## 2 Probing the Vacuum Process
### Laser-Induced Birefringence and Diffraction of X-rays  
An XFEL can work as a useful probe on the vacuum process induced at the focal spot of a high-power laser. Since both lasers provide real photons, this is another real process corresponding to a different center-of-mass photon energy \(\sqrt{\omega_{1}\omega_{2}}=100\) eV with \(\omega_{1}=10\) keV and \(\omega_{2}=1\) eV. The high-power laser is often referred to as a pump laser in analogy to a conventional pump-probe experiment where the laser excites a sample and the probe studies the response. The change of the probe's wave vector induced by the scattering is called \"vacuum diffraction\" [30]. Furthermore, polarization flipping--which occasionally occurs in the scattering--adds tiny ellipticity to the linearly-polarized probe, and is called \"vacuum birefringence\" [31]. Figure 5(left) shows the principle of the experiment where the pump laser is radiated in a counter-propagating geometry. The probe and pump are focused on the same spot to enhance the diffraction signal that is scattered in a forward direction with a tiny angle. An efficient scheme of detecting photons that have both properties of flipped polarization and a finite diffraction angle was proposed in Reference [32]. In addition, a practical setup using an X-ray focusing lens and crossed polarizers can be found in Reference [9]. While there are many theoretical studies and proposals, no experiments have been put into practice.  
Regardless of detecting diffraction or birefringence, the key point of the experiment is the way to synchronize the probe and pump with respect to space and time. As for the timing adjustment, there is a working method using the fast electronic response of a GaAs film that can tune the timing within \(\sim\)1 ps [33]. However, to our knowledge, there is so far no efficient method to spatially overlap the pump and probe with both focused to \(\sim\)10 \(\upmu\)m, and thus a dedicated method needs to be found and established.  
For this purpose, we carried out a test measurement at SACLA in 2016. A probe XFEL and a 2.5-TW pump laser were focused to about 10 \(\upmu\)m by composite refractive lenses and an off-axis parabolic mirror, respectively. A zinc film with a thickness of 20 \(\upmu\)m was placed at the collision point to check their overlap (the detailed setup and procedures are described in Reference [34]). Their energy deposit on the film created a characteristic pattern of deformation that could be checked later by an optical microscope. Figure 5(right) shows a microscope image of the film after the timing adjustment and some iteration of the spatial overlapping. The XFEL was shot on the film from the back side of the image. The larger hole with a diameter of about 500 \(\upmu\)m was made by the focused pump laser, whereas a smaller one at the center was made by the penetration of the probe. The deviation between the two centers was found to be less than \(\sim\)10 \(\upmu\)m, showing the accuracy of this method [34].  
A PW-laser facility is under installation at SACLA and is scheduled to be available to users in 2018 [29]. The expected sensitivity to \(\sigma_{\gamma\gamma}\) is shown in Figure 4(right). By the time the laser system  
Figure 5: **(Left)** Principle of the experiment probing vacuum diffraction. **(Right)** Microscope image of a zinc film after the temporal and spatial adjustments. The rectangular hole at the center is made by the penetration of X-rays (40(H) \(\times\) 20(V) \(\upmu\)m). The large deformation overlapping the hole is made by a focused 2.5-TW laser (12(H) \(\times\) 10(V) \(\upmu\)m).  
becomes ready for use, our effort will be directed to the establishment of a synchronization scheme feasible for a higher pump power.