  The full information about the interaction between a quantum emitter and an
arbitrary electromagnetic environment is encoded in the so-called spectral
density. We present an approach for describing such interaction in any coupling
regime, providing a Lindblad-like master equation for the emitter dynamics when
coupled to a general nanophotonic structure. Our framework is based on the
splitting of the spectral density into two terms. On the one hand, a spectral
density responsible for the non-Markovian and strong-coupling-based dynamics of
the quantum emitter. On the other hand, a residual spectral density including
the remaining weak-coupling terms. The former is treated nonperturbatively with
a collection of lossy interacting discrete modes whose parameters are
determined by a fit to the original spectral density in a frequency region
encompassing the quantum emitter transition frequencies. The latter is treated
perturbatively under a Markovian approximation. We illustrate the power and
validity of our approach through numerical simulations in three different
setups, thus offering a variety of scenarios for a full test, including the
ultra-strong coupling regime.
