Optical comb enables generation of elusive terahertz frequencies

October 19, 2017 //By Jean-Pierre Joosting
Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) are exploring the possibility of using an infrared frequency comb to generate elusive terahertz frequencies -– which lie in the electromagnetic spectrum between radio waves and infrared light.

Terahertz frequencies have long promised to transform communications and sensing but are very challenging to source. By harnessing a recently discovered laser state, SEAS researchers have discovered an infrared frequency comb in a quantum cascade laser that offers a new way to generate terahertz frequencies.

Dubbed a harmonic frequency comb, this new system produces a spectrum of teeth with spacing tens of times larger than traditional frequency combs. The large but precise spacing allows these modes of light to beat together to produce extremely pure terahertz tones.

"The discovery of the harmonic state of quantum cascade lasers is surprising from a laser physics point of view," said Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering and senior author of the paper in Nature Photonics.

"Until recently, it was thought that multimode lasers would normally lase on all the possible frequencies of the cavity. In the harmonic state, many cavity frequencies are skipped. Even more remarkable is that this discovery opens up unforeseen opportunities in unused regions of the electromagnetic spectrum, the terahertz."

This is an image of optical frequency combs generated in quantum cascade lasers. The discovered harmonic comb regime produces a spectrum with an intermodal spacing that is 10 to 100 times larger than that observed in fundamental frequency combs (right) enabling completely new applications in this platform. Both types of frequency combs can be generated using the same type of device. Image courtesy of Jared Sisler/Havard SEAS).