Superstrong Coupling of a Microwave Cavity to YIG Magnons

TL;DR

This paper demonstrates ultra-strong and super-strong coupling between microwave cavity modes and YIG magnons using innovative cavity designs, achieving coupling strengths surpassing mode spacing at millikelvin temperatures.

Contribution

It introduces a novel cavity design with multiple posts to realize discrete Whispering Gallery and Fabry-Perot-like modes for enhanced magnon-photon coupling.

Findings

Achieved ultra-strong coupling of 1.84 GHz

Reached super-strong coupling regime with 1.1-1.5 GHz

Expanded mode structure using D4 symmetric eight-post cavity

Abstract

Multiple-post reentrant 3D lumped cavity modes have been realized to design the concept of discrete Whispering Gallery and Fabry-Perot-like Modes for multimode microwave Quantum Electrodynamics experiments. Using a magnon spin-wave resonance of a submillimeter-sized Yttrium-Iron-Garnet sphere at milliKelvin temperatures and a four-post cavity, we demonstrate the ultra-strong coupling regime between discrete Whispering Gallery Modes and a magnon resonance with strength of 1.84 GHz. By increasing the number of posts to eight and arranging them in a D$_4$ symmetry pattern, we expand the mode structure to that of a discrete Fabry-Perot cavity and modify the Free Spectral Range (FSR). We reach the superstrong coupling regime, where spin-photon coupling strength is larger than FSR, with coupling strength in the 1.1 to 1.5 GHz range.