#### Author ORCID Identifier

https://orcid.org/

**0000-0001-5729-6402**

#### Date of Award

Fall 12-13-2021

#### Degree Type

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics and Astronomy

#### First Advisor

Vadym Apalkov

#### Abstract

Spasers are devices based on the effects of a plasmonic field and emulate a laser phenomenon in a nanoscale. A spaser consists of a resonator, which is a metal, and a gain, which is usually a semiconductor. The topological properties of materials are independent of geometrical deformations and are invariant under structural changes and perturbations. The electronic properties of a group called transition metal dichalcogenides(TMDCs), a type of two-dimensional(2D) materials, exhibit robustness around certain symmetry points called valleys, where the dipole-transitions are most probable.

Topological nanospaser consists of a silver nanospheroid and a gain 2D monolayer TMDC placed atop of it. The metallic spheroid acts as a nanoresonator for the plasmonic field. It supports two dipole modes rotating in the opposite directions with surface plasmon resonance frequency ($\omega_{sp}$). When transition frequency in the gain matches to the $\omega_{sp}$ of nanospheroid there is a coupling between the plasmonic modes and the chiral valleys ($\mathrm{K}$ and $\mathrm{K^\prime}$) which results in the generation of plasmons. Here, we selectively pump a single valley and study the dynamics of a nanospaser for different radii of the gain flake. When the radius of TMDC nanoflake is less than the radius of the nanospheroid, the plasmons generated are only those which match the chirality of the pumped valley and plasmons with the opposite chirality are absent. However, for a large enough flake size, the plasmonic field outside the footprint of the spheroid polarizes the unpumped valley and the generation of these mismatched plasmonic modes becomes highly probable. In addition, we also analyze the far-field radiation due to this nanospaser.

We, further, study the effects of inter-level relaxation in a spaser system of spherical nanoparticle embedded inside a sphere composed of dye. Assuming gain to be a three-level model, we will explain the effects due to relaxation in the higher energy levels in the generation of plasmons. Contrary to a two-level system spaser where the dependence of plasmons on excitation is linear, we observe a quadratic relationship.

Both these nanospasers have tremendous potential uses in the different areas of infrared spectroscopy, sensing, probing, and mainly biomedical treatment.

#### Recommended Citation

Ghimire, Rupesh, "Theory of nanospaser: the role of topology and inter-level relaxation.." Dissertation, Georgia State University, 2021.

https://scholarworks.gsu.edu/phy_astr_diss/139

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