[Paper Review] Electromagnetic coupling between subradiant plasmons and dye molecular excitons analyzed by spectral changes in ultrafast surface-enhanced fluorescence

Tamitake Itoh, Yuko S. Yamamoto|arXiv (Cornell University)|Mar 20, 2026

Gold and Silver Nanoparticles Synthesis and Applications0 citations

TL;DR

The paper develops a method to evaluate electromagnetic coupling between subradiant plasmons and molecular excitons using enhancement factors derived from ultrafast surface-enhanced fluorescence in silver nanoparticle dimers, revealing spectral and temporal signatures of the coupling.

ABSTRACT

Electromagnetic (EM) coupling between molecular exciton and plasmon has been studied using in Rayleigh scattering or extinction spectroscopy. However, evaluating EM coupling involving subradiant plasmon is challenging because this resonance does not manifest clearly in far-field spectra. In this study, we developed a method to evaluate such coupling using EM enhancement factors (FR) derived from ultrafast surface-enhanced fluorescence (ultrafast SEF). This SEF, which appears as a broad background in surface-enhanced resonant Raman scattering (SERRS) spectra, were measured using silver nanoparticle dimers containing dye molecules within their nanogaps. Our results show that the spectral peaks of FR for subradiant resonances appear near the dips in Rayleigh scattering spectra. Furthermore, these FR peaks exhibit blue-shifts during the quenching processes of both ultrafast SEF and SERRS. We examined these static and temporal spectral properties using a coupled oscillator model composed of radiant plasmons, subradiant plasmons, and molecular excitons. The static properties were reproduced by increasing the linewidths of the radiant plasmon resonance, while the temporal properties were captured by decreasing the EM coupling energies between the exciton and both plasmon oscillators. These findings indicate that this methodology is a powerful tool for evaluating EM coupling between subradiant plasmons and molecular excitons.

Motivation & Objective

• Motivate and quantify electromagnetic coupling between subradiant plasmons and molecular excitons, which is difficult to access by far-field spectra.
• Develop a practical method to extract electromagnetic enhancement factors from ultrafast surface-enhanced fluorescence.
• Link static and dynamic spectral properties to a coupled oscillator model involving radiant plasmons, subradiant plasmons, and molecular excitons.

Proposed method

• Use silver nanoparticle dimers with dye molecules in nanogaps to measure ultrafast SEF as a broad background in SERRS.
• Extract electromagnetic enhancement factors (FR) from spectral data.
• Compare FR spectral peaks to Rayleigh scattering dips to identify coupling signatures.
• Apply a coupled oscillator model with radiant plasmon, subradiant plasmon, and molecular exciton to reproduce static and temporal features.
• Show that static properties are captured by increasing radiant plasmon linewidths, while temporal features require decreasing exciton–plasmon coupling energies.

Experimental results

Research questions

• RQ1 How can EM coupling involving subradiant plasmons be quantified when subradiant resonances are not clearly visible in far-field spectra?
• RQ2 What spectral and temporal signatures in ultrafast SEF reveal coupling strength and dynamics between plasmons and molecular excitons?
• RQ3 Can a coupled oscillator model reproduce both static and ultrafast dynamical aspects of the exciton–plasmon system?

Key findings

• FR spectral peaks align near dips in Rayleigh scattering spectra for subradiant resonances.
• FR peaks blue-shift during quenching of ultrafast SEF and SERRS.
• Static properties are reproduced by increasing the radiant plasmon linewidth.
• Temporal properties are captured by decreasing the EM coupling energies between the exciton and both plasmon oscillators.
• The methodology provides a tool to evaluate EM coupling between subradiant plasmons and molecular excitons.