Broad-band plasmonic isolator compatible with low-gyrotropy magneto-optical material

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.