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Attosecond metrology


The generation of ultrashort pulses is a key to exploring the dynamic behaviour of matter on ever-shorter timescales. Recent developments have pushed the duration of laser pulses close to its natural limit—the wave cycle, which lasts somewhat longer than one femtosecond (1 fs = 10-15 s) in the visible spectral range. Time-resolved measurements with these pulses are able to trace dynamics of molecular structure, but fail to capture electronic processes occurring on an attosecond (1 as = 10-18 s) timescale. Here we trace electronic dynamics with a time resolution of ≤ 150 as by using a subfemtosecond soft-X-ray pulse and a few-cycle visible light pulse. Our measurement indicates an attosecond response of the atomic system, a soft-X-ray pulse duration of 650 ± 150 as and an attosecond synchronism of the soft-X-ray pulse with the light field. The demonstrated experimental tools and techniques open the door to attosecond spectroscopy of bound electrons.

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Figure 1: The principle of measuring cross-correlation between light electric field and X-ray intensity with attosecond resolution by means of ‘two-colour’ photoionization.
Figure 2: Diagram of the principal physical effects and processes preceding our light-field-controlled X-ray photoemission experiment.
Figure 3: Kr 4p photoelectron spectra produced by 90-eV soft-X-ray pulses in the presence of a strong visible light field at two different delays td of the X-ray pulse.
Figure 4: Cross-correlation of X-ray pulse with few-cycle laser pulse.
Figure 5: Spectral width ΔW of the Kr 4p photoelectron spectra as a function of td.
Figure 6: Calculated far-field, near-axis temporal intensity profile of a soft-X-ray pulse.
Figure 7: Calculated (line) and measured (dots) instantaneous frequency of the few-cycle light field.

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We thank M. Uiberacker for assistance with measurements, and Y. Lim and U. Kleineberg for manufacturing the X-ray multilayer mirror. Discussions with M. Ivanov are gratefully acknowledged. This work was supported by the Austrian Science Fund and by the European ATTO network.

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Correspondence to F. Krausz.

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Hentschel, M., Kienberger, R., Spielmann, C. et al. Attosecond metrology. Nature 414, 509–513 (2001).

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