In 2014, University of Utah engineering mentor Ajay Nahata and his mates
demonstrated an inkjet printer could fabricate metasurfaces, or horizontal
arrays of plasmonic resonators. Metasurfaces are the two-dimensional version of
metamaterials, and metasurfaces for optical holography are usually made of gold
and silver. However, when the frequency of the impinging rays is in the
terahertz region, graphene and other semiconductors can wrangle plasmons as
well. Noble-metal plasmonic resonators require expensive, custom
microfabrication techniques, while the lowly inkjet printer can lay BK7 window
down printer with
sufficient resolution-and with spatially varying electrical ktp crystal
subwavelength nanostructures. In new experiments, Nahata and his team printed
dipole arrays, with dimensions in the few hundreds of microns, onto clear
plastic substrates. The printed dipoles looked identical to the undressed eye,
but contained varying proportions of silver and carbon inks. The researchers
then measured the metasurfaces' answers to polarized rays at 0. 1, 0. 15 and 0.
3 THz. The team then embedded random grayscale “quick-response” (QR) codes-the
familiar two-dimensional smartphone-readable designs-into the same pattern.
Under monochromatic illumination, a simple rectangular-dipole metasurface could
distinguish among nine different numbers of dull, while an array of prism
(cross-shaped resonators) could distinguish among 36 values. When caf2
the researchers designed a
metasurface responsive to three different frequencies of light, with four
distinct gray levels per station, they produced an encoded image with 64
possible colors per pixel.