By precisely etching hundreds of tiny triangles onto the surface of a microscopic film of zinc oxide, nanophotonics pioneer Naomi Halas and her colleagues have created a “metalene” 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 and has historically been expensive to use, in part because it is absorbed by nearly all types of glass used to make conventional lenses.

“This work is particularly promising in light of recent demonstrations that chipmakers can scale up the production of metasurfaces with CMOS-compatible processes,” said Halas, co-corresponding author of a demonstration metalens study published in Scientists progress. “This is a fundamental study, but it clearly points to a new strategy for high-throughput manufacturing of compact VUV optical components and devices.”

The Halas team showed that their microscopic metalenes could convert 394 nanometer UV into a focused 197 nanometer VUV output. The disc-shaped metal is a transparent sheet of zinc oxide thinner than a sheet of paper and barely 45 millionths of a meter in diameter. During the demonstration, a 394-nanometer UV-A laser was shot through the back of the disk, and the researchers measured the light emerging from the other side.

The study’s co-first author, Catherine Arndt, a graduate student in applied physics in the Halas research group, said the main feature of metalene is its interface, a front surface dotted with concentric circles of tiny triangles.

“The interface is where all the physics happens,” she said. “We actually impart a phase shift, changing both the speed at which light travels and the direction in which it travels. We don’t have to collect the emitted light because we use electrodynamics to redirect it to the interface where we generate it.”

Violet light has the lowest wavelength visible to humans. Ultraviolet has even lower wavelengths, which range from 400 nanometers to 10 nanometers. Vacuum UV, with wavelengths between 100 and 200 nanometers, is so named because it is strongly absorbed by oxygen. Using VUV light today typically requires a vacuum chamber or other specialized environment, as well as machinery to generate and focus the VUVs.

“Conventional materials generally don’t generate VUVs,” Arndt said. “It is made today with non-linear crystals, which are large, expensive and often export controlled. The result is that VUV is quite expensive.”

In earlier work, Halas, Rice’s physicist Peter Nordlander, Rice’s former Ph.D. student Michael Semmlinger and others have demonstrated that they can transform 394 nanometer UV to 197 nanometer VUV with a zinc oxide metasurface. Like the metalens, the metasurface was a transparent film of zinc oxide with a patterned surface. But the required pattern wasn’t as complex because it didn’t need to focus the light output, Arndt said.

“Metalenses takes 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 through water. That’s why you get reflections off the surface of a pond. The surface of the water is the interface, and when the sunlight hits the interface, a small part is reflected.”

Previous work has shown that a metasurface can produce VUVs by converting long-wave UVs through a frequency-doubling process called second harmonic generation. But the VUV is expensive, in part because it’s expensive to handle after it’s produced. Commercially available systems for this can fill cabinets as large as refrigerators or compact cars and cost tens of thousands of dollars, she said.

“For a metalens, you’re trying to both generate light and manipulate it,” Arndt said. “In the field of visible wavelengths, metalens technology has become very effective. Virtual reality headsets use it. Metalens have also been demonstrated in recent years for visible and infrared wavelengths, but no one haven’t done it at shorter wavelengths. And a lot of materials absorb VUV. So for us, it was just an overall challenge to see, ‘Can we do that?'”

To fabricate the metalenes, Arndt worked with co-corresponding author Din Ping Tsai of City University of Hong Kong, who helped produce the complex surface of the metalenes, and with three co-first authors: Semmlinger, a Rice graduate in 2020, Ming Zhang, a 2021 Rice graduate, and Ming Lun Tseng, assistant professor at National Yang Ming Chiao Tung University in Taiwan.

Tests at Rice showed that the metalens could focus their 197-nanometer output into 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 tell if the technology can compete with state-of-the-art VUV systems.

“It’s really fundamental at this point,” she said. “But it has a lot of potential. It could be made much more effective. With this first study, the question was, ‘Does it work?’ In the next phase, we will ask ourselves: “How much better can we do?”

Halas is the Stanley C. Moore Professor of Electrical and Computer Engineering at Rice, Director of the Smalley-Curl Institute at Rice, and Professor of Chemistry, Bioengineering, Physics, and Astronomy, as well as Materials Science and of nano-engineering. Nordlander, co-author of the study, is a Wiess professor and professor of physics and astronomy, and a professor of electrical and computer engineering, materials science and nanoengineering.

Additional study co-authors include Benjamin Cerjan and Rice’s Jian Yang; Tzu-Ting Huang and Cheng Hung Chu from Academia Sinica in Taiwan; Hsin Yu Kuo of National Taiwan University; Vin-Cent Su of United National Taiwan University; and Mu Ku Chen from City University of Hong Kong.

The research was funded by the Ministry of Science and Technology of Taiwan (107-2311-B-002-022-MY3, 108-2221-E-002-168-MY4, 110-2636-M-A49-001 ), National Taiwan University (107-L7728, 107-L7807, YIH-08HZT49001), Shenzhen Science and Technology Innovation Commission (SGDX2019081623281169), University Grants Committee/Research Grants Council of Administrative Region Hong Kong Special Committee in China (AoE/P-502/20), Department of Science and Technology of Guangdong Province of China (2020B1515120073), Department of Electrical Engineering, City University of Hong Kong (9380131) , Taiwan Ministry of Education Yushan Young Scholar Program, Taiwan Academia Sinica Applied Science Research Center, Robert A. Welch Foundation (C-1220, C-1222), National Science Foundation (1610229, 1842494), Air Force Office of Scientific Research (MURI FA9550-15-1-0022) and Defense Thre at Reduction Agency ( HDTRA1-16-1-0042).