Strong to ultrastrong coherent coupling measurements in a YIG/cavity system at room temperature

We present an experimental study of the strong to ultrastrong coupling regimes at room temperature in frequency-reconfigurable three-dimensional reentrant cavities coupled with a yttrium-iron-garnet slab. The observed coupling rate, defined as the ratio of the coupling strength to the cavity frequency of interest, ranges from 12% to 59%. We show that certain considerations must be taken into account when analyzing the polaritonic branches of a cavity spintronic device where the rf field is highly focused in the magnetic material. Our observations are in excellent agreement with electromagnetic finite-element simulations in the frequency domain.