New tools for QCL-based THz Spectroscopy

Optical resonators are well-established tools commonly used in spectroscopy, they have widespread applications over the whole electromagnetic spectrum, while record-level optical finesses were achieved in the visible/near-IR. In this context, the THz portion of the electromagnetic spectrum is still lacking of such tools. One reason is certainly that, for many years, the THz range has been an underexploited region. However, recent advances in generation and detection of THz radiation, as well as the advent of novel THz-emitting laser sources, such as quantum cascade lasers (QCLs), and the constantly evolving technology of new materials, are now making THz light emerge as a new promising frontier for interdisciplinary research areas. However, among the number of different applications of THz radiation, a central role is played by molecular spectroscopy. In order to further increase the sensitivity of a spectroscopic system, high-finesse cavity resonators represent an attractive tool as they give access to much longer interaction lengths between light and absorbing medium, and could also provide a narrow reference for a QCL, allowing a reduction of its free-running linewidth.We report on two different resonant cavities designs, injected by a continuous-wave quantum cascade laser emitting at 2.55 THz [1]. The two cavities are a V-shaped and a ring shaped resonator, the input/output couplers of both cavities are wire grid polarizers, acting as highly reflective mirrors, and each cavity is equipped with a shifting mirror, in order to tune the cavity length. The achieved ring-cavity finesse is about 63, corresponding to a Q-factor of 2.6×10-5. Due to its geometrical configuration, the V-shaped resonator produces an optical feedback to the QCL, when in resonance. This optical feedback (OF) prevents an accurate finesse measurements, as it broadens the resonance peak profiles. In order to better understand this effect, we injected both the resonators at the same times; the V-shaped cavity is swept across its resonances, while the ring cavity is kept in a position corresponding to the center of a resonance side. In this conditions, any shift of the laser frequency induced by OF will result in a variation of the signal transmitted by the ring cavity. Experimental data have confirmed this effect, that in the future can be exploited for optically locking the QCL frequency to the V-shaped cavity resonance.