Self-compressed waveform-stable light transients enabling water-window attosecond spectroscopy

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.