Dissertations, Theses, and Capstone Projects

Date of Degree

2-2025

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Vadim Oganesyan

Committee Members

Enrique Pujals

Vladimir Rosenhaus

Tobias Schaefer

David Schwab

Subject Categories

Condensed Matter Physics | Physics

Keywords

High-harmonic Generation, HHG, Condensed Matter Physics, Computational Physics

Abstract

High-harmonic generation (HHG) is one of the most prominent phenomena in ultrafast science, offering technological applications and rich fundamental insights. However, the majority of HHG studies have focused on electronic systems. In this thesis, we develop a framework where HHG emerges from magnetic excitations through spin-phonon coupling and the hybridization effect, arising from complex non-equilibrium steady state patterns. The variations of spin-emitted power versus spin-phonon coupling strengths, continuous-wave terahertz laser characteristics, and model parameters are explored numerically. Furthermore, analytical expressions within the weak spin-phonon coupling regime are proposed. These findings provide rigorous groundwork for the formation of spin HHG in a magnetophononic system, with potential applications in laser technologies, spintronics, and quantum technologies. This dissertation consists of five chapters:

In Chapter 1, we present a brief introduction to two key topics: non-equilibrium steady states and high-harmonic generation. We discuss their significance in scientific research and their relevance to technological advancements and industrial applications. Additionally, we provide an overview of a series of studies on these subjects and outline our methodology, which is based on the magnetophonics mechanism and dynamical systems approach.

In Chapter 2, we introduce the model describing our system of interest: a dimerized spin chain model corresponding to the target material CuGeO3, driven by a continuous-wave laser whose electric field couples to the lattice vibrations. The driven phonons subsequently excite the magnetic sector, as prescribed by the magnetophononic mechanism. We present the Hamiltonian terms that describe this system. Next, we explain how implementing the Lindblad formalism allows us to model the dissipation of pumped energy into the phononic bath. By solving the Heisenberg master equation, we derive the equations of motion governing the dynamics of both the spin and phonon sectors.

In Chapter 3, we present our results based on the numerical solutions of the equations of motion. We discuss the non-equilibrium steady state (NESS) that appears after a sufficient time and identify the high harmonics emerging in the frequency spectrum of the spin-emitted power, originating from magnetic excitations. We describe the system settings and parameters, and explain how we explore the parameter space to examine their effects on the generated harmonics. We analyze the impact of spin-phonon coupling strengths, drive characteristics such as laser amplitude and drive frequency, and dissipation effects. Finally, we propose an experimental perspective for this setup.

In Chapter 4, we propose analytical expressions for the weak spin-phonon coupling regime, considering phonon frequencies both within and outside the two-triplon band. The analytical expressions successfully explain the time evolution of the observables. We then proceed to demonstrate the agreement between these analytical results and the numerical ones.

In Chapter 5, we provide a summary of our work, highlighting the key findings and their implications. We also discuss potential future directions based on our results and their relevance to the wider scope of the field.

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