Valentina Utrio Lanfaloni, Federico Vismarra, Emir Ardalı, Nicholas R. Monahan, Joss Wiese, Th. Köpp, Fernando Ardana‐Lamas, Giuseppe Fazio, Leonardo Redaelli, Yoann Pertot, Kristina S. Zinchenko, Tadas Balčiūnas, Hans Jakob Wörner
The demonstration of soliton self-compression in the terawatt-level regime using large-core hollow capillary fibres and long-wavelength driving pulses has opened new possibilities for tabletop ultrafast spectroscopy experiments. Here we report the creation of phase-stable sub-cycle self-compressed light transients, as well as their field- and phase-resolved optical field sampling. We demonstrate the direct in situ measurement of self-compressed light transients, reaching durations down to 2.5 ± 0.2 fs, which is half of an optical cycle at a centroid wavelength of 1,366 nm, and determine their waveform phase offset. We apply these transients to soft X-ray high-harmonic generation and attosecond X-ray absorption spectroscopy. Attosecond transient absorption spectroscopy at 250 eV demonstrates the utility of the sub-cycle light transients for experiments with ultimate temporal resolution. The advances reported in this work merge the deep sub-cycle temporal resolution offered by self-compressed, phase-characterized light transients and showcase their application for water-window attosecond X-ray absorption spectroscopy, pushing the boundaries of achievable temporal resolution.