Researchers at ITMO University in Russia have created a “laser paintbrush” capable of creating localized color on a metallic canvas, using their method to create miniaturized replicas of various works of art—including Vincent van Gogh’s The Starry Night. Their technique even makes it possible to change or erase or rewrite the colors several times. They described their work in a new paper published in the journal Optica.

Traditional paints get their colors from various pigments (often derived from minerals), but there are many examples of structural color in nature, like the bright colors in butterfly wings. As we’ve reported previously, those colors don’t come from pigment molecules but from how the wings are structured. The scales of chitin (a polysaccharide common to insects) are arranged like roof tiles. Essentially, they form a diffraction grating, except these naturally occurring photonic crystals only produce certain colors, or wavelengths, of light, while a diffraction grating will produce the entire spectrum, much like a prism. Alter the structure by changing the size of the tiles, and the crystals become sensitive to a different wavelength. And the perception of color doesn’t depend on the viewing angle.

Manmade “nanopillars” can also be used to generate structural colors, for instance, by illuminating nanopillar arrays with white light to produce specific colors (red, blue, and green light), simply by varying the sizes (widths) of the nanopillars. In fact, last year, scientists at the National Institute of Science and Technology (NIST) used millions of nanopillars in an array to control both the color and intensity of incident light, projecting a faithful reproduction of Johannes Vermeer’s Girl With a Pearl Earring as proof of concept.

Like the aforementioned “nano painting,” this new laser paintbrush is another form of structural color, arising from the effect on white light by the interference of two distinct waves: one reflecting from the top of a very thin metal oxide film that forms on the metal during heating, and the other reflecting from the metallic surface of the titanium canvas. “Depending on the thickness of such films, waves with different wavelengths interfere, so we can see different colors,” co-author Yaroslava Andreeva told Ars. For instance, “Increasing the thickness, one can see yellow, orange, red, purple, and blue colors in a row.”

The trick to creating these thin films lies in heating the metal to temperatures above the oxidation point; it’s different for different metals, but it is usually roughly half the metal’s melting point temperature. The scientists let oxidation occur for a specified time—depending on how thick they want the final film to be—and the oxide film will form as the metal cools down. The longer the cooling time, the thicker the resulting film. The ITMO team uses the laser for this, along with a galvanometric scanner, since it gives them better control not just over the temperature but over the time duration, thereby producing films of precisely the desired thickness for the desired color.

The nanosecond ytterbium laser’s scanning speed is linked to the cooling time: a faster scan means the metal cools faster, while a slower scan speed corresponds to a slower cooling time. “The faster we process the material, the thinner layer we heat and the higher gradients of temperature occurs [between the surface and the volume],” said Andreeva. “So the heat from the surface can spread faster into the volume, and the surface cools faster.”

It’s basically akin to the so-called “Mpemba effect,” in which hot water freezes faster than lukewarm or cool water. (It’s named after Erasto Mpemba, who, as a schoolboy in Tanzania in 1963, noticed that his homemade ice cream froze faster than his schoolmates’ batches if he didn’t cool the boiled milk before placing it in the freezer. Physicists still hotly debate the underlying mechanisms.)

The current study grew out of the team’s prior research, which used lasers to create different colors on titanium it seeded, as well as stainless steel surfaces, which proved to be extremely resistant to environmental and chemical impacts, with no need for special storage requirements. “We wanted to do more than offer a wide palette of stable colors,” said co-author Galina Odintsova. “Thus, we worked to create a convenient tool for applying them more like an artist’s brush. Increasing the laser heating range enough to create the evaporation process makes our color strokes reversible, rewritable, erasable and much more efficient. Our marking speed is more than 10 times faster than reported before.”

They were able to create nine different colors in all, as well as being able to erase or change colors with a second pass of the laser at a faster scan rate. According to Andreeva, the color change is the result of the evaporation of the previous oxide layer and subsequent reoxidization, producing a new layer of a different thickness and, thereby, a new color.

“With this approach, an artist can create miniature art that conveys complex meaning not only through shape and color but also through various laser-induced microstructures on the surface,” said co-author and research team leader Vadim Veiko. In addition to the 3 x 2 inch version of The Starry Night, the team used its technique to create its own original artworks on titanium in various styles—pop art, minimalism, and abstract art—inspired by the work of other artists.

Ultimately, Veiko et al. would like to produce a handheld tool based on their laser paintbrush system, possibly adding advanced nanostructured and hybrid materials for even more optical effects. “We hope that laser painting will attract the attention of modern artists and lead to the creation of a completely new type of art,” said Andreeva. “The approach can also be used for modern design and to create color markings on various products.”

DOI: Optica, 2021. 10.1364/OPTICA.420074  (About DOIs).

Listing image by Yaroslava Andreeva

https://arstechnica.com/?p=1758287