What will affect the properties of high-order harmonic?

Due to the great high photon energy, which can ionize the outer valence electron of most atoms and molecules, the extreme ultraviolet (XUV) has been widely investigated in atomic, molecular, and optical physics. However, the conventional radiation sources of XUV are large scale facilities and they are expensive to be maintained.

The high-order harmonic generation (HHG) is a novel platform as an XUV source. HHG can convert the light at low frequency into an XUV with an attosecond pulse in the time domain. In practical applications, the XUV with both high intensity and attosecond pulse duration is of great significance such as multiphoton ionization and pump probe.

Therefore, the focusing XUV has been developed to satisfy the requirements of practical applications. Although various methods have been proposed, broadband focusing with a high demagnification factor is still a challenge.

A research team led by Prof. Dr. Maria Hoflund from Lund University proposed a method to focus the XUV radiation in broadband range associated with high demagnification factor. The results were published in Ultrafast Science.

According to the researchers, the multiterawatt Ti: Sapphire laser system was used as a pump source to excite argon gas cells for HHG that was focused on by the Wolter optics. The knife was placed around the focus, and the beam was detected by a flat-field spectrometer.

The theoretical model was also established to explain the fundamental physics, including three steps of tunneling ionization, electron acceleration by the laser field, and recombination. And the results indicated that the simulation with the varying focal length agreed well with the experiment.

The spatiotemporal intensities were finally estimated, the pulse energy was 1.6 nJ with the harmonic pulse duration was 20 fs, implying the maximum XUV peak intensity of 10¹³ W/cm² can be obtained. Besides, the different experimental conditions were also considered, indicating the chromatic aberrations induced by the dipole phase affect strongly the temporal profile and intensity of the focused attosecond pulses.

This work reveals the influence of how the generation geometry affects the intensity and duration of the focused attosecond pulses.

This work was supported by the Swedish Research Council, the European Research Council (advanced grant QPAP), the Knut and Alice Wallenberg Foundation, and the Crafoord Foundation.