RLE PI Qing Hu and graduate students David Burghoff, Tsung-Yu Kao, Ningren Han, Chun Wang Ivan Chan, Xiaowei Cai and Yang Yang have published their breakthrough research on terahertz laser frequency combs, and are featured as the cover story of the June 2014 issue of Nature Photonics. This significant research has applications expanding from cancer detection to explosive sensing.
The Team coaxed terahertz quantum cascade lasers to act as frequency combs, a type of light source consisting of a large number of evenly-spaced frequencies, much like the lines of a ruler. The frequency combs are useful for broadband spectroscopy as they contain many colors. Since the colors are evenly-spaced, their presence can be easily identified. Until now, laser frequency combs have been restricted to the more technologically-mature shorter wavelengths. This breakthrough will enable the creation of compact, portable spectrometers that can reliably detect many types of molecules using terahertz light.
The MIT researchers collaborated with Darren Hayton and Jian-Rong Gao at the SRON Netherlands Institute for Space Research, as well as John Reno at the Center for Integrated Nanotechnology, Sandia National Laboratories, New Mexico. The work at MIT was supported by NASA and the NSF.
Terahertz light has a wavelength that falls between the wavelengths of infrared light and microwaves on the electromagnetic spectrum. Terahertz light is especially promising for spectroscopy, the detection of chemicals by optical means. Compared with infrared light, terahertz light has a much lower frequency, and it can be used to probe the rotational and vibrational modes of many different kinds of complex molecules. Terahertz spectroscopy can be used to elucidate structural information about an unknown chemical (e.g., detecting different polymorphs of pharmaceuticals or detecting explosives). Challenges remain as terahertz light is difficult to generate, and most of the sources available in this range are bulky and low-powered.