**6. Outlook**

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 current laser technology.

**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].

OPCPAs are scalable in terms of peak and average power and directly benefit from the availability of turn-key, industrial-grade ps-pump lasers. For more than two decades, power‐ ful, cost-effective ps pump sources have been unavailable. Nowadays, diode-pumped ytterbium-doped lasers in the thin-disk geometry are able to deliver 1 ps scale pulses at kilowatt-scale average power, in combination with terawatt-scale peak powers. By merging these two existing technologies, and comprising OPCPAs driven by ytterbium-based pump lasers, the new generation of femtosecond technology will combine terawatt-scale peak powers with kilowatt-scale average powers in ultrashort optical pulse generation opening new path in the generation of isolated attosecond pulses with higher flux and photon energies.

The increased photon flux will greatly expand the applicability of attosecond spectroscopy to scrutinizing phenomena where current-generation sources delivered signal near or below the noise level, whereas shorter wavelengths provide direct access to electronic motions on increasingly shorter length scales from nanostructures toward atomic dimensions.

The availability of attosecond X-ray pulses could lead to resolve the spatiotemporal motion of electrons in their ultimate resolution, picometer-attosecond resolution, via attosecond X-ray diffraction spectroscopy (**Figure 8**).
