QCL. Tuning the frequency of the multiplied microwave reference and keeping the IF fixed allows for precise frequency control of the laser. We identify ten methanol transitions between 3443.5 and 3447.6 GHz and quantify the transition frequencies and broadening parameters with high precision. These measurements are performed without liquid cryogens in a compact setup.