Upper bounds on collective light-matter coupling strength with plasmonic meta-atoms

TL;DR

This paper theoretically establishes the fundamental upper bounds on the collective light-matter coupling strength achievable with plasmonic meta-atoms, highlighting optimal geometries and implications for quantum technologies.

Contribution

It provides the first theoretical analysis of the universal upper limits of collective plasmon-photon coupling in dense assemblies of plasmonic meta-atoms.

Findings

Universal upper bounds on normalized coupling strength $g/\omega_0$ for spherical meta-atoms.

Elongated spheroidal meta-atoms maximize coupling strength.

Results inform design of plasmonic structures for enhanced light-matter interactions.

Abstract

Ultrastrong coupling between optical and material excitations is a distinct regime of electromagnetic interaction that enables a variety of intriguing physical phenomena. Traditional ways to ultrastrong light-matter coupling involve the use of some sorts of quantum emitters, such as organic dyes, quantum wells, superconducting artificial atoms, or transitions of two-dimensional electron gases. Often, reaching the ultrastrong coupling domain requires special conditions, including high vacuum, strong magnetic fields, and extremely low temperatures. Recent report indicate that a high degree of light-matter coupling can be attained at ambient conditions with plasmonic meta-atoms -- artificial metallic nanostructures that replace quantum emitters. Yet, the fundamental limits on the coupling strength imposed on such systems have not been identified. Here, using a Hamiltonian approach we theoretically analyze the formation of polaritonic states and examine the upper limits of the collective plasmon-photon coupling strength in a number of dense assemblies of plasmonic meta-atoms. Starting off with spheres, we identify the universal upper bounds on the normalized collective coupling strength $g/\omega_0$ between ensembles of plasmonic meta-atoms and free-space photons. Next, we examine spheroidal metallic meta-atoms and show that a strongly elongated meta-atom is the optimal geometry for attaining the highest value of the collective coupling strength in the array of meta-atoms. The results could be valuable for the field of polaritonics studies, quantum technology, and modifying material properties.