## 4 Recent Progress in Beamlines  
### Optical Laser Pump with Various Wavelengths and XFEL Probe Experiments  
The femtosecond X-ray scattering (FXS) endstation at the hard X-ray beamline is dedicated to time-resolved X-ray diffraction experiments for single-crystal materials [20]. The optical laser pump and X-ray probe experiments enable the studying of ultrafast dynamics of the four fundamental physical degrees of freedom (lattice, charge, spin, and orbital), for which behaviors in the time domain bear intriguing physics associated with photoexcited states of matter. The fact that these physical degrees of freedom are coupled to one another makes it challenging to identify the individual origin accounting for the photoinduced change of the X-ray diffraction signal. An effective way to resolve this difficulty is employing an optical laser pump with different wavelengths that allows for selective photoexcitation for the individual degrees of freedom. For an insulating material with an electronic band gap of 1.55 eV, the laser beam with wavelength (\(\lambda_{\text{optical}}\)) = 800 nm excites the valence electrons across the band gap. On the other hand, for a material with an optical phonon state at 82 meV, the beam with \(\lambda_{\text{optical}}\) = 15 \(\upmu\)m dominantly provokes the phonon mode. Therefore, it becomes possible to discriminate the trigger of the transient X-ray diffraction and unveil the underlying physics. The FXS endstation has made a continuous effort to establish optical laser sources in a wide range of wavelengths (\(\lambda_{\text{optical}}\)). Figure 6a summarizes the optical laser wavelengths and their maximum energies realized at the FXS endstation so far [43]. The optical laser provides an 800 nm wavelength fundamental beam (the maximum energy = 3 mJ/pulse), which is also exploited to produce the second (400 nm) and third (266 nm) harmonic beams. In addition to these wavelengths, an optical parametric amplifier (TOPAS Prime, Light Conversion Ltd., Lithuania) is used in combination with a frequency mixer and a difference frequency generation unit to generate the laser beam in the \(\lambda_{\text{optical}}\) range between 240 nm and 15 \(\upmu\)m. In this case, the beam energy becomes orders of magnitude smaller (several or tens of microjoules) than the fundamental \(\lambda_{\text{optical}}\), but the maximum fluence of \(\sim\)10-10\({}^{2}\) mJ/cm\({}^{2}\) is still achievable by focusing the laser spot. The FXS endstation has a future plan to extend the available \(\lambda_{\text{optical}}\) range further up to several hundreds of microns based on the plasma source [44], along with optical rectification using a LiNbO\({}_{3}\) crystal [45]. This \(\lambda_{\text{optical}}\) range corresponds to the energy region where antiferromagnets typically host magnon excitations. This is considered essential for studying the influence of the magnon on the physical degrees of freedom mirrored in the X-ray diffraction signals. Figure 5: Measured intensities of two-color FELs. In parallel with establishing a variety of optical laser sources, it is important to effectively deliver the laser beam to the sample on a goniometer. In particular, for the laser beam with an \(\lambda_{\text{optical}}\) of tens or hundreds of microns, the beam intensity is substantially reduced by air scattering or absorption by water molecules in the air. A vacuum sample chamber or a beam path with flowing dried gas is required in the design while avoiding any collision with the four-circle diffractometer. In addition, one needs to place other optical components close to the sample in order to manipulate the optical laser path and the spot size feasible for the pump-probe experiment. The FXS endstation uses an optical table near the diffractometer and installs several motorized stages to optimize the position of the optical components (Figure 6b).