Probing the nonlinear response of strongly coupled plasmon-WSe₂ system

Plasmonic nanostructures have been previously shown to facilitate strong light-matter interactions in a wide variety of systems. With the recent progress in our understanding of 2D transition metal dichalcogenides (TMDs), studies on plasmon-exciton coupling in these systems have increasingly attracted considerable attention, demonstrating evidence of strong light-matter coupling in scattering spectra. However, more convincing evidences of strong coupling are still needed, including probing in nonlinear optical interactions, to give a direct access for studying near-field properties of the coupled system.
In this work, gold nanorods and WSe₂ flakes (TMD material) are successfully assembled to investigate the strong light-matter interaction. Rabi splitting and anti-crossing phenomena in dark-field spectra is utilized as signatures of strong-coupled candidates. Interestingly, simulation based on the classical oscillator model predicts that second harmonic generation (SHG) will show a pronounced spectral splitting in this regime, which can be further verified by SHG experiments with a femtosecond pulsed laser. Thus, we suggest that probing in the nonlinear regime may give a clearer evidence of strong coupling that could not be observed in linear measurements.