Plasmon-Enhanced Raman Sensing with Metal-Insulator-Metal Metasurfaces

Metal-insulator-metal (MIM) plasmonic metasurfaces provide a powerful platform for enhancing light-matter interactions; however, achieving simultaneous spectral tunability, fabrication reproducibility, and ultrasensitive detection remains a challenge. Here, we present the rational design, simulation, and lithographic fabrication of three distinct MIM metasurfaces (bowtie, honeycomb, and nanotriangle) optimized for plasmon-enhanced Raman spectroscopy (PERS). Finite-difference time domain (FDTD) simulations reveal localized surface plasmon resonances with electric field enhancement factors (EF) exceeding |E|² ∼ 1600, supported by experimental reflection spectra and the fidelity of nanofabrication. Raman sensing of molecular probes (R6G, 4-ATP, 4-CTP) demonstrates analytical enhancement factors reaching 10⁷ and detection limits as low as ∼10^-15 M. This is made possible by the designed nanogap resonances, and the broadband localized surface plasmon resonance (LSPR) overlap with the excitation and scattering bands. Our findings establish the lithographically defined MIM metasurfaces as reliable, tunable, and ultrasensitive surface-enhanced Raman spectroscopy (SERS) platforms, making them suitable for next-generation portable chemical and biological sensing systems.