Evolutionary Optimized, Monocrystalline Gold Double Wire Gratings as a SERS Sensing Platform

Abstract: Achieving reliable and quantifiable performance in large-area surface-enhanced Raman spectroscopy (SERS) substrates has long been a formidable challenge. It requires substantial signal enhancement while maintaining a reproducible and uniform response. Conventional SERS substrates are typically made of inhomogeneous materials with random resonator geometries and distributions. As a result, they exhibit several or broadened plasmonic resonances, undesired absorptive losses, and inhomogeneous field enhancement. These limitations diminish the signal strength and hamper reproducibility, making it difficult to conduct comparative studies with high sensitivity. In this study, we propose an approach that utilizes monocrystalline gold flakes to fabricate well-defined gratings composed of plasmonic double-wire resonators, which are fabricated through focused ion-beam lithography. The geometry of the double wire grating substrate (DWGS) was evolutionary optimized to achieve efficient enhancement for both excitation and emission processes. The use of monocrystalline material minimizes absorption losses while enhancing the shape fidelity during the nanofabrication process. The DWGS shows notable reproducibility (RSD=6.6%), repeatability (RSD=5.6%), and large-area homogeneity over areas $>10^4\,\mu$m$^2$. Moreover, it provides a SERS enhancement factor of $\approx 10^6$ for 4-Aminothiophenol (4-ATP) analyte and detection capability for sub-monolayer coverage. The DWGS demonstrates reusability, as well as long-term stability on the shelf. Experimental validation with various analytes, in different states of matter, including biological macromolecules, confirms the sensitive and reproducible nature of DWGSs, thereby establishing them as a promising SERS substrate design for future sensing applications.