When light hits many ordered structures in subtle objects, reactions such as refraction, diffuse reflection, and diffraction will occur, resulting in structural colors, such as the wing hues of insects and birds.

A Chinese research team from Westlake University has drawn inspiration from the phenomenon and developed full-color inkless printing technology.

In a study recently published in the journal Nature Communications, the research team has developed a composite film composed of ultra-hard ceramic materials and used an ultrafast laser to fabricate micro-nano structures on the film's surface, providing a new idea for the industrial application of inkless laser color-printing technology.

The annual global sales of printers have reached hundreds of millions. The widely used inkjet or laser color printers require a lot of ink or toner. The ink contains volatile harmful substances such as lead, cadmium, and mercury. The toner releases small particles that can be absorbed by the human body, causing harm to the environment and human health.

The structural color, however, features long durability and high resolution and is environmentally friendly, according to the study. It is hence becoming an attractive technique in many applications. But there are problems in these applications, such as narrow color gamut, limited materials, and easy fading.

The research team developed a novel "paper," a composite ceramic film about 110 nanometers thick, one thousandth the thickness of a human hair.

The film has three layers. The bottom layer is gold-like titanium nitride, which acts as a reflective layer to block light and increase brightness. The intermediate layer is a high-loss aluminum-titanium nitride dielectric, which can regulate the absorption of natural light. The top layer is aluminum oxide.

When an ultrafast laser is applied to the surface of the aluminum-titanium nitride, an additional transparent thin film of aluminum oxide is formed, which, together with the aluminum-titanium nitride, regulates the natural light absorbed.

The team applied the laser to the film and controlled the energy or scanning speed of the laser to change the thickness of the aluminum oxide film and the aluminum-titanium nitride film.

After the thickness changes, the natural light will form specific reflected colors through the complex interference effect between the three layers, thus forming different colors.

Using various techniques, researchers performed a material analysis of the laser-colored areas and confirmed that the observed colors were from the laser-induced oxide layer, said the study.

The technology can achieve full-color inkless printing with high speed and high resolution, presenting nearly 90 percent of the standard RGB, better than current mainstream laser coloring technology.