Author ORCID Identifier

https://orcid.org/0000-0002-6233-7552

Date of Award

7-8-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Vadym Apalkov

Second Advisor

Unil Perera

Third Advisor

Brian D. Thoms

Fourth Advisor

Mukesh Dhamala

Abstract

The dissertation aims to study the light-matter interactions in graphene quantum dots (GQDs), investigating high-order harmonic generation (HHG), an extreme nonlinear frequency up-conversion process. We extended the study incorporating the variation of shape, size, geometry, and type of edges in GQD systems. We theoretically modeled the quantum dots system and developed the formulation employing quantum master equations and a density matrix approach. We explored ultrafast electron dynamics and optical nonlinearities in the system of GQDs, completing the four research projects: (i) Generation of high harmonics and its dependence on the relaxation process, (ii) HHG in triangular GQDs governed by edge states, (iii) Ellipticity dependence of HHG in GQDs (iv) HHG in GQDs with monovacancy or divacancy. We addressed nonlinearity in hexagonal GQDs placed in a short, linearly polarized optical pulse. At short finite dephasing times, the ultrafast electron dynamics show significant irreversibility with a significant residual population of the excited quantum dot levels. When dephasing time increases, intensities correspond to a low-frequency boost, while the cutoff energy decreases regarding the high harmonic spectra. In zigzag-edged triangular GQDs, the intensities of high harmonics show a strong dependence on the initial electron population of the edge states of the quantum dot. If a zigzag triangular quantum dot possesses an even number of edge states, then even high harmonics are strongly suppressed when half of the edge states of the quantum dots are populated before the pulse. The odd and even harmonics are of comparable intensities for any other populations of the edge states. The elliptically polarized ultrashort pulse interacts with the system of quantum dots to reveal unique nonlinear behavior different from the linearized polarized optical field. The generated high harmonics are sensitive to pulse ellipticity, frequency, amplitude, and GQDs’ symmetry. Furthermore, we unraveled the influence of defects in HHG in GQDs with monovacancy or divacancy. Vacancy uplifts energy levels, provides additional channels for multiphoton excitation, boosts even order harmonics, and disrupts symmetry-related suppression under circular polarization. These studies motivate the researchers to enhance, control, and optimize HHG in GQDs and other 2D materials.

DOI

https://doi.org/10.57709/37369742

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