Multimode vibrational coupling across the insulator-to-metal transition in 1T-TaS$_{2}$ in THz cavities

TL;DR

This study demonstrates how embedding 1T-TaS$_{2}$ in THz cavities reveals multimode vibrational coupling and its suppression across the insulator-to-metal transition, highlighting the influence of free charges on light-matter interactions.

Contribution

It provides the first experimental observation of cavity-mediated multimode vibrational strong coupling in a correlated material and shows its suppression due to free charge screening.

Findings

Strong collective vibrational coupling in the insulating phase

Hybridization of vibrational modes into polaritons

Suppression of coupling in the metallic phase due to free charges

Abstract

The use of optical cavities on resonance with material excitations allows controlling light-matter interaction in both the regimes of weak and strong coupling. We study here the multimode vibrational coupling of low energy phonons in the charge-density-wave material 1T-TaS$_{2}$ across its insulator-to-metal phase transition. For this purpose, we embed 1T-TaS$_{2}$ into THz Fabry-P\'erot cryogenic cavities tunable in frequency within the spectral range of the vibrational modes of the insulating phase and track the linear response of the coupled phonons across the insulator-to-metal transition. In the low temperature dielectric state, we reveal the signatures of a multimode vibrational strong collective coupling. The observed polariton modes inherit character from all the vibrational resonances as a consequence of the cavity-mediated hybridization. We reveal that the vibrational strong collective coupling is suppressed across the insulator-to-metal transition as a consequence of the phonon-screening induced by the free charges. Our findings emphasize how the response of cavity-coupled vibrations can be modified by the presence of free charges, uncovering a new direction toward the tuning of coherent light-matter interaction in cavity-confined correlated materials.