Photon-photon polaritons in chi(2) microresonators

TL;DR

This paper explores the strong coupling regime in chi(2) microresonators, introducing polariton quasi-particles, and predicts spectral modifications, resonance splitting, and parametric thresholds using a dressed-resonator approach.

Contribution

It introduces the concept of photon-photon polaritons in chi(2) microresonators and applies a dressed-resonator approach to analyze their dynamics and spectral properties.

Findings

Dominance of Hermitian operator parts in strong coupling regime

Introduction of polariton quasi-particles and their effective masses

Prediction of spectral response modifications and parametric thresholds

Abstract

We consider a high-Q microresonator with $\chi^{(2)}$ nonlinearity under conditions when the coupling rates between the sidebands around the pump and second harmonic exceed the damping rates, implying the strong coupling regime (SC). Using the dressed-resonator approach we demonstrate that this regime leads to the dominance of the Hermitian part of the operator driving the side-band dynamics over its non-Hermitian part responsible for the parametric gain. This has allowed us to introduce and apply the cross-area concept of the polariton quasi-particles and define their effective masses in the context of $\chi^{(2)}$ ring-microresonators. We further use polaritons to predict the modified spectral response of the resonator to a weak probe field, and to reveal splitting of the bare-resonator resonances, avoided crossings, and Rabi dynamics. Polariton basis also allows deriving a discrete sequence of the parametric thresholds for the generation of sidebands of different orders.