| Attosecond pulses have been realized successfully in the XUX and soft x-ray region by using high-order harmonics, opening the way to attosecond science. This is the regime that is not accessible by traditional synchrotron radiation or even emerging free electron lasers. Another important aspect of attosecond pulses is the exploring of nonlinear optics in the XUV and soft x-ray region because the peak intensity of attosecond harmonics is superior to those of synchrotron radiation or perhaps free electron lasers. Nonlinear optics naturally give us the chance of the simplest pump-probe experiment in the attosecond regime. We have applied two-photon ionization to the characterization of harmonic pulses in the time domain. We observed two-photon ionization in rare gases by 14-eV harmonic photons for the first time and applied it to autocorrelation in 1998. We characterized attosecond pulses by autocorrelation at 28 eV in 2004. More recently, we determined the pulse shape and phase of attosecond pulses uniquely by frequency-resolved optical gating at 28 eV in 2006.We will report in the workshop on the efforts to generate intense harmonics, to observe photo-electrons created by nonlinear process efficiently and to extend nonlinear optics to the higher energy range. |
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