The multiplexed holographic colour print described in a recent Nature Communications paper, “Holographic colour prints for enhanced optical security by combined phase and amplitude control” consists of multi-layer pixels that effectively overlay a structural colour element (in the shape of carefully dimensioned nano-pillar arrays) on top of phase plates of carefully designed thickness so as to encode holograms through light phase modulation.
In effect, the authors report that such a dual layer design is able to control both the phase and amplitude of light at pixel level, with phase and amplitude controlled independently based on individual pixel design. Thanks to the light amplitude (wavelength) control layer, an array of pixels can be designed to appear as a colour image under non-coherent white light.
Yet, under the monochromatic illumination of a red, green or blue laser, the light phase control bottom layer of the same array of RGB pixels can be designed so as to shine three different holograms (each colour-filtered through the top structural colour filter layer).
The researchers implemented their idea through a single lithographic process by 3D direct laser writing into a photoresist, effectively growing various heights of nano-pillars 0.4μm in diameter on top of phase plates of varying thicknesses. Each individual 3×3μm2 pixel held a square array of nano-pillars patterned at a 1μm pitch.
The authors report that by yielding a dielectric polymer with a refractive index ranging from 1.54 to 1.58 in the visible region, the process allowed them to achieve a full 2π phase modulation for red, green, and blue light by simply varying the bottom phase plate thickness over a 1.2μm range.
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.
Singapore University of Technology and Design (SUTD) – www.sutd.edu.sg
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