抄録
Non-linear optical effects have provided us with an abundance of phenomena that are not achievable in the range of conventional linear optics. Second harmonic generation (SHG) is a second-order non-linear optical effects that doubles the frequency of light (from hν to 2hν) and occurs with broken inversion symmetry, such as surfaces/interfaces. Taking advantage of the characteristics, in the range of visible light, it has been used not only as a technique of the up-conversion method but also as a tool for probing surfaces/interfaces selectively.
With the advent of X-ray Free Electron Laser (XFEL) such as SACLA, SHG experiments have been extended into the X-ray region [1]. By utilizing the inner-core shell excitation resonance, element selectivity was demonstrated by soft X-ray SHG spectroscopy measurements [2]. Although the soft X-ray SHG method has been expected to be valuable for materials science, it is difficult to observe the weak soft X-ray SHG signals because of the contamination by external effects, such as second-order light generated at the XFEL beamline. In this research, we developed a new detection system for soft X-ray SHG spectroscopy employing an ellipsometry method [3] that opens a new research field of non-linear X-ray spectroscopy [4,5], for example in Li-ion batteries [6], solar cells [7], and spintronics [8].
This system was constructed at soft X-ray beamline BL1 at SACLA (See Figures). The incident light was guided through various filters that reduce the intensity to prevent the sample damage (Al filter) and cut off the second-order light from the beamline (Si filter). The beam was focused before irradiation on the sample and reflected at an angle of 45o from the surface plane. The 2hν component of the beam from a sample was selectively reflected by a Ru/B4C multilayer mirror and subsequently detected at a multi-channel plate. A unit of the mirror and the detector rotates with a rotation angle θ around the optical path to selectively detect X-rays with specific polarization [9]. To demonstrate the system, a GaAs crystal was selected, and the incident photon energy was set to hν = 75 eV to meet the resonance condition with the As M-edge. By choosing the X-ray polarization, it was elucidated that pure SHG signals can be observed without specific filters that remove the second-order light from the beamline. Furthermore, the detection system becomes a highly sensitive SHG measurement apparatus when it is combined with a Wolter mirror that focuses soft X-ray beam into sub-μm size [10]. In this presentation, we explain the newly developed system in detail and introduce state-of-the-art soft X-ray non-linear spectroscopy research.
[1] A. Zong et al., Nat. Rev. Mater. 8, 224 (2023). [2] Sh. Yamamoto et al., Phys. Rev. Lett. 120, 223902 (2018). [3] T. Sumi et al., e-J. Surf. Sci. Nanotech. 20, 31 (2021). [4] E. Berger et al., Nano Lett. 21, 6095 (2021). [5] C. B. Uzundal et al., Phys. Rev. Lett. 127, 237402 (2021). [6] C. Woodahl et al., Nat. Mater. 22, 848 (2023). [7] M. Horio et al., Appl. Phys. Lett. 123, 031602 (2023). [8] T. Sumi et al., Appl. Phys. Lett. 122, 171601 (2023). [9] M. Araki et al., e-J. Surf. Sci. Nanotech. 18, 231 (2020). [10] S. Egawa et al., Opt. Express 23 33889 (2019).
