Colour printing at 100,000 dpi for light manipulation

September 10, 2018 // By Julien Happich
Subwavelength-sized silicon nanostructures, known as Mie resonators, feature unique resonance property driven by the simultaneous excitation of electric and magnetic multipoles when incident light is trapped and confined inside them.

Notably, a nanostructure's resonance wavelengths can be tuned by altering its geometry, ensuring it only scatters specific wavelengths. This particular aspect is attracting researchers willing to achieve subwavelength printing resolutions by encoding a distribution of hard-Mie resonators as fixed colour pixels. Such high resolution structures could be designed for security certification and optical data storage, but also as a mean to accurately manipulate light in transmission devices such as flat lenses and filters.

Until now, a drawback of Mie resonators was that although pure colours were easily achieved with large homogeneous arrays, single isolated resonators would suffer from non-negligible resonant wavelength shifts when designed in proximity to distinct neighboring structures, due to the coupling of adjacent electromagnetic fields. This meant that unexpected colours would occur when one would shrink the gap between individual Mie resonators to create subwavelength resolution colour images (with individual Mie resonators pixels less than 100nm apart).

Oblique Structured illumination microscopy (SIM)
of Cr-capped Si Mie resonators.

Now through numerical analysis, researchers from the University of Osaka have revealed that adding Cr masks on top of Si Mie resonators, it was possible to suppress unwanted color changes caused by adjacent nanostructures. This means that a single pixel Si Mie resonator capped with chromium Cr now exhibits the same color as it does in an array, unaffected by neighboring nanostructures.

Reporting their findings in the ACS Photonics journal under the title "Metal-Masked Mie-Resonant Full-Color Printing for Achieving FreeSpace Resolution Limit", the researchers experimentally proved their results by fabricating Cr-capped monocrystalline Si nanostructures tuned to generate various vivid colors including RGB. They systematically experimented with various structure parameters (height, diameter, periodicity) to map the CIE1931 colour space.

Colour chart: an optical microscope image of the fabricated Cr-masked Mie-resonator arrays through a 20× objective irradiated with linear-polarized white light. The diameter d and periodicity P are changed from 70 to 250 nm in increments of 5nm and from 150 to 350 nm in increments of 20nm, respectively. Each individual color area is 10x10 μm2. Scale bar = 20 μm.


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