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Fine-tuning photons to capture fleeting electron motions

Cornell researchers have discovered a way to accelerate photons using four orders of magnitude less energy than existing methods, paving the way for ultraviolet lasers that can capture processes lasting a quintillionth of a second.

Photon acceleration boosts the energy of a photon, allowing physicists to fine-tune the color of laser beams, making them useful for various functions. Traditionally, photon acceleration has required extreme-intensity laser pulses and gas plasmas.

A team led by Gennady Shvets, professor of applied and engineering physics, used an ultrathin nanostructure known as a metasurface to accelerate photons to the same output level achieved using gas plasmas, but using a fraction of the energy.

“People would use ultrahigh power lasers to strip off electrons from gas molecules to accelerate photons,” Shvets said, “but this process is too violent to be practical.”

“We’ve built an analog of what’s happening at these big lasers, but at a tabletop setting,” said Maxim Shcherbakov, a postdoctoral associate in Shvets’ lab and first author of “Photon Acceleration and Tunable Broadband Harmonics Generation in Nonlinear Time-Dependent Metasurfaces,” which published March 22 in Nature Communications. “We can apply less power to the system and the photons get accelerated at the same level.”

Now, the researchers hope to build on this finding to generate some of the world’s shortest laser pulses, which would produce bursts of light as short as a few attoseconds – one quintillionth of a second. “These will help to study basic condensed matter phenomena in many different settings and materials; there are numerous applications of such ultra-short pulses,” Shcherbakov said.

For example, a super-short burst of light could create snapshots of the movement of an electron inside an atom, or in the process of hopping from one atom to another in a crystal lattice. “Currently people generate attosecond pulses by using large gas chambers and very powerful lasers. We want to change those by a process with a very small footprint, and requiring very low laser energy,” he said.

Photons – particles representing a quantum of light – generally have a certain color, corresponding to the amount of energy they contain. Photons are important tools in transferring information, such as over the internet, but their colors – and energy levels – are very difficult to change.

One way to change photons’ energy is through harmonics, in which several photons can combine to form a single photon with a higher energy. Previously, researchers were only able to combine discrete photons, but Shvets and colleagues discovered that metasurfaces make it possible to combine three photons to create one with 3.1 times the energy.

“This makes the process much easier and more tunable, so you can actually fine-tune the final color of your laser beam with this process,” Shcherbakov said.

Combining the acceleration of photons using metasurfaces and the ability to boost their energy using harmonics offers numerous practical applications, and opens up new research directions. “Lasers at the visible and the near-infrared range are abundant,” Shcherbakov said, “but there are certain ranges where you would love to have affordable lasers, but the technology just isn’t there.”

One example is lasers in the extreme ultraviolet range, which can’t be easily produced with existing technology but which would have important uses in physics, materials science, chemistry and biology. The researchers’ findings could make this possible.

“This is where combining photons and accelerating them will become useful,” Shcherbakov said. “And most importantly, these lasers will be producing very short bursts of energy, which is either very expensive or unavailable. This could enable more discoveries, relating to anything that moves or any process that takes place in these very short amounts of time.”

The paper is co-authored by postdoctoral researcher Zhiyuan Fan, and researchers from the Ohio State University.

The work was supported by the Office of Naval Research, the Air Force Office of Scientific Research and the Cornell Center for Materials Research, with funding from the National Science Foundation. Part of the study was carried out at the Cornell NanoScale Science and Technology Facility, which is also supported by the NSF.

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Jeff Tyson