A team of researchers from Harvard University’s John A. Paulson School of Engineering and Applied Sciences (SEAS; Cambridge, MA), DEVCOM Army Research Lab (Houston, TX) and DRS Daylight Solutions (San Diego, CA) recently created a compact demonstrates a terahertz laser that operates at room temperature and can generate 120 individual frequencies in the range of 0.25 to 1.3 THz – far more than previous terahertz sources.
The terahertz frequency range, which lies between microwaves and infrared light in the electromagnetic spectrum, remains difficult for applications to exploit because most terahertz sources are either bulky, inefficient, or rely on low-temperature equipment to tune these elusive frequencies with limited tuning to create.
However, the team’s new terahertz laser is “a pioneering technology for generating terahertz radiation,” says Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and one of the inventors of the quantum cascade laser. “Thanks to its compactness, efficiency, wide tuning range and room temperature operation, this laser has the potential to become a key technology to close the terahertz gap for imaging, security or communications applications.”
Even greater vocal range
Back in 2019, the Capasso Group worked with the Massachusetts Institute of Technology (MIT; Cambridge, MA) and the US Army to develop a prototype that proved terahertz frequency sources to be compact, operable at room temperature, and widely tunable by combination can use a quantum cascade laser pump with a nitrous oxide molecular laser.
This time, instead of nitrous oxide, the team uses methyl fluoride—a molecule that interacts strongly with optical fields (see figure). “This compound is really good at absorbing infrared and emitting terahertz,” notes Arman Amirzhan, a PhD student at SEAS. “By using non-toxic methyl fluoride, we increased the laser’s efficiency and tuning range.” In fact, this work more than triples the tuning range of this prototype by more than three times.
Methyl fluoride has been used as a gain material for terahertz lasers for nearly 50 years, “but it produces only a few lasing frequencies when pumped by a large carbon dioxide laser,” says Henry Everitt, chief optical science technologist for the US Army. “The two innovations we are reporting on, a compact laser resonator pumped by a quantum cascade laser, combine to give methyl fluoride the ability to lase on hundreds of lines.”
As for the basic physical concepts, the first is the quantization of the energy level of molecules, which is described by quantum mechanics. “The energy difference corresponding to a rotational motion of the molecule corresponds to frequencies in the terahertz range (50 GHz to 2 THz, with discrete lines spaced about 50 GHz for the methyl fluoride molecule),” explains Paul Chevalier, SEAS scientist and principal investigator of the team. “The difference in energy that corresponds to a vibration of molecules corresponds to frequencies of the order of 30 THz: corresponding to infrared light with a wavelength of about 10 mm.”
The second concept is that the molecule has a large dipole moment, which means it interacts strongly with light. “Therefore, it is able to absorb infrared light at a wavelength of around 10 mm – across many discrete transitions,” adds Chevalier. “And when placed in a properly designed laser cavity, it emits light in the terahertz range, and the exact frequency depends on the pumped infrared frequency chosen.”
The coolest part of this job? “For almost every frequency of the 120 individual lines that we had to lase, we’re probably the first to see them lasing,” says Chevalier. “This is made possible by the continuous tunability of the quantum cascade laser. In the past there have been lasers that used the methyl fluoride molecule, but these used a line-tunable carbon dioxide laser and therefore only a handful of those lines could be obtained at the time.”
Biggest challenge? Not being able to see the actual light coming out of the laser. “That makes optimizing the laser much more difficult because we rely on instruments that can see this terahertz light,” says Chevalier. “These tools have their own limitations and we have to deal with them.”
The group’s laser has the potential to be one of the most compact terahertz lasers ever developed, and they hope to further reduce its footprint.
“It can potentially reach the size of a shoebox,” says Capasso. “More specifically, I think that with the right optimization, we can get the laser itself down to the size of a cube, about 15 × 15 × 15 cm.”
Such a device, much smaller than a cubic foot, will allow researchers to use this frequency range for more applications – including short-range communications, short-range radar, biomedicine and imaging.