Microcavity phonon polaritons from weak to ultrastrong phonon-photon coupling

TL;DR

This study explores the transition from weak to ultrastrong coupling between phonons in hexagonal boron nitride and microcavity photons, revealing the limits and potential of cavity phonon polaritons for material property modification.

Contribution

It demonstrates the evolution of phonon-photon coupling from weak to ultrastrong in microcavities with hBN, including the achievement of strong coupling in nanometer-thin layers.

Findings

Strong coupling achieved with 10 nm hBN layers.

Ultrastrong coupling yields dispersion matching bulk phonon polaritons.

Maximum light-matter coupling limited by bulk material properties.

Abstract

Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material. Here, we study the much less explored coupling between lattice vibrations (phonons) and microcavity modes. Embedding thin layers of hexagonal boron nitride (hBN) into classical microcavities, we demonstrate the evolution from weak to ultrastrong phonon-photon coupling when the hBN thickness is increased from a few nanometers to a fully filled cavity. Remarkably, strong coupling is achieved for hBN layers as thin as 10 nm. Further, the ultrastrong coupling in fully filled cavities yields a cavity polariton dispersion matching that of phonon polaritons in bulk hBN, highlighting that the maximum light-matter coupling in microcavities is limited to the coupling strength between photons and the bulk material. The tunable cavity phonon polaritons could become a versatile platform for studying how the coupling strength between photons and phonons may modify the properties of polar crystals.