Symmetry-Controlled Ultrastrong Phonon-Photon Coupling in a Terahertz Cavity

TL;DR

This study demonstrates how a structural phase transition in lead halide perovskites can reversibly control ultrastrong phonon-photon coupling in terahertz cavities, enabling dynamic tuning of hybrid light-matter states.

Contribution

It introduces a method to reversibly tune ultrastrong phonon-photon coupling using symmetry changes in perovskites, advancing control over light-matter hybridization.

Findings

Observation of three polariton branches above Tc

Emergence of an additional branch below Tc due to a new phonon mode

All normalized coupling strengths remain in the ultrastrong regime

Abstract

Optical cavities provide a powerful means to engineer light-matter hybrid states by coupling confined electromagnetic fields with matter excitations. Achieving in situ control of the coupling strength is essential for investigating how such hybridization evolves with the coupling strength. In this work, we use a symmetry-changing structural phase transition in lead halide perovskites to reversibly tune the phonon-photon coupling strength, leveraging the fact that their phonon frequencies and oscillator strengths are dictated by lattice symmetry. Terahertz time-domain spectroscopy of MAPbI3 embedded in nanoslot cavities reveals three polariton branches above the critical temperature Tc = 162.5 K, and the emergence of an additional branch below Tc, activated by a new phonon mode in the low-temperature phase. The full dispersion is accurately reproduced using a multimode Hopfield model, confirming that all normalized coupling strengths remain in the ultrastrong coupling regime. These results demonstrate symmetry-controlled tuning of ultrastrong coupling via phonon engineering in optical cavities.