Quantum-hydrodynamic modal perspective on plasmonic gap structures

TL;DR

This paper presents a quantum hydrodynamic modal analysis of plasmonic gap structures, revealing detailed mode transitions and damping effects, which enhances understanding of their extreme light confinement for various applications.

Contribution

It introduces a QHT-based quasinormal mode analysis to study plasmonic mode transitions, providing new insights into nonlocal damping and charge-transfer efficiency in gap structures.

Findings

Mode transition detailed through touching regime

Nonlocal damping significantly affects mode evolution

Charge-transfer index characterizes plasmon efficiency

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.