Investigation of Pre-Pulse Effects on Ultrashort-Pulse Laser Interaction with Structured Targets

Abstract

The role of pre-plasma in the efficient generation of protons by intense laser-matter interaction from structured targets is investigated. Optimal energy coupling between laser and plasma is found by varying the fluence and arrival time of an independently controllable ultrashort pre-pulse with respect to the main interaction pulse. The coupling is evaluated based on the energy of the accelerated protons. The accelerated proton energy is maximized at optimal pre-pulse delay and fluence conditions. Plasma emission spectrum and Particle-in-Cell simulations provide a possible explanation of the obtained experiment results.

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], nanospheres [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 [8,17,18,19]. 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 subject of 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 Zigler 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.

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 ≈ 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 (≈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 (≈2 mm) on the compression is compensated by adjusting the grating distances in the compressor to achieve a pulse width of 40 ± 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 ≈10⁻⁷, deemed adequate for the current work. However, the maximum output energy of the pulse is reduced to ≈25 ± 2 mJ on target, thereby limiting the main interaction pulse power to ≈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 ≈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 ≈67.5° (±0.5) 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 ≈4.0 ± 0.2 $\mathsf{\mu }$m at focus (measured with a CMOS camera and an appropriate imaging system). The APP beam diameter on the parabola is ≈18 ± 1 mm, and the spot has a FWHM of ≈11 ± 0.3 $\mathsf{\mu }$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. 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° 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° with respect to the target normal. The distance from the focal point on the target to the CR39 surface along the TN is ≈3 cm at its minimum and can be adjusted to longer distances to sample the protons over 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 ≈32° 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° 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 ≈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 ±1 $\mathsf{\mu }$m. The APP fluence and the MP intensity are calculated by accounting for the 67.5° projection on the target surface. Figure 1d shows the enlarged structured target design. Spikes are grown on the base pedestal, which is 75 $\mathsf{\mu }$m × 75 $\mathsf{\mu }$m × 30 $\mathsf{\mu }$m; the spike dimensions are 0.8 $\mathsf{\mu }$m × 0.8 $\mathsf{\mu }$m × 15 $\mathsf{\mu }$m, and the edge-to-edge distance between the spikes is 1.2 $\mathsf{\mu }$m. Each spike is grown on a small 1.4 $\mathsf{\mu }$m × 1.4 $\mathsf{\mu }$m × 1 $\mathsf{\mu }$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 $\mathsf{\mu }$m by the depth of field of the objective, and the smallest resolvable feature is ≈1 $\mathsf{\mu }$m in extent. The FWHMs of the MP and APP are also shown in Figure 1e for reference.

3. Experiment Results

Figure 2a–c show typical optical spectrum plots of the plasma emission from the laser target interaction at APP delay timings of −2, −5, and −14 ns, respectively, with APP fluence ranges of (I) no APP, (II) 0.1–0.2, (III) 0.4–0.6 and (IV) 2–5 J/cm². It is observed that at no-APP or low-APP fluence (conditions I and II), a strong narrow peak at ≈402 ± 2 nm is dominant at all time delays. At the highest APP fluence (condition IV), multiple broad peaks with wavelengths (>650 nm) start to dominate while the peak at 402 nm is attenuated. When the pre-pulse delay $\mathsf{\Delta }$t is set to −5 ns, the 402 nm peak remains dominant, but a distinct peak at wavelength 719 nm is also visible at moderate (III) and high fluences (IV) (Figure 2b). The spectral profile in case of $\mathsf{\Delta }$t 5 ns and moderate APP fluence (III) is shown in detail in Figure 2d. Apart from the broadened signal at 402 nm and signal close to 719 nm (wavelengths between 725 and 925 nm are filtered out), there is a distinct signal observed at ≈493 nm.

CR39 solid-state nuclear track detector plates were used as a proton energy (PE) diagnostic tool in the experiment due to their ability to provide both the energy and the spatial distribution of the protons. The use of overlapping aluminum-net filters enables proton energy discrimination, albeit coarse, to a certain extent. Figure 3a shows a typical aluminum-net filter attached to the surface of a 5 cm × 5 cm CR39 plate of 850 $\mathsf{\mu }$m thickness. The filter set consists of a base filter covering the entire surface of the CR39 and a grid net structure consisting of aluminum strips of 3 mm width. By choosing different combinations of aluminum foil thickness, both the range and resolution of proton energy measurement can be controlled. In a typical experiment, the delay between the APP and the MP is kept fixed, with the MP energy at ≈25 ± 2 mJ. Six CR39 plates are mounted on a rotating motor, and only the intended plate is exposed to the proton beam path, which is drawn schematically in Figure 1b, with the others being shielded. The APP fluence is varied between 0 (no APP) and ≈15 J/cm², centered around 0.4 J/cm²—the spikes optical damage threshold was found to be close to the snow optical damage threshold obtained by Shavit et al. [43]. The procedure is repeated at different time delays. Post-experiment, the CR39 plates are developed by etching. Figure 3b shows the result of an experiment at $\mathsf{\Delta }t\approx -5$ ns and APP fluence of 0.5 J/cm², where protons with energies in the range 2.5 MeV ≤ PE < 2.8 MeV were obtained, the highest in this work with MP energy of 25 mJ. The adjacent SEM image in Figure 3b shows typical holes created by the protons in detail (post-development). A counting algorithm implemented in MATLAB estimated the average flux of protons in this energy range to be ∼${10}^4$ cm⁻² per shot.

