**Acknowledgements**

using different crystals in different amplification stages. In addition to the presented systems, different harmonics of the Yb:YAG amplifier can be used to pump few-cycle pulses in visible

The capabilities of the current high harmonic generation sources, based on CPA Ti:Sa tech‐ nology, are limited to energies around a few hundred eV and to pulse durations of several tens of attoseconds. This limitation originates from a deficiency of the current laser technology that can either provide pulses with ultrahigh (petawatt) peak powers at relatively low repetition rates or moderate peak power (gigawatt) pulses at kHz repetition rates. Scaling attosecond pulses to high repetition rates and photon energies as high as several keV demands few-cycle laser systems with high peak and average power, which is beyond the performance of the

**Figure 8.** Attosecond X-ray diffraction: as a coherently induced charge oscillation takes place in an atom or molecule, an incident X-ray pulse takes a diffraction snapshot of the electron distribution at the time of interaction; changing the time delay between the source of the excitation and the attosecond pulse allows for the temporal evolution of the charge density to be directly measured in time and space. The figure represents the simulated dynamic of hydrogen atoms when they are exposed to 100-as, X-ray pulses and are excited into the 1S-2P coherent superposition state. As shown, the electron dynamic can be reconstructed by means of attosecond X-ray diffraction spectroscopy [3].

or mid-infrared spectral range [16].

68 High Energy and Short Pulse Lasers

**6. Outlook**

current laser technology.

I wish to thank Prof. Ferenc Krausz for fruitful discussions about the material of this chapter.
