  We study the effect of ferromagnetic metals (FM) on the circularly polarized
modes of an electromagnetic cavity and show that broken time-reversal symmetry
leads to a dichroic response of the cavity modes. With one spin-split band, the
Zeeman coupling between the FM electrons and cavity modes leads to an
anticrossing for mode frequencies comparable to the spin splitting. However,
this is only the case for one of the circularly polarized modes, while the
other is unaffected by the FM, allowing for the determination of the
spin-splitting of the FM using polarization-dependent transmission experiments.
Moreover, we show that for two spin-split bands, also the lifetimes of the
cavity modes display a polarization-dependent response. The change in photon
lifetimes can be understood as a suppression due to level attraction with a
continuum of Stoner modes with the same wavevector. The reduced lifetime of
modes of only one polarization could potentially be used to engineer and
control circularly polarized cavities.
