Differential molecule-cavity mode coupling in soft-cavities

TL;DR

This study compares the coupling of dye molecules to two different modes in a dielectric microsphere cavity, revealing strong coupling with whispering gallery modes but not with dark-field scattering modes, using experiments and simulations.

Contribution

It demonstrates the differential molecule-cavity coupling strengths in a soft-cavity supporting multiple modes, highlighting the importance of mode type in strong coupling phenomena.

Findings

Strong coupling observed with whispering gallery modes.

No strong coupling detected with dark-field scattering modes.

Experimental results supported by coupled oscillator model and finite-element simulations.

Abstract

The way molecules absorb, transfer, and emit light can be dramatically modified by coupling them to optical cavities. The extent of the modification is often defined by the cavity-molecule coupling strength. Evaluating this coupling strength for different types of modes supported by a cavity is crucial in designing cavities for molecule-cavity coupling. Here we probe a unique multimode cavity, a dielectric microsphere, also called a soft-cavity, which supports two distinct types of mode, dark-field scattering (DFS) modes and whispering gallery modes (WGM). Though seemingly similar, these modes show different characteristics such as spatial electric field profile, resonance line-width etc. We investigated coupling of a mono-layer of J-aggregated dye molecules and a dielectric plastic microsphere using two techniques, far-field excitation and evanescent excitation to generate DFS modes and WGMs respectively. We found that using WGMs we observe a clear signature of strong coupling, whereas with DFS modes we do not. We compared our experimental data to a simple coupled oscillator model and performed finite-element method based numerical simulations to provide a clearer understanding of our experimental findings.