Cavity-enhanced thz molecular spectroscopy
We have realized and characterized two different designs for resonant THz cavities, based on wire-grid polarizers as input/output couplers, and injected by a continuous-wave quantum cascade laser (QCL) emitting at 2.55 THz. The resonators performances experimentally achieved, in terms of finesse and coupling efficiency, agree well with theoretical calculations based on the measured mirror losses. With achieved quality factor Q≈2.5×10<sup>5</sup>,these cavities show resonant peaks as narrow as few MHz, comparable with the typical Doppler linewidth of THz molecular transitions and slightly broader than the free-running QCL emission spectrum. The effects of the optical feedback from one cavity to the QCL have been examined by using the other cavity as a frequency reference and an optical lock of the QCL frequency to the resonant cavity might be feasible.