  Compact electro-optic (EO) modulators with large extinction ratios,
low-switching energies, and high operation speeds are desirable for integrated
photonic and linear optical computing. Traditional 3D semiconductors and
dielectrics are unsuitable for achieving such modulators due to the small
magnitude of EO effects in them. Excitonic 2D semiconductors present a unique
opportunity in this regard given their large and tunable optical constants near
the excitonic resonances. However, strategies for confining and electrically
tuning the excitons into compact EO modulators have not been realized thus far.
Here, we design and simulate an ultra-compact, plexcitonic (strongly-coupled
exciton-plasmon) electro-absorption modulator (EAM) with a sub-micron linear
footprint operating close to the excitonic peak of the WS2 monolayer (641 nm)
hybridized with the plasmon mode of a silver slot waveguide. Electrostatically
injected free carriers in WS2 modulate the light-matter interaction via Coulomb
screening of the excitons as well as promoting the formation of charged
excitons (trions). For our optimized designs, the EAM is expected to achieve a
9.1 dB extinction ratio, concurrently with a 7.6 dB insertion loss in a 400 nm
lateral footprint operating with a predicted < 3 fJ/bit switching energy at 15
GHz for 3-dB bandwidth modulation. Our work shows the potential of plexcitonic
quasi-particles for integrated optical modulators.
