  Plasmonic waveguides based on 2D materials, which enable the formations of
guided modes confined around few-layered material, are promising plasmonic
platforms for the miniaturization of photonic devices. Nonetheless, such
waveguides support modes that are evanescent in the waveguide core with the
majority of the fields concentrated around waveguide edges, which are different
from those supported by 3D dielectric waveguides where the modal fields are of
oscillatory nature and peak at the center. As a result, many photonic devices
and functionalities that can be achieved within 3D dielectric waveguides based
on total-internal-reflation modes cannot be realized using 2D material-based
plasmonic structures. In this work, we propose and demonstrate how to leverage
anisotropy in 2D materials to tailor of modal fields supported by 2D material
waveguide for the first time. By regulating material absorption of the
constituent 2D materials, the modal fields of these 2D modes can be tailored to
localize around the waveguide center, which in turn can improve the
efficiencies of coupling-based photonic functions using 2D materials, from
in-plane multimode-interference couplers to out-of-plane optical radiation.
Using natural anisotropic 2D materials such as black phosphorus, these pivotal
functions can expand existing device capabilities that are typically achieved
in 3D dielectrics but using 2D materials, thus allowing for the implementation
of 2D plasmonic circuits with no need to relying on 3D layers.
