Author ORCID Identifier

0000-0003-1390-6154

Date of Award

Fall 12-11-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Vadym Apalkov

Second Advisor

Brian D. Thoms

Third Advisor

Sidong Lei

Abstract

We study theoretically nonlinear dynamics of a graphene nanoflake placed in the field of an ultrashort optical pulse. We describe the graphene system within an effective model with infinite mass boundary conditions. For an optical pulse with a duration of just a few femtoseconds, the electron dynamics is coherent and is described by a time-dependent Schro ̈dinger equation. For graphene nanoring, if the optical pulse is circularly polarized, then two valleys of graphene are populated differently, resulting in a finite valley polarization of the system after the pulse. Such valley polarization is a unique property of graphene nanoscale systems, while for a graphene monolayer, a circularly polarized pulse does not produce any valley polarization. The valley polarization of a graphene nanoring depends on the parameters of the system, such as inner and outer radii. With the system’s size increasing, the valley polarization monotonically decreases, converging to its zero value for the infinite graphene monolayer. Furthermore, we theoretically study the generation of high harmonics in disk shape graphene quantum dots placed in (i) linearly and (ii) and subsequently, a transition to circularly polarized short pulses. The length gauge was used to describe the interaction of quantum dots with an optical pulse. The generated radiation spectra of graphene quantum dots as a result of a linearly polarized pulse can be controlled by varying the quantum dot size, i.e., its radius. With increasing the quantum dot radius, the intensities of low harmonics mainly decrease, while the cutoff frequency increases. The sensitivity of the cutoff frequency to the QD size increases with the intensity of the pulse. Moreover, the transition from linearly to circularly polarized pulse suppresses the harmonics and cutoff frequency in radiation spectra. The cutoff frequency is independent of the size of a quantum dot when a completely circularly polarized pulse is shined. The cutoff frequency and intensity of harmonics can be tuned by the ellipticity and frequency of the laser pulse.

DOI

https://doi.org/10.57709/36355275

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