Extreme-UV light of 10–121 nm wavelength is incredibly useful for performing ultrafast spectroscopy and imprinting ever-smaller transistors on computer chips, yet it is also incredibly difficult to harness. The radiation is strongly absorbed by a wide variety of materials because its energy surpasses that of the electronic transitions of the elements it interacts with. To steer the radiation, researchers generally direct it at glancing angles using unwieldy mirrors. Now Marcus Ossiander, Federico Capasso, Martin Schultze, and their colleagues from Harvard University and Graz University of Technology in Austria have devised a method to efficiently transmit and focus extreme-UV radiation.
The new technique is a twist on the development of flat metamaterials called metasurfaces, which manipulate specific wavelengths of light via a periodic pattern of subwavelength-size components. A subset of those surfaces, known as metalenses, scatter light in such a way that it gets focused much as it would have if it had passed through a traditional optical lens. (See, for example, Physics Today, October 2022, page 19.) Generally, a metasurface’s minute components come in the form of pillars, disks, or other slightly raised features. Those weren’t an option for Ossiander’s team because they would simply absorb the UV radiation. Instead, the researchers decided to use air, in the form of nanosized holes drilled into a material, to serve as a waveguide. By embedding the holes in a material with a refractive index less than air’s value of 1, the surrounding substrate would channel the incoming radiation without interacting with it directly.
For their metalens, the researchers chose silicon, which has a refractive index of about 0.77 for 50 nm light. They numerically simulated perforated silicon’s interaction with the radiation and evaluated numerous arrangements of holes of various sizes. After identifying an optimal orientation, the researchers etched a 1-mm-diameter silicon wafer with hundreds of millions of 20–80 nm holes to create a lens with a 10 mm focal length. Ossiander and colleagues put their creation, shown in the photo above, to the test by firing a near-IR laser at a gas of argon atoms. The laser photons ionized the atoms and generated attosecond pulses of extreme UV, which then passed through the metalens. The resulting UV beam was focused to a minimum diameter of about 0.7 µm, less than a factor of 2 from the diffraction limit.
The metalens wasn’t perfect. Although it focused nearly half the radiation that passed through it, nearly 90% of the incoming radiation was absorbed or otherwise lost. Still, any lens in the extreme-UV regime could prove useful for applications such as semiconductor lithography. The researchers hope to develop a microscopy technique with the resolution to reveal attosecond events that take place at nanometer scales. (M. Ossiander et al., Science 380, 59, 2023.)
