Hexagonal nanodimers as plasmonic metasurface for SERS exhibiting ultrahigh electromagnetic enhancement

In the era of highly sensitive single-molecule detection, Surface-Enhanced Raman Spectroscopy (SERS) has proven to be one of the most promising surface-sensitive techniques, enabled due to the highly localized plasmonic field at the nanoscaled rough metallic surfaces. Here, we report a theoretical study of systematic design and analysis of SERS-based closely spaced periodic gold hexagonal nanodimers using finite-difference time-domain simulations, which induce strong localized surface plasmon resonance resulting in the highly localized hotspots at the sharp edges of the structure and hence higher-order electromagnetic coupling at the center of dimers. Moreover, we explore the influence of geometric parameters, such as nanopattern spacing, polarization, and surrounding environment, on the SERS enhancement factor (EF) and spectral response. Through calculations, the maximum EF is achieved up to the order of ∼ 2.7 × 1011 and 1.46 × 1012 for incident laser wavelengths of 632 nm and 785 nm, respectively. In addition, due to the cavity quantum electrodynamic effect, a high Purcell factor of ∼ 3.8 × 106 and corresponding EF of ∼ 1.5 × 1013 are attained, facilitating the possibility of nearly single molecule detection. The proposed device is found to be highly sensitive to changes in the surrounding refractive index from 1.330 to 1.400, proving its applicability for biomedical monitoring. The proposed structure showed a potential for designing SERS substrates for real-world applications, including biomedical diagnostics, environmental monitoring, chemical analysis, and quantum plasmonics.