# Investigation of Pre-Pulse Effects on Ultrashort-Pulse Laser Interaction with Structured Targets
## 2 Experiment Details  
The experiments are performed at the High-Intensity Laser Laboratory in the Hebrew University of Jerusalem in Israel. The block diagram in Figure 1a shows the overview of the setup. The heart of the ultrashort-pulse laser system consists of a titanium-sapphire-based Spectra-Physics Solstice ACE frontend, capable of emitting both stretched and compressed pulses simultaneously. The stretched main interaction pulse (MP) (pulse-width \(>\) 150 ps, energy \(\approx\) 8 mJ) is amplified by a Nd:YAG pumped 4-pass Amplifier and compressed via a grating-based Compressor (both constructed in-house) to generate a pulse of width 40 fs (FWHM) (central wavelength, \(\lambda_{0}\approx\) 805 nm) with 100 mJ energy per pulse on target (\(\approx\)2.5 TW peak power) at a repetition rate of 10 Hz. Due to leakages and misalignments in the regenerative amplifier of the ACE frontend, the nanosecond temporal profile of a typical pulse consists of various pre-pulses and a pedestal of contrast as low as 10\({}^{-4}\), despite the presence of an external pulse cleaning Pockels cell before the 4-pass amplifier, as shown in Figure 1a. To understand the role of a single controllable pre-pulse with minimal influence from other pre-pulses or the pedestal, additional cleaning of the pulse is performed by placing a Thorlabs FGL850S filter after the high-power compressor, which acts like a single pass saturable absorber (SA). The effect of the filter thickness (\(\approx\)2 mm) on the compression is compensated by adjusting the grating distances in the compressor to achieve a pulse width of 40 \(\pm\) 2 fs. No significant change in the spectral properties of the compressed pulse is observed. The saturable absorber significantly improves the nanosecond and sub-nanosecond contrast of the MP, with the highest pre-pulse now having a contrast of \(\approx\)10\({}^{-7}\), deemed adequate for the current work. However, the maximum output energy of the pulse is reduced to \(\approx\)25 \(\pm\) 2 mJ on target, thereby limiting the main interaction pulse power to \(\approx\)0.6 TW for this work. An externally controllable artificial pre-pulse (APP) is derived from the 2 mJ compressed output of the ACE frontend. The timing of the pulse is adjusted with respect to the compressed main pulse (MP) using bounces on multiple mirrors and a long delay line (blocked together as Delay Line in Figure 1a) so that a delay of \(-\)14 ns to +0.5 ns can be achieved between the APP and MP (negative delay indicates that the APP arrives before the MP). The pulse width of the APP after the long delay line is \(\approx\)150 fs (after compensation by the compressor inside the ACE frontend). The APP energy is controlled using thin neutral density filters, which do not significantly affect the pulse width. The APP is made to co-propagate along the same path as the MP after the compressor by injecting it through an 8% pellicle beam splitter (PBS) that also allows simultaneous shot-to-shot monitoring of the MP and APP outside the interaction chamber (Figure 1a). Post PBS, both pulses are steered by the same optics into the interaction chamber (IC) of 60 cm diameter, which has various ports for interaction beam injection and optical diagnostics. Figure 1b shows the experimental setup inside the vacuum interaction chamber (IC). The MP and APP are guided and focused onto the target at an angle of incidence of \(\approx\)67.5\({}^{\circ}\) (\(\pm\)0.5\({}^{\circ}\)) with respect to the target normal (TN) using an f/1.5 (with respect to the MP) Off-Axis Parabola of 7.5 cm effective focal length. The MP had a full width at half maxima (FWHM) of \(\approx\)4.0 \(\pm\) 0.2 \(\upmu\)m at focus (measured with a CMOS camera and an appropriate imaging system). The APP beam diameter on the parabola is \(\approx\)18 \(\pm\) 1 mm, and the spot has a FWHM of \(\approx\)11 \(\pm\) 0.3 \(\upmu\)m at the MP focus. The target (described later) is mounted on a multi-axis motorized stage having micron-level precision and high repeatability for accurate alignment at the laser focus.