Quantitative single-shot terahertz spectroscopy using supercontinuum-based spectral encoding in dynamic refractive media

Single-shot terahertz (THz) spectroscopy probes sub-picosecond, non-repetitive events by combining the advantages of laser absorption techniques with the phase-sensitive detection of interferometric methods. However, its usage as a quantitative tool is hindered by the chirp penalty of the spectral encoding scheme, where a narrow probe bandwidth distorts the THz signal, limiting the bandwidth and spectral resolution of THz measurements. We categorize this distortion into a temporal-Fraunhofer and temporal-Fresnel regimes, analogous to spatial diffraction, defined by the characteristic dispersion length and distortion factor, and use this framework to unify existing time- and frequency-domain THz measurement techniques across a range of detection architectures. We then introduce Supercontinuum-Enhanced Terahertz Spectroscopy (SETS), a method that leverages a broadband supercontinuum probe to overcome these challenges. With SETS, we show an increase in usable THz bandwidth from 1.5 THz to 2.3 THz, reducing signal distortion by 50% and offering a scalable pathway to extend bandwidth further. Numerical models of spectral encoding highlight the flexibility of SETS, achieving high spectral resolution (< 4 GHz with a 250 ps supercontinuum pulse) while maintaining a usable bandwidth > 1 THz for amplitude and phase spectra. Experiments on argon plasma and water vapor, complemented by theoretical validation, generalization, and extrapolation, show the capability of SETS to measure electron density, collision frequency, and absorption spectra simultaneously with tailored measurement accuracy and resolution. By addressing a critical limitation in single-shot diagnostics, SETS enables high-resolution, non-intrusive, quantitative measurements for complex reactive flows and dynamic refractive media.