By applying a voltage to the outer graphene layers, we generate an electric field that tunes the absorption of the two MoS 2 layers. The vertical separation between the electrons and the holes gives rise to a static electric dipole. The main idea is to use transitions based on electrons and holes that reside in different MoS 2 layers – so-called interlayer excitons. Our device consists of a double layer of MoS 2 sandwiched between an insulator and top and bottom electrodes made from the electrical conductor graphene. We have created a novel structure that shows optical transitions with strong absorption and wide tunability in the visible range. A large out-of-plane dipole moment requires a vertical separation of the electron and hole. In a single layer MoS 2, absorption can reach up to 100 %, but the optical transition cannot be electrically tuned in such devices, as the excitons have essentially no out-of-plane dipole moment due to their confinement to a single layer. This can in principle be achieved via the quantum-confined Stark effect with an electric field applied perpendicular to the sample plane. The ability to additionally tune their transition energies is essential for various interesting opto-electronic applications based on light emission, detection, modulation and manipulation. Their optical properties are governed by excitons – electrons and holes bound by Coulomb attraction – that remain stable at room temperature. B 103, L201114 (2021)Ī strong and electrically tunable optical resonance in a two-dimensional semiconductorĪtomically thin transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS 2), strongly interact with light. “Symmetry indicators for inversion-symmetric non-Hermitian topological band structures”, Phys. "Exceptional Topological Insulators”, Nat. Such a scenario is a natural result of strong electron-phonon interaction, paving the road for a future material discovery. For instance, the ETI phase emerges in a Weyl semimetal, when quasiparticles at the two Weyl nodes acquire finite but distinct lifetimes. The ETI can be induced universally in gapless solid-state systems and metamaterials, thereby setting a paradigm for non-Hermitian topological matter. Even though it does not require any symmetry to be stabilized, we explain how this non-Hermitian topological phase can also be inferred using symmetry-indicators of the bulk Hamiltonian. It covers the bulk energy point gap as a single sheet of complex eigenvalues or with a single exceptional point. Like the single surface Dirac electron, the exotic surface state of the ETI cannot be regularized in purely two dimensions. Here we introduce the exceptional topological insulator (ETI) realizing a surface anomaly - akin to the three-dimensional topological insulator - that can only exist within the topological bulk embedding. Non-Hermiticity does not only give rise to new bandstructure features such as point gaps or exceptional points but also enriches the world of topological phases. One of the reasons is that most experimental platforms are in fact either accidentally or tuneably lossy, such that their effective description involves a non-Hermitian Hamiltonian. Following the success of topological phases in solid-state systems, non-Hermitian physics has recently attracted a lot of interest.
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