by Ingrid Fadelli , Phys.org

Terahertz radiation (THz), electromagnetic radiation with frequencies ranging from 0.1 and 10 THz, is central to the functioning of various technologies, including imaging, sensing and spectroscopy tools. While THz radiation waves have been manipulated in different ways over the past decades, controlling their direction in air has so far remained a challenge.

Researchers at Ecole Polytechnique (CNRS) at Institut Polytechnique de Paris recently demonstrated the steering of laser-produced THz radiation in air, using a recently introduced technique dubbed "flying focus." Their paper, published in Physical Review Letters, could open new possibilities for the manipulation of THz electromagnetic waves, which could in turn be leveraged to develop new technologies.

"My group has been working on the generation of THz radiation by laser-induced filaments in air for almost 20 years," Aur¨¦lien Houard, senior author of the paper, told Phys.org. "A major advantage of these filaments is that they can be generated at a large distance from the laser in the atmosphere. However, the THz emission remained confined close to the laser axis, which is not convenient for remote detection."

As part of their study, Houard and his colleagues set out to reliably control the direction of laser-produced THz radiation in air, using a newly introduced technique called flying focus. This technique is designed to control the group velocity of focused femtosecond pulses,

"This group velocity is the parameter that determines the angular distribution of the THz radiation in the filaments," said Houard. "We then decided to test it with plasma filaments in air to see if we could control the ionization velocity and the resulting THz radiation angle."

Essentially, the technique employed by the researchers works by adjusting a laser's ionization front (i.e., the site where air molecules are stripped of electrons). By directing this point, it can direct THz waves, causing them to steer at specific angles or even backwards.

"By shaping the different frequency components of the laser pulse, the flying focus technique allows you to remotely control the direction and the velocity of the plasma generated at the focus of the laser beam," explained Houard. "In this way, you can increase the intensity of the radiation produced by the plasma and choose the direction in which it is emitted."

The initial experiments carried out by Houard and his colleagues yielded very promising results, highlighting the potential of flying focus approaches for steering THz waves in air. In the future, the team's findings could inspire other research groups to employ the new technique and assess its potential for specific applications, which could in turn lead to the development of new technologies, including remote THz spectroscopy tools to analyze materials in detail from afar.

"Our study demonstrates that the flying focus can be used to reverse the direction of secondary radiation to produce backward THz emission," added Houard. "We now plan to test different methods to improve the THz emission from the filament and to try to apply the same technique to other secondary radiations from laser filaments."

More information: Silin Fu et al, Steering Laser-Produced THz Radiation in Air with Superluminal Ionization Fronts, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.045001. On arXiv: DOI: 10.48550/arxiv.2407.18579