Abstract
This Letter presents an extremely compact graphene-based plasmonic isolator with a two-port multimode interference (MMI) structure. The proposed structure utilizes the non-reciprocal phase shift effect to provide optical isolation. A new approach is used for MMI-based isolator design in terms of $m$th overall self-imaging. The influence of the device geometries and $m$ parameter on the isolation ratio (ISR), insertion loss, operation bandwidth (BW), and required magnetic field strength $B$ and its appropriate direction is investigated. The device has an ultra-small footprint compared to the operating wavelength of $\lambda = 8\,\,\unicode{x00B5}{\rm m}$ with high ISR. An MMI isolator with a size of $0.5\,\,\unicode{x00B5}{\rm m} \times 4.02\,\,\unicode{x00B5}{\rm m}$ ($\approx {\lambda ^2}/32$) and ${\rm ISR} = {18.35}\;{\rm dB}$ and an MMI isolator with a size of $0.7\,\,\unicode{x00B5}{\rm m} \times 7.44\,\,\unicode{x00B5}{\rm m}$ ($\approx {\lambda ^2}/12$) with ${\rm ISR} = {27.1}\;{\rm dB}$ are achieved. Moreover, the non-reciprocal MMI shows ultra-wide 20 dB isolation BW of 0.75–2.34 THz.
© 2020 Optical Society of America
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