# Investigation of Pre-Pulse Effects on Ultrashort-Pulse Laser Interaction with Structured Targets
## 1 Introduction  
The rapid scientific and technological progress in the field of high-power ultrashort-pulse lasers has enabled the investigation of light interaction with matter under extreme conditions [1, 2, 3, 4, 5, 6]. Research towards increasing laser absorption in solid targets has looked at tailoring the laser pulses and engineering the target morphology to control the scale length of the pre-plasma. In experiments, various target morphologies have been explored in this regard, like grating targets [7], nanostructured targets [8], metal nanoparticles [9], microstructures [10, 11], nanostructured foils [12], near-critical-density targets [13], nano-spheres [14], and others. Exotic targets like bacterial coating and amorphous snow grown on a substrate have also shown enhanced laser absorption compared with a plane target under similar laser conditions [15, 16]. Analytical models and numerical simulations have been developed to explain the enhanced ion acceleration in structured targets [17, 18, 19]. In most of the abovementioned cases, the experimental systems had high temporal contrast that allowed them to neglect the pre-plasma effects caused by the amplified spontaneous emission (ASE) and picosecond pedestal on the interaction. However, recent studies with controllable pre-plasma have demonstrated significant influence on electron heating and consequent changes in the production of secondary particles [20, 21, 22, 23] and radiation generation [6, 24].  
The effect of the ASE on proton acceleration has been extensively studied both theoretically [25, 26, 27, 28] and experimentally [29, 30, 31, 32, 33, 34] in the flat-foil targets. High ASE leads to expansion of the plasma at the rear surface of the target, which distorts the sharp solid-vacuum boundary and inhibits the accelerating field, leading to lower proton energies in thin-foil targets. When the thickness of the foil increases protons can benefit from ASE preheating [35]. However, the impact of pre-plasma on structured targets remains a subjectof ongoing research, with contradictory conclusions reported by various groups. Some experimental results [13,20] observed enhanced proton energy with pre-plasma, while in other works, it leads to inhibition of laser-plasma energy coupling and reduced proton energy [36,37].  
It appears that a more synergetic approach involving simultaneous control of the laser parameter and target morphology is required for better optimization of the laser absorption in the target. The series of works by [PERSON] et al. demonstrated that efficient energy coupling to the pre-plasma is possible from the front surface of a fractal-like amorphous snow target [15,38,39,40,41]. However, the fractal structure of amorphous snow remains too intricate for analysis and is strongly affected by the water vapor deposition conditions [42], affecting the shot-to-shot repeatability and proton beam divergence [40,43]. Therefore, in this study, we focused on understanding how the pre-pulse creates conditions that favor the optimal pre-plasma in targets with simplified, well-reproducible, and regular microstructures. This is performed by scanning the energy fluence of the controllable pre-pulse and its arrival time with respect to the main interaction pulse. We use optical emission spectroscopy to better comprehend the different pre-plasma regimes and their impact on proton acceleration. Particle-in-Cell (PIC) simulations are included to validate our experimental observations and offer potential explanations for the observed phenomena.