Tailoring ultrahigh index plasmonic combinatorial metamaterials for SEIRA and SERS by tuning the fill fraction

Plasmonic materials with strong mid-infrared resonances allow enhanced molecular sensing, but obtaining the required high field enhancements in easily constructed geometries is challenging. Multilayer aggregates (MLaggs) of close-packed gold nanoparticles (AuNPs) exhibit strong extinction resonances that enhance both surface-enhanced Raman (SERS) and IR absorption (SEIRA) spectroscopies. Here, we show that these simply fabricated MLaggs behave as low-loss high refractive index (n) metamaterials in the mid-infrared and that they can reach a refractive index of n > 10. By admixing silver nanoparticles (AgNPs) with AuNPs and co-aggregating them, we demonstrate combinatorial metamaterials with near-perfect absorption and controlled linewidth. By selective dissolution, we are able to control the aggregate fill fraction and tune both the SEIRA resonance and the refractive index. Dissolution of the AgNPs leads to more porous aggregates, which enable adsorption and detection of larger analytes, making MLaggs useful for healthcare sensing applications.