*2.2. Laser*

The laser shots were performed with the Hephaïstos facility at PIMM (Laboratory for Processes and Engineering in Materials and Mechanics, Paris, France). The platform is equipped with a Gaïa HP laser from THALES (Elancourt, France). It is composed of two Nd:YAG synchronized or delayed lasers, both emitting at 532 nm. It delivers up to 14 J with a Gaussian temporal profile with a full width half-maximum (FWHM) of 9 ns. The laser spot diameter is 3 mm through a diffractive optical element (DOE) and lens with a focal length of 148 mm [14,15]. The parameters chosen and the

range of laser intensity covered by this study are standard parameters for laser peening applications such as fatigue behavior improvement and the reduction of crack propagation by the induction of compressive residual stress. Energy and pulse duration were measured shot by shot. The spatial profile was measured every day.

Figure 2 shows a typical image and corresponding profile of the laser spot at the location of the target, demonstrating a uniformity of intensity below 6%. With these laser parameters, the power density reached up to 13 GW/cm<sup>2</sup> to achieve the breakdown threshold for all materials, which is the physical limit of the process.

**Figure 2.** (**a**) Spatial shape and (**b**) intensity profile for a 3-mm laser spot with a diffractive optical element (DOE) at the Hephaïstos facility.
