High-Resolution Single-Shot Terahertz Spectroscopy Using Probe Multiplexing for Plasmas

Single-shot terahertz (THz) spectroscopy is crucial for capturing ultrafast phenomena in plasmas, such as dynamic absorption features and density changes. However, traditional methods using chirped probe pulses face an inherent conflict: high spectral resolution (detailing the THz spectrum) comes at the expense of poor temporal resolution (capturing rapid changes). Chirping stretches the probe pulse in time, improving the range of detectable frequencies but blurring features within the pulse. This work proposes a novel single-shot THz modulation technique using probe multiplexing to overcome this limitation. Here, a single probe pulse is pulse-split to create a pulse train, which samples the THz signal at different time delays simultaneously using electro-optic modulation. This eliminates the need for chirped pulses, enabling high spectral resolution without sacrificing time resolution (or THz bandwidth in spectral domain). The technique can be implemented with various detection methods depending on desired time resolution. For moderate bandwidths (1-2 THz), a streak camera can achieve sub-picosecond resolution. For even higher resolution, the streak camera can be coupled with a spectrometer, allowing for detailed characterization of the THz time-domain pulse with improved time resolution. Using single-shot THz detection, we measure the amplitude and phase of transmitted THz at a time resolution of 141 fs. We use the phase shift to calculate the plasma density of an argon inductively coupled plasma source (300 mTorr, 50 W). With a phase uncertainty less than 0.1 rad, we also test it on an inductive plasma source to quantify temporally varying plasma properties within radiofrequency cycles.