The driving laser determines the upper limit of the attosecond laser light source

The driving laser determines the upper limit of the attosecond laser light source.
At present, attosecond pulse lasers are mainly generated through high-order harmonic generation (HHG) driven by strong fields. The essence of their generation can be understood as electrons being ionized, accelerated, and re-combining to release energy, thereby emitting attosecond XUV pulses.
Therefore, the output of attosecond pulses is extremely sensitive to the pulse width, energy, wavelength, and repetition frequency of the driving laser: shorter pulse widths are conducive to isolating attosecond pulses, higher energy improves ionization and efficiency, longer wavelengths raise the cutoff energy but significantly reduce the conversion efficiency, and higher repetition frequencies improve the signal-to-noise ratio but are limited by the single-pulse energy.
Different applications focus on different key indicators of attosecond lasers, thus corresponding to the design choices of different types of driving laser sources.
For applications such as ultrafast dynamics research and electron microscopy, stable isolation of attosecond pulses (IAP) usually requires short-pulse driving pulses and good carrier envelope phase (CEP) control to achieve effective time gating and waveform controllability;
For experiments such as pump-probe spectroscopy and multi-photon ionization, high-energy or high-flux attosecond radiation helps improve excitation/absorption efficiency, which is usually achieved under higher driving energy and higher average power conditions through HHG, and requires maintaining acceptable phase matching and beam quality under high ionization conditions;
To generate attosecond radiation in the X-ray window (which is of great value for coherent imaging and time-resolved X-ray absorption spectroscopy), mid-infrared long-wavelength driving is often used to increase the harmonic cutoff energy and obtain higher photon energy coverage;
In measurements that are sensitive to statistical accuracy, such as counting and photoelectron spectroscopy, higher repetition frequencies can significantly improve the signal-to-noise ratio and data acquisition efficiency, while lower single-pulse charge/energy helps reduce the limitation of spatial charge effects on energy spectrum resolution.
The correspondence between driving laser parameters, attosecond pulse laser characteristics, and application requirements is shown in Figure 1. Overall, the demands of applications continuously drive the further improvement of attosecond pulse laser parameters, and thereby drive the continuous development of the architecture and key technologies of ultrafast laser systems.