Proton energy ranges at various APP to MP time delays and APP fluence conditions were measured, and the lower bound value (e.g., 2.5 MeV for protons in the range 2.5 MeV ≤ PE < 2.8 MeV) was considered for statistics. Figure 3c shows the variation in proton energy with APP fluence (between 0.1 and 15 J/cm²) at selected time delays: (i) −2 ns (red square with dashed line), (ii) −5 ns (blue circle with dash-dot line), (iii) −8 ns (green diamond with dash-dot line). The data points are connected by a Bezier curve to aid in discerning the trend. It is observed that in the absence of APP, the proton energy is always below 250 keV, and is excluded from the semi-log plot. Each data point in the plot is the mean of five experiments repeated under similar experimental conditions. The error in proton energy measurement can be attributed to three main factors: (i) fluctuation in MP intensity due to alignment error on the morphology of the spike target (drawn schematically in Figure 1e), (ii) shot-to-shot instability of the MP and APP energies, and (iii) discrete proton energy measurement by aluminum filters ($\mathsf{\Delta }\approx 200$ keV). Figure 3d shows the variation in mean (and corresponding maximum) proton energy with time delay. The plot is generated by extracting the peak proton energy value (mean and maximum of 5 experiments) at each time delay. The plot shows a fast rise in the mean PE as the negative delay is increased, reaching a maximum at −5 ns, and then slowly decreasing. At $\mathsf{\Delta }t\approx -\infty$, i.e., when the MP arrived long after the APP had interacted with the target, the mean PE was found to be ∼250 keV.

4. Simulations

The experiment results indicate that plasma emission spectra and proton acceleration are sensitive to the initial conditions and temporal evolution of the pre-plasma. To validate the existence of optimal pre-plasma conditions and gain insight into the process, EPOCH-PIC [44] simulations are employed. A detailed numerical simulation of the pre-plasma generation by the pre-pulse is beyond the scope of this work. Hence, we focus on the verification of the proton acceleration with different pre-plasma conditions. In order to simulate the plasma expansion with various delay times, we assumed a constant pre-plasma temperature T_e = 0.28 eV during the expansion period, which leads to the 2.5 $\mathsf{\mu }$m/ns expansion speed for plastic targets. Thus, in a simplified hemispherical pre-plasma model, for a fixed APP fluence, the various time delays between APP and MB will change the size of the pre-plasma cloud and the scale length of pre-plasma, while maintaining the constant number of particles in the pre-plasma with fixed expansion velocity. We consider only the optimal fluence of 0.4 J/cm² for the total number of particles that can be estimated from the femtosecond laser ablation model described by Gamaly et al. [45]. The densities and plasma expansion are in good agreement with the experimental measurements of pre-plasma density with similar pre-pulse conditions (−4.7 ns, 10¹³ W/cm² [46]).

Figure 4a shows the 2D geometry setup in EPOCH to simulate the MP laser interaction with the structured target with a localized pre-plasma. The red arrow indicates the direction of the laser. The target structure is tilted so that the MP subtends an angle of ${60}^{\circ }$ with respect to the target normal. The structured target is modeled by CH₂ rectangular spikes identical to the structure shown in Figure 1d. The target is assumed to be pre-ionized to $H^{+}$ and $C^{2+}$, with a solid electron density of $n_e=80n_c$, where $n_c$ is the critical plasma density. We compared these results with simulations using the same target without pre-plasma, as well as with a flat, thick-foil target. The differences in the electron heating processes in the simulations from the flat target, structured target bulk, and pre-plasma are shown in Figure 4b. The electron temperature in the flat target is lower compared with that in the structured target. The simulations of the structured target, both with and without pre-plasma, showed similar bulk electron temperatures. However, the temperature of pre-plasma electrons in structured targets is much higher compared with the bulk of structured targets. This leads to the acceleration of electrons from the pre-plasma into the tips of the structured targets and stronger charge accumulation on the tips, which leads to the local field enhancement and generation of a stronger average accelerating field that is shown in Figure 4c. The figure shows that the accelerating field in the figure with a structured target is stronger and remains longer compared with that in the case without pre-plasma. This field enhancement mechanism, which is shown in Figure 4b,c, is described in more detail in the work of Schleifer et al. [41].

In order to emulate the effects of different delay times and compare them with the experiment results, we considered various plasma expansions (from no pre-plasma up to 33 $\mathsf{\mu }$m, which corresponds to the delay of ≈13 ns between MP and APP). The resulting proton energy cutoffs are shown in Figure 4d. The energy qualitatively aligns with the observed experimental results, showing an optimum pre-plasma delay close to −4 ns, characterized by a sharp rise at shorter delay times and a gradual decline after the peak is reached.

