Ultra-compact plexcitonic electro-absorption modulator

Abstract: 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.