Harvard, MIT, Duke, US Army Team Up on Terahertz Technology

Researchers have created a new terahertz radiation emitter with coveted frequency adjustment capability. The compact source could enable the development of futuristic communications, security, biomedical, and astronomical imaging systems.

The experimental setup showing the different components of the system and highlighting the path followed by the quantum cascade laser light (red) and terahertz radiation (blue). Courtesy of Arman Amirzhan, Harvard SEAS.

Terahertz electromagnetic frequencies have been sought after for their range of applications, such as high bandwidth, high resolution, long-range sensing, and ability to visualize objects through materials. However, the costliness, bulk, inefficiencies, and lack of tunability of traditional terahertz emitters has stymied growing markets.

This new combined laser terahertz source, made possible through the collaboration of researchers at Harvard, MIT, Duke, and the U.S. Army, paves the way for future technologies, from T-ray imaging in airports and space observatories, to ultrahigh-capacity wireless connections.

“Existing sources have limited tunability, not more than 15 to 20% of the main frequency, so it’s fair to say that terahertz is underutilized,” said co-senior author Federico Capasso from Harvard University. “Our laser opens up this spectral region, and in my opinion, will have a revolutionary impact.”

Capasso is the inventor of a compact tunable semiconductor laser, the quantum cascade laser (QCL), which is used commercially for chemical sensing and trace gas analysis. The QCL emits mid-infrared light, the spectral region where most gases have their characteristic absorption fingerprints, to detect low concentrations of molecules.

At a conference in 2017, Capasso met Henry Everitt, senior technologist with the U.S. Army and adjunct professor at Duke University. Through their discussion the idea was birthed to apply the widely tunable QCL to a laser with terahertz ability.

Everitt, alongside Steven Johnson’s group at MIT, theoretically calculated that terahertz waves could be emitted with high efficiency from gas molecules held within cavities much smaller than those currently used on the optically pumped far-infrared (OPFIR) laser — one of the earliest sources of terahertz radiation. Like all traditional terahertz sources, the OPFIR was inefficient with limited tunability. But, guided by the theoretical calculations, Capasso’s team was able to use the QCL to dramatically increase the terahertz tuning range of a nitrous oxide OPFIR laser.

“The same laser is now widely tunable,” Capasso said. “It’s a fantastic marriage between two existing lasers.”

In initial experiments with the shoebox-size QCL pumped molecular laser (QPML), the researchers demonstrated that the terahertz output could be tuned to produce 29 direct lasing transitions between 0.251 and 0.955 THz.

It was Johnson and Everitt’s theoretical models that highlighted nitrous oxide as a strongly polar gas with predicted terahertz release in the QPML. Similarly, a whole menu of other gas molecules have been predicted for terahertz generation at different frequencies and tuning ranges.

“This is a universal concept, because it can be applied to other gases,” Capasso said. “We haven’t quite reached one terahertz, so next thing is to try a carbon monoxide laser and go up to a few terahertz, which is very exciting for applications.”

Capasso and Everitt are hoping to use their laser to look skyward and identify unknown spectral features in the terahertz region. The team is now developing high-power terahertz QPMLs for astronomical observations.