  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.

Optics;Mesoscale and Nanoscale Physics