Plasmonic Color-Graded Nanosystems with Achromatic Subwavelength Architectures for Light Filtering and Advanced SERS Detection.

Plasmonic color-graded systems are devices featuring a spatially variable plasmonic response over their surface. They are widely used as nanoscale color filters; their typical size is small enough to allow integration with miniaturized electronic circuits, paving the way to realize novel nanophotonic devices. Currently, most plasmonic color-graded systems are intrinsically discrete because their chromatic response exploits the tailored plasmon resonance of microarchitectures characterized by different size or geometry for each target color. Here, we report the realization of multifunctional plasmon-graded devices where continuously graded chromatic response is achieved by smoothly tuning the composition of the resonator material while simultaneously maintaining an achromatic nanoscale geometry. The result is a new class of versatile materials: we show their application as plasmonic filters with a potential pixel size smaller than half of the exciting wavelength but also as multiplexed surface-enhanced Raman spectroscopy (SERS) substrates. Many more implementations, such as photovoltaic efficiency boosters or color routers, await and will benefit from the low fabrication cost and intrinsic plasmonic flexibility of the presented systems.