  We present an experimental study of the strong to ultra-strong coupling
regimes at room temperature in frequency-reconfigurable 3D re-entrant cavities
coupled with a YIG 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.
