  In 1H monolayer transition metal dichalcogenide, the inversion symmetry is
broken, while the reflection symmetry is maintained. On the contrary, in the
bilayer, the inversion symmetry is restored, but the reflection symmetry is
broken. As a consequence of these contrasting symmetries, here we show that
bilayer WS$_2$ exhibits a quantum confined Stark effect (QCSE) that is linear
with the applied out-of-plane electric field, in contrary to a quadratic one
for a monolayer. The interplay between the unique layer degree of freedom in
the bilayer and the field driven partial inter-conversion between intra-layer
and inter-layer excitons generates a giant tunability of the exciton oscillator
strength. This makes bilayer WS$_2$ a promising candidate for an atomically
thin, tunable electro-absorption modulator at the exciton resonance,
particularly when stacked on top of a graphene layer that provides an
ultra-fast non-radiative relaxation channel. By tweaking the biasing
configuration, we further show that the excitonic response can be largely tuned
through electrostatic doping, by efficiently transferring the oscillator
strength from neutral to charged exciton. The findings are prospective towards
highly tunable, atomically thin, compact and light, on chip, reconfigurable
components for next generation optoelectronics.

Mesoscale and Nanoscale Physics;Materials Science;Applied Physics