  The ability to modulate light using 2-dimensional (2D) materials is
fundamentally challenged by their small optical cross-section leading to
miniscule modal confinements in diffraction-limited photonics despite
intrinsically high electro-optic absorption modulation (EAM) potential given by
their strong exciton binding energies. However the inherent polarization
anisotropy in 2D-materials and device tradeoffs lead to additional requirements
with respect to electric field directions and modal confinement. A detailed
relationship between modal confinement factor and obtainable modulation
strength including definitions on bounding limits are outstanding. Here we show
that the modal confinement factor is a key parameter determining both the
modulation strength and the modulator extinction ratio-to-insertion loss
metric. We show that the modal confinement and hence the modulation strength of
a single-layer modulated 2D material in a plasmonically confined mode is able
to improve by more than 10x compared to diffraction-limited modes. Combined
with the strong-index modulation of graphene the modulation strength can be
more than 2-orders of magnitude higher compared to Silicon-based EAMs.
Furthermore modal confinement was found to be synergistic with performance
optimization via enhanced light-matter-interactions. These results show that
there is room for scaling 2D material EAMs with respect to modal engineering
towards realizing synergistic designs leading to high-performance modulators.
