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Since its first demonstration in 1964 [1,2], two-photon photoemission (2PPE) spectroscopy at surfaces has become an efficient combination of femtosecond lasers with conventional angle-resolved photoelectron
spectroscopy (ARPES), allowing direct access to the ultrafast electron dynamics with energy and momentum resolution [3,4]. In strong contrast to conventional ARPES as well as the advanced time-
resolved ARPES measurements, in the 2PPE experiments the photon energy is specifically chosen to be lower than the surface work function in order to greatly suppress the one-photon photoemission process
[5]. As a consequence, each photoelectron can only obtain sufficient energy to escape from the surface upon the absorption of two photons. Besides the clear signatures of valence band electronic structure in the 2PPE spectra [6], the two-photon excitation pathway can go through unoccupied electronic states, thereby providing the straightforward detection of conduction band electronic structure of semiconductors [7], topological insulators [8], and strongly correlated materials [9]. In this talk, 2PPE spectroscopy and our recent development of first 2PPE experiments in Taiwan will be
presented. As a specific example, the momentum patterns of photoelectrons from the 2PPE processes on NiTe2 single crystals, which have been kindly provided by Prof. Chin Shan Lue and Dr. Chia-Nung Kuo at the Department of Physics, National Cheng Kung University, will be demonstrated [10]. Hereby we have identified the momentum patterns of the spin-orbit split Te 5px,y valence bands in NiTe2. As outlook, higher-
order photoemission processes involving three photons will be discussed [11].
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[2] M. C. Teich, J. M. Schroeer, and G. J. Wolga, Phys. Rev. Lett. 13, 611 (1964).
[3] H. Petek and S. Ogawa, Prog. Surf. Sci. 56, 239 (1997).
[4] M. Weinelt, J. Phys. Condens. Matter 14, R1099 (2002).
[5] J. H. Bechtel, W. L. Smith, and N. Bloembergen, Phys. Rev. B 15, 4557 (1977).
[6] A. Li et al., Phys. Rev. B 105, 075105 (2022).
[7] H. Tanimura et al., Phys. Rev. B 100, 035201 (2019).
[8] J. A. Sobota et al., Phys. Rev. Lett. 111, 136802 (2013).
[9] K. Gillmeister et al., Nat. Commun. 11, 4095 (2020).
[10] M. Singh et al., Appl. Phys. Lett. 128, 031601 (2026).
[11] F. Bisio et al., J. Phys.: Condens. Matter 23, 485002 (2011).