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Bridging the terahertz gap

Optical frequency combs are widely-used, high-precision tools for measuring and detecting different frequencies — a.k.a., colors — of light. Unlike conventional lasers, which emit a single frequency, these lasers emit multiple frequencies simultaneously. The equally spaced frequencies resemble the teeth of a comb. Optical frequency combs are used for everything from measuring the fingerprints of specific molecules to detecting distant exoplanets.

Recently, our group has explored the possibility of using an infrared frequency comb to generate elusive terahertz frequencies. These frequencies — which lie in the electromagnetic spectrum between radio waves and infrared light — have long promised to transform communications and sensing but are very challenging to source. By harnessing a recently discovered laser state, we have demonstrated 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.

This work appears in Nature Photonics:

“Self-starting harmonic frequency comb generation in a quantum cascade laser”, D. Kazakov, M. Piccardo, Y. Wang, P. Chevalier, T. S. Mansuripur, F. Xie, C. Zah, K. Lascola, A. Belyaning and F. Capasso, Nat Photon, 2017, DOI: 10.1038/s41566-017-0026-y