This book documents novel techniques in the area of field-resolved spectroscopy (FRS) in the terahertz range of the electromagnetic spectrum. In FRS, the electric field of a—typically very broadband—long-wavelength test wave is scanned using non-linear gating processes with short-wavelength light pulses. Measuring the oscillating field of the test wave provides direct, time-resolved insights into its formation, and into light-matter interactions. The first part of the thesis surveys the conceptual background of the technologies used, introducing frequency combs, interferometry, FRS and terahertz spintronics. In the second part, the dissertation addresses the challenge of precisely calibrating the delay axis during the scanning process for FRS. Employing a novel, multi-color interferometric arrangement, the author’s implementation surpasses the temporal precision of state-of-the-art terahertz-FRS by approximately one order of magnitude. The last part of the thesis deals with spintronic terahertz emitters as a broadband alternative to photoconductive switches. One highlight is the development, demonstration and characterization of fiber-tip spintronic terahertz emitters—the world’s’ smallest terahertz emitters. The author then integrated these emitters into an s-SNOM system and successfully introduced a spatiotemporally resolved terahertz emission nanoscopy with nanometer spatial and femtosecond temporal resolution, opening new avenues in optical sensing.

Felix Paries is a physicist specializing in ultrafast photonics and field resolved spectroscopy. He earned a German Diplom/M.Sc. in 2020 with a thesis on attosecond precision timing jitter metrology, and completed a Ph.D. in 2025, during which he captured the nanoscale spatiotemporal evolution of in-plane terahertz spintronic photocurrents and developed interferometric delay calibration for optical sampling systems. Both degrees were awarded by RPTU University Kaiserslautern-Landau.
His current research advances field resolved spectroscopy toward biomedical use. At present, he dedicates the majority of his lab-time to two key pillars: interferometric delay tracking of high end optical sampling systems, enabling ultraprecise measurement of electric field signatures, and increasing FRS-system compactness and robustness to transfer the technology from the lab to clinical and industrial environments. 
Felix Paries serves as postdoctoral researcher at the Laboratory for Lightwave Metrology and is affiliated with Fraunhofer ITWM, RPTU University Kaiserslautern-Landau, and Leibniz IPHT. 
His dissertation received the Springer Theses Award.