Ultrafast control of spin, valley, and excitons in 2D semiconductors

Abstract

This thesis demonstrates ultrafast optical control over spins, valleys, and excitons in 2D semiconductors transition metal dichalcogenides (TMDs), advancing their potential for valleybased information processing. Modern scientific research and technology thrives on the ability to control and manipulate quantum degrees of freedom for information storage and transfer. With the discovery of 2D van der Waals materials, “valleys” — the band structure extrema in the momentum space—have emerged as a new degree of freedom for information transfer. TMDs stand out as promising valleytronic materials with valley-selective optical selection rules, spin/valley locking, and excitons with large binding energies. Our key idea is to leverage the two-dimensionality of TMDs, which allows easy manipulation of the valley degree of freedom through techniques such as interfacial engineering and mechanical strain.

The first group of results in this thesis relates to ultrafast tunneling and depolarization of spin/valley-polarized excitation in TMD heterostructures probed by time-resolved Kerr rotation spectroscopy. We demonstrate spin-conserving charge transport across a TMD heterostructure interface and establish control over ultrafast spin relaxation dynamics through Rashba interactions. In TMD heterostructures, interlayer excitons — layer-separated electron-hole pairs with permanent out-of-plane dipoles — serve as a straightforward tool to control Rashba interactions through a self-induced electric field. By systematically varying excitation fluence, sample temperature, and external electric fields in a MoS2/MoSe2 heterostructure, we establish Rashba interactions as a dominant spin relaxation mechanism for T > 70 K, with the spin/valley depolarization rate tunable by an order of magnitude.

Next, we bring together fields of optics and nanomechanics to identify the momentum configuration of excitons, discovering previously inaccessible intervalley excitons associated with the Γ and Q valleys in monolayers of WSe2 and WS2. We demonstrate that ‘strain fingerprinting’ can be used as a general tool to determine the valley configuration of quasiparticles in 2D semiconductors. We also reveal a new class of valley-polarized hybrid excitons in monolayer TMDs with their electronic wavefunctions delocalized across K, K’, Q, and other valleys. By modulating the intervalley energy separation through strain, we achieve a hundredfold reduction in the valley depolarization rate and up to a fivefold increase in the steady state valley polarization for the valley-hybridized excitons compared to previously studied excitons.

To summarize, we advanced ultrafast control of spins and valleys in TMDs through interfacial engineering and mechanical strain. Our results reveal the emergence of new, robust valleybased information carriers, extending TMD valleytronics beyond the conventional K and K’ valleys, paving ways for realizing alternative degrees of freedom in a diverse class of 2D semiconductors.