Probing valley and magnetic photoexcitations in 2D crystals and their heterostructures
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Layered materials are excellent systems for investigating physics in two dimensions. Understanding the optical response of 2D layered materials and their heterostructures at the atomically thin limit is an important aspect of this field. Photoexcitations, such as excitons, critically impact future optoelectronic technologies, such as next-generation solar cells, light-emitting diodes, lasers, and single-photon sources. Moreover, they can provide deep insights into the rich electronic properties of the host crystals, especially in two dimensions, where stronger electron confinement, symmetry changes, and interfacial effects are often very influential. The 2D semiconducting transition metal dichalcogenides, for example, have garnered tremendous excitement for their strong light-matter interactions, which involve tightly bound excitons with intriguing spin-valley physics. Furthermore, newly discovered 2D magnets are unlocking new opportunities to explore magnetic photoexcitations in the atomically thin limit. This dissertation presents optical spectroscopy experiments that probe the fundamental photoexcitations within 2D semiconducting transition metal dichalcogenides, magnetic chromium triiodide (CrI3), and their van der Waals heterostructures. First, we show how second-harmonic generation spectroscopy serves as a powerful probe of excitons and trions in monolayer WSe2 and demonstrate an electrical exciton-based second-harmonic generation switch. We then add MoSe2 to the picture, forming MoSe2/WSe2 heterobilayers in which we reveal the valley-contrasting physics of long-lived free and trapped interlayer excitons. Next, we introduce atomically thin CrI3 and describe our observation of spontaneous circularly polarized photoluminescence. We also unravel its ligand-field and charge-transfer-dominated photoresponse, which broadens the landscape of 2D material photoexcitations beyond excitons. Finally, we combine ultrathin layers of CrI3 with monolayer WSe2, where we discover unprecedented control of valley excitons in monolayer WSe2 by magnetic proximity to CrI3.
- Physics