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Twist-engineering of the electronic structure of van-der-Waals layeredmaterials relies predominantly on band hybridization between layers. Band-edgestates in transition-metal-dichalcogenide semiconductors are localized aroundthe metal atoms at the center of the three-atom layer and are therefore notparticularly susceptible to twisting. Here, we report that high-lying excitonsin bilayer WSe2 can be tuned over 235 meV by twisting, with a twist-anglesusceptibility of 8.1 meV/{\deg}, an order of magnitude larger than that of theband-edge A-exciton. This tunability arises because the electronic statesassociated with upper conduction bands delocalize into the chalcogenide atoms.The effect gives control over excitonic quantum interference, revealed inselective activation and deactivation of electromagnetically inducedtransparency (EIT) in second-harmonic generation. Such a degree of freedom doesnot exist in conventional dilute atomic-gas systems, where EIT was originallyestablished, and allows us to shape the frequency dependence, i.e. thedispersion, of the optical nonlinearity.

Twist-angle engineering of excitonic quantum interference and optical nonlinearities in stacked 2D semiconductors