Tunable terahertz laser promises ‘T-ray vision’ at room temps
Terahertz waves have frequencies higher than microwaves and lower than infrared and visible light. Where optical light is blocked by most materials, terahertz waves can pass straight through, similar to microwaves. Compact terahertz lasers could enable security systems to see through clothing, book covers, and other thin materials, to produce crisp, higher-resolution images than microwaves while being much safer than X-rays.
While prior art theories about terahertz laser implied the design of large bulky setups with low pressure meters-long lasing cavities pumped with large infrared lasers, a new theory developed by researchers at MIT was able to explain some earlier high-pressure terahertz gas laser experiments, leading to a shoebox-sized unit working at room temperature. The device was built from commercial, off-the-shelf parts and is designed to generate terahertz waves by spinning up the energy of molecules in nitrous oxide.
The theory published in a paper titled “Widely tunable compact terahertz gas lasers” in Science reveals that almost any rotational transition of almost any molecular gas can be made to lase. In their experiments, the authors were able to tune the nitrous oxide terahertz laser over 37 lines spanning 0.251 to 0.955 terahertz, each with kilohertz linewidths. But according to their theory, laser lines spanning more than 1 terahertz with powers greater than 1 milliwatt would also be possible from many molecular gases pumped by quantum cascade lasers.
Steven Johnson, professor of mathematics at MIT, says that in addition to T-ray vision, terahertz waves can be used as a form of wireless communication, carrying information at a higher bandwidth than radar, for instance, and doing so across distances that scientists can now tune using the group’s device.
“By tuning the terahertz frequency, you can choose how far the waves can travel through air before they are absorbed, from meters to kilometers, which gives precise control over who can ‘hear’ your terahertz communications or ‘see’ your terahertz radar,” Johnson explains. “Much like changing the dial on your radio, the ability to easily tune a terahertz source is crucial to opening up new applications in wireless communications, radar, and spectroscopy.”
The team searched through libraries of gases to identify those that were known to rotate in a certain way in response to infrared light, eventually landing on nitrous oxide. Since these initial experiments, the researchers have extended their mathematical model to include a variety of other gas molecules, such as carbon monoxide and ammonia, providing scientists with a menu of different terahertz generation options with different frequencies and tuning ranges, paired with an infrared quantum cascade laser matched to each gas.
(green squiggles) by using a special infrared laser (red)
to rotate molecules of nitrous oxide packed in a pen-sized
cavity (grey).
Courtesy of Chad Scales, US Army Futures Command.
The group’s theoretical tools also enable scientists to tailor the cavity design to different applications. They are now pushing toward more focused beams and higher powers, with commercial development on the horizon. Johnson says scientists can refer to the group’s mathematical model to design new, compact and tunable terahertz lasers, using other gases and experimental parameters.
“These gas lasers were for a long time seen as old technology, and people assumed these were huge, low-power, non-tunable things, so they looked to other terahertz sources,” Johnson notes. “Now we’re saying they can be small, tunable, and much more efficient. You could fit this in your backpack, or in your vehicle for wireless communication or high-resolution imaging. Because you don’t want a cyclotron in your car.”
This research was supported in part by the U.S. Army Research Office and the National Science Foundation.
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