THz-frequency cavity magnon-phonon-polaritons in the strong coupling regime

TL;DR

This paper demonstrates the strong coupling of magnons and phonons to terahertz electromagnetic waves within a photonic crystal cavity, creating new hybrid modes with potential applications in THz spintronics and quantum electrodynamics.

Contribution

The study reports the first experimental observation of cavity magnon-phonon-polaritons at THz frequencies in a hybrid ferroelectric and magnetic crystal system.

Findings

Normal-mode splitting observed in the frequency spectrum

Avoided crossing in the temperature-frequency plot

Cavity mode volume of approximately 0.5 cubic micrometers

Abstract

We demonstrate the strong coupling of both magnons and phonons to terahertz (THz) frequency electromagnetic (EM) waves confined to a photonic crystal (PhC) cavity. Our cavity consists of a two-dimensional array of air-holes cut into a hybrid slab of ferroelectric lithium niobate (LiNbO$_3$) and erbium orthoferrite (ErFeO$_3$), a canted antiferromagnetic crystal. The phonons in LiNbO$_3$ and the magnons in ErFeO$_3$ are strongly coupled to the electric and magnetic field components of the confined EM wave, respectively. This leads to the formation of new cavity magnon-phonon-polariton modes, which we experimentally observe as a normal-mode splitting in the frequency spectrum and an avoided crossing in the temperature-frequency plot. The cavity also has a mode volume of $V=3.4\times10^{-3}\lambda^3\simeq0.5(\lambda/n)^3$ $\mu$m$^3$ and can achieve a Q-factor as high as 1000. These factors facilitate the pursuit of the fields of THz cavity spintronics and quantum electrodynamics.