Frequency modulation spectroscopy with a terahertz quantum-cascade laser

Many physical phenomena have characteristic energies, which correspond to terahertz (THz) frequencies. For example, high-resolution spectroscopy allows for the investigation of the structure and the energy levels of molecules and atoms. THz quantum-cascade lasers (QCLs) are promising radiation sources for such a type of spectroscopy, because they are frequency tunable, and they exhibit mW output powers as well as a narrow line width. So far, absorption spectroscopy with QCLs employed modulation of the QCL frequency on the order of kHz and phase-sensitive detection (1). We describe a spectrometer based on a QCL using frequency modulation (FM) spectroscopy with frequencies up to 50 MHz. The condition for FM spectroscopy is achieved when the modulation frequency is large compared to a characteristic width of the spectral feature of interest and only one sideband probes the spectral feature. Potentially, the method is very sensitive, because the modulation frequency is well above the frequencies of the most important noise sources (2)