This new material is claimed to offer possibilities 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, a member of the Berkeley team and co-author on a paper published in Optica.
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.
Controlling light with structures rather than traditional optics is not new with diffraction gratings routinely used in astronomy to direct light and spread it into its component colours. Efforts to control colour with this technique, however, have proved impractical due to optical losses.
The authors of the Optica paper applied a similar principle by etching 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.
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 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,” Chang-Hasnain said in a statement.
Earlier efforts to develop a flexible, colour-shifting surface had deficiencies with easily etched metallic surfaces only reflecting 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 overcame 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 thin, flat, and easy to manufacture with the desired surface properties. This produces materials that reflect precise and very pure colours and that are highly efficient, reflecting up to 83 per cent of the incoming light.
Their initial design, subjected to a change in period of 25nm, created colours that could be shifted from green to yellow, orange, and red - across a 39nm range of wavelengths. Future designs, the researchers believe, could cover a wider range of colours and reflect light with even greater efficiency.
For consumers, this chameleon-like material could be used in a new class of display technologies, 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. Further applications could include sensors that would change colour to indicate that structural fatigue was stressing critical components on bridges, buildings, or the wings of airplanes.
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