Abstract
The advent of ultrashort laser pulses unveils a remarkable opportunity to explore the fundamental microscopic dynamics that underpin intricate macroscopic phenomena in quantum materials. This advancement also facilitates ultrafast photonic control of the state of matter to fulfill the increasing demand for novel functionalities in various areas of applications. In this regard, terahertz wave has been firmly established as an outstanding tool for probing and optical manipulation of quantum materials, on account of the exceptional coherence and negligible laser heating effect it provides.
In Chapter 1 of this thesis, we begin by providing a concise overview of light-induced phenomena in both solid and gaseous media. A general introduction details the phenomenon of photo-induced phase transitions in quantum materials and summarizes notable advancements in the research field recently enabled by coherent terahertz waves. Chapters 2 and 3 thoroughly visit the fundamental principles of optical spectroscopy and nonlinear optics, thereby establishing a foundation for the subsequent experimental investigation. Chapters 4 and 5 meticulously detail the development of optical methodology for coherent manipulation of the air-plasma-based THz radiation, in terms of intensity, center frequency, bandwidth, and carrier-envelope phase. Finally, Chapter 6 presents the application of THz radiation in ultrafast dynamic study of the archetypal Type-II Weyl semimetal Td-WTe2, in which we secured spectroscopic evidence for photo-induced Td-1T’ topological phase transition. The transition was found to be nonthermal and in a percolative manner, as evidenced by the emergence of a remarkable universal dynamic scaling behavior.
The growing demand for terahertz technology is expected to drive advanced research into the spatiotemporal control of terahertz radiation, while strong-field effects from coherent terahertz excitation promise innovative applications in the functionalization of quantum materials.