MIT researchers reinvent terahertz lasers, make them tunable

November 18, 2019 //By Julien Happich
Improving on previous mathematical theories to explain the physics behind infrared-pumped terahertz molecular gas lasers, researchers at MIT were able to design a compact terahertz laser, orders of magnitude smaller than today’s designs, while being easily frequency-tunable at the turn of a knob.

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.”

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