Author ORCID Identifier
Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
In this dissertation, we theoretically study the nonlinear optical response of Transition metal dichalcogenide (TMDC) Quantum dots to an ultrafast optical pulse. The applied ultrafast laser pulse has a duration of a few femtoseconds, with the electric field amplitude being of the order of a few V/nm. The electron systems of the disk-shaped Quantum dots studied, are described within a low-energy effective k.p model Hamiltonian with infinite mass boundary conditions and applied separately at symmetry-points known as K and K’ valleys. The electron dynamics generated after interaction with the pulse, is investigated and found to be highly reversible for the range of applied field amplitudes. Based on the generated electronic populations, two nonlinear optical processes are investigated, viz., Valley polarization and High harmonic generation.
The ultrafast valley polarization is shown to be induced through time-reversal symmetry breaking of the K and K’ valleys using a circularly polarized optical pulse. For different TMDC materials, valley polarization is shown to depend on the extent of spatial confinement within the quantum dots, as well as on the electric field amplitude of the applied optical pulse. Valley polarization increases monotonically with dot radius while exhibiting non-monotonic behavior with a local maximum at a finite dot radius. The variation in behavior between materials is attributed to differences in the respective spin-resolved density of states and average energy densities of the single-particle electronic energy states.
Further, the populations of individual energy states after interaction with the pulse lead to the dipole moments for electronic transitions. The nonlinear radiation of high harmonics of the fundamental frequency ��o of the incident linearly and elliptically polarized pulses, can then be related to the corresponding Fourier transformed dipole moments. Harmonic cutoff frequencies and intensities of the generated odd high harmonics increase with dot radius and pulse intensity, for a linearly polarized pulse, while increasing pulse frequency increases harmonic intensity and decreases cut-off frequency. Ellipticity of high harmonics transform monotonically with pulse ellipticity, while cut-off frequency decreases with the latter, for an elliptically polarized incident pulse.
Mitra, Aranyo, "Ultrafast Field-Induced Nonlinear Optical Processes In Two Dimensional Transition Metal Dichalcogenide Quantum Dots." Dissertation, Georgia State University, 2023.
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