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Compact integrated optical isolation based on extraordinary dichroic transmission through a magnetoplasmonic waveguide grating

Using rigorous magneto-optical waveguide modelling, we have calculated the dichroic transmission of the fundamental TM waveguide mode through a magnetoplasmonic waveguide grating. The ferromagnetic metallic grating material is a CoFe alloy that is magnetized parallel to the grating. When deposited on top of a standard III-V waveguide with a thin top cladding layer and thus placed in the evanescent tail of the guided TM ground mode, it induces both plasmonic and magneto-optic effects in the transmission of this waveguide grating. Due to the direction of the magnetization - perpendicular to the light propagation and parallel to the waveguide layer interfaces - the integrated transverse magnetooptic Kerr effect induces non-reciprocal dichroic transmission for the guided TM light. We have numerically studied the TM ground mode dichroism (for a telecom wavelength of 1300nm) as a function of the cladding layer thickness and the grating parameters, namely its duty cycle, period and thickness. This study has revealed that there exist clear grating designs where the dichroic transmission is resonantly enhanced as compared to the case where the ferromagnetic metal is a continuous film. A detailed study of the field maps associated to these points reveals that the guided TM ground mode resonantly couples to a vertical cavity plasmonic resonance in the air slots of the CoFe grating. This behaviour is reminiscent of extraordinary optical transmission but here in an integrated non-reciprocal version. We have previously reported experimentally strong integrated and forward transparent optical isolation based on this TM dichroism but using a continuous film [1,2]. The present design study indicates that the extraordinary magnetoplasmonic effects taking place in a properly designed CoFe grating improves the performance of this device by at least a factor 4.