Abundance of cavity-free polaritonic states in resonant materials and nanostructures

TL;DR

This paper demonstrates that cavity-free polaritonic states naturally occur in resonant materials and nanostructures, challenging the necessity of external optical cavities for strong light-matter coupling and expanding potential applications.

Contribution

It reveals that electronic and vibrational polaritons are inherent eigenstates of resonant materials, eliminating the need for external cavities in strong coupling phenomena.

Findings

Cavity-free polaritons are natural eigenstates in bulk and nanostructured resonant materials.

Examples include excitonic slabs and water droplets showing strong coupling without external cavities.

The abundance of such polaritons questions their practical relevance in polaritonic chemistry and related fields.

Abstract

Strong coupling between various kinds of material excitations and optical modes has recently shown potential to modify chemical reaction rates in both excited and ground states. The ground-state modification in chemical reaction rates has usually been reported by coupling a vibrational mode of an organic molecule to the vacuum field of an external optical cavity, such as a planar Fabry-P\'erot microcavity made of two metallic mirrors. However, using an external cavity to form polaritonic states might: (i) limit the scope of possible applications of such systems, and (ii) be unnecessary. Here we highlight the possibility of using optical modes sustained by materials themselves to self-couple to their own electronic or vibrational resonances. By tracing the roots of the corresponding dispersion relations in the complex frequency plane, we show that electronic and vibrational polaritons are natural eigenstates of bulk and nanostructured resonant materials that require no external cavity. Several concrete examples, such as a slab of excitonic material and a spherical water droplet in vacuum are shown to reach the regime of such cavity-free self-strong coupling. The abundance of cavity-free polaritons in simple and natural structures questions their relevance and potential practical importance for the emerging field of polaritonic chemistry, exciton transport, and modified material properties.