A New Widely Tunable Terahertz Laser

Illustration of the quantum cascade laser-pumped THz laser showing the QCL beam (red) and the THz beam (blue) along with rotating molecules inside the cavity. (Source: A. Amirzhan, Harvard U.)

The terahertz frequency range offers the potential for high-bandwidth communi­cations, ultrahigh-resolution imaging, precise long-range sensing for radio astronomy, and much more. But this section of the electro­magnetic spectrum has remained out of reach for most appli­cations. That is because current sources of terahertz frequencies are bulky, ineffi­cient, have limited tuning or have to operate at low temperature. Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences SEAS, in colla­boration with MIT and the US Army, have developed a compact, room temperature, widely tunable terahertz laser.

“This laser outperforms any existing laser source in this spectral region and opens it up, for the first time, to a broad range of appli­cations in science and tech­nology,” said SEAS researcher Federico Capasso. “There are many needs for a source like this laser, things like short range, high bandwidth wireless communi­cations, very high-resolution radar, and spectro­scopy,” said Henry Everitt, senior technologist with the Army CCDC Aviation & Missile Center.

While most electronic or optical terahertz sources use large, ineffi­cient and complex systems to produce the elusive frequencies with limited tuning range, Capasso, Everitt and their team took a different approach. Previous terahertz molecular lasers used optical pumps but they were limited in their tunability to just a few fre­quencies, meaning the elevator only went to a small number of floors. The break­through of this research is that the team used a highly tunable, quantum cascade laser as their optical pump. These powerful, portable lasers are capable of efficiently producing widely tunable light. In other words, this quantum elevator can stop at every floor in the building.

The theory to optimize the operation of the new laser was developed by Steven Johnson, Professor of Applied Mathematics and Physics at MIT, his graduate student Fan Wang and Everitt. “Molecular THz lasers pumped by a quantum cascade laser offer high power and wide tuning range in a sur­prisingly compact and robust design,” commented Nobel laureate Theodor Hänsch of the Max Planck Institute for Quantum Optics in Munich, who was not involved in this research. “Such sources will unlock new appli­cations from sensing to funda­mental spectro­scopy.”

“What’s exciting is that concept is universal,” said Paul Chevalier, a postdoctoral fellow at SEAS. “Using this framework, you could make a terahertz source with a gas laser of almost any molecule and the appli­cations are huge.” The researchers combined the quantum cascade laser pump with a nitrous oxide laser. “By opti­mizing the laser cavity and lenses, we were able to produce frequencies spanning nearly 1 THz,” said Arman Amirzhan, a graduate student in Capasso’s group.

“This result is one of a kind,” said Capasso. “People knew how to make a terahertz laser before but couldn’t make it broadband. It wasn’t until we began this colla­boration, after a seren­dipitous encounter with Henry at a conference, that we were able to make the connection that you could use a widely tunable pump like the quantum cascade laser.” This laser could be used in everything from improved skin and breast cancer imaging to drug detection, airport security and ultra­high-capacity optical wireless links.

“I’m particularly excited about the possibility of using this laser to help map the inter­stellar medium,” said Everitt. “Molecules have unique spectral finger­prints in the terahertz region, and astro­nomers have already begun using these finger­prints to measure the compo­sition and temperature of these primordial clouds of gas and dust. A better ground-based source of terahertz radiation like our laser will make these measure­ments even more sensitive and precise.” (Source: Harvard U.)

Reference: P. Chevalier et al.: Widely tunable compact terahertz gas lasers, Science 366, 856 (2019); DOI: 10.1126/science.aay8683

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

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