Strong coupling of plasmonic bright and dark modes with two eigenmodes of a photonic crystal cavity

TL;DR

This paper demonstrates strong coupling between plasmonic dark modes and photonic crystal cavity modes via a bright mode mediator, revealing four polariton modes and advancing understanding of light-matter interactions in metamaterials.

Contribution

It introduces a novel experimental observation of strong coupling mediated by a bright mode between dark modes and cavity photons in the terahertz range.

Findings

Observation of frequency splitting indicating strong coupling

Identification of four polariton modes due to mode interactions

Reproduction of results with a four coupled harmonic oscillator model

Abstract

Dark modes represent a class of forbidden transitions or transitions with weak dipole moments between energy states. Due to their low transition probability, it is difficult to realize their interaction with light, let alone achieve the strong interaction of the modes with the photons in a cavity. However, by mutual coupling with a bright mode, the strong interaction of dark modes with photons is possible. This type of mediated interaction is widely investigated in the metamaterials community and is known under the term electromagnetically induced transparency (EIT). Here, we report strong coupling between a plasmonic dark mode of an EIT-like metamaterial with the photons of a 1D photonic crystal cavity in the terahertz frequency range. The coupling between the dark mode and the cavity photons is mediated by a plasmonic bright mode, which is proven by the observation of a frequency splitting which depends on the strength of the inductive interaction between the plasmon bright and dark modes of the EIT-like metamaterial. In addition, since the plasmonic dark mode strongly couples with the cavity dark mode, we observes four polariton modes. The frequency splitting by interaction of the four modes (plasmonic bright and dark mode and the two eigenmodes of the photonic cavity) can be reproduced in the framework of a model of four coupled harmonic oscillators.