Bridging the Terahertz Gap

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 enabling completely new applications in this platform. (Source: J. Sisler, Havard SEAS)

Optical frequency combs are widely-used, high-precision tools for measuring and detecting different fre­quencies 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. Now, 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.

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, the 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. “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.”

In tradi­tional frequency combs, teeth are separated by a small frequency dictated by the charac­teristic length of the laser cavity. The harmonic frequency comb, however, can use a larger multiple of that frequency. “With this new comb regime we can bypass the strict limi­tations set by the cavity length and reach an unpre­cedented degree of flexibility in the realm of quantum cascade laser frequency combs,” said Marco Piccardo, a post­doctoral fellow in the Capasso lab.

Key to the research was proving that these largely-spaced teeth were indeed equi­distant. Using another reference comb, the team was able to study the harmonic frequency comb spectrum at very high reso­lution. “We show that the lines are equidistant with an uncer­tainty of only 300 hertz, that quanti­fies the relative precision of this measure­ment to five parts per trillion,” said Dmitry Kazakov, a visiting research intern in the Capasso group. “It is as if one could measure the distance from Earth to the Moon and be off by less than the thickness of a human hair.”

Most current tera­hertz gene­rators use large, complex optical systems operating at very low tempera­tures to produce terahertz frequen­cies. The harmonic frequency comb operates at room tempera­ture, uses commer­cial quantum cascade lasers, and is self-starting, meaning that the laser can automati­cally switch to this regime when electrical current is injected into the device. “This opens up com­pletely new appli­cations for frequency combs, especially in wireless communi­cations,” Capasso said. “We foresee that in the near future this comb regime will enable a new class of chip-scale modem operating at terahertz fre­quencies, accommo­dating the ever-in­creasing consumer demand for high data rate digital communi­cation.“ (Source: SEAS)

Reference: D. Kazakov et al.: Self-starting harmonic frequency comb generation in a quantum cascade laser, Nat. Phot., online 16 October 2017; DOI: 10.1038/s41566-017-0026-y

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences SEAS, Harvard University, Cambridge, USA

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