We demonstrate a new plasmonic approach to high-density optical data storage; using
dual-color plasmonic nano-pixels to encode two information sets into the same unit
area using single arrays of two-state metal nano-apertures.
The ability to effectively separate discrete colors from white-light lies at the heart
of how we record and view optical information; whether that be the arrangement of
colored inks in painting and printing applications, or the spectral filters that enable
many modern image display and recording technologies. In each case, color
separation is typically provided by organic compounds; dyes and pigments that absorb
and scatter particular wavelengths of light, leading to their distinct color profiles.
Recently, structural color systems based on engineered nanophotonic materials have
emerged as an appealing alternative to absorptive dyes [1]. Among these examples are
color filters based on plasmonics. Plasmonic filters hold several dimensional and
stability advantages over their micro-scale, dye-based counterparts. As a result, they
have been positioned as new technological solutions for sub-wavelength color
printing [1], RGB splitting for image sensors [2], anti-counterfeiting measures [3], and
optical data storage [4]; thus representing one of the most promising, commercially
relevant areas of current plasmonic research activity.
Here, we demonstrate a method for patterning full-color images and codes that
exhibit polarization-dependent information states. Our individual pixels are comprised
of asymmetric cross-shaped nano-apertures in a thin film of aluminum; each aperture
engineered to exhibit 2 independent plasmonic color resonances that can be
individually tuned across the sRGB spectrum. This enables us to encode 2 arbitrary
information sets into the same unit area using the same array of nano-pixels. We show
that using a standard optical microscope, color separation can be controlled down to
2x2 nano-pixels while retaining polarization selectivity. This defines our maximum
data storage capability; each 2x2 pixel area acting as a 2-state data bit that can be read
optically. The maximum data density we can achieve using this technique is
approximately 1.46 Gb/cm2, with the added ability to further encode each of those
pixels using the full visible-color spectrum.