As a proof of concept, the researchers first designed a 480×480 pixel dual colour QR code, only 1.44mm2 but able to store a 2620-bit message at error correction level H. The code could be retrieved under white light using a QR-code scanner. But since each QR code colour pixel itself consists of a 4×4 block of holographic colour pixels (with varying phase plates under the same colour structures), the same QR code under red and blue laser illumination projected two distinct holographic patterns (a red Chinese seal and a blue Penny Black stamp), about 10cm across on a wall at a 1m projection distance. Under ambient room lighting, even a low power laser pointer was enough to create a visible far field projection along the axis of laser illumination, the authors report.
The team from SUTD then increased the number of structured colour filters to 6, which allowed them to create a full colour reproduction of Luigi Russolo’s painting "Perfume", albeit shrinking it to less than 1.5mm2 while multiplexing three distinct grayscale holograms under red, green and blue illumination.
While the colour filters collectively form the colour image under white light, they also control the transmission of red, green, and blue laser lights through the pixels of the underlying multiplexed holograms.
Because phase and amplitude control is purely structural at pixel level, the researchers anticipate that customised masters manufactured through nanoscale 3D printing could encode whole multiplexed holographic colour prints as mere surface relief profiles that could then easily be mass produced by nanoimprint lithography.
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