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.

We report resonant inelastic x-ray scattering (RIXS) spectra of neon atoms interacting with intense x-ray pulses generated using an x-ray free-electron laser (XFEL). We find that an unexpected peak emerges near the 𝐾⁢𝛼 line of Ne, which does not coincide with any physical resonances of neon ions. We perform theoretical calculations based on a quantum-state-resolved rate-equation approach with x-ray-induced processes including possible resonant excitations. Our dynamics simulations demonstrate that a sequence of multiple resonant photoabsorption events are involved and the interplay of those multiple resonances in combination with the relatively large spectral bandwidth of XFEL radiation leads to the emergent resonance-like structure at a position where no resonances exist. Our finding offers critical guidance for future applications of high-intensity RIXS at XFEL facilities.

Isolated attosecond pulses (IAPs) in the water window are critical for attosecond spectroscopy of chemical and biochemical dynamics. Their creation via high-harmonic generation (HHG) typically demands carrier-envelope phase (CEP) stabilization and complicated pulse compression of the ultrashort HHG driving pulses. This study describes field-resolved measurements of single-cycle pulses obtained through soliton self-compression of short-wave-infrared (1485 nm) femtosecond pulses. Pulse compression down to 1.1 optical cycles, corresponding to a duration of 5.05 fs, is demonstrated for 1.175 mJ pulses with 0.325 mJ within the full width at half-maximum of the pulse. High-harmonic generation in the water window is achieved with these pulses, and its dependence on the pulse compression is studied. It is also shown that IAPs are obtained for nearly any value of the CEP, which demonstrates that single-cycle pulses obtained from soliton self-compression enable the creation of IAPs without the requirement of a stable CEP. The results of this work open the path to attosecond time-resolved measurements in the water window, benefiting from the higher flux obtained with signal pulses (compared to idler pulses) of widely used optical parametric amplifiers.

The carrier-envelope offset phase (CEP) of a few-cycle optical pulse is commonly used to control electron dynamics on the attosecond timescale, whereas lasing spectra from transitions between rotational states are generally emitted over much longer durations, typically nanoseconds. Here, we demonstrate CEP control of the rotational lasing spectra corresponding to the transition from B2Σu+(0) to X2Σg+(1) in N2+ cations, transforming its lineshape from a symmetric Lorentzian profile to an asymmetric Fano type–and vice versa. This lineshape modulation arises from the interference between the B-X coherence initiated by the main pulse and the supercontinuum (self seed) by self-phase modulation, resembling an “f-to-3f" interferometry. Additionally, for lasing lines with lower rotational quantum numbers, we observe a stronger coupling between adjacent lasing peaks, which originates from the amplification of both even- and odd-order rotational coherent emission lines. Our study presents a general framework for controlling lasing lineshapes and provides new insights into sub-optical-cycle dynamics in air lasing.