  In this work we propose and theoretically characterize optical isolators
consisting of an all-dielectric and non-magnetic resonator featuring an
intensity-dependent refractive index and a strong coherent field propagating in
a single direction. Such devices can be straightforwardly realized in
state-of-the-art integrated photonics platforms. The mechanism underlying
optical isolation is based on the breaking of optical reciprocity induced by
the asymmetric action of four-wave mixing processes coupling a strong
propagating pump field with co-propagating signal/idler modes but not with
reverse-propagating ones. Taking advantage of a close analogy with fluids of
light, our proposed isolation mechanism is physically understood in terms of
the Bogoliubov dispersion of collective excitations on top of the strong pump
beam. A few most relevant set-ups realizing our proposal are specifically
investigated, such as a coherently illuminated passive ring resonator and
unidirectionally lasing ring or Taiji resonators.
