Engineering Bimodal Surface Lattice Resonances in Asymmetric Aluminum Metasurfaces for Multiplexed SERS Applications: A Simulation Study

The surface lattice resonance (SLR) of plasmonic metasurfaces is a powerful mechanism to enhance the molecule-specific spectral fingerprint in Raman spectroscopic applications. SLR produces ultranarrow and sharpened spectrum line shapes with significant electric field localization, overcoming the limitations of radiative losses associated with localized surface plasmon resonance (LSPR) in metallic nanostructures. Yet, SLR-supporting metasurfaces are rarely employed in practical surface-enhanced Raman spectroscopy (SERS) applications owing to the limited multiplexing capabilities of a single metasurface, which require high fabrication complexities. Herein, we propose a simulation study to engineer an aluminum (Al)-based plasmonic metasurface for future potential multiplexed SERS applications. Aluminum offers significant advantages over gold and silver for visible-range applications owing to economic and complementary metal-oxide-semiconductor (CMOS) compatibility, along with higher plasma frequency, which enables strong SLR excitation in the visible spectrum where traditional noble metals are least efficient. We employed rectangular lattice periodicity along the Al nanorod axes to break the C4 symmetry and generate distinctive parallel first-order SLRs along (±1, 0) and (0, ±1) diffraction orders (DOs), simultaneously in a single metasurface. The proposed Al metasurface is designed to generate bimodal SLRs at 536 and 636 nm, matching the two commonly used Raman pumps of 532 and 633 nm in the visible spectrum, respectively, with wider SERS applications. The proposed metasurface exhibits robust SLRs with minimal sensitivity to variations in the surface-bound refractive index, demonstrating competence to produce SLR-induced high spectral fingerprints over a wide range of biomolecules. In addition, the SERS enhancement factor for the model molecules radiative at 532 and 633 nm Raman lasers has been modeled using the Lorentz oscillator formalism to decipher the SLR-induced strong electric field enhancement at the designed SLR wavelengths. The proposed geometry shows SERS EF in the order of 105 at both SLR wavelengths. This computational research demonstrates an efficient bimodal SLR-based scalable and wavelength-selective platform that could be utilized for potential multiplexed SERS-based biosensing experiments.