A 3000-year-old metallurgy technique of heating metal to create vibrant colors creates a nanostructured surface that can act as a perfect light absorber, researchers from Case Western Reserve University have found. The team applied its findings to create a nickel thin-film that perfectly absorbs red light. “We showed that perfect light absorption could be realized using a simple thin-film with the right combination of oxide and metallic substrate,” said professor Giuseppe Strangi. “This combination naturally occurs with certain metals like the nickel and titanium we used in this study.” A thermally painted subwavelength metasurface produces an array of colors that do not change with viewing angle. An oxide layer that is a few nanometers thick was grown at the edge of the metal at high temperatures. This created the optical metasurface that perfectly absorbs certain wavelengths of light. Courtesy of Giuseppe Strangi, Case Western Reserve University. To demonstrate their technique, the researchers deposited 150 nm of nickel or titanium on silicon and then heated the films for 20 to 40 minutes at 400 °C to form an oxide layer. An analysis of the samples’ absorption properties showed that when the nickel films were baked for 40 minutes they absorbed approximately 99.94 percent of red light. The researchers further demonstrated that the light absorption could be tuned across visible and NIR wavelengths by modifying the heating duration, causing changes to the thickness of the oxide layer. Perfect light absorption occurs in the nickel oxide (NiO) metasurface because light rays emerging from the oxide layer and the metal substrate come together in such a way that they cancel each other out. Courtesy of Giuseppe Strangi, Case Western Reserve University. The researchers said that perfect light absorption occurs in the heated metal because light rays emerging from the oxide layer and the metal substrate come together in such a way that they cancel each other out — a phenomenon known as total destructive interference. Any remaining light is absorbed inside the metal substrate. “This method of creating perfect absorption is very practical due to its simplicity and reproducibility,” Strangi said. “The oxide layer makes the surface scratch-resistant and protects it from further oxidation.” The researchers plan to perform additional experiments to determine if high-resolution patterns can be formed by growing metal oxide layers. They are also working to develop gas sensors using the perfect light absorbers. The research was published in Optical Materials Express, a publication of OSA, The Optical Society (http://dx.doi.org/10.1364/OME.9.001386).