Laser-Created Plasma Beam Antenna Transmits Radio Waves for the First Time

Engineers at North Carolina State University have achieved something that has long existed more in the realm of science fiction than in working laboratory hardware: a functioning antenna made not of metal, but of glowing plasma suspended in mid-air. The team demonstrated a technique that uses a focused laser to ionize a thin column of air, producing a luminous plasma filament that behaves like a conventional antenna and can transmit radio waves. The demonstration, described as the first of its kind, is published open access in the IEEE Journal of Microwaves, and it opens a path toward antennas whose length, and therefore operating frequency, can be reshaped on demand simply by adjusting a beam of light.

The visual effect is striking. “The plasma beam antenna looks like a lightsaber and is tunable, meaning we should be able to transmit across a broad range of frequencies,” says Prya Darshni, corresponding author of the journal article and a Ph.D. student at NC State. That lightsaber-like shaft is not merely a novelty; it is a precisely engineered structure. By firing a laser beam of a specific power and diameter, the researchers ionize a thin beam of air, creating a defined shaft of plasma known as a plasma filament. Within that filament, free electrons respond to electromagnetic fields in much the same way that electrons in a metal conductor do, allowing the column of ionized gas to radiate radio signals into space.

The importance of the achievement becomes clear when one considers how fundamental antenna length is to radio engineering. The length of an antenna controls the frequencies at which radio waves can be efficiently transmitted and received, which is why antennas for different bands come in such different sizes. Physically manipulating antenna length to sweep a desired range of frequencies can be extremely challenging in certain applications, and space exploration technologies are a prime example. Deployable booms, telescoping rods and mechanical reconfiguration mechanisms add mass, complexity and failure modes to spacecraft, where every gram of payload matters. “One of the questions we wanted to explore with this work was whether it would be possible to create plasma antennas using lasers, which would allow us to generate antennas at whatever length was needed,” Darshni explains. “And we have now shown that it is possible.”

Creating the plasma filament, however, was only half of the problem. To make the plasma antenna a practical tool, the researchers also needed a way to connect it to radio technology so that a signal could actually be transmitted. A plasma column floating in air has no wire to attach a feed line to, and touching it with metallic hardware would defeat much of the purpose of a reconfigurable, structure-free antenna. The team’s solution was to develop and demonstrate a contactless antenna-feed: a metal ring that serves as a capacitor. The laser passes through the ring, creating a plasma filament that is surrounded by the capacitor. By generating an electromagnetic field with the capacitor, the researchers can interact with the plasma beam without ever making physical contact with it.

The complete system operates in a straightforward sequence. A radio frequency generator feeds a signal into the capacitor ring; this generates the appropriate electromagnetic field around the plasma column; and that field then causes the plasma filament antenna to transmit radio waves at the appropriate frequency. In the published demonstration, the team transmitted a 30 MHz signal in the VHF band, a frequency range used for applications from FM radio broadcasting to aviation communications. The contactless coupling scheme is a critical piece of engineering in its own right, because it solves the fundamental interface problem between solid-state radio electronics and a gaseous conductor that exists only where the laser has ionized the air.

Tunability is where the concept becomes genuinely powerful. “By controlling the parameters of the laser, you can control the characteristics of the plasma filament – including its length,” says Darshni. Because antenna length dictates resonant frequency, a laser-defined filament whose length can be adjusted on the fly is effectively a broadband, continuously tunable antenna. “This is valuable for applications where you need an antenna that can sweep all frequencies,” she notes. But frequency agility is not the only degree of freedom the technique offers. “There are also applications where it is important to be able to control the angle of the antenna, in order to target the direction of radar sweeps or to improve the strength of a signal you want to pick up,” Darshni says. “The technique we’ve demonstrated here would allow users to control the angle of the plasma filament antenna via beam steering – simply shifting the direction of the laser.” In other words, pointing the antenna becomes as simple as pointing a beam of light, with no motors, gimbals or phased-array electronics required.

The long-term implications extend well beyond the laboratory bench. Satellites and space exploration technologies are one potential area of interest, because both payload mass and the ability to scan across a wide range of frequencies are important considerations for orbital systems. An antenna that materializes on demand, changes length electronically and steers by laser could dramatically reduce the mechanical complexity of communications and radar packages on future spacecraft. Crucially, the physics does not require the vacuum of space to be abandoned as a working environment. “In low earth orbit, there is sufficient air to form a plasma,” says Paul Franzon, co-author of the paper and the Cirrus Logic Distinguished Professor of Electrical and Computer Engineering at NC State. The residual atmosphere at those altitudes, thin as it is, provides enough molecules for a laser to ionize into a usable filament.

Franzon also highlights the practical appeal of the design from a systems engineering standpoint. “This is an exciting new concept that enables one to be able to have a customized antenna without complex mechanical deployment mechanisms,” he says. For missions where unfolding a large antenna structure is one of the riskiest moments of deployment, the idea of generating the radiating element out of thin air, literally, is an attractive alternative. The same properties could eventually matter on Earth as well, in scenarios where rapidly reconfigurable, steerable antennas are needed for radar, communications or spectrum management, although the researchers emphasize that the current work is an early-stage demonstration rather than a field-ready product.

The team is careful to be precise about what has and has not been shown. While the plasma filament has now been proven to transmit radio waves, the researchers have not yet demonstrated its ability to serve as an antenna that can receive signals. Even so, Darshni sees no fundamental obstacle on that front. “There’s no reason to believe it wouldn’t also work as a receiver,” she says, since the reciprocity that governs conventional antennas suggests a structure that radiates efficiently at a given frequency should also collect signals at that frequency. Establishing reception experimentally, along with improving efficiency, power handling and filament stability, now forms the roadmap ahead.

“This is the first step, but it is a big step – it is the first time anyone has ever demonstrated that plasma-filament antennas can work,” says Darshni. “Now that we’ve shown it is possible, we can begin improving its performance.” The article, “Laser-Induced-Plasma-Filament Antenna Transmitting 30 MHz VHF,” was published with co-author Arthur Dogariu of Texas A&M University and Princeton University, who assisted with measurements, and the researchers acknowledge the NC State Instrument Shop team of Chris Hewett, Byron Goode and Joe McElveen for manufacturing the experimental components. From a glowing column of ionized air to a working radio transmitter, the demonstration marks the moment plasma antennas moved from theory into demonstrated reality – and the beginning of an effort to make light-sculpted antennas a practical tool for communications on Earth and in orbit.

Subject of Research: Laser-induced plasma filament antennas for radio transmission

Article Title: Researchers successfully demonstrate first-ever plasma beam antenna

Article References: Researchers successfully demonstrate first-ever plasma beam antenna. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: plasma antenna, laser-induced plasma, plasma filament, radio transmission, VHF, beam steering, tunable antenna, contactless antenna feed, NC State University, IEEE Journal of Microwaves, space communications, photonics