Session P05: Attosecond Light Sources and Their Applications

High-power Attosecond Hard X-ray Pulse Generation at the European XFEL

The generation of high-power attosecond hard X-ray pulses is pivotal for probing the structural and electronic dynamics of matter with unprecedented precision. Leveraging a novel self-chirping method, we propose to generate attosecond pulses of exceptional intensity at angstrom wavelengths using X-ray free-electron lasers (FEL). This presentation will discuss the theoretical foundations of this method and the proof-of-principle experiments conducted at the European XFEL. Our findings demonstrate the successful generation of high-power, single-mode hard X-ray FEL pulses.   

Quantum trajectory selector: clocking recollision physics

The core principle behind strong field atomic phenomena is the process where electrons, freed by an intense laser field, are driven back to recollide with the core. This process, also referred to as the three-step model, results in the generation of attosecond radiation bursts, high-energy electrons, and multiple charged ions. Traditionally, these phenomena have been interpreted as the coherent sum of individual electron quantum trajectories. In this talk, I will discuss our recent work on the quantum trajectory selector (QTS), a method designed to isolate and examine individual quantum trajectories. Utilizing an XUV attosecond pulse train with a shaped bandwidth, we generate near-zero energy electron wave packets that simulate the initial conditions of tunnel ionization in the three-step model. This allows us to accurately determine the moment the electron enters the continuum and analyze the subsequent rate of electron emission and double ionization, driven by a phase locked near-infrared field. We measure a delay-dependent rate for both rescattered electron emission and double ionization and precisely clock their corresponding ionization time with attosecond precision calibrated using the streaking of the direct electrons. The QTS offers an innovative framework for increasing our understanding of recollision-driven physics and attosecond science in atoms, molecules and solids.   

Attosecond Imaging of Electronic Wave Packets

Quantum coherence among electronic states forms dynamical electronic wave packets. An electronic wave packet has significant spatial evolution besides its temporal evolution, due to the delocalized nature of composing electronic states. However, the spatial evolution was not previously accessible to experimental investigations at the attosecond time scale. We show attosecond spatial imaging of wave packet motion can be achieved with the phase-resolved two-electron-angular-streaking (PR-2eAS) method. We show the technique can distinguish the two electrons arising from double ionization processes to image the first frame of an ultrafast spin-orbit wave packet in the krypton cation. Furthermore, we capture the motion of an even faster wave packet in the xenon cation for the first time: an electronic hole being re-filled 1.2 fs after it was produced, and the hole-filling was observed in the opposite side where the hole was born.   

Attosecond x-ray free-electron lasers

High-harmonic generation has revolutionized ultrafast science, creating the experimental basis for a new field of investigation: attosecond science and technology.

With the development of attosecond pulses at the Linac Coherent Light Source (LCLS), XFELs have emerged as the brightest sources of attosecond x-ray pulses, with a peak brightness that surpasses table-top high-harmonic generation by seven orders of magnitude in the soft x-ray region. In my talk I will discuss the development of attosecond x-ray science capabilities at the LCLS and other XFEL facilities: from the first demonstration of isolated attosecond pulses to the observation of electronic dynamics with two-color pump/probe experiments. Finally, I will discuss the future of attosecond XFELs and the unique opportunities enabled by the high-repetition rate LCLS-II upgrade and advanced plasma-based electron sources.   

Pulse Characterization via Two-Photon Auto- and Cross-Correlation Signals

Characterization of ultrashort vacuum and deep ultraviolet laser pulses is crucial for spectroscopy and dynamic imaging of atoms, molecules, and materials.

We present a self-consistent method, extended from the technique proposed in [1], for fully characterizing ultrashort pulses and pulse trains via photoionization spectra.

In this method, analytic solutions for two-photon ionization of atoms by Gaussian pulses are used to characterize the pulse as a superposition of multiple Gaussians at multi-color central frequencies. This approach allows one to use auto- and cross-correlation signals to characterize the time-dependent amplitude, phase, and chirp variation of the electric field from the pulse. This method is demonstrated with vacuum and deep ultraviolet pulses and pulse trains obtained from numerical simulations of macroscopic high harmonic spectra [2].

[1] S. Walker, R. Reiff, A. Jaron-Becker, and A. Becker. Characterization of vacuum and deep ultraviolet pulses via

two-photon autocorrelation signals. Opt. Lett., 46(13):3083–3086, Jul 2021.

[2] R. Reiff, J. Venzke, A. Jaron-Becker, and A. Becker. Interference effects in harmonic generation induced by focal

phase distribution. OSA Continuum, 4(7):1897–1906, Jul 2021.   

Toward Real-Space Imaging of Electronic Motions with Attosecond X-Ray Scattering

 Ultrafast X-ray scattering has emerged as a groundbreaking technique for directly imaging the evolution of molecular structures, providing insights into complex processes such as conformational changes, and bond breakage at atomic-scale spatial and temporal resolutions. Ultrafast scattering experiments have thus far utilized pulses from X-ray free-electron lasers to probe atomic motion on the scale of tens of femtoseconds. Extending this approach into the attosecond time domain would enable imaging of coherent dynamics of electronic densities and currents.  

However, this regime introduces complexity in experimental setup and analysis, necessitating a different set of tools than those traditionally employed. This is due to the nature of scattering information, which encompasses both elastic and inelastic scattering and their cross-terms, rendering the simple Fourier relationship between the intensity detected and the charge density invalid.

Here, we outline a methodology for utilizing attosecond hard X-ray scattering to recover electronic charge densities and currents in real space and time from an experimental perspective. We delve into several critical aspects necessary to accomplish a "movie" of electronic motion. These aspects include the application of pump-probe phase tagging through single-shot characterization of attosecond hard X-ray pulses, strategies for controlling the detection bandwidth to highlight scattering from specific electronic states, and methods for distinguishing between densities and currents by leveraging the anisotropy and symmetry properties inherent to the attosecond scattering process. Additionally, we describe an inversion technique required to achieve sub-angstrom resolution in the reconstruction process.  Together, these steps facilitate capturing a comprehensive view of electronic coherent motions within molecules, marking a significant advancement in the field of imaging.