Wei Cao

The retrieval of attosecond and femtosecond pulses from attosecond streaking spectrograms is a high-dimensional ill-posed inverse problem. The traditional FROG CRAB (frequency-resolved optical gating for complete reconstruction of attosecond bursts) method relies on the central momentum approximation, while supervised deep learning is constrained by the scarcity of labeled samples and lacks interpretability. In this paper, we propose a self-supervised collaborative reconstruction framework incorporating physics-based constraints, which integrates light-field representation, a differentiable physical model, and a prior regularization module. This framework requires only a small number of unlabeled samples for training and performs direct inference, thereby achieving collaborative high-precision retrieval of attosecond extreme ultraviolet (XUV) and femtosecond infrared pulses. Reconstructions from simulated and measured streaking spectrograms demonstrate that, compared to FROG CRAB, our self-supervised approach is more accurate even for extremely low signal-to-noise ratios, offering a novel, intelligent reconstruction solution for attosecond metrology with limited data and low photon flux.

A non-linear wave-mixing process between an extreme ultraviolet (XUV) pulse and near-infrared laser fields in helium atoms was numerically simulated. Beyond the dipole approximation, three-waving mixing (TWM) signals associated with the electric quadrupole transition, although negligibly small, were generated in the far field. By appropriately choosing the pulse configuration, the TWM signal can interfere with the four-wave mixing signal permitted by the dipole transition, thus observing the oscillation of the signal strength with delay. This provides a clear signature of the electric quadrupole effect in an XUV transient wave-mixing measurement.

Transient absorption spectroscopy using soft X-ray coherent light sources as ultrafast probes holds significant potential applications in chemistry, biology, and materials science. This article presents the design of a transient absorption apparatus based on desktop soft X-ray light sources. A commercial femtosecond laser system (4.4 mJ, 25 fs, 800 nm, 1 kHz) drives an optical parametric amplifier, generating a 900 μJ, 28 fs, 1440 nm short-wavelength infrared (SWIR) pulse. This SWIR pulse is spectrally broadened and temporally compressed into a few-cycle pulse (400 μJ, 16.5 fs, 1530 nm) by a hollow-core fiber compressor. Then, few-cycle SWIR pulse drives the generation of attosecond soft X-ray high-order harmonic radiation, with the maximum photon energy extending into the water window region (>300 eV). The spectral resolution of the soft X-ray spectrometer is determined to be 334 meV at 243 eV. The remaining 800 nm pump pulse from the OPA system is combined with the high-order harmonic soft X-ray probe by using a hole mirror, forming a Mach-Zehnder interferometer with a time jitter of less than 10 fs during the one-hour data acquisition. This setup demonstrates the feasibility of performing time-resolved soft X-ray spectroscopy in a compact experimental configuration. Preliminary studies of transient absorption near the argon L-edge and carbon K-edge are conducted, demonstrating that this system can be used as a powerful tool for element-specific, time-resolved, and transition-channel-resolved investigations of electron dynamics.

High-harmonic generation (HHG) is capable of producing attosecond pulses, which find widespread usage in observing ultrafast dynamics in atoms, molecules, and solids.^{1 – 8} Given their short pulse width, high peak intensity, and broad spectral width, few-cycle pulses serve as common drivers for generating attosecond pulses.⁹ Consequently, the accurate characterization of the few-cycle electric field is essential for comprehending and effectively utilizing attosecond pulses in subsequent research.

High-order harmonic generation (HHG), characterized by its highly nonlinear nature, often exhibits a complex spatio-temporal profile that poses challenges for practical applications. In this study, we unveil a method for manipulating the spatio-spectral distribution of HHG by guiding the recollision electron trajectory in the spatio-temporal domain using a control field. The resulting far-field high harmonic (HH) radiation inherits the intricate spatio-temporal characteristics of the control field, showcasing diverse features including spatial tilting, spectral shifting, and emission angle deflection. Using the relative delay between the control field and the driving pulse as the primary control parameter, we achieve precise tailoring of the high harmonics in the spatio-spectral domain. This controllability in HH benefits ultrafast metrology and imaging applications in the extreme ultraviolet (XUV) regime.

We have demonstrated the capability of spectral multiplexing in multi-distance diffractive imaging, enabling the reconstruction of samples with diverse spectral responses. While previous methods such as ptychography utilize redundancy in radial diffraction data to achieve information multiplexing, they typically require capturing a substantial amount of diffraction data. In contrast, our approach effectively harnesses the redundancy information in axial diffraction data. This significantly reduces the amount of diffraction data required and relaxes the stringent requirements on optical path stability.