Quantum-hydrodynamic modal perspective on plasmonic gap structures

Abstract

Plasmonic gap structures are among the few configurations capable of generating extreme light confinement, finding applications in surface-enhanced spectroscopy, ultrasensitive detection, photocatalysis and more. Their plasmonic response undergoes a dramatic, quantum effect-driven transition as the gap size approaches zero. Modal analysis can reveal insights into the mechanisms governing this process, which are otherwise obscured by nonlocal damping effects. Here, we offer a fresh modal perspective on the transition of the plasmonic response using quantum hydrodynamic theory (QHT)-based quasinormal mode (QNM) analysis. Focusing on the bonding dipolar and charge-transfer plasmons of a nanosphere dimer, we examine the detailed mode transition through the touching regime as well as the asymptotic behavior compared with the classical results as the constituent nanoparticles either separate or overlap. The complex eigenfrequency particularly provides accurate information on the linewidth and quality factor of the plasmon modes. We introduce an index to characterize charge-transfer efficiency, especially for the charge-transfer plasmon. The significant role of nonlocal damping in the mode evolution is elucidated by our mode-resolved QHT-QNM analysis. The insights from our theoretical study provide an integrated understanding of mode evolution in plasmonic gap structures, which can further advance gap structure-based applications.

References in corpus (6)

• Plasmons in nearly touching metallic nanoparticles: singular response in the limit of touching dimers
• Nonlocal optical response in metallic nanostructures
• Resonance shifts and spill-out effects in self-consistent hydrodynamic nanoplasmonics
• Ab-initio nanoplasmonics: The impact of atomic structure
• Nonlocal quasinormal modes for arbitrarily shaped three-dimensional plasmonic resonators
• Nonlocal study of ultimate plasmon hybridization