By precisely etching hundreds of tiny triangles on the surface of a microscopic film of zinc oxide, the study’s co-author Naomi Halas and colleagues said they created a metalens that transforms incoming long-wave UV (UV-A) into a focused output of vacuum UV (VUV) radiation.
VUV is used in semiconductor manufacturing, photochemistry and materials science but can be costly to work with, partly because it's absorbed by almost all types of glass used to make conventional lenses.
“This work is particularly promising in light of recent demonstrations that chip manufacturers can scale up the production of metasurfaces with CMOS-compatible processes,” Halas said in a statement. “This is a fundamental study, but it clearly points to a new strategy for high-throughput manufacturing of compact VUV optical components and devices.”
According to the team, its microscopic metalens could convert 394nm UV into a focused output of 197nm VUV. The disc-shaped metalens is a transparent sheet of zinc oxide thinner than a sheet of paper — just 45 millionths of a metre in diameter.
In the demonstration, a 394nm UV-A laser was shined at the back of the disc, and researchers measured the light that emerged from the other side.
Study co-first author Catherine Arndt, an applied physics graduate student in Halas’ research group, said the metalens’ key feature is its interface, a front surface studded with concentric circles of tiny triangles.
“The interface is where all of the physics is happening,” she said. “We’re actually imparting a phase shift, changing both how quickly the light is moving and the direction it’s traveling. We don’t have to collect the light output because we use electrodynamics to redirect it at the interface where we generate it.”
Violet light has the lowest wavelength visible to humans. Ultraviolet has even lower wavelengths, ranging from 400nm to 10nm. Vacuum UV, with wavelengths between 100-200nm, is so-named because it is strongly absorbed by oxygen. Using VUV light today typically requires a vacuum chamber or other specialised environment, as well as machinery to generate and focus VUV.
Arndt explained that conventional materials don’t usually generate VUV, made today with nonlinear crystals which are bulky, expensive and often import-controlled.
In previous work, Halas and colleagues demonstrated that they could transform 394nm UV into 197nm VUV with a zinc oxide metasurface. Like the metalens, the metasurface was a transparent zinc oxide film with a patterned surface, but the required pattern wasn’t as complex as it didn’t need to focus the light output, Arndt said.
“Metalenses take advantage of the fact that the properties of light change when it hits a surface,” she said. “For example, light travels faster through air than it does through water. That’s why you get reflections on the surface of a pond. The surface of the water is the interface, and when sunlight hits the interface, a little of it reflects off.”
Prior work showed a metasurface could produce VUV by upconverting long-wave UV via a frequency-doubling process called second-harmonic generation. But VUV is costly, partly because it is expensive to manipulate after it’s produced.
Commercially available systems for that can fill cabinets as large as refrigerators or compact cars and costs tens of thousands of dollars, Arndt added.
“For a metalens, you’re trying to both generate the light and manipulate it,” she said, explaining that metalens technology has become very efficient in the visible wavelength regime.
“Virtual reality headsets use that. Metalenses have also been demonstrated in recent years for visible and infrared wavelengths, but no one had done it at shorter wavelengths. And a lot of materials absorb VUV.”
Tests at Rice showed the metalens could focus its 197nm output onto a spot measuring 1.7 microns in diameter, increasing the power density of the light output by 21 times. Arndt said it’s too early to say whether the technology could compete with state-of-the-art VUV systems, but that it has ‘a lot of potential’.