Attosecond Streaking in the Water Window: A New Regime of Attosecond Pulse Characterization

We report on the first streaking measurement of water-window attosecond pulses generated via high-harmonic generation, driven by sub-2-cycle, carrier-to-envelope-phase-stable, 1850-nm laser pulses. Both the central photon energy and the energy bandwidth far exceed what has been demonstrated thus far, warranting the investigation of the attosecond streaking technique for the soft-x-ray regime and the limits of the frogcrab retrieval algorithm under such conditions. We also discuss the problem of attochirp compensation and issues regarding much lower photoionization cross sections compared with the extreme ultraviolet in addition to the fact that several shells of target gases are accessed simultaneously. Based on our investigation, we caution that the vastly different conditions in the soft-x-ray regime warrant a diligent examination of the fidelity of the measurement and the retrieval procedure.

Popular Summary

Attosecond-duration x-ray pulses are bursts of light so brief ($10^{-18}$ s) that they can freeze-frame the motion of electrons, thus reaching the ultimate time scale of the processes that govern chemistry and the functionality of materials. The usefulness of such an “electron camera” is virtually without limits. It can be used to capture how a solar cell absorbs light and produces electricity, see where the bottlenecks are when electrons move in tiny electronic circuits, or understand how electrons change chemical bonding and trigger biological change. Before taking such snapshots, however, researchers need to fully understand just how fast these “cameras” operate. Using methods from modern laser and atomic physics, we report the first characterization of attosecond pulses in the water-window soft-x-ray regime.

Our measurements combine ultrafast temporal resolution with the element selectivity of soft x rays in the water window, a region of the spectrum in which water is transparent to x rays. Thus, we are able to pinpoint the flow of electrons with atomic-site specificity inside complex materials. Our results show that the characterization of such pulses is more complex than in the routinely accessed extreme UV regime. We discuss the challenges and aspects for a credible characterization in the soft-x-ray range, which will require a much higher level of care and diligence for broadband spectra and temporal transients approaching the atomic unit of time.

These results herald a new era of attosecond science in which subfemtosecond temporal resolution is paired with element selectivity to investigate a wide range of fundamental processes in nature.

Figure 1

Photoionization cross sections [58] for the typically used target gases in attosecond streaking experiments. Helium $1s$ cross section is included in dark blue in each plot as a point of reference.

Figure 2

Attochirp as a function of driving wavelength, 800 nm (red curve) and 1850 nm (blue curve).

Figure 3

Schematic layout of the attosecond streaking beam line. Five interconnected vacuum chambers make up the attosecond streaking beam line. Following the beam propagation direction, from right to left, the first two chambers are only rough pumped to facilitate the high gas pressures needed for phase-matched HHG, and the last three chambers are turbopumped to minimize reabsorption of the x rays and ensure the required pressure environments needed by the diagnostics in the last chamber (left).

Figure 4

Streaking trace (linear scale) from the water-window attosecond pulse recorded in Kr.

Figure 5

Streaking traces performed using the same laser source, but a different HHG gas (argon), which offers higher flux at the expense of photon energy (centered 100 eV). The trace on the left for a randomly selected value of CEP clearly shows more than one attosecond pulse (highlighted with the dotted black and magenta curves). By selecting a value of CEP that results in a spectral continuum and maximum cutoff, the trace on the right is recorded, where only one attosecond pulse is detected. Argon gas is used primary for alignment, as the photon yield is higher and traces are acquired quicker; i.e., the full traces are acquired in 5 and 20 min, respectively.

Figure 6

Time-of-flight spectrum measured in krypton ionized by the soft-x-ray continuum. The presence of ${\mathrm{Kr}}^{2+}$, ${\mathrm{Kr}}^{3+}$, and ${\mathrm{Kr}}^{4+}$ clearly indicates the occurrence of Auger relaxation mechanism after the initial photoionization. The isotope structure of Kr is also visible.

Figure 7

Calculated dipole emission phase for Kr $3d$ as a function of photon energy [72].

Figure 8

frogcrab retrievals of the raw data shown in Fig. 4. Top: Two cycles from 15–27 fs; bottom: two cycles from 22–34 fs. Left-hand plots show the measured traces which have had some filtering applied. Middle plots show the frogcrab reconstructed traces and the right-hand plots show their temporal profiles.

Figure 9

Spatiotemporal phase-matching maps as a function of HHG target pressure in Ne. Calculated on-axis phase mismatch as a function of propagation position and time within the pulse for 300-eV radiation generated in neon for our experimental conditions and target pressures of 1–4 bar. The intensity in our target only has a field strength sufficient to generate 300 eV radiation between $-1$ to 1 mm in the propagation direction.

Figure 10

Noise-free streaking field intensity simulations. Top: Low intensity; bottom: high intensity. Left: No attochirp simulated; right: $2500{\mathrm{as}}^2$ simulated.

Figure 11

Statistical noise analysis of an attosecond pulse with similar bandwidth to our experimental data. Top: $1250{\mathrm{as}}^2$ simulated; middle: $2500{\mathrm{as}}^2$ simulated; bottom: $5000{\mathrm{as}}^2$ simulated.

Figure 12

Bandwidth retrievals. Left-hand column (theoretically generated streaking spectrograms): Plots are annotated at the bottom left, with the simulated bandwidth, simulated applied attochirp, and corresponding simulated pulse duration. Middle (frogcrab algorithm) and right-hand (lsgpa algorithm) columns are annotated with the retrieved attochirp and the retrieved pulse duration. Top four rows: ${10}^{11}\mathrm{W}/{\mathrm{cm}}^2$. Bottom row: ${10}^{12}\mathrm{W}/{\mathrm{cm}}^2$.