Colour microprints project multiple authentication patterns under laser exposure

January 10, 2019 // By Julien Happich
Stepping up on conventional anti-counterfeit optical markings such as microscopic colour prints and holograms, a team of researchers from the Singapore University of Technology and Design (SUTD) have managed to combine both approaches into one, effectively encrypting multiple hidden holographic patterns within what looks like a simple colour QR code.

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 holographic colour print (left) consists of a structural colour filters integrated on top of light phase encoded holograms. The colour filters act as colour pixels in a colour image under white light but also control the transmission of red, green, and blue monochromatic laser lights, yielding distinct holographic projections, each independent of the colour image and of the other projections.

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.

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