Symmetry-Controlled Ultrastrong Phonon-Photon Coupling in a Terahertz Cavity
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.
Cite
@article{arxiv.2511.16285,
title = {Symmetry-Controlled Ultrastrong Phonon-Photon Coupling in a Terahertz Cavity},
author = {Dasom Kim and Maxime Dherbécourt and Sae R. Endo and Geon Lee and Ayush Agrawal and Sunghwan Kim and Wen-Hua Wu and Aditya D. Mohite and Minah Seo and David Hagenmüller and Junichiro Kono},
journal= {arXiv preprint arXiv:2511.16285},
year = {2025}
}