Frequency-Resolved Optical Gating of Isolated Attosecond Pulses in the Extreme Ultraviolet

The pulse shape and phase of isolated attosecond extreme ultraviolet (XUV) pulses with a duration of 860 asec have been determined simultaneously by using frequency-resolved optical gating based on two-photon above-threshold ionization with 28-eV photons in He. From the detailed characterization, we succeeded in shaping isolated XUV pulses on an attosecond time scale by precise dispersion control with Ar gas density or by changing the driving pulse width. These results offer a novel way to excite and observe an electron motion in atoms and molecules.

Figure 1

(a) Energy diagram of two-photon above-threshold ionization in He. (b) Electron spectrum by two-photon above-threshold ionization. The horizontal axis is the electron energy plus the ionization potential of He. (c) Measured time-frequency map of the spectrum versus time delay between two replicas of ninth harmonic pulses. The signal intensity is expressed by contours with 10 steps from the top to the ground. (d) Time-frequency map retrieved from (c) by the iterative Fourier-transform algorithm with generalized projection. The root-mean-square error between (c) and (d) is 2.6%.

Figure 2

Retrieved pulse shapes and phases. The dashed curves show the transform-limited pulse shapes. Thick solid curves in the phase correspond to the entire pulse and are used to determine the quadratic chirp coefficients. (a), (b), and (c) correspond to Ar gas densities of 5.7, 7.2, and 9.4 torr, respectively, with a fixed driving pulse width of 15 fs. (d) corresponds to a driving pulse width of 19 fs with an Ar density of 7.2 torr. (a), (b), and (c) show the dependence of the pulse shape and phase on Ar gas density. (b) and (d) show the dependence on driving pulse width.

Figure 3

(a) Harmonic spectra at Ar densities of 9.4 torr (thick solid carve), 7.2 torr (dot-dashed carve), 5.7 torr (dashed carve), and 3.5 torr (thin solid carve), respectively. The dotted lines show the spectral range corresponding to 9 times the spectral band of the driving pulse. (b) The dependence of measured (solid line) and transform-limited pulse durations (dotted line) on Ar gas density. (c) The quadratic chirp coefficients depending on Ar gas density.