Engineers have created an incredibly thin, chameleon-like material that can change colour - on demand - by simply applying a minute amount of force.
The material paves the way for an entirely new class of display technologies, colour-shifting camouflage, and sensors that can detect otherwise imperceptible defects in buildings, bridges, and aircraft.
“This is the first time anybody has made a flexible chameleon-like skin that can change colour simply by flexing it,” said Connie J Chang-Hasnain from the University of California at Berkeley.
By precisely etching tiny features - smaller than a wavelength of light - onto a silicon film one thousand times thinner than a human hair, the researchers were able to select the range of colours the material would reflect, depending on how it was flexed and bent.
Researchers etched rows of ridges onto a single, thin layer of silicon. Rather than spreading the light into a complete rainbow, however, these ridges - or bars - reflect a very specific wavelength of light.
By “tuning” the spaces between the bars, it’s possible to select the specific colour to be reflected. Unlike the slits in a diffraction grating, however, the silicon bars were extremely efficient and readily reflected the frequency of light they were tuned to.
Since the spacing, or period, of the bars is the key to controlling the colour they reflect, the researchers realised it would be possible to subtly shift the period - and therefore the colour - by flexing or bending the material.
“If you have a surface with very precise structures, spaced so they can interact with a specific wavelength of light, you can change its properties and how it interacts with light by changing its dimensions,” said Chang-Hasnain.
Earlier efforts to develop a flexible, colour shifting surface fell short on a number of fronts. Metallic surfaces, which are easy to etch, were inefficient, reflecting only a portion of the light they received.
Other surfaces were too thick, limiting their applications, or too rigid, preventing them from being flexed with sufficient control.
The researchers were able to overcome both these hurdles by forming their grating bars using a semiconductor layer of silicon approximately 120 nanometres thick.
Its flexibility was imparted by embedding the silicon bars into a flexible layer of silicone. As the silicone was bent or flexed, the period of the grating spacings responded in kind.
The semiconductor material allowed the team to create a skin that was incredibly thin, perfectly flat, and easy to manufacture with the desired surface properties.
The study was published in The Optical Society’s journal Optica.