A mechanism that transfers energy from nitrogen to argon enables a cascading bidirectional laser effect in atmospheric air

To produce light, lasers typically use optical cavities, pairs of face-to-face mirrors that amplify light by bouncing it back and forth. Recently, some physicists have been studying the generation of “laser light” in open air without using optical cavities, a phenomenon known as “cavityless lasing” in atmospheric air.

Researchers from the University of California, Los Angeles (UCLA) and the Max Born Institute have recently revealed a physical mechanism that leads to this phenomenon. This mechanism, described in an article published in Physical Exam Lettersconsists of a photon energy transfer from nitrogen (N₂) in argon (Ar).

“We noticed that there appeared to be a previously unknown reduction in the Ar ionization rate in the high-field ionization regime (using a 261-nm pump laser) compared to that predicted by PPT theory or the time-dependent Schrödinger equation,” Chan Joshi, co-author of the paper, told Phys.org. “We wanted to know if 3-photon resonant absorption of 261-nm photons in Ar could play a role in the reduction.”

The recent study by Joshi’s colleagues builds on the team’s previous experimental efforts. In their new experiments, the team observed that 3-photon absorption of 261-nm photons by Ar atoms is followed by the emission of a cascade of superfluorescence, specifically a bidirectional, cavity-free, laser-like emission.

“In addition, we unexpectedly found that the cascade superfluorescence changed wavelength if we used air containing 1% Ar,” said Zan Nie, the study’s lead author. “Further investigation of this curious effect uncovered a new mechanism of laser radiation in air that facilitates the transfer of radiative energy from nitrogen to Ar.”

The new mechanism discovered by Joshi and colleagues allows for bidirectional, two-color, cascading lasing in atmospheric air. This mechanism could open new avenues for generating retrograde lasing in air, a long-standing research goal in the physics community.

“Since ambient air contains different components, we investigated this problem by first mixing argon with different components of ambient air, such as the most abundant components: nitrogen and oxygen,” Joshi explains. “It turned out that mixing nitrogen and argon gave the same results as using ambient air, while mixing other gases such as oxygen or helium did not give the same results. Therefore, through this comparison experiment, we can deduce that the origin of the air laser was due to the coupling between argon and nitrogen.”

Joshi and colleagues also showed that N₂ Molecules in an electronically excited state exhibit a nonlinear 3-photon absorption at 261 nm at frequencies slightly shifted relative to Ar. This shift serves as a higher excited state for the cascading superfluorescence the team observed. In their paper, the researchers present a theoretical model that explains superfluorescence and its underlying mechanisms.

“The quest for an efficient cavity-free laser in open air has been going on for over a decade,” said Misha Ivanov, a co-author of the paper. “The main goal, and a rather difficult one, is to achieve a two-way laser. In other words, you want to shoot a laser into the air and have the air return a laser-like burst of light to you. This would be very useful for remote sensing, but this is just mind-blowing.”

This recent study by Nie, Ivanov, Joshi and colleagues has unveiled a previously unknown photon-mediated mechanism that transfers energy from N₂ to Ar, eventually enabling a bidirectional cascading laser effect in atmospheric air. In the future, this mechanism could be exploited to achieve backward laser effect in air, which could open new opportunities for the development of remote sensing technologies.

“Our future research project is to study the physics of this mechanism, such as quantum beating, in more detail,” Nie added. “Simply put, the simultaneous excitation of multiple Ar levels produces time-dependent charge density oscillations. The frequencies of these oscillations can reveal the existence of previously unknown levels not only of Ar but also of vibrational-rotational levels of nitrogen that are important in the radiative coupling process.”

“We also have ideas for increasing the efficiency of the retrograde air laser to bring this technique closer to real-world remote sensing applications.”

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