Second harmonic generation by strongly coupled exciton-plasmons: the role of polaritonic states in nonlinear dynamics

TL;DR

This paper explores how strongly coupled exciton-plasmon polaritonic states influence second harmonic generation in nano-structured aluminum arrays, revealing their role in nonlinear optical processes and potential applications in THz sources.

Contribution

It demonstrates the impact of polaritonic states on SHG through a combined numerical and quantum model, highlighting their role in nonlinear dynamics.

Findings

Observation of Rabi splitting in SHG spectrum due to strong coupling.

Identification of polaritonic states' contribution to nonlinear optical response.

Agreement between quantum model and numerical simulations.

Abstract

We investigate second harmonic generation (SHG) from hexagonal periodic arrays of triangular nano-holes of aluminum using a self-consistent methodology based on the hydrodynamics-Maxwell-Bloch approach. It is shown that angular polarization patterns of the far-field second harmonic response abide to three-fold symmetry constraints on tensors. When a molecular layer is added to the system and its parameters are adjusted to achieve the strong coupling regime between a localized plasmon mode and molecular excitons, Rabi splitting is observed from occurrence of both single- and two-photon transition peaks within the SHG power spectrum. It is argued that the splitting observed for both transitions results from direct transitions between lower and upper polaritonic states of the strongly coupled system. This interpretation can be accounted by a tailored three-level quantum model, with results in agreement with the unbiased numerical approach. Our results suggest the hybrid states formed in strongly coupled systems directly contribute to the nonlinear dynamics. This opens new directions in designing THz sources and nonlinear frequency converters.