[PhD Thesis Public Presentation_C209_ zoom is also available] ‐Vivek Pareek– “Unraveling the Nature of Excitons and their Interactions through Time-Resolved Photoemission and Optical Spectroscopy”

Presenter: Mr. Vivek Pareek

Supervisor: Prof. Keshav Dani

Unit: Femtosecond Spectroscopy Unit

Title: Unraveling the Nature of Excitons and their Interactions through Time-Resolved Photoemission and Optical Spectroscopy

Abstract:

Unraveling the Nature of Excitons and their Interactions through Time-Resolved Photoemission and Optical Spectroscopy
The exciton – a coulomb-bound electron-hole pair, was first conceptualized by Frenkel and Wannier in the 1930s. Since then, it has been integral to understanding the optoelectronic response in semiconductors, particularly in low-dimensional semiconductors. Therein, excitons have large binding energies due to quantum confinement and reduced dielectric screening, thus dominating the optical response of the material even at room temperature. Despite their impor- tance, a critical fundamental property of excitons remains inaccessible – the momentum of the constituent electrons and holes! Such a measurement would immediately reveal valuable information, such as their direct or indirect nature, their wave function, their size, and the nature of their interactions. Resolving the momentum coordinates of excitons requires the development of a new instrumentation platform that probes the excitons in time, energy, space, and momentum. This thesis describes the need for such an instrumentation platform and its development, namely the development of time-resolved momentum microscopy. It then describes three studies on the nature of excitons and their interactions. First, we study the interlayer excitons in a WSe2/MoS2 heterostructure. Using time-resolved Momentum Microscopy, we resolve the momentum coordinates of the constituent electrons and holes within the interlayer exciton, directly measuring its size and confinement within the moir unit cell. Next, we demonstrate Floquet effects in monolayer WS2, in the absence of optical fields, resulting from the time-periodic oscillations in the electron self-energy due to excitons. The strong amplitude of the time-periodic perturbation allows us to observe the hybridization of the original band structure with the exciton-dressed one. Finally, we use traditional μ-TAS to study the exciton-exciton annihilation process in bilayer black phosphorus. We show that it is possible to alter the dimensionality of the exciton-exciton annihilation process from one dimensional-like to two dimensional-like by tuning the exciton density and temperature. In conclusion, this thesis answers some fundamental questions about excitons and their interactions in two dimensional semiconductors and paves the way for uncovering novel non-equilibrium phenomena in two dimensional materials.