Spatiotemporal isolation of attosecond soft X-ray pulses in the water window

Abstract

Attosecond pulses at photon energies that cover the principal absorption edges of the building blocks of materials are a prerequisite for time-resolved probing of the triggering events leading to electronic dynamics such as exciton formation and annihilation. We demonstrate experimentally the isolation of individual attosecond pulses at the carbon K-shell edge (284 eV) in the soft X-ray water window with pulse duration below 400 as and with a bandwidth supporting a 30-as pulse duration. Our approach is based on spatiotemporal isolation of long-wavelength-driven harmonics and validates a straightforward and scalable approach for robust and reproducible attosecond pulse isolation.

Figure 1. Experimental set-up and conditions.

Top: schematic of the experimental set-up. (a) Spatiospectral profile in the gas jet focus without WFR. (b) Corresponding calculated electric field. (c) Measured HHG spatiospectral profile generated in 1-bar Argon without WFR. (d) Spatiospectral profile in the gas jet focus with WFR. (e) Corresponding calculated electric field. (f) Measured HHG spatiospectral profile generated in 1-bar Argon with WFR, revealing a 4-mrad separation between beamlets.

Figure 2. CEP control of emission angle.

(a) Spatial profile of the emitted radiation in function of CEP, revealing a clear control of the emission angle. (b) Spatial profile for three CEPs. The spectra are integrated from 200 to 400 eV.

Figure 3. Isolated spatiotemporal profiles for three different emission angles

(a–c). Shown is the result of the measurement for three different relative CEP values of Φ, Φ+π/3 and Φ+2π/3 with the spectrograph imaging the focal plane of the ellipsoidal mirror along the energy axis. The dotted lines indicate the 2.5-mrad-integration region for the spectra on the right.

Figure 4. Single-atom response WFR simulation results.

Shown in the top row (a–c) are results including short and long trajectories, whereas the bottom row (d–f) shows results for identical simulation values, but with the long trajectory numerically removed. Panels a,d show the generated spatiotemporal profile in focus; (b,e) angular-spectral profile in the far field, for an angular range of 5.5 mrad; (c,f) spectrogram of the pulse, calculated for the electric field along the dotted line in a. The case for long trajectories is only displayed in the Supplementary Information for completeness.

Figure 5. Photon yield and calculated pulse profiles.

(a) Comparison of photon yields on target. Chirped mirror reflection profile from (b). Simulated pulse profile in the case without wavefront rotation, after transmission through a 400-nm Al filter, leading to a pre/post-pulse contrast of 1:5. (c) Simulated pulse profile in the case of wavefront rotation, leading to an increased pre/post-pulse contrast of 1:20.