# Investigation of Pre-Pulse Effects on Ultrashort-Pulse Laser Interaction with Structured Targets
## 2 Experiment Details
The generated protons are characterized using plates of CR39 solid-state nuclear track detector (referred to as CR39 henceforth) in conjunction with aluminum-foil filters, mounted together on a motorized wheel. The normal to the CR39 surface (CRN) subtends an angle of 18\({}^{\circ}\) with respect to the target normal (TN), which is shown on Figure 1b. This is based on results from earlier experiments that showed the energetic protons are ejected at an angle between 15-20\({}^{\circ}\) with respect to the target normal. The distance from the focal point on the target to the CR39 surface along the TN is \(\approx\)3 cm at its minimum and can be adjusted to longer distances to sample the protons over  
Figure 1: **(a)** Block diagram schematic of the experimental setup. MP: main pulse, APP: artificial pre-pulse, SA: saturable absorber, PBS: pellicle beam splitter, IC: interaction chamber. **(b)** The experimental setup inside the vacuum interaction chamber. TN: target normal, CRN: normal to the CR39 proton detector. **(c)** SEM image of the printed structured target. Red-ellipse: MP projection, yellow-dashed-ellipse: APP projection, dark-red arrow: laser polarization axis. **(d)** Enlarged part of planned target design with focus on spike. **(e)** The laser focusing geometry on the spike target showing the typical feature size on which the laser is focused, as well as the error in the alignment due to the depth of field of the imaging objective with respect to its focal plane. The red dashed lines demonstrate the FWHM of APP and MP. a larger detector area. The alignment of the target features at the laser focus is monitored by a target imaging objective inside the vacuum chamber, along with an imaging system located outside. Optical plasma emissions are collected at an angle of \(\approx\)32\({}^{\circ}\) with respect to the TN by a lens of 15 cm focal length and passed through an 800 nm dielectric mirror with a transparent back surface to filter out the radiation between 725 and 925 nm, and subsequently guided outside the chamber to an optical spectrometer. Structured targets are 3D printed on the sapphire substrate via two-photon polymerization (TPP) on a Photonic Professional Nanoscribe 3D Laser Lithography System utilizing hydrocarbon-based photoresist polymer Ip-S. Figure 1c shows a typical scanning electron microscope (SEM) image of a structured target in an Environmental-SEM (FEI-Quanta 200). The typical MP (red-ellipse) and APP (yellow-dashed-ellipse) focal area projections (corresponding to an angle of incidence of 67.5\({}^{\circ}\) on the substrate) on the target are shown in the same figure. The dark-red arrow indicates the laser polarization direction, as shown in Figure 1c. The angle-corrected APP area is \(\approx\)7.6 times larger than the corresponding MP area, thereby ensuring their overlap, notwithstanding the shot-to-shot spatial jitter, which is usually less than \(\pm\)1 \(\upmu\)m. The APP fluence and the MP intensity are calculated by accounting for the 67.5\({}^{\circ}\) projection on the target surface. Figure 1d shows the enlarged structured target design. Spikes are grown on the base pedestal, which is 75 \(\upmu\)m \(\times\) 75 \(\upmu\)m \(\times\) 30 \(\upmu\)m; the spike dimensions are 0.8 \(\upmu\)m \(\times\) 0.8 \(\upmu\)m \(\times\) 15 \(\upmu\)m, and the edge-to-edge distance between the spikes is 1.2 \(\upmu\)m. Each spike is grown on a small 1.4 \(\upmu\)m \(\times\) 1.4 \(\upmu\)m \(\times\) 1 \(\upmu\)m pedestal for stability. Figure 1e shows the typical laser focus alignment on the target that is attempted before each shot. The accuracy of the target alignment at the laser focus is limited to 3.6 \(\upmu\)m by the depth of field of the objective, and the smallest resolvable feature is \(\approx\)1 \(\upmu\)m in extent. The FWHMs of the MP and APP are also shown in Figure 1e for reference.