Controlling and Characterizing Quantum Material Dynamics via Nonlinear Terahertz Spectroscopy

Author: Hsieh, Pin-Hsun

Year: 2027

Degree: Dissertation (Ph.D.)

Advisor: Blake, Geoffrey A.

Committee Members: Okumura, Mitchio; Cushing, Scott K.; See, Kimberly; Blake, Geoffrey A.

Option: Chemistry

DOI: 10.7907/8xqc-w987

Abstract

This dissertation explores the development and application of advanced terahertz spectroscopy with nonlinear optical techniques to characterize and manipulate quantum materials and complex structural dynamics. First, a sensitive imaging method is presented for characterizing nonlinear terahertz beam profiles. This technique optimizes χ(3) light-matter interactions and facilitates the realization of two-dimensional terahertz spectroscopy using multiple emitters. Such advancements enable the direct observation of fast picosecond dynamics in liquids and solids that remain difficult to probe in single-emitter configurations. Second, the research investigates the driver of the phase transition in the candidate excitonic insulator Ta₂NiSe₅. A novel geometric gating technique is introduced to disentangle structural and electronic orders using intense sub-picosecond terahertz pulses. By intentionally misaligning the incident electric field from the highly anisotropic tunneling trajectory, adiabatic carrier generation is suppressed. This transitions the system into a direct Impulsive Stimulated Raman Scattering regime and allows for the coherent pumping of the structural shear mode while preserving the many-body excitonic condensate. Driving this macroscopic lattice deformation without destroying the electronic condensate proves that the spontaneous structural distortion origins from an intrinsic lattice instability, establishing a robust framework for the optical control of quantum materials. Finally, the thesis addresses the necessity of dynamic terahertz polarization control for breaking time-reversal symmetry in Floquet engineering. Through multidimensional Terahertz-Terahertz-Raman spectroscopy, the ultrafast nonlinear optical response of (110)-cut cubic zinc sulfide is analyzed. The study attributes anomalous low-frequency spectral oscillations to a macroscopic phase mismatch between the co-propagating terahertz pump and optical probe. Exploiting this temporal walk-off gives the direct extraction of a highly anisotropic transient refractive index. By tuning the crystallographic azimuthal angle, the terahertz-induced birefringence satisfies the conditions for a broadband zero-order terahertz quarter-waveplate. Relying on a transient nonlinear response rather than physical thickness, this proposed platform provides a powerful pathway for generating circularly polarized terahertz pulses for the on-demand optical manipulation of quantum materials.

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