Apparent Resonance Splitting in Self-Coupled Excitonic Systems

TL;DR

This study investigates the nature of optical resonances in excitonic WS$_2$ films, distinguishing between apparent resonance splitting and true polariton formation, and demonstrates how substrate and thickness influence light-matter coupling.

Contribution

The paper clarifies the conditions under which true exciton-photon polaritons form in WS$_2$ films, highlighting the roles of film thickness and substrate reflection phase.

Findings

Resonance splitting in ultrathin films is not due to polariton formation.

Thicker films exhibit clear polariton signatures in both reflectance and photocurrent.

Substrate material and film thickness can tune the strong coupling and hybridization phenomena.

Abstract

Thin films of high-refractive-index excitonic materials enable self-coupling by simultaneously supporting intrinsic excitonic transitions and optical resonances. These optical resonances take the form of Fabry-Perot resonances in thick films and absorption resonances in ultrathin films placed on metallic substrates. Here, we investigate whether these optical resonances lead to true exciton-photon hybridization. Using far-field reflectance and spectrally resolved photocurrent measurements, we study tungsten disulfide (WS$_2$) flakes on both metallic and dielectric substrates across a range of thicknesses. While reflectance spectra for ultrathin flakes exhibit resonance splitting between excitons and absorption resonances, our photocurrent measurements reveal only excitonic peaks, indicating that no polaritons are formed. In contrast, thicker flakes exhibit Fabry-Perot resonances that strongly couple to excitons, resulting in clear splitting in both reflectance and photocurrent spectra, and providing evidence of polariton formation. We further show that the polariton resonances can be tuned through the reflection phase at the WS$_2$-substrate interface by changing the substrate material. In addition to coupling with the strong A-exciton, we observe polariton formation involving the weaker B-exciton at shorter wavelengths, as well as higher-order hybridization where both excitons interact simultaneously with a single Fabry-Perot resonance. These findings clarify the distinction between apparent and true strong coupling in excitonic materials and demonstrate how reflection phase and flake thickness can be used to engineer light-matter interactions.