Metamaterial Absorbers for Surface-Enhanced Infrared Absorption Spectroscopy of Ultrathin Molecules and Proteins Detection

Abstract

Metamaterial absorbers provide a powerful platform for manipulating light– matter interactions in the mid-infrared regime, enabling vibrational fingerprints of ultrathin molecules and proteins to be detected with exceptional sensitivity. This dissertation develops two complementary strategies—strong-coupled vertical-nanogap absorbers and porous-enhanced plasmonic architectures—to overcome the fundamental cross-section limit of conventional IR spectroscopy. First, a metal–insulator–metal absorber with a vertical nanogap is engineered to maximize near-field confinement and to achieve strong spatial and spectral overlap with protein vibrational modes. The undercut formed SiO₂ spacers and nanogap tailoring reduce the effective cavity volume, driving a highly confined optical resonator in which the plasmonic resonance effectively couples to the Amide I and II modes. Using a coupled harmonic oscillation model, the coupling strengths are quantitatively extracted, and new indicators for SEIRA performance are defined. With an aptamer–thrombin bioassay, this platform enables label-free sensing with a limit of detection of 267.38 pM, demonstrating its capability for ultrasensitive protein sensing. Second, a hybrid porous metasurface absorber is developed through Ag/Au alloying–dealloying process, with its morphology numerically reproduced using spinodal decomposition model based on the Cahn-Hilliard equation. The resulting porous structure preserves a central bulk plasmonic backbone while embedding high-density pores above and below it, producing intense near-field hot spots in simulation. This hybrid architecture substantially amplifies SEIRA responses, experimentally yielding a 41.87% spectral difference for monolayer analytes, surpassing conventional metamaterial absorbers. Together, these advances establish metamaterial absorbers as versatile, scalable, and high-performance vibrational sensing platforms. The combined strong- coupled and porous-enhanced approaches open pathways toward rapid molecular diagnostics, protein structural analysis, and integrated mid-IR photonic devices for chemical and biological sensing.