  Optical isolator remains one of the main missing elements for photonic
integrated circuits despite several decades of research. The best solutions up
to now are based on transverse magneto-optical effect using either narrow-band
resonators or high-gyrotropy magneto-optical materials with difficult
integration on usual photonic platforms. We propose in this paper a radically
new concept which enables performing broad-band non-reciprocal transmission
even in the case of low-gyrotropy material. The principle explores the
separation of back and forth light paths, due to the magneto-biplasmonic
effect, i.e., the coupled mode asymmetry induced in plasmonic slot waveguides
loaded with a magneto-optical (MO) layer. We show numerically that such a
metal-MO dielectric-metal slot waveguide combined with suitable side-coupled
lossy rectangular nanocavities gives more than 18 dB isolation ratio on several
tens of nanometers bandwidth, with only 2 dB insertion losses. We propose an
analytical approach describing such a magneto-plasmonic slot waveguide to
identify the involved physical mechanisms and the optimization rules of the
isolator. Additionally, we show that low-gyrotropy material (down to ~0.005)
can be considered for isolation ratio up to 20 dB, opening the road to a new
class of integrated isolators using easy-to-integrate hybrid or composite
materials.
