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Researchers generate terahertz lasers with nitrous oxide



  New laser opens up a large, underutilized area of ​​the electromagnetic spectrum
Image of the experimental setup showing the different system components and highlighting the path followed by QCL light (red) and THz radiation (blue). Picture credits: Arman Amirzhan, Harvard SEAS

In the electromagnetic midfield between microwaves and visible light are terahertz radiation and the promise of "x-ray vision."

Terahertz waves have frequencies that are higher than microwaves and lower than infrared and visible light. Where optical light is blocked by most materials, similar to microwave terahertz waves can be transmitted directly. If made into lasers, terahertz waves could provide "T-ray vision," with the ability to see through clothing, book covers, and other thin materials. Such a technology could produce crisp, higher-resolution images than microwaves and be far safer than X-rays.

The reason why we do not see X-ray equipment in airport security and medical imaging equipment, for example, is the following. Generation of terahertz radiation requires very large, bulky equipment or devices that produce terahertz radiation at a single frequency ̵

1; not very useful. because a wide frequency range is required to penetrate various materials The US Army has built a compact device the size of a shoebox that produces a terahertz laser whose frequency can be adjusted over a wide range. The device is made of commercially available components designed to generate terahertz waves by spinning the energy of molecules in nitrous oxide or, as is well known, nitrous oxide.

Steven Johnson, a professor of math at MIT, says that in addition to X-ray, terahertz waves can be used as a form of wireless communication that transmits information at a higher bandwidth than radar, for example, over distances scientists now have With the can set group & # 39; s device.

"By setting the terahertz frequency, you can choose how far the waves can travel through the air before they are absorbed, from meters to kilometers, to precisely control who" hears "your terahertz communication or" "Like Terahertz Radar," says Johnson, "Similar to changing your radio's dial, simply setting a terahertz source is critical to unlocking new applications in wireless communications, radar, and spectroscopy." [19659008] Johnson and his colleagues have published their findings in the Journal Science co-authored by MIT postdoc fan Wang, as well as Paul Chevalier, Arman Armizhan, Marco Piccardo and Federico Capasso of Harvard University, and Henry Everitt of the US Army Combat Capabilities Development Command Aviation and Missile Center.

  New laser opens large, unexploited area of ​​el
Artistic representation of the QCL-pumped THz laser with the QCL beam (red) and the THz beam (blue) as well as rotating N2O molecules (nitrous oxide) within the cavity. Picture credits: Arman Amirzhan, Harvard SEAS

Space for Molecular Respiration

Since the 1970s, scientists have experimented with the generation of terahertz waves using molecular gas lasers – arrangements in which a high-power infrared laser is shot into a large tube filled with gas ( typically methyl fluoride) whose molecules vibrate and eventually rotate. The rotating molecules can jump from one energy level to the next, the difference of which is emitted as a kind of residual energy in the form of a photon in the terahertz range. The more photons accumulate in the cavity, the more a terahertz laser is created.

According to the researchers, the improvement in the design of these gas lasers has been hampered by unreliable theoretical models. In small cavities at high gas pressure, the models predicted that from a certain pressure, the molecules would be too "cramped" to spin and emit terahertz waves. Partly for this reason, terahertz gas lasers typically used meter-long cavities and large infrared lasers.

In the 1980s, however, Everitt found that he could produce terahertz waves in his laboratory with a gas laser that was much smaller than traditional devices, at pressures far higher than those models was possible. This discrepancy has never been completely clarified, and work on terahertz gas lasers has fallen by the wayside in favor of other approaches.

A few years ago, Everitt mentioned this theoretical puzzle to Johnson, as the two MIT collaborators in other work collaborated on the Institute for Soldier Nanotechnologies. Together with Everitt, Johnson and Wang took up the challenge and finally formulated a new mathematical theory to describe the behavior of a gas in a molecular gas laser cavity. The theory has also successfully explained how terahertz waves can be emitted even from very small high pressure cavities. Johnson says that gas molecules can vibrate in response to an infrared pump with multiple frequencies and rotation rates, but earlier theories disregarded many of these vibrational states and instead assumed that a handful of vibrations were ultimately crucial to generating a terahertz wave , If a cavity were too small, previous theories suggested that molecules that vibrate in response to an incident infrared laser more often collide with each other and release their energy, rather than building it further to produce terahertz.

Instead, The New The model tracked thousands of relevant vibrational and rotational states among millions of groups of molecules in a single cavity, using new computer tricks to make such a big problem understandable on a laptop computer. It was then analyzed how these molecules would react to incident infrared light, depending on their position and direction within the cavity.

"If you include all those other vibrational states that were ejected by humans, you get a buffer," says Johnson. "In simpler models, the molecules spin, but if they collide with other molecules, they lose everything, and if you include all of these other states, it does not happen anymore, and these collisions can transfer energy to other vibrational states and give way, as it were." You have more freedom to continue spinning and generate terahertz waves.

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When the team determined that its new model accurately predicted what Everitt observed decades ago, they collaborated with Capasso's group at Harvard to create a new type of compact terahertz To develop the generator by combining the model with new gases and a new type of infrared laser.

For the infrared source, researchers used a quantum cascade laser or QCL – a newer type of laser that is compact and tunable.

"Sie can rotate a command dial and change the frequency of the input laser, and the hope was that we could use that to change the frequency of the terahertz output, "Johnsonsays.

The researchers have teamed up with Capasso, a pioneer in the Development of QCLs teamed up, which provided a laser that produced a power range that, in their theory, with a cavity of the Gr size of a pen (about 1/1000 the size of a conventional cavity). The researchers then searched for a gas that could be turned.

The team searched libraries of gases to identify those who were known to turn in a certain way in response to infrared light and eventually land on nitrous oxide or nitrous oxide as an ideal and accessible candidate for their experiment.

They ordered lab-grade nitrous oxide, which they pumped into a pen-sized cavity. When they sent infrared light from the QCL into the cavity, they found that they could produce a terahertz laser. As they tuned QCL, the frequency of terahertz waves also shifted over a wide range.

"These demonstrations confirm the universal concept of a molecular terahertz laser source that is tunable with continuous pumping throughout its rotational states, tunable QCL," says Wang.

Since these first experiments, researchers have extended their mathematical model to a variety of other gas molecules, including carbon monoxide and ammonia, to provide scientists with a menu of different terahertz generation options with different characteristics of frequencies and tuning ranges, paired with one for each gas matching QCL. The group's theoretical tools also allow scientists to tailor the cavity design to different applications. They are now pushing for more focused beams and higher performance, with commercial development emerging.

According to Johnson, scientists can use the group's mathematical model to design new, compact and tunable terahertz lasers using other gases and experimental parameters. [19659005] "For a long time, these gas lasers were considered old technology, and people believed that they were huge, power-efficient, and un-tunable things, so they focused on other terahertz sources," says Johnson. "Now we say they can be small, tuneable and much more efficient – you can put them in your backpack or in your vehicle for wireless communication or high-resolution imaging because you do not want a cyclotron in your car." , "


Researchers almost double the continuous output power of a terahertz laser type


Further information:
"Widely Tunable Compact Terahertz Gas Lasers" Science (2019). science.sciencemag.org/cgi/doi… 1126 / science.aay8683

Provided by
Massachusetts Institute of Technology




Quote :
Researchers generate nitrous oxide terahertz laser (November 14, 2019)
retrieved on November 14, 2019
from https://phys.org/news/2019-11-terahertz-laser-gas.html

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