Date of Award

12-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Vadym Apalkov

Abstract

In this dissertation, we theoretically explore the ultrafast nonlinear process in nanoflakes of 2D materials placed in the field of a femtosecond-long optical pulse. The duration of a pulse is of the order of a few femtoseconds and its amplitude is in the range of 0.1-1.5 V/A. To be more specific, we study ultrafast electron dynamics in transitional metal dichalcogenides (TMDCs) quantum dots. Different sizes of quantum dots (QD) of a hexagonal shape are considered. Taking into account that the valence and conduction bands of TMDC materials are mainly formed by d orbitals of the corresponding metal atoms, we consider a three-band tight binding model of TMDC, where the spin-orbit coupling is also included into the model. The energy spectra were calculated for different sizes of TMDC QDs and, for all cases, the anticipated in-gap edge states were observed. With increasing the QD size, the band gap decreases and asymptotically approaches the band gap of the corresponding TMDCs monolayer. In the thesis, two nonlinear characteristics of electron dynamics in TMDC QDs are considered. The first one is the absorption of an optical pulse by TMDC QDs. Such an absorption is determined by the residual electron population of the excited QD states, i.e., population after the pulse. As a function of the field amplitude, the absorption first increases at low amplitudes, shows a peak at an intermediate amplitude, and then converges to a constant value at high amplitudes. The second characteristic of nonlinear electron dynamics is the high harmonic generation, which is determined by the time dependence of the generated electron dipole moment during the pulse. The generation of high optical harmonics was studied for different TMDC materials, different QD sizes, and different characteristics of the optical pulse. One of the characteristics of the corresponding radiation spectrum is the cutoff frequency, which is the highest-order harmonics that can be generated for the pulse of a given intensity. As a function of the QD size, the cutoff frequency shows the maximum value at intermediate QD sizes, which is realized for TMDC QD consisting of around 61 metal atoms. Such nonmonotonic dependence of the radiation spectra on the parameters of TMDC QD shows that the strongest nonlinear optical response of TMDC QDs occurs at QD of a finite size.

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