Nonlinear Attosecond Metrology by Intense Isolated Attosecond Pulses

For the future progress of nonlinear attosecond science, one of the most important issues is the development of high-energy isolated attosecond pulse sources. In this paper, we review our research on the energy-scaling method for isolated attosecond pulse generation, which combines infrared two-color gating and an energy-scaling method of high-order harmonic generation. By establishing the robust energy-scaling method, the maximum pulse energy of the isolated attosecond pulse reaches 1.3 μJ/pulse in the XUV region, which is almost 100-fold higher than the values ever reported before. The pulse duration of this isolated attosecond pulse is directly measured to be 500 as by the autocorrelation method using the nonlinear interaction of N₂. This nonlinear experiment clearly shows that our developed isolated attosecond pulse has enough pulse energy to make a breakthrough for nonlinear attosecond metrology as well as for attosecond-pump/attosecond-probe experiments investigating electronic processes. In addition, we further extend the high-energy harmonic continuum by using the infrared two-color gating method up to the soft-x-ray region. With the aim to temporally characterize an isolated attosecond pulse in the soft-x-ray region, we demonstrate the first observation of above-threshold ionization of He with two-photon absorption of multiple higher harmonics in the soft-x-ray region.