Optimization of high-order harmonic generation for the time-resolved ARPES.

We experimentally investigated the optimized phase matching condition for high order harmonic generation as a source of time-resolved Angle-resolved photoemission spectroscopy (TR-ARPES) applications. In the loose focusing scheme, we find that the divergence of harmonics decreases with the increase of gas cell length, while the maximum intensity is obtained with 10–15 mm gas cell. Our result shows that stable beam condition with best temporal resolution can be realized for TR-ARPES by using a longer gas cell (longer than 25 mm in our experiment), and an appropriate gas cell length can provide balanced condition for good beam intensity and good temporal resolution. For the experiments carried out on HHG-based TR ARPES, the wavefront tilt of harmonic beam is inevitably introduced by the grating used in monochromator, leading to the boarding of pulse width and the dramatic decrease of the temporal resolution. A simple way to decrease the wavefront tilt is by optimizing the beam quality of high order harmonics. In our work, we aim to decrease the divergence of harmonic beam while keeping the enough intensity. We have experimentally investigated the intensity and beam divergence of high order harmonics generated in Ar for different gas cell condition. The divergence of harmonics decreases with the increase of gas cell length which is shown in (a), while the maximum intensity is obtained with 10 15 mm gas cell shown in (b). Our result c d shows that stable beam condition with best temporal resolution can be realized for TR ARPES by using a longer gas cell, and an appropriate gas cell length can provide balanced condition for good beam intensity and good temporal resolution. [ABSTRACT FROM AUTHOR]