## 2 Experimental Setup
The shadows present on the images come from objects placed between the target and the Spectralon plate. (**c**) Images of the transmitted laser beam on another Spectralon plate during DPM shots: (**c1**) calibration shot (without target), (**c2**) shot on a 10 nm Si foil, and (**c3**) shot on a 50 nm Si foil. Colormaps are normalized to the maximum count in the calibrated shot shown in (**c1**). Figure 2: (**a**) Laser focal spot measurement (at low power) on a DPM shot. The intensity is calculated from the spatial integration of the energy in each pixel, assuming that the full laser energy is contained in the image and taking into account the laser pulse duration. (**b**) Encircled laser energy distribution. The laser irradiated the PMs at 45\({}^{\circ}\) incidence angle, