  By considering a single lossy three-dimensional cavity mode and using a
quasinormal mode expansion for the photonic medium, we show that the
frequency-dependence of the coupling between the cavity and its reservoir is
dependent on both the cavity contents and the gauge used. For the case of
coupling to a single quantum dipole, we identify the form of the spectral
density, revealing a $\sim \omega^{-1}$ prefactor scaling, as well as a
spatially-dependent contribution. We thus establish the correct quantum form
for the cavity-reservoir interaction and show its significant impact on
broadband strong coupling.
