Research

Sub-THz Cavity Enhanced Absorption

While molecular beam experiments are ideal for generating and studying transient species, low number densities and short pathlengths require highly sensitive spectroscopic methods. Unfortunately, in the THz / sub-THz region, the lack of strong sources combined with relatively insensitive detectors significantly impacts sensitivities. In the optical and Near-IR, fluorescent photon counting is possible, while in the longer microwave and RF regions, exquisitely sensitive heterodyne techniques are available. Not only are the sources and detectors better in these other electromagnetic regions, they may also be combined with cavities to dramatically increase the effective sample pathlengths, e.g., Cavity Enhanced Absorption Spectroscopy. Robustly coupling radiation in and out of cavities has proven to be problematic in the THz / sub-THz region due to the lack of high reflectivity / partially transmissive concave dichroic mirrors at these wavelengths. A partial solution has been to employ multipass or unconventional cavity geometries.

We have demonstrated that it is possible to create a pass-through confocal Fabry–Pérot resonator at sub-THz wavelengths; the novelty being the simple conventional layout. To do this we have created, the equivalent of dichroic mirrors with concave wire-grid polarizers on a supporting plastic substrate. The resulting signals are beautiful and dramatic (see Fig. 1). We obtain cavity Qs on the order of 100,000, a 70-fold sensitivity increase and use it to demonstrate the detection of ClO radicals and even the weak magnetic dipole allowed transitions of O2 near 60 GHz. As the signals are coherent, we model the line shapes using a modified Flygare et al. solution to the Optical Bloch equations.