THz Spectroscopy

Most of the molecules observed in space have been detected via their rotational transitions through radio telescope observations.  In particular, observations in the far-infrared, or THz, regime are most commonly used to detect complex organic molecules that are important in prebiotic chemistry.  Hardware for this type of spectrometer (i.e., light sources, detectors, etc.) has been historically limited.  However, the recent development of several far-infrared observatories combined with the technological push forward for telecommunications and security applications has led to commercial production of components that enable new spectroscopy to be explored both in the lab and in space.

The standard approach for measuring laboratory rotational spectra is a simple absorption experiment with one pass of light through the sample.  While this approach works well for stable molecules, it is often not of sufficient sensitivity to detect radicals, ions, or reactive species.  We therefore are always seeking new methods that will improve spectral sensitivity so that we can use these techniques to study interesting astrochemical species.

One common approach to increase sensitivity is to increase the number of passes that the light travels through the sample. We have implemented multipass spectrometers that have helped in this regard.  But to truly push our capabilities forward, we must explore options that increase the pathlength by orders of magnitude rather than by just a factor of a few.  To do this, we need to use sensitive cavity-enhanced spectroscopic techniques involving high-finesse Fabry-Perot resonators.  Such resonators are widely used in the microwave (i.e. FTMW) and near-infrared (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.