Similar to its perhaps better-known cousin origami – the Japanese art of folding paper – kirigami is an art form in which paper is both folded and cut. The jaw-dropping three-dimensional designs that kirigami artists create, inspired a team of physicists from the University of Amsterdam to design an equally spectacular type of material: one that smoothly changes its colour when it is stretched.

In ordinary materials, colour is a result of what the material is made of. Think of pigments in paint, or dyes in fabric. Once a material is chosen, the color is therefore also fixed. In the new material, it is the structure of the material that determines its colour. This type of colour, fittingly called “structural colour”, has the enormous advantage that it can easily be changed at any desired moment.

In the lab, materials can be designed to exhibit tailored properties. These so-called metamaterials are built out of tiny structures much smaller than the thickness of a human hair, and their colour comes from the shape and arrangement of these tiny structures – indeed very similar to the art of kirigami. When the material is stretched, its microscopic patterns move and rotate, changing how light reflects from the surface. As a result, during the stretching, the colour of the light reflected from this ‘nanoscale chameleon skin’ shifts smoothly from green to yellow and finally to red.

The design of the material was not a straightforward process, recalls first author Freek van Gorp: “At first, we faced a major challenge: the material we wanted to use, silicon, is quite brittle at the macroscopic scale. We initially tried to place tiny silicon particles on or inside a flexible substrate, but the substrate itself introduced new complications. The real breakthrough came when we began to question whether we needed a substrate at all. By turning the silicon into a thin, patterned mesh, we could make the material both flexible and functional. This approach made it possible to combine optical metasurfaces with mechanical metamaterials in a single design, enabling the color-tuning effect we demonstrated in this work.”

Jorik van de Groep, group leader of the 2D nanophotonics lab in which the research was performed, adds: “The crucial novelty lies in the multifunctionality of the structure. By nanopatterning the thin silicon membrane, we were able to make it simultaneously function as a mechanical metamaterial that governs the internal rotations and displacements, as well as an optical metasurface that uses resonant light scattering by the structure, together giving rise to the tunable structural color.”

With the design of the material finished, the researchers are currently working on bringing the concept from simulation to reality by fabricating an actual flexible metasurface in the cleanroom at nearby research institute AMOLF. Now that it has been shown how light can be controlled by motion rather than by chemistry, implementing the idea in a fully functional metamaterial is the obvious next step, opening possibilities in the near future for tunable color coatings, smart sensors, and lightweight optical devices that adapt to their environment.