Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers

We report that the refractive index of transition metal-dichacolgenide (TMDC) monolayers, such.as MoS2, WS2, and WSe2, can be substantially tuned by >60% in the imaginary part and >20% in the real part.around excitoa,resonances using complementary metal-oxide-semiconductor (CMOS) compatible electrical gating. This giant tunablility is rooted in the dominance of excitonic effects in the refractive index of the monolayers. and the strong susceptibility of the excitons, to the influence of injected charge carriers. The tunability mainly results from thg effects of injected charge carriers to broaden the spectral width of excitonic interband transitions and to facilitate the interconversion of neutral and charged excitons. The other effects of the injected charge-carriers, such as renormalizing bandgap and changing exciton binding, energy, 'only play negligible roles: We also demnnstrate that the atomically thin monolayers, when combined with photonic structures, can. enable the efficiencies of optical absorption (reflection) tuned from 40% (60%) to 80%.(20%) due to the giant tunability of the refractive index. This work may pave the way toward the development of field-effect photonics in which the optical functionality can be controlled with CMOS-circuits.