Scientists at the University of California Berkeley have developed a novel material that can change color simply by flexing it. The material offers intriguing possibilities for an entirely new class of display technologies, color-shifting camouflage, and sensors that can detect otherwise imperceptible defects in buildings, bridges, and aircraft.
Li Zhu et al. created a chameleon-like artificial ‘skin’ that shifts color on demand.
The colors we typically see in paints, fabrics, and other natural substances occur when white, broad spectrum light strikes their surfaces.
The unique chemical composition of each surface then absorbs various wavelengths of light. Those that aren’t absorbed are reflected back, with shorter wavelengths giving objects a blue hue and longer wavelengths appearing redder and the entire rainbow of possible combinations in between.
Changing the color of a surface, such as the leaves on the trees in autumn, requires a change in chemical make-up.
Recently, scientists have been exploring another approach, one that would create designer colors without the use of chemical dyes and pigments.
Rather than controlling the chemical composition of a material, it’s possible to control the surface features on the tiniest of scales so they interact and reflect particular wavelengths of light.
This type of ‘structural color’ is much less common in nature, but is used by some butterflies and beetles to create a particularly iridescent display of color.
University of California scientists applied a similar principle, though with a radically different design, to achieve the color control they were looking for.
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 a complete rainbow, however, these ridges reflect a very specific wavelength of light.
By tuning the spaces between the bars, it’s possible to select the specific color 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 of the bars is the key to controlling the color they reflect, the scientists realized it would be possible to subtly shift the period – and therefore the color – by flexing or bending the material.
The team, led by Dr Connie Chang-Hasnain of the University of California Berkeley’s Department of Electrical Engineering and Computer Science, was able to overcome both these hurdles by forming their grating bars using a semiconductor layer of silicon 120 nm 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, perfectly flat, and easy to manufacture with the desired surface properties.
This produces materials that reflect precise and very pure colors and that are highly efficient, reflecting up to 83 percent of the incoming light.
Their initial design, subjected to a change in period of a mere 25 nm, created brilliant colors that could be shifted from green to yellow, orange, and red – across a 39-nm range of wavelengths.
Future designs, Dr Chang-Hasnain and her colleagues believe, could cover a wider range of colors and reflect light with even greater efficiency. For this demonstration, they created a 1-cm square layer of color-shifting silicon.
The results appear online in the journal Optica.
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Li Zhu et al. 2015. Flexible photonic metastructures for tunable coloration. Optica, vol. 2, no. 3, pp. 255-258; doi: 10.1364/OPTICA.2.000255
