Measurement of the intensity-dependent atomic dipole phase of a high harmonic by frequency-resolved optical gating

Abstract

Attosecond pulse generation is one of the attractive topics in the field of high harmonic generation and ultrafast optics. For the generation of ultrashort pulses in the attosecond regime, precise phase measurement and manipulation are requisite. However, the phase of high harmonics has never been measured. In this paper, we propose the frequency-resolved optical-gating (FROG) technique' by using two-photon ionization (TPI) for the measurement of the extreme ultraviolet ( X W ) ultrashort pulses and demonstrate the phase measurement and compensation of the fifth harmonic of Tksapphire (Tis) laser pulses for the first time.Since there are no transparent solid materials in XUV region, it is impossible to make FROG measurement by using the nonlinear optical processes such as second harmonic generation (SHG), self-diffraction, and Kerr effect. However, TPI process of atomic gas is free from such a restriction. The nonlinear autocorrelation-type signals can he obtained by resolving the kinetic energy and number of the photoelectrons generated hy TPI, corresponding to the photon energy and -300 -200 -100 0 100 200 300 Delay Time (fs) -10 0 -15 -302 -200 -100 0 100 200 300 Time (fs) CMQZ Fig. 2. a) TPI FROG trace with the use of a 2-mm thick CaF, plate. h) The solid and dotted lines are the intensity and phase profiles of the retrieved pulse, respectively.spectral intensity of SHG, respectively. Since the transition dipole moment due to the nonresonant TPI is proportional to E(t)E(tz), where E(t) is the electric field at time t and z is the delay time, the FROG trace can be analyzed by the same process as the SHG-FROG. Figure 1 shows the experimental setup. Spatially divided Tis laser pulses were focused on the Xe gas to generate the fifth harmonic (7.75 eV) separated from the other harmonics and the fundamental by using two beamsplitters, a CaF, filter, and a concave dichroic mirror for the fifth harmonic. The pulse energy, the pulse duration, and the peak intensity were 7 mJ, 40 fs, and 6 x W/cmZ, respectively. The CaF, plates with the thicknesses of 0.5,2, and 4 mm were used to change the optical dispersion. The fifth harmonic pulses were focused on the Xe gas, of which ionization energy is 12.13 eV. The ejected photoelec--200 0 200 Time Ifs) CMQZ Fig. 3. Measured (solid lines) and calculated (dotted lines) pulse profiles and phases with 0.5-(a) and 4-mm (b) thick plates. The calculated pulse profiles are based on the pulse measured with a 2-mm thick CaF, plate. The phases are shifted vertically.trons were detected after passing through a magnetic bottle electron spectrometer at each delay time. Figure 2 shows the 128 x 128 FROG trace obtained with the 2-mm thick CaF, plate and the retrieved intensity and phase of the pulse. The generalized projection algorithm was used to retrieve the fifth harmonic pulse.' The FROG error was 3%. Figure 3 shows the retrieved pulses with the 0.5-and 4-mm thick plates, which are consistent with the calculated pulses from the measured pulses with the 2-mm thick plate. The pulse durat...

High harmonic generation in Ne and H_2 gas mixtures

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Extreme-ultraviolet pump–probe studies of one-femtosecond-scale electron dynamics

“…Nonlinear (NL) XUV processes constitute the ideal tool for the study of such dynamics. Attosecond pulse trains 7-9 have reached intensities sufficient to induce two-XUV-photon processes [10][11][12][13][14] . However, isolated attosecond pulses, requisite for XUV-pump-XUV-probe experiments, have not yet attained the required parameters for an observable two-XUV-photon process.…”

Temporal characterization of attosecond pulses emitted from solid-density plasmas

“…This approach requires measurement of frequency-resolved two-XUV-photon ionization traces, which can be recorded applying energy-resolved PE spectroscopy. While second-order AC-based FROG-type measurements of individual harmonics [52,53] and mode-resolved AC measurements of an as-pulse train generated in a gas-phase medium have been demonstrated [32], FROG-type measurements of attosecond trains have not yet been successfully demonstrated. In well-established femtosecond metrology, the FROG trace corresponds to the mapping of the phase-amplitude distribution of the pulse to be characterized [54].…”