Highly sensitive spectroscopy techniques in the microwave and infrared spectral regions have allowed the study of many transient molecules. These experiments have primarily been conducted using cavity-enhanced spectroscopoic techniques such as Fourier Transform Microwave (FTMW) Spectroscopy and Cavity Ringdown Spectroscopy (CRDS). FTMW covers frequencies below 50 GHz, while IR-CRDS covers frequencies above 1000 cm-1. These frequency limits are dictated almost entirely by the availability of radiation sources. The spectral region between these two windows is the submillimeter, or terahertz, frequency window, which has been dubbed the "gap in the electromagnetic spectrum."
Another limitation in THz spectroscopy is the lack of mirrors that enable the coupling of THz radiation into the cavity while maintaining high-reflectivity. One promising option is the use of wire-grid polarizers instead of traditional dielectric-coated or metal-coated mirrors. We are currently developing a THz-CRDS instrument using these polarizers. This instrument should rival the sensitivity of FTMW and CRDS spectrometers: