  Coulomb interactions in atomically thin materials are uniquely sensitive to
variations in the dielectric screening of the environment, which can be used to
control quasiparticles and exotic quantum many-body phases. A static
approximation of the dielectric response, where increased dielectric screening
is predicted to cause an energy redshift of the exciton resonance, has been
until now sufficient. Here, we use charge-tunable exciton resonances to study
screening effects in transition metal dichalcogenide monolayers embedded in
materials with dielectric constants ranging from 4 to more than 1000. In
contrast to expectations, we observe a blueshift of the exciton resonance
exceeding 30 meV for larger dielectric constant environments. By employing a
dynamical screening model, we find that while the exciton binding energy
remains mostly controlled by the static dielectric response, the exciton
self-energy is dominated by the high-frequency response. Dielectrics with
markedly different static and high-frequency screening enable the selective
addressing of distinct many-body effects in layered materials and their
heterostructures, expanding the tunability range and offering new routes to
detect and control correlated quantum many-body states and to design
optoelectronic and quantum devices.
