Not every university shoots laser pulses powerful enough to burn paper and skin down a hallway. But that’s exactly what happened at UMD’s Energy Research Facility, an unremarkable-looking building on the northeast corner of campus. Now when you visit the utilitarian white and gray hall it appears like any other university hall – unless you peek behind a cork panel and discover the metal panel covering a hole in the wall.
But for a few nights in 2021, UMD physics professor Howard Milchberg and his colleagues turned the hallway into a laboratory: the glossy surfaces of doors and a water fountain were masked to avoid potentially blinding reflections; Connecting corridors were cordoned off with signs, barrier tape and special laser-absorbing black curtains; and scientific equipment and cables typically inhabited open walking space.
As the team members went about their work, a cracking sound warned of the dangerously powerful path the laser was cutting down the hallway. Sometimes the beam’s journey ended at a block of white ceramic that filled the air with louder pops and a metallic whiff. Each night, a researcher would sit alone at a computer in the adjoining lab with a walkie-talkie and make the desired adjustments to the laser.
Their efforts consisted of temporarily converting thin air into a fiber optic cable — or more accurately, an air waveguide — that would carry the light tens of meters. Like one of the fiber optic internet cables that provides efficient highways for optical data streams, an air waveguide dictates a path for light. These air waveguides have many potential applications related to collecting or transmitting light, such as: B. detecting light emitted by air pollution, long-distance laser communications, or even laser weapons. With an airwave conductor, there is no need to pay out a fixed cable and deal with the limitations of gravity; instead, the cable quickly forms cantilevered in the air. In an article accepted for publication in the journal Physical Check X The team described how they set a record by guiding light down 45-meter-long airwaveguides and explained the physics behind their method.
The researchers performed their record-breaking atmospheric alchemy at night so as not to bother (or zap) colleagues or unsuspecting students during the workday. They had to have their security procedures approved before they could repurpose the hallway.
“It was a really unique experience,” says Andrew Goffin, a UMD electrical and computer engineering student who worked on the project and is the lead author of the resulting journal article. “Shooting lasers outside of the lab involves a lot of work that you don’t have to bother with when you’re in the lab — like putting up curtains to protect your eyes. It was definitely exhausting.”
All the work was to see how far they could take the technique. Previously, Milchberg’s lab had shown that a similar method works for distances less than a meter. But the researchers encountered an obstacle in extending their experiments to tens of meters: their lab is too small and moving the laser is impractical. A hole in the wall and a hallway become a laboratory space.
“There were major challenges: the massive scale to 50 meters forced us to rethink the basic physics of airwaveguide generation, and the desire to send a high-power laser down a 50-meter public corridor naturally raises major safety concerns. ‘ says Milchberg. “Fortunately, we have received excellent cooperation from both the Physics Department and the Maryland Environmental Safety Agency!”
Without fiber optic cables or waveguides, a beam of light—whether from a laser or a flashlight—continuously expands as it travels. If allowed to propagate unhindered, a ray’s intensity can drop to unusable levels. Whether you’re attempting to recreate a sci-fi laser blaster or detecting pollutant levels in the atmosphere by pumping energy with a laser and capturing the light released, it pays to ensure efficient, concentrated delivery of the light.
Milchberg’s potential solution to this light containment challenge is additional light—in the form of ultrashort laser pulses. This project built on earlier work from 2014 in which his lab demonstrated that they can use such laser pulses to shape waveguides in the air.
The short-pulse technique uses a laser’s ability to deliver such high intensity along a path called a filament that it forms a plasma — a phase of matter in which electrons have been torn from their atoms. This energetic path heats the air so it expands, leaving a path of low-density air in the wake of the laser. This process is similar to a tiny version of lightning and thunder, in which the energy of the lightning turns the air into a plasma, which explosively expands the air and creates the thunderbolt. The cracking sounds the researchers heard along the beam path were thunder’s tiny cousins.
But these low-density filament paths alone weren’t what the team needed to guide a laser. The researchers wanted a high-density core (like internet fiber optic cables). So they created an array of multiple low-density tunnels that naturally diffuse and transition into a trench surrounding a denser core of undisturbed air.
The 2014 experiments used a fixed array of just four laser filaments, but the new experiment used a novel laser setup that automatically upscales the number of filaments depending on the laser energy; the filaments are naturally distributed around a ring.
The researchers showed that the technique could increase the length of the airwaveguide and increase the power they could deliver to a target down the hall. At the end of the laser’s journey, the waveguide had blocked about 20% of the light that would otherwise have been lost from its target area. The distance was about 60 times further than their record from previous experiments. The team’s calculations suggest they are not yet close to the theoretical limit of the technique, and they say much higher managerial efficiencies should be easily achievable using the method in the future.
“If we had a longer corridor, our results show that we could have adjusted the laser for a longer waveguide,” says Andrew Tartaro, a UMD physics graduate student who worked on the project and is the paper’s author. “But we got our guide spot on for the hallway we have.”
The researchers also carried out shorter eight-meter tests in the laboratory, in which they examined the physical processes taking place in more detail. For the shorter test, they managed to deliver about 60% of the light that might have been lost to their target.
The popping sound of plasma formation was put to practical use in their tests. Not only was it a clue as to where the beam was, it also provided researchers with data. They used an array of 64 microphones to measure the length of the waveguide and how strong the waveguide was along its length (more energy going into making the waveguide results in a louder bang).
The team found that the waveguide only lasted hundredths of a second before disappearing into thin air again. But that’s eons for the bursts of lasers the researchers sent through it: light can travel more than 3,000 km in that time.
Based on the researchers’ findings from their experiments and simulations, the team plans experiments to further improve the length and efficiency of their air waveguides. They also plan to guide different colors of light and investigate whether a faster filament pulse repetition rate can create a waveguide to channel a high-power continuous beam.
“Reaching the 50-meter scale for airwaveguides literally paves the way for even longer waveguides and many applications,” says Milchberg. “Based on new lasers that we will be getting soon, we have the recipe to extend our guidance to a kilometer and beyond.”