Not every university sends laser pulses powerful enough to burn paper and skin blazing down a hall. But that’s what happened in the UMD Energy Research Facility, an impenetrable building on the northeast corner of campus. If you visit the utilitarian white and gray hall now, it looks like any other university hall — as long as you don’t peak behind a cork board and see the metal plate covering a hole in the wall.
But for a few nights in 2021, UMD Physics Professor Howard Milchberg and his colleagues transformed the hall into a laboratory: The shiny surfaces of the doors and a water fountain were covered to avoid potentially blinding reflections; the connecting halls were blocked off with signs, warning tape and special black laser-absorbing curtains; and scientific equipment and cables with normal open walking space.
As members of the team went about their work, a screeching sound warned of the dangerously powerful path the laser blazed down the hall. Sometimes the beam’s journey ended at a white ceramic block, filling the air with louder pops and a metallic tang. Each night, a researcher sat alone at a computer in the nearby laboratory with a walkie-talkie and made requested adjustments to the laser.
Their efforts were to temporarily transform thin air into a fiber optic cable — or, more specifically, an air waveguide — that would guide light for thousands of meters. Like one of the fiber optic internet cables that provide efficient highways for optical data streams, an air waveguide prescribes a path for light. These air waveguides have many potential applications related to the collection or transmission of light, such as light emitted by atmospheric pollution, long-range laser communication or even laser weapon detection. With air wave directional, there is no need to untangle a solid cable and worry about the constraints of gravity; instead, the cable forms quickly without support in the air. In a paper accepted for publication in the journal Physical Review X the team described how they set a record by guiding light in air guides 45 meters long and explained the physics behind their method.
The researchers performed atmospheric alchemy to set a record at night to avoid inconveniencing (or zapping) their colleagues or unsuspecting students during the working day. They had to approve their safety procedures before they could use the hall again.
“It was a truly unique experience,” says Andrew Goffin, a UMD electrical and computer engineering graduate student who worked on the project and is lead author of the resulting journal article. “There’s a lot of work that goes into shooting a laser outside the lab that you don’t have to deal with when you’re in the lab – like putting up curtains for eye safety. It was definitely tiring.”
All the work was to find out how far they could push the technique. Milchberg’s lab had previously shown that a similar method worked for distances of less than a meter. But the researchers hit a roadblock in extending their experiments to thousands of meters: Their lab is too small and it’s impractical to move the laser. Therefore, a hole in the wall and a hall becoming a laboratory space.
“There were big challenges: the huge scale of up to 50 meters forced us to rethink the basic physics of air-directed generation, as well as trying to send a high-powered laser down a public hall 50 meters long, this naturally leads to major safety issues,” says Milchberg. “Fortunately, we got excellent cooperation from the physics and environmental safety office of Maryland!”
Without fiber optic cables or waveguides, a beam of light — whether from a laser or a flash — will expand as it travels. If allowed to spread unchecked, beam intensity can drop to unusable levels. Whether you’re trying to recreate a science fiction laser blaster or detect pollutant levels in the atmosphere by pumping them full of energy with a laser and capturing the light emitted, it pays to ensure efficient, concentrated delivery of light.
Milchberg’s possible solution to this challenge of keeping light confined is additional light – in the form of ultra-short laser pulses. This project built on previous work from 2014 where his lab showed they could use such laser pulses to sculpt waveguides in air.
The short pulse technique uses laser power to deliver such high intensity along a path, called a filament, that it creates a plasma — a phase of matter in which electrons are torn free from their atoms. This energetic path heats the air, so it expands and leaves a path of low-density air in the wake of the laser. This process is like a miniature version of lightning and thunder where the energy of the lightning bolt turns the air into plasma which expands the air explosively, creating the thunder; the popping sounds the researchers heard along the path of the beam were the tiny cousins of thunder.
But these low-density filament paths alone were not what the team needed to guide lasers. The researchers wanted a high-density core (similar to fiber optic internet cables). So, they created an arrangement of multiple low-density tunnels that would naturally diffuse and merge into a mass around a denser core of undisturbed air.
The 2014 experiments used a fixed arrangement of four laser filaments, but the new experiment used a new laser arrangement that automatically increases the number of filaments depending on the laser energy; the filaments distribute themselves naturally around a ring.
The researchers showed that the technique could extend the length of the air wave, increasing the power they could deliver to a target at the end of the hall. At the end of the laser’s journey, the waveguide retained about 20% of the light that would otherwise have been lost from its target area. The distance was about 60 times longer than their record from previous experiments. The team’s calculations indicate that they are not yet close to the theoretical limit of the technique, and say that much higher guidance efficiencies should be possible with the method in the future.
“If we had a longer hall, our results show that we could adjust the laser for a longer waveguide,” says Andrew Tartaro, a UMD physics graduate student who worked on the project and is an author on the paper. “But we found our right direction for the hall we have.”
The researchers also carried out shorter eight meter tests in the laboratory where they investigated in more detail the physics at play in the process. For the shorter test they managed to deliver about 60% of the light that could have been lost to their target.
The popping sound of the plasma formation was put to practical use in their tests. Apart from being an indication of where the beam was, it also provided the researchers with data. They used a line of 64 microphones to measure the length of the waveguide and how strong the waveguide was along its length (more energy to make the waveguide translates into a louder pop).
The team found that the waveguide lasted for hundreds of seconds before being sent back into thin air. But that’s no news for the laser blasts the researchers were sending through: Light can travel more than 3,000 km in that time.
Based on what the researchers learned from their experiments and simulations, the team is planning experiments to further improve the length and efficiency of their air wave guides. They also plan to direct different colors of light and investigate whether a faster filament pulse repetition rate can produce a waveguide to guide a continuous high-power beam.
“Reaching the 50-meter scale for air waveguides literally blazes the path for even longer waveguides and many applications,” says Milchberg. “Based on a new laser that we will soon have, we have the recipe to extend our guidance to one kilometer and beyond.”