A nonlinear metalens, made using nanoimprint lithography, is able to change the wavelength of laser light while at the same time functioning as a normal, light-focusing lens. When 800-nm infrared (IR) light is sent through the metalens, 400-nm visible light emerges on the other side. The ability of the metalens to half the wavelength of incident light is made possible by combining the nanostructures that comprise the metalens with lithium niobate, a material that supports the nonlinear optical effect. This effect is not limited to a defined laser wavelength, making it useful for various applications. The metalens can be used in nonlinear microscopy and spectroscopy, nonlinear holography for anti-counterfeiting technology, and tunable flat optics, for example. The metalens was designed and fabricated by a team of physicists at ETH Zurich (the Swiss Federal Institute of Technology). The team, led by professor Rachel Grange, developed a crystalline lithium niobate platform and optimized the platform for soft nanoimprinting lithography (SNIL) to fabricate nonlinear nanostructures. Infrared light passes through the metal lens and is converted into violet light and focused in a focal point. This is due to the properties of the material and the special surface structures, enlarged here in the magnifying glass. Courtesy of ETH Zurich/Ü. Talts. A nanoscale pattern is stamped into the lithium niobate material. “The solution containing the precursors for lithium niobate crystals can be stamped while still in a liquid state,” researcher Ülle-Linda Talts said. Once the material is heated to 600° Celsius, it takes on crystalline properties that enable the conversion of light. According to the researchers, this fabrication method is suitable for mass production because an inverse mold can be used multiple times, allowing as many metalenses as needed to be printed. The nanoimprinted metalenses achieved second-harmonic focusing over a broad spectral range, from near-ultraviolet to near-infrared, while increasing the nonlinear signal intensity by up to 34 times. The metalenses exhibited nearly vertical side-walls and aspect ratios of up to 6. Miniaturization of nonlinear optical components is essential for integrating advanced light manipulation into the compact photonic devices that will enable scalable, cost-effective applications. Size constraint is an inherent feature of classic lens design. The nanoimprinting process for creating the metalens could expand the field of nonlinear metasurfaces by providing a low-cost, highly scalable method for fabricating nonlinear nanostructures. Metalenses and similar hologram-generating nanostructures could be used to make banknotes and securities counterfeit-proof and guarantee the authenticity of artwork. The nanostructures that comprise the lenses are too small to be seen using visible light, while their nonlinear material properties allow highly reliable authentication. The nonlinear metalenses could also be used with camera detectors to modify and steer laser emissions to convert IR light to visible. They could reduce the equipment needed for deep-UV light patterning in state-of-the-art electronics fabrication. To date, the exceptional stability and hardness of lithium niobate have limited the use of this material in fabricated nanostructures. Direct nanoimprint lithography could make it possible to use lithium niobate platforms for high-quality, nonlinear optical nanostructures with outstanding frequency conversion efficiency across a broad transparency range. The ease of nanostructuring this material, combined with its high second-order nonlinearity, could enable cost-efficient, highly scalable fabrication of nonlinear metasurfaces. The development of metasurfaces is a relatively young branch of research encompassing physics, materials science, and chemistry. “We have only scratched the surface so far and are very excited to see how much of an impact this type of new, cost-effective technology will have in the future,” Grange said. The research was published in Advanced Materials (www.doi.org/10.1002/adma.202418957).