Non-Thermal Optical Engineering of Strongly-Correlated Quantum Materials

Citation

Shan, Junyi (2022) Non-Thermal Optical Engineering of Strongly-Correlated Quantum Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0shs-fa90. https://resolver.caltech.edu/CaltechTHESIS:05112022-213102696

Abstract

This thesis develops multiple optical engineering mechanisms to modulate the electronic, magnetic, and optical properties of strongly-correlated quantum materials, including polar metals, transition metal trichalcogenides, and copper oxides. We established the mechanisms of Floquet engineering and magnon bath engineering, and used optical probes, especially optical nonlinearity, to study the dynamics of these quantum systems.

Strongly-correlated quantum materials host complex interactions between different degrees of freedom, offering a rich phase diagram to explore both in and out of equilibrium. While static tuning methods of the phases have witnessed great success, the emerging optical engineering methods have provided a more versatile platform. For optical engineering, the key to success lies in achieving the desired tuning while suppressing other unwanted effects, such as laser heating.

We used sub-gap optical driving in order to avoid electronic excitation. Therefore, we managed to directly couple to low-energy excitation, or to induce coherent light-matter interactions. In order to elucidate the exact microscopic mechanisms of the optical engineering effects, we performed photon energy-dependent measurements and thorough theoretical analysis. To experimentally access the engineered quantum states, we leveraged various probe techniques, including the symmetry-sensitive optical second harmonic generation (SHG), and performed pump-probe type experiments to study the dynamics of quantum materials.

I will first introduce the background and the motivation of this thesis, with an emphasis on the principles of optical engineering within the big picture of achieving quantum material properties on demand (Chapter I). I will then continue to introduce the main probe technique used in this thesis: SHG. I will also introduce the experimental setups which we developed and where we conducted the works contained in this thesis (Chapter II). In Chapter III, I will introduce an often overlooked aspect of SHG studies -- using SHG to study short-range structural correlations. Chapter IV will contain the theoretical analysis and experimental realizations of using sub-gap and resonant optical driving to tune electronic and optical properties of MnPS₃. The main tuning mechanism used in this chapter is Floquet engineering, where light modulates material properties without being absorbed. In Chapter V, I will turn to another useful material property: magnetism. First I will describe the extension of the Floquet mechanism to the renormalization of spin exchange interaction. Then I will switch gears and describe the demagnetization in Sr₂Cu₃O₄Cl₂ by resonant coupling between photons and magnons. I will end the thesis with a brief closing remark (Chapter VI).

Thesis Files