Coupled metamaterial-phonon terahertz range polaritons in a topological insulator

TL;DR

This study demonstrates strong coupling between phonons and metamaterial resonances in a topological insulator, creating tunable phonon polaritons in the terahertz range with potential for manipulating material properties.

Contribution

It introduces a method to achieve and observe strong phonon-metamaterial coupling in topological insulators using terahertz spectroscopy, enabling control over material properties.

Findings

Strong light-matter coupling observed in THz regime.

Hybrid phonon polaritons demonstrated with a normalized coupling strength of ~0.09.

Tuning the metamaterial resonance controls phonon interactions.

Abstract

We report terahertz time-domain spectroscopy (TDTS) experiments demonstrating strong light-matter coupling in a terahertz (THz) LC-metamaterial in which the phonon resonance of a topological insulator (TI) thin film is coupled to the photonic modes of an array of electronic split-ring resonators. As we tune the metamaterial resonance frequency through the frequency of the low frequency $\alpha$ mode of (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ (BST), we observe strong mixing and level repulsion between phonon and metamaterial resonance. This hybrid resonance is a phonon polariton. We observe a normalized coupling strength, $\eta$ = $\Omega_R$/$\omega_c$ $\approx$ 0.09, using the measured vacuum Rabi frequency and cavity resonance. Our results demonstrate that one can tune the mechanical properties of materials by changing their electromagnetic environment and therefore modify their magnetic and topological degrees of freedom via coupling to the lattice in this fashion.