Optical spectroscopy of emergent excitonic states in atomically thin heterotrilayer and bilayer semiconductors
Keywords: Humanities and Sciences.
Physics.

For years, scientists have imagined the possibilities of 2D materials, pondering how they might behave differently from their thicker counterparts, with predictions pointing towards a paradigm shift in physics when transitioning from the bulk to the atomically thin. Such speculations proposed that the reduced dimensionality would lead to a host of unprecedented physical properties, in particular not observed before half-integer quantum Hall effect and many more. Yet, it was not until the seminal discovery of graphene in 2004—a single layer of carbon atoms — that the exploration of two-dimensional Van der Waals (vdW) materials truly gained momentum. The defining characteristic of these materials lies in their structure, where each layer is interconnected by weak out-of-plane vdW forces, facilitating the exfoliation to smooth single layers, without dangling bonds. These layers exhibit remarkable electronic, thermal, and optoelectronic properties previously unseen. Initial investigations were primarily aimed at deciphering these groundbreaking properties. As the field matured, the intrigue surrounding 2D monolayers evolved, focusing on the possibility of engineering novel materials and exploring new physics by layering different monolayers, each with distinct properties. This method has been particularly successful in the exploration of transition metal dichalcogenides (TMDs), where early research into the properties of monolayer TMDs and their heterostructures has led to significant advancements in fields such as valleytronics, exciton condensates, and the development of novel optoelectronic devices. More recently, the ability to adjust the twist angle between layers introduced the concept of moiré superlattices, presenting even more fascinating physics, including Mott insulating states and Wigner crystal phases. Despite these advancements, the intricate process of fabricating such heterostructures poses considerable challenges, leaving the exploration of more complex moiré heterostructures as a relatively untapped area with the potential to reveal unprecedented new physics. In this thesis, I explore the optical characterization of novel complex TMD heterostructure devices influenced by moiré superlattices. In this thesis, I first present the first conclusive demonstration of so-called quadrupolar excitons in symmetrically stacked vdW heterotrilayers, predicted by theory to exhibit new excitonic states and novel quantum phases. By using sophisticated sample fabrication techniques - including precise allignment of TMD layers, in-situ second harmonic generation and edge contacts—we assemble angle-aligned trilayer structures composed of WSe2/WS2/WSe2 monolayers. By probing these devices by methods such as electrostatic gating, reflectance contrast spectroscopy, photoluminescence spectroscopy and lifetime measurements, we demonstrate the existence of quadrupolar excitons in such hetero-structures. Next, I explore the dynamics of resident electron's spin in highly aligned MoSe2/WS2 moiré superlattices. By using pump-probe technique, we measure reflectance magnetic circular dichroism signal from the heterostructure, from which we get valley polarization and spin relaxation times of resident electrons. Finally, I summarize the new findings and talk about potential future directions.