２．Status of SARPES
Spin and angle-resolved photoemission spectroscopy (SARPES) is a powerful experimental
technique to reveal various information about the occupied electronic states in solids, including theirenergy, momentum, and spin. In recent years, the demand for spin-resolved measurements has beengetting high because of much interest in Rashba surface states and topological materials with spinpolarized electronic structures derived from strong spin-orbit interactions. These experiments require
high energy resolution and enough photoelectron yield rates to detect their small energy scales(~several meV). Following these demands and requirements, we have developed a high-energy
resolution SARPES apparatus using a vacuum-ultraviolet (6.994-eV) laser and very-low-energyelectron-diffraction (VLEED) type spin detectors at the Laser and Synchrotron Research Center(LASOR) in the Institute for Solid State Physics (ISSP) [1]. Our SARPES apparatus is currently
utilized to obtain precise information on fine distributed spin-dependent electronic structures near theFermi level in solids. We started the project to construct the laser-SARPES apparatus in FY 2014 andjoint research at this station started in 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 a custom-made ScientaOmicron DA30-L, modified to attach the VLEED type spin detectors.
The electrons are currently excited by 6.994-eV photons, yielded by the 6th harmonic of a high-power
Nd:YVO4 quasi-continuous wave laser. A helium discharge lamp (VG Scienta, VUV5000) is alsoavailable as a photon source. At the MBE chamber, samples can be heated by a direct current heatingor electron bombardment. The surface evaluating and preparing instruments, such as evaporators, lowenergy electron diffraction, sputter-gun, and quartz microbalance can be installed. At the carouselchamber, 16-samples can be stocked in the UHV environment. Spin-polarized states were investigatedin both bulk and surface of various topological materials including magnetic and superconducting ones,atomic layers, and ferromagnetic compounds.
Spin-resolved technique with highly efficient photoelectron yield rates has advantages not only inhigh-resolution measurements but also in time-resolved

Figure 1: The laser-SARPES apparatus at LASOR
in ISSP [1]. The double VLEED spin detectors areattached to a hemispherical analyzer (DA30-L,ScientaOmicron). Currently, 7-eV laser for highenergy resolution measurement and 10.7eV pulsed
laser for pump- probe measurements are available.

measurements based on pump-probe methods. A combination of SARPES with pump-probe laser
techniques makes it possible to image the optically excited spin-polarized electron population in theunoccupied bands in energy-momentum space and track the dynamics of charge and spin in theultrafast time-domain, which is useful for studying novel optical responses of many materials such as
topological and spin-valley materials. Since FY2018, we have started to upgrade our laser-SAPRES
apparatus by combining a pomp-probe laser, setting up the system named time-resolved SARPES (trSARPES). This project requires a short pulse laser, and thus we newly installed a pulsed 10.7-eV laser
system [2,3] developed by the Kobayashi group at ISSP’s LASOR. This fundamental laser system isytterbium fiber-based, achieving a 270-fs, 1-MHz repetition rate, and high power by a chirped pulse
amplification technique, which allows the pump-probe measurements with high photoelectron yieldrates. In addition to a great capability of the time-resolution, our 10.7-eV laser gives more wide
momentum information in contrast to the low photon energy sources such as 7-eV.
The development progressed, and now tr-SARPES measurements have been realized. The 10.7-eVlaser as the probe photon is obtained as the third harmonic generation in Xe gas, and a unique Xe gaschamber has been developed for the system up to the monochromatization and the focusing system.
In addition, the light polarization of the 10.7-eV laser can be selectively controlled by MgF2 half-wave
plate. The pump photon is selectable from 1.19-eV or 2.38-eV for a wide variety of band-gap materials.
The beamline is designed as a vacuum chamber after 10.7-eV generation, and it is connected to theUHV analyzer chamber through a differential pumping system without a vacuum window.
Furthermore, some mirror and lens in the vacuum beamline are movable, allowing the system to switch
between 7-eV and 10.7-eV beamlines. In this way, this system has made tr-SARPES possible, which
is demonstrated in Fig. 2. This state-of-the-art tr-SARPES apparatus can be widely used for studying
unoccupied electronic states in spintronics materials and their ultrafast carrier/spin dynamics.

Figure 2: Time-resolved ARPES (a) andtr-SARPES mapping (b) of unoccupied
topological surface state in Sb2Te3,observed by 10.7-eV laser after 1.19-eVlaser pumping.

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).