Excitons in Two-Dimensional Materials: from Many-Body Physics to Quantum Technologies
Two-dimensional monolayers (ML) of Transition Metal Dichalcogenides (TMDs) are atomically thin, direct bandgap semiconductors that have emerged as one of the preferred solid-state platforms of study for both technology and physics applications. Their optical signatures include the photoluminescence emission from the recombination of bound electron-hole pairs, or excitons. Their unique band structure gives rise to rich excitonic spectra where the excitons possess large binding energies in the order of meVs due to the 2d nature of the material. In this Thesis I fi rst explore complex exciton species within a particular TMD, WSe₂, that arise due to charge-mediated interactions. In particular, biexcitons are bound exciton pairs that hold great interest from a fundamental standpoint as the simplest building block in coherent multi-exciton phenomena and for applications such as sources of entangled photons. In this Thesis I show direct experimental evidence of two fundamental biexciton complexes in a monolayer of high-quality ML-WSe₂ by performing polarisation-resolved, gate-controlled and magnetic-fi eld dependent photoluminescence (PL) measurements. Furthermore, I investigate additional lines that arise in the PL signal of ML-WSe₂, and provide tentative explanations for them. As well as free excitons, MLs of WSe₂ show PL emission of localised excitons, or excitons that are quantum con ned to a particular spatial location, resulting in single-photon emitters (SPE). Here, I explore the deterministic generation of SPEs in ML-WSe₂ and another TMD, ML-WS₂, by using pre-patterned substrates. The attractiveness of TMDs and their heterostructures as hosts of single-photon emitters (SPEs) stems from their potential for lack of surface dangling bonds, the easy interfacing with cavities and waveguides, and the high photon extraction efficiency due to their two-dimensional nature. I present the efforts to study the origin of SPEs in TMDs and provide future directions including the charging of the localised traps to serve as optically active qubits for Quantum Technologies. All previous excitons are spatially direct, insofar both the electrons and the holes are present in the same ML. Spatially indirect excitons, or interlayer (IX) excitons, arise due to the type-II band alignment of two different ML-TMDs. They possess an electron in one ML-TMD, bound with a hole in another, different ML-TMD in close proximity, resulting in a net non-zero electric dipole moment. Through PL experiments I show the spatial con nement of IX excitons in a ML-WSe₂/ML-WS₂ heterostructure. Furthermore, I show lifetimes of free IX excitons approaching 200 μs, an order of magnitude longer than any previous IX and three orders for any TMD IX, consistent with the high-purity layered material used. The three ingredients - confi nement, long lifetime, and non-zero dipole moment allowing for long-range dipolar interactions - evidence the potential to create arbitrary trapping pro les for long-lived dipolar particles within a highly con gurable platform, providing a unique avenue for probing exotic states of matter in degenerate gases and arti cial lattices across a range of geometries for Quantum Simulations. In this Thesis I thus present the deterministic manipulation of excitons in atomically thin semiconductors, unveiling the potential of ML-TMDs to be used in Quantum Technologies.