Dynamic plasmonics enable chameleon-like camouflage
They then crafted a mechanical chameleon covered with plasmonic cells and equipped with sensors so as to tune the individual cells’ colours to its background, providing an artificial, active camouflage.
Each plasmonic cell consists of an array of highly ordered Au-core/Ag-shell nanodomes covered by a gel electrolyte containing Ag+ ions, and sandwiched between two electrodes. The plasmonic properties can be modulated through an electrochemical bias, effectively shifting the light absorption and final colour rendering in the full-visible region of 430 to 650 nm, through an electrodepositing/stripping process activated by the reversible redox reaction of silver in contact with the gel. The egg-shaped nanodomes are about 25nm in diameter on the X-Y plane and vary in the range 0–6nm depending on the electrochemical bias, while their height above the X-Y plane varied from 0 to 30nm.
Fig. 1: Structure and function of the plasmonic cell devices. (a) Schematic diagram of the plasmonic cell. The double-layered hemiellipsoids represent nanodomes with different Ag shell thickness. (b) SEM image of the SiO2 nanohole array formed after etching and removal of AAO. Scale bar: 100 nm. Inset: Cross-sectional SEM image of one SiO2 nanohole. Scale bar: 50 nm. (c) Top-view SEM image of the Au nanodome array. Scale bar: 100 nm. (d) Formation of the working plasmonic cell including electrodes, gel electrolyte, and sealing. (e) Microscopic image of the device’s color in RGB color. Scale bar: 50 μm. (f) Transmission of 600 nm light as a function of electrodeposition voltage. Scan rate: 0.2 V/s. Inset: Photo of the plasmonic cell device at the starting and ending point.
Here, by electrodepositing and stripping Ag shells on the plasmonic Au nanodomes (designed on ITO glass), the researchers produced a reversible plasmonic cell covering the entire visible spectrum, without the need for colour filters or multiple colour-mixing layers.
To demonstrate a practical camouflage application with these plasmonic cells, they designed a mechanical chameleon covered with fish-scale-like colour patches (the plasmonic cells) coupled with a colour sensor on each side of the chameleon, controlling the colour patches for a realistic active camouflage (through applying a voltage of 1.5V across the different cells for a determined number of seconds).
Fig. 2: Demonstration of the plasmonic chameleon and display. The front and back parts of the chameleon are able to change colour independently with respect to their colour backgrounds.
Although this part of the experiment was more approximate, a more elaborate model could use image sensors and analyse the robot’s surroundings to precisely map the colours to its plasmonic scales.
The researchers also see low-powered electronic papers and display units as another potential application for their plasmonic cells, with a total refresh speed under 1 second (they would require further miniaturisation).
See their full paper on the American Chemical Society Nano website
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