2. Status of spin- and angle-resolved photoelectron spectroscopy
with laser light at Laser and Synchrotron Research Laboratory
Spin-and angle-resolved photoelectron spectroscopy (SARPES) is known as a powerful technique
for studying spin-dependent electronic states in solids. Up to the 1990s, utilization of this technique
was strongly limited into mainly ferromagnetic materials; the main target was their large exchange
splitting (~several hundred meV) because the efficiency of spin detections was so low that one needs
to sacrifice both energy and angular resolutions. However, the recent progress of the spin detector
improves the efficiency and enables us to precisely investigate small spin-structures in non-magnetic
spin-orbit coupled materials and even its unique texture in momentum space, such as Rashba spinsplit systems and topological insulators. Recently, we have developed the SARPES apparatus using
vacuum-ultraviolet (h=6.994eV) lasers at Laser and Synchrotron Research Center (LASOR) in theInstitute for Solid State Physics (ISSP) [1]. Our SARPES machine is currently utilized to obtain
precise information on spin-dependent electronic structures near the Fermi level in solids (~severalmeV). We have started a project to construct the laser-SARPES machine from FY 2014 and joint
researches at this station have started from FY2015.
Our laser-SARPES apparatus consists of an analysis chamber, a carousel sample-bank chamberconnected to a load-lock chamber, and a molecular beam epitaxy (MBE) chamber. All of thesechambers are connected via UHV gate valves. Figure 1 represents the hemispherical electron analyzer,
which is custom-made ScientaOmicron DA30-L, modified to attach the very-low-energy-electrondiffraction (VLEED) type spin detectors. The electrons are currently excited by 6.994-eV photons,yielded by 6th harmonic of a high-power Nd:YVO4 quasi-continuous wave laser. A helium discharge
lamp (VG Scienta, VUV5000) is also available as a photon source. At the MBE chamber, samples canbe heated by a direct current heating or electron bombardment. The surface evaluating and preparing

Figure 1: The laser-SARPES machineat LASOR at ISSP [1]. The double
VLEED spin detectors are attached to ahemispherical analyzer (DA30-L,ScientaOmicron). Currently, 7-eV laser
for high energy resolution measurement
and 10.7eV pulsed laser for pumpprobe measurement are available.

instruments, such as evaporators, low energy electron diffraction, sputter-gun and quartz microbalance,
can be installed. At the carousel chamber, 16 samples can be stocked in the UHV environment. InFY2019, eleven research proposals from outside of the institute were accepted and conducted. Spinpolarized states were investigated in both bulk and surface of various topological materials includingmagnetic and superconducting ones, atomic layers, and ferromagnetic compounds.
Since FY2018, we have started to upgrade our laser-SAPRES apparatus to combine with a pompprobe laser technique. The laser system utilized for this project is a pulsed 10.7eV laser system,
developed by Kobayashi group in LASOR at ISSP [2, 3]. This laser system is based on the Yb-doped
fiver with the high repetition rate (1 MHz) with a pulse length of 270 fs. The light polarizations of
10.7eV can be selectively controlled by MgF2 half-wave plate. In addition to a great capability of thetime-resolution, our 10.7eV laser gives more wide momentum information in contrast to the low
photon energy source such as 7eV, which is demonstrated in Figure 2. Thus, this state-of-the-art pumpprobe SARPES apparatus can be widely used for studying unoccupied electronic states in spintronics
materials and their ultrafast carrier/spin dynamics.

Figure 2: The Fermi surface mapping
and band structure mapping for Bi thinfilms grown on Si(111) substrate,
observed by 10.7eV laser [2, 3]. Such aphoton energy can cover widemomentum information even aroundthe Brillouin zone corner.

References:[1] K. Yaji, A. Harasawa, K. Kuroda, S. Toyohisa, M. Nakayama, Y. Ishida, A. Fukushima, S.
Watanabe, C.-T. Chen, F. Komori and S. Shin, Rev. Sci. Instrum. 87, 053111 (2016).
[2] Z. Zhao and Y. Kobayashi, Opt. Exp. 25, 13517 (2017).
[3] Z. Zhao, K. Kuroda, A. Harasawa, T. Kondo, S. Shin and Y. Kobayashi, Chin. Opt. Lett. 17, 051406(2019).