Tuning Surface-Enhanced Raman Scattering (SERS) via Filling Fraction and Period in Gold-Coated Bullseye Gratings

Surface-enhanced Raman scattering (SERS) is a highly sensitive analytical technique capable of single-molecule detection, owing to its exceptional chemical specificity, sensitivity, and reproducibility. In this study, we developed a robust SERS platform based on long-range ordered bullseye plasmonic nano-gratings, fabricated via a combination of electron beam lithography and reactive ion etching. The resulting nanostructured arrays—comprising concentric bullseye patterns with tunable period and filling fraction—were uniformly coated with a thin gold film to support strong surface plasmon resonances, generating intense electromagnetic field enhancements across the substrate. Using this platform, we demonstrated quantitative detection of small molecules such as Rhodamine 6G at low concentrations, achieving enhancement factors on the order of 105. Interestingly, we found that the geometric configuration yielding the strongest local electric field did not correspond to the highest SERS enhancement, which we attribute to a mismatch between the field orientation and molecular polarizability alignment. This study provides insights for optimizing plasmonic substrates for sensitive molecular detection.