Time-, spin- and angle-resolved photoemission spectroscopy with a 10.7-eV laser at 1-MHz repetition rate

Spin- and angle-resolved photoemission spectroscopy (SARPES) is a powerful experimental technique that enables the complete determination of the spin structure in energy and momentum spaces. Combining SARPES with pump-probe laser techniques allows for imaging optically excited electron populations in the unoccupied bands and tracking ultrafast charge and spin dynamics in the time domain. However, in pump-probe SARPES, balancing high-energy photon flux with SARPES efficiency is challenging. High-energy photons are necessary to cover a wide k range, but probe photons for pump-probe SARPES are typically generated by high harmonic generation at kHz-class repetition rates using Ti-Sapphire lasers. Even if these photon fluxes are high enough to overcome low spin-detection efficiency, they lead to significant space charge effects in photoemission experiments [1]. As a result, photon flux must be reduced, causing the photoelectron accumulation time to become unrealistically long.

Here, I will present our state-of-the-art pump-probe SARPES setup. This apparatus is based on a SARPES system equipped with highly efficient very-low-energy-electron-diffraction (VLEED) spin detectors [2]. We combine this with bright 10.7-eV laser pulses at a high repetition rate (1 MHz), driven by harmonic generation of a Yb-based chirped-pulse amplified laser [3]. This setup enables us to obtain large SARPES signals sufficient to observe unoccupied spin-polarized bands in various materials and track their ultrafast electron and spin dynamics with the high-energy probe photon.