Multimode Phonon-Polaritons in Lead-Halide Perovskites in the Ultrastrong Coupling Regime

TL;DR

This paper demonstrates multimode ultrastrong coupling of phonons in lead-halide perovskites using nanoslot cavities, enabling control over phonon emission and interactions for advanced quantum and optoelectronic applications.

Contribution

It introduces a theoretical framework for multimode ultrastrong phonon coupling in perovskites via nanoslot cavities, a novel approach for phonon control.

Findings

High coupling strengths achieved in nanoslot resonators.

Effective phonon mode interactions mediated by the cavity.

Prediction of superthermal phonon bunching in thermal equilibrium.

Abstract

Phonons play a central role in fundamental solid-state phenomena, including superconductivity, Raman scattering, and symmetry-breaking phases. Harnessing phonons to control these effects and enable quantum technologies is therefore of great interest. However, most existing phonon control strategies rely on external driving fields or anharmonic interactions, limiting their applicability. Here, we realize multimode ultrastrong light--matter coupling and theoretically show the modulation of phonon emission. This regime is realized by coupling two optical phonon modes in lead halide perovskites to a nanoslot array functioning as a single-mode cavity. The small mode volume of the nanoslots enables high coupling strengths in the phonon-polariton system. We show theoretically that the nanoslot resonator mediates an effective interaction between phonon modes, leading to superthermal phonon bunching in thermal equilibrium between distinct modes. Our findings are well described by a multimode Hopfield model. This work establishes a pathway for engineering phononic properties for light-harvesting and light-emitting technologies.