Gap-Controlled Plasmon Dephasing and Near-Field Confinement in Metal–Insulator–Metal Nanostructures

Plasmonic nanostructures confine light into nanoscale volumes via localized surface plasmon resonances, enhancing local electromagnetic fields and excitation probabilities of nearby molecules. However, their optical confinement time, represented by the plasmon dephasing time (T2), is inherently short due to radiative damping. To overcome this limitation, coupled plasmonic nanostructures have been designed to generate out-of-phase plasmon modes within nanogaps, which suppress radiative losses and enhance electromagnetic field localization. Nevertheless, the relationship among the gap distance that efficiently excites out-of-phase plasmons, the resulting T2, and near-field intensity remains unclear. Here, we employ a minimal metal–insulator–metal (MIM) architecture consisting of Au nanostructures separated from a continuous Au film by an Al2O3 spacer to elucidate these relationships. Experimental and simulation results show that excitation of an out-of-phase plasmon mode between the upper Au nanoblock and its image dipole in the underlying film suppresses radiative losses and extends T2. The spacer-thickness dependence demonstrates that thicker dielectric layers yield narrower excitation line widths and longer T2. Under the present experimental conditions, a thickness of 15–20 nm provides an optimal condition, achieving strong near-field enhancement with prolonged coherence and offering practical guidelines for designing MIM structures with controllable optical coherence.