Research

mm–waves to Terahertz

What are mm-waves and why do we use them?

In our research, we use pure rotational spectroscopy to probe reaction dynamics.  Oddly enough, while rotational spectroscopy is arguably one of the simplest and highest resolution forms of spectroscopy, it is also one of the most underutilized and least known forms of spectroscopy.  While spectroscopists have been conducting high resolution studies of small molecules with these wavelengths for decades, the technology to generate and detect the appropriate radiation (microwave, millimeter wavelength, submillimeter wavelength, and terahertz radiation, see figure above) has required highly specialized knowledge of microwave and radio frequency technology.  As a result, an unnatural divide has existed between low frequency forms of spectroscopy and so called, “optical” forms of spectroscopy such as infrared, visible, and UV. 

Recently, new simple-to-implement technology has become available for generating millimeter / submillimeter wavelength (mm/submm-waves) radiation (see section on “What’s an Armadillo?”).  This technology is so simple to use that it can now be easily integrated into more complicated, “reaction dynamics” sorts of experiments.   Furthermore, at these frequencies it is actually better to propagate the radiation through “free space”, rather than with waveguides.  This allows the use of plastic lenses to pass the radiation through the experiment. 

We take advantage of already assigned and cataloged transition frequencies (see JPL and Cologne databases) of small molecules to determine the quantum state distribution of the reaction products.  In addition to the ultrahigh resolution afforded by the technology, the spectroscopy may be used to probe a wide variety of product molecules.  The gross selection rule for pure rotational spectroscopy is simply that the molecules are polar.  This means the products / parent molecules may be ions, radicals, neutrals, transient intermediate, or even clusters. 

The figure above gives the dominant sources and detectors used between microwave and IR wavelengths as well as examples of transition frequencies and intensities of HCN at supersonic molecular beam and room temperatures.  The technologies are constantly changing, and as a result, the figure should be considered a rough guide; difference frequency IR techniques now routinely get into the low Mid–IR and THz sources are a hot area of research.  Commercially available multiplier technology from Virginia Diodes now exists up to 1.7 THz (green region below).