Weak Molecular Signal Detection Mechanism Based on Cavity Plasmon

Optical interrogation of weak infrared molecular signatures from hazardous small molecules is important for environmental monitoring and industrial safety. However, these weak molecular responses are often difficult to distinguish using conventional far-field spectroscopy. Here, we theoretically investigate an Ag metal–insulator–metal (MIM) nanocavity as a cavity-length-engineered plasmonic platform for molecular resonance-plasmon coupling. The molecular response is incorporated through an effective-medium dielectric model for a surface-confined molecular population within the 1 nm nanogap. By tuning the cavity length, the bare cavity plasmon resonance can be matched to a selected infrared-active molecular resonance, converting weak molecular absorption responses into a finite-contrast spectral splitting in the simulated spectrum. SO2, COCl2, and H2O are first investigated as representative systems, with optimized cavity lengths of 545, 404, and 190 nm, respectively. Using a unified 5% normalized dip-depth criterion, we identify the onset of spectral splitting and obtain local threshold molecular number densities of 4.47 × 1024, 1.48 × 1025, and 1.84 × 1026 m−3, respectively. The same cavity-matching and threshold-evaluation procedure is further applied to NO2, NH3, PH3, HF, HCN, and H2S. These results provide a comparative theoretical framework for molecule-specific cavity design in MIM nanocavities.