New material can selectively reflect light like a chameleon skin

(a) Schematic of the color display from the HCM embedded flexible membrane. (b) Schematic of a one-dimensional HCM. Credits: Li Zhu et al. University of Berkeley. Paper published on Optics InfoBase

Researchers at Berkeley, UC, have created an extremely thin material that can reflect different wavelengths of light, that is different colours, depending on the way it is deformed by pressure and bending.

When we say that a given object is red we would be more accurate i stating that such objects absorbs all the visible spectrum wavelengths with the exception of the red one. It is the fact that the red wavelength (656 nm for pure red) is reflected that brings the red colour to our eyes!

The absorption of wavelength, and their reflection, depends on the surface of the object. What researchers at Berkeley have managed to do is to create a sort of grating where the distance between the ridges and the valleys absorb or reflect specific wavelength. By changing the distance the wavelength reflected changes, and therefore the colour perceived.

The results have been published on a paper published on Optica.

They have managed to create a High Contrast Megastructure, HCM, based on a very thin (120nm) layer of silicon with etched bars deposited on a silicone substrata that provides the desired flexibility. The grating (formed by the sequence of bars) can reflect wavelengths at a very precise value, around 20nm. This creates a pure colour reflection with a high yield: about 83% of the light at that specific wavelength is reflected, all the others are absorbed.

By flexing the layer you are increasing or decreasing the distance between the bars leading to a change in the reflected wavelength.

This can be used for fun, as a sort of chameleon skin, or as a sort of display or, even more interesting as a precise sensor to detect minute variation (a few nm displacement is enough to change the colour of the reflected light). 

Using this "skin" as an extremely precise sensor can make detection of minute stress on bridges and building visible.  This is just another example of the progress we are seeing in the area of smart materials, one of the pillars of change in the next decade.

Author - Roberto Saracco

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