Engineers from the University of California at Berkeley have created a chameleon-like material that can change colour when touched. By etching microscopic features 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.
As described in journal Optica, the new material offers possibilities for an entirely new class of display technologies, colour-shifting camouflage, and sensors that can detect otherwise unnoticable defects in buildings, bridges, and aeroplanes.
Controlling light with structures rather than traditional optics is not new. In astronomy, for example, diffraction gratings are used to disperse broad white light into its component wavelengths.
Efforts to control colour with this technique, however, have proved impractical because the optical losses are simply too great.
The UC Berkeley scientists applied a similar principle to achieve colour control. In place of slits cut into a film, they instead etched rows of ridges onto a single, thin layer of silicon. Rather than spreading the light into its component colours, these ridges reflect a very specific wavelength of light. Therefore, by tuning the spaces between the bars, it’s possible to select the specific colour to be reflected.
In addition, the silicon bars are much more efficient than the slits in a diffraction grating.
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 Connie Chang-Hasnain, a member of the Berkeley team and co-author of the research paper.
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 Berkeley researchers were able to overcome both of these hurdles by forming their grating bars using a semiconductor layer of silicon approximately 120nm 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 also allowed the team to create a skin that was incredibly thin, flat, and easy to manufacture with the desired surface properties. This produces materials that are highly efficient, reflecting up to 83 per cent of the incoming light.
According to the researchers, the design of the material could still be improved to reflect a wider range of colours and reflect light with an even greater efficiency− the current material can be shifted from green to yellow, orange, and red, across a 39nm range of wavelengths.
And, developments still need to be made so that the material can be produced on larger scales, so that it could be used in applications such as entertainment, security, and monitoring. For this research, the size of the material the team produced was one-centimetre square.
Once manufactured on larger scales, the chameleon-like substance could be used in a new class of display technologies, for example, adding brilliant colour presentations to outdoor entertainment venues.
It also may be possible to create an active camouflage on the exterior of vehicles that would change colour to better match the surrounding environment.
More day-to-day applications could include sensors that would change colour to indicate structural fatigue in bridges, buildings, or the wings of airplanes.