Sensitive cavity-enhanced spectroscopic techniques involving high-finesse Fabry-Perot resonators are widely used in the microwave (i.e. FTMW) and near-infrare (i.e. CRDS) spectral regimes, but hardware limitations have hindered their extension to far-infrared (THz) wavelengths. Thus, until recently, no spectroscopic technique with sensitivities comparable to FTMW or CRDS existed for the THz range, resulting in the frequency region being described as "the gap in the electromagnetic spectrum."
The success and sensitivity of any high-finesse technique depends inherently on radiation coupling efficiency and on the reflectivity of the optical elements that define the cavity. At THz frequencies, neither the equipment nor the methodologies used in either the microwave or the near-infrared region are appropriate. We are therefore investigating a promising alternative to traditional cavity mirrors where wire grid polarizers are used to both couple and trap the THz radiation. Our optimized design uses three polarizers to minimize background signal at the detector and maximize alignment efficiency. An intracavity focusing optic is included to minimize diffraction losses upon successive passes through the cavity.
With this design, we observe sharp cavity modes indicating cavity quality factors on the order of 105 at 250 GHz. From the design performance, we are investigating the feasibility of extending high-finesse cavity techniques to THz frequencies.