By Josh Perry, Editor
[email protected]

Researchers from Case Western University (Cleveland, Ohio) demonstrated that heating a metal to create a vibrant color, which remains the same color regardless of the viewing angle, creates nanostructures on the surface that acts as a perfect light absorber.

A thermally painted subwavelength metasurface produces a dazzling array of colors that do not change with viewing angle. (Giuseppe Strangi, Case Western Reserve University)

According to a report from the Optical Society of America (OSA), perfect light absorbers can absorb more than 99 percent of a specific color. The Case Western researchers applied this technique to build a nickel thin film that absorbs red light.

“Scientists have previously demonstrated perfect light absorption using ultrathin absorptive materials on metals or with highly-engineered nanostructures,” the report indicated. “However, these materials require at least two material depositions using nanolithography fabrication methods that are expensive, time consuming and hard to reproduce.”

Researchers placed 150 nanometers of nickel or titanium on a silicon substrate and heated the films for 20-40 minutes at 400°C. After 40 minutes of baking, the nickel film absorbed 99.94 percent of red light.

“The researchers also demonstrated that the light absorption could be tuned across visible and near-infrared wavelengths by modifying the heating duration, which changes the thickness of the oxide layer,” the article continued.

It added, “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.”

The research was recently published in Optical Materials Express. The abstract stated:

“Heating reflective metals is known to produce a wide range of colors due to oxidation of the metal surface. In fact, the most vibrant colors used in the pre-industrial era came from oxides, acetates and carbonates of metal ores and minerals.

“In this work, we show that heating low reflectivity metals, e.g., Ni and Ti, creates structural colors through perfect light absorption. We tune the absorption across the visible and NIR spectrum by changing the heating duration and, consequently, the oxide thickness. We demonstrate experimentally angle-insensitive perfect and near-perfect absorption in the visible and NIR regimes up to ±60?±60?.

“The absorption is insensitive to the incidence angle due to the relatively high refractive index of the formed oxides, which create iridescent free coloration. We demonstrate that the oxide layer thickness, with refractive index n, is <?/4?<λ/4n due to non-trivial phase change at the oxide/metal interfaces, which makes these systems the simplest example of meta-surfaces based on thin films. The results show that oxidized metals can have applications beyond producing vibrant colors.”