  Interfacing solid-state emitters with photonic structures is a key strategy
for developing highly efficient photonic quantum technologies. Such structures
are often organised into two distinct categories: nanocavities and waveguides.
However, any realistic nanocavity structure simultaneously has characteristics
of both a cavity and waveguide, which is particularly pronounced when the
cavity is constructed using low-reflectivity mirrors in a waveguide structure
with good transverse light confinement. In this regime, standard cavity quantum
optics theory breaks down, as the waveguide character of the underlying
dielectric is only weakly suppressed by the cavity mirrors. By consistently
treating the photonic density of states of the structure, we provide a
microscopic description of an emitter including the effects of phonon
scattering over the full transition range from waveguide to cavity. This
generalised theory lets us identify an optimal regime of operation for
single-photon sources in optical nanostructures, where cavity and waveguide
effects are concurrently exploited.
