Abstract
We propose a robust and efficient way of controlling the optical spectra of two-dimensional materials and van der Waals heterostructures by quantum cavity embedding. The cavity light-matter coupling leads to the formation of exciton–polaritons, a superposition of photons and excitons. Our first-principles study demonstrates a reordering and mixing of bright and dark excitons spectral features and in the case of a type II van-der-Waals heterostructure an inversion of intra- and interlayer excitonic resonances. We further show that the cavity light-matter coupling strongly depends on the dielectric environment and can be controlled by encapsulating the active two-dimensional (2D) crystal in another dielectric material. Our theoretical calculations are based on a newly developed nonperturbative many-body framework to solve the coupled electron–photon Schrödinger equation in a quantum-electrodynamical extension of the Bethe-Salpeter approach. This approach enables the ab initio simulations of exciton–polariton states and their dispersion from weak to strong cavity light-matter coupling regimes. Our method is then extended to treat van der Waals heterostructures and encapsulated 2D materials using a simplified Mott-Wannier description of the excitons that can be applied to very large systems beyond reach for fully ab initio approaches.
Highlights
Excitons dominate the optical properties of two-dimensional semiconductors
We propose a robust and efficient way of controlling the optical spectra of two-dimensional materials and van der Waals heterostructures by quantum cavity embedding
Single layers of transition metal dichalcogenides (TMDs) can be stacked in multilayer heterostructures allowing for device engineering with a high degree of freedom.[7−9] Among other designs, bilayers of TMDs with a type II band alignment enables the creation of interlayer excitons, bound electron−hole pairs where the charges are physically confined in two different layers, which show a great potential in photovoltaic applications.[10,11]
Summary
Single layers of transition metal dichalcogenides (TMDs) have been under intense investigation for their excitonic properties.[1−3] Their weak electronic screening,[4,5] a consequence of the reduced dimensionality, leads to the formation of strongly bound bright and dark excitons which play a fundamental role in a large variety of optoelectronic, spintronic, and valleytronic properties.[6] single layers of TMDs can be stacked in multilayer heterostructures allowing for device engineering with a high degree of freedom.[7−9] Among other designs, bilayers of TMDs with a type II band alignment enables the creation of interlayer excitons, bound electron−hole pairs where the charges are physically confined in two different layers, which show a great potential in photovoltaic applications.[10,11] Because of their strong coupling to electromagnetic radiation, TMD excitons represent ideal candidates to study strong light-matter coupling in optical cavities.[12−16] In an optical resonator, excitons interact with the quanta of light, generated by the spatial confinement of the cavity, resulting in the formation of new hybrid states with partial matter and partial light character, the “exciton−polaritons”.17−19 These states are inherently different from the bare excitonic states in the material and a variety of novel phenomena can be expected. We show that the ab initio QED-BSE results can be described by an alternative approach based on the MottWannier model, which we refer to as MW-QED, that represents a computationally inexpensive method able to simplify the QED-BSE approach by simplifying the description
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