Plasmonic colour-graded systems are devices featuring a spatially variable plasmonic response over their surface. They are widely used as nanoscale colour 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 colour-graded systems are intrinsically discrete, as their chromatic response exploits the tailored plasmon resonance of micro-architectures characterized by different size and/or geometry for each target colour.Here we report the realization of multifunctional plasmon-graded devices where continuouslygraded 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, like photovoltaic efficiency boosters or colour routers await, and will benefit from the low fabrication cost and intrinsic plasmonic flexibility of the presented systems.

In this work, porous silicon photonic crystals (PSi PhCs) covered by silver nanoparticles (AgNPs) were prepared as surface-enhanced Raman scattering (SERS) substrate to detect methylene blue (MB) at low concentrations. The limit of MB detection in water by the SERS substrate is evaluated to be 10\(^{-10}\) mol/L. The SERS signal intensities of 446 cm\(^{-1}\) and 1623 cm\(^{-1}\) peaks in SERS spectra of MB are fit in exponential functions of concentrations ranging from 10\(^{-4}\) to 10\(^{-10}\) mol/L. These results show that the AgNPs on PSi PhCs substrates could be applied in environmental sensing.