ETH Zurich scientists have invented the first-ever lithium niobate metalenses that can convert invisible infrared light into visible green light. The team’s production process is easily scalable, with one researcher comparing it to a metalens “printing press.”
With precisely engineered nanostructures several times thinner than glass lenses, these extremely thin lenses, which focus light waves like traditional lenses and convert light into alternate wavelengths (what researchers call the nonlinear optical effect), could help create counterfeit-proof currency, assist art authentication, and help design next-generation microscopes and telescopes.
Metalenses That Turn Invisible Light Visible
The cameras in modern mobile phones are so small and powerful that NASA chose them for its recent Mars Ingenuity Helicopter rather than spending money on improving the technology. Nonetheless, the lenses in those cameras are still made of glass, often accounting for the thickest part of the phone.
In recent years, scientists have turned to specially engineered nanostructures called metalenses that can control and focus light like a glass lens but are 40 times thinner than a human hair. Metalenses are also lighter than glass lenses, adding to their versatility.
In another field called nonlinear optics, researchers study the ability of certain materials to convert light from one wavelength to another. For example, scientists have used this effect to create contact lenses that allow people to see in the dark. The light from a laser pointer starts out as invisible infrared light, then passes through a crystalline structure that converts the light’s wavelength to visible green light.
“When infrared light with a wavelength of 800 nanometres is sent through the metalens, visible radiation with a wavelength of 400 nanometres emerges on the other side and is directed at a designated point,” the statement announcing the new metalenses explains.
Curious if they could combine the structural benefits of metalenses with the emerging field of nonlinear optics, the ETH Zurich researchers turned to lithium niobate. According to the team’s statement, lithium niobate is already used in the telecommunications industry to create components that convert light into different wavelengths at the interface of electronics and optical fibers.
“It Works in a Similar Way to Gutenberg’s Printing Press”
To test the theory, team leader Rachel Grange, a professor at the Institute for Quantum Electronics at ETH Zurich, attempted to create the first ever metalenses made of lithium niobate in the lab. The team settled on a process that combined precision nanoengineering and chemical synthesis. Specifically, they mixed the lithium niobate precursor material for their lenses in a liquid. Because their lenses are fabricated from a liquid solution, they can be stamped and then allowed to solidify in a desired shape.
“The solution containing the precursors for lithium niobate crystals can be stamped while still in a liquid state,” explained Ülle-Linda Talts, a doctoral student working with Grange and a co-author on the paper detailing the team’s findings. “It works in a similar way to Gutenberg’s printing press.”
Once the lenses are formed and stamped, they are heated to 600°C. At this temperature, the material takes on unique crystalline properties, like the crystals in laser pointers, which imbue it with the nonlinear optical effect. After the structures are allowed to cool, the result is a batch of ultrathin metalenses that can turn invisible light visible.
Because lithium niobate nanostructures are difficult to produce due to their exceptional stability and hardness, the ability to shape the metalenses in a liquid form before hardening is particularly valuable. According to the researchers, “this technique is suitable for mass production as an inverse mould can be used multiple times, allowing the printing of as many metalenses as needed.”
Counterfeit Detection and Other Applications
Although the current versions of the metalenses only turn invisible infrared light into visible green light, the research team says the effect is “not limited to a defined laser wavelength.” This versatility and its cost-effective fabrication method mean the metalenses could one day be adapted for several technological applications, including counterfeit-proof currency and artwork authentication.
“Their exact structures are too small to be seen using visible light, while their nonlinear material properties allow highly reliable authentication,” the team explains.
Whether the team’s new metalenses are used for authentication, security, or reducing equipment needs for deep-UV light patterning used in several state-of-the-art electronics fabrication applications, they believe that their first-ever lithium niobate metalenses are just a small first step in a rapidly growing effort to combine nanofabrication and chemistry to create advanced materials with customized properties.
“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.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.