US academic-military group creates tunable terahertz laser

20 Nov 2019

Harvard SEAS, MIT and US Army develop compact, room-temperature THz source; promising better imaging, communications.

Tunable terahertz: Experimental setup showing different components.

Researchers from Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with the Massachusetts Institute of Technology and the U.S. Army, have developed a compact, room-temperature, widely tunable terahertz laser. The research is published in Science.

The terahertz frequency range offers the potential for high-bandwidth communications, ultrahigh-resolution imaging, and precise long-range sensing for radio astronomy, to name a few applications.

“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,” commented Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and co-senior author of the paper.

Many applications

“There are many needs for a source like this laser, such as short-range, high-bandwidth wireless communications, very high-resolution radar, and spectroscopy,” said Henry Everitt, senior technologist with the U.S. Army CCDC Aviation & Missile Center and co-senior author of the paper. Everitt is also an adjunct professor of physics at Duke University.

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.

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 have taken a different approach.

The team says that their “breakthrough” was achieved by the use of a highly tunable, quantum-cascade laser (QCL) as the optical pump. These powerful, portable lasers, co-invented by Capasso and his group at Bell Labs in the 1990s, are capable of efficiently producing widely tunable light.

The theory to optimize the operation of the new laser was developed by Steven Johnson, professor of applied mathematics and physics at MIT, and his graduate student Fan Wang. The researchers combined the quantum-cascade laser pump with a laser based on nitrous oxide.

“By optimizing the laser cavity and lenses, we were able to produce frequencies spanning nearly 1THz,” said Arman Amirzhan, a graduate student in Capasso’s group and co-author of the paper.

Artistic view of the QCL-pumped terahertz laser (click for more).

“Molecular THz lasers pumped by a quantum-cascade laser offer high power and wide tuning range in a surprisingly compact and robust design,” said Nobel laureate Theodor Hänsch of the Max-Planck Institute for Quantum Optics in Munich, Germany, who was not involved in this research. “Such sources will unlock new applications from sensing to fundamental spectroscopy.”

“What’s exciting is that the concept is universal,” said Paul Chevalier, a postdoctoral fellow at SEAS and first author of the paper. “Using this framework, you could make a terahertz source with a gas laser of almost any molecule and the applications are huge.”

'One of a kind'

Capasso added, “This result is one-of-a-kind. People knew how to make a terahertz laser before but could not make it broadband. It wasn’t until we began this collaboration, after a serendipitous encounter with Henry Everitt at a conference, that we were able to make the connection that you could use a widely tunable pump like the quantum-cascade laser.”

“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.”