Terahertz Laser Generated with Laughing Gas

Researchers from Massachusetts Institute of Technology (MIT), Harvard University, and the U.S. Army have built a compact device able to produce a terahertz laser that can be tuned over a wide range.

The device was built with commercial off-the-shelf parts and is designed to generate terahertz waves by spinning up the energy of molecules in nitrous oxide, more commonly known as laughing gas. With new information gained from novel computational techniques, the researchers were able to create a tunable terahertz laser

“These gas lasers were for a long time seen as old technology, and people assumed these were huge, low-power, nontunable things, so they looked to other terahertz sources,” Steven Johnson of MIT said. “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 communications or high-resolution imaging. Because you don’t want a cyclotron in your car.”

The design builds upon work done in the 1980s, when Henry Everitt of the U.S. Army Combat Capabilities Development Command Aviation and Missile Center found that he was able to produce terahertz waves in his laboratory using a gas laser much smaller than traditional devices, and at pressures far higher than the theoretical models of the time suggested were possible.

Those models discounted a number of vibrational states, assuming that only a handful of vibrations were what ultimately mattered in producing a terahertz wave. Previous theories suggested that if a cavity were too small, molecules vibrating in response to an incoming infrared laser would collide more often and release their energy rather than building it up further to spin and produce terahertz waves.

Instead, the new model, devised by Everitt, Johnson, and Fan Wang of MIT, tracked thousands of relevant vibrational and rotational states among millions of groups of molecules within a single cavity. The model then analyzed how those molecules would react to incoming infrared light, depending on their position and direction within the cavity.

“We found that when you include all these other vibrational states that people had throwing out, they give you a buffer.” Johnson said. “In simpler models, the molecules are rotating, but when they bang into other molecules they lose everything. Once you include all these other states, that doesn’t happen anymore.”

With their now computational model having confirmed Everitt’s observation, the researchers collaborated with Federico Capasso’s group at Harvard University to design a new type of terahertz generator.

The researchers chose a quantum cascade laser (QCL) as the infrared source.

“You can turn a dial and it changes the frequency of the input laser, and the hope was that we could use that to change the frequency of the terahertz coming out,” Johnson said.

The team sifted 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. The gas was pumped into a cavity about the size of a pen. When stimulated with the QCL, the researchers found that they were able to produce a tunable terahertz laser at a much smaller size than previously thought possible.

Since their 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 a QCL matched to each gas. 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.