5. Analysis and Discussion

The proton energy plot in Figure 3c shows the optimal fluence, which is strong enough to produce the pre-plasma with a density that will affect the main beam, without destroying the fine structure of the target. For all APP delay times, the optimum fluence is close to the optical damage threshold 0.4–0.6 J/cm². By selecting the best time delay $\mathsf{\Delta }t=-5$ ns (Figure 3d), which results in the highest proton energy cutoff, we can compare experimental data with reconstructed numerical energy spectrum Figure 4d.

There is a good qualitative agreement between the experimental results and numerical simulations. Both exhibit similar behavior, with a sharp rise (likely caused by the rapid expansion of the pre-plasma in the early delay times) followed by a peak that is gradually decaying. Accurately capturing the fast dynamics of the early stages of interaction between the structured target and pre-plasma in simulations is challenging due to the complex expansion dynamics and sharp density gradients. However, the results clearly show lower proton energy cutoffs for the structured target without pre-plasma, as well as lower average accelerating field, as shown in Figure 4c. In experiments, the proton energy was gradually decreasing with longer pre-pulse delay times (≈6–12 ns); this trend was not fully reproduced by the numerical simulations. This decrease is likely due to the recombination and cooling of the pre-plasma over longer timescales, this was not included in the PIC pre-plasma model.

The observed optical spectrum provides insights into plasma dynamics with low and high APP fluences and serves as a possible indicator of nonlinear effects associated with different heating mechanisms, such as J × B heating [47] and resonance absorption [48,49]. Optimal heating of the pre-plasma electrons should be accompanied by strong nonlinear signals, which we anticipate observing in the spectrum.

The peak observed at 402 nm can be associated with the second harmonic signal, which may suggest the presence of a short sub-micron pre-plasma scale length, with a sharp vacuum-solid boundary. In such conditions, resonance absorption is the dominant laser–plasma interaction mechanism [49,50]. The multiple peaks at wavelengths greater than 650 nm can be primarily attributed to Stimulated Raman Scattering (SRS). It is interesting to note that the TPD signals (if any) are rather weak compared with SRS-related peaks, even though TPD has a lower intensity threshold [51,52]. This may be attributed to the fact that the angle corrected critical plasma density value $\left(n_{c}co{s}^2\left(\theta \right)\approx 2.6\times {10}^{20}{\mathrm{cm}}^{-3}\right)$ is lower than the quarter-critical density value $\left({\displaystyle \frac{n_c}{4}}\approx 4.4\times {10}^{20}{\mathrm{cm}}^{-3}\right)$, where the critical plasma density $n_c\approx 1.7\times {10}^{21}{\mathrm{cm}}^{-3}$ for ${\lambda }_0\approx 805$ nm. This implies that the obliquely incident MP would propagate through the low-density pre-plasma prior to interacting with an overdense structured target.

The optical spectrum data indicates the existence of an optimum pre-plasma scale length conducive to efficient laser–plasma interaction. Under optimal APP fluence conditions (case III in Figure 2b), the signature from SRS is visible, especially at the $\mathsf{\Delta }t$ of −5 ns, which requires the plasma density lower than the quarter critical density [49]. The spectral profile under clear SRS conditions of $\mathsf{\Delta }t$ and APP fluence is shown in detail in Figure 2d. Apart from the broadened second harmonic signal at 402 nm and part of the SRS signal close to 719 nm (wavelengths between 725 and 925 nm are filtered out), a distinct signal is observed at approximately 493 nm. To achieve optimal conditions, it is crucial to find a balance between the effects of the pre-plasma and the structured target. If the pre-pulse fluence is too low, the pre-plasma impact will be minimal; conversely, excessive pre-pulse fluence will destroy the microstructure of the target. In high-fluence cases, the microstructure of the target was destroyed before the main pulse arrived, despite the rich parametric instability and nonlinear mixing processes observed in the optical emission spectrum (case IV). This is validated by proton energy diagnostics. This allows for the identification of an optimal condition where both the pre-plasma and structured target effects can coexist and be utilized effectively.

6. Conclusions

We have experimentally investigated the role of the fluence and arrival delay time of an independently controllable ultrashort pre-pulse in creating appropriate pre-plasma conditions for optimized energy coupling. A distinct optimal condition is found for the pre-pulse fluence and arrival delay. At optimum pre-pulse conditions, proton energy peaks ≈2.5 MeV, with a relatively low main pulse power of 0.6 TW on target. Analysis of the plasma emission optical spectrum shows a distinct feature that differentiates between different APP regimes. PIC simulations show good qualitative agreement with the experimental data, while the analysis of the accelerating field and the electron energy spectrum of the pre-plasma provides a deeper understanding of the mechanisms responsible for the observed effects. The simulations confirm that the optimal size and density profile of the pre-plasma are critical in enhancing energy coupling between the main pulse and the target.