  Transition metal dichalcogenides (TMDs) constitute a versatile platform for
atomically thin optoelectronics devices and spin-valley memory applications. In
monolayers optical absorption is strong, but the transition energy is not
tunable as the neutral exciton has essentially no out-of-plane electric dipole.
In contrast, interlayer exciton transitions in heterobilayers are widely
tunable in applied electric fields, but their coupling to light is considerably
reduced. Here, we show tuning over 120 meV of interlayer excitons with high
oscillator strength in bilayer MoS2. These shifts are due to the quantum
confined Stark effect, here the electron is localised to one of the layers yet
the hole is delocalised across the bilayer. We optically probe the interaction
between intra- and interlayer excitons as they are energetically tuned into
resonance. This allows studying their mixing supported by beyond standard
density functional theory calculations including excitonic effects. In MoS2
trilayers our experiments uncover two types of interlayer excitons with and
without in-built electric dipoles, respectively. Highly tunable excitonic
transitions with large oscillator strength and in-built dipoles, that lead to
considerable exciton-exciton interactions, hold great promise for non-linear
optics with polaritons.
