  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 exciton resonances using
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 the 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
demonstrate 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 refractive
index. This work may pave the way towards the development of field-effect
photonics in which the optical functionality can be controlled with CMOS
circuits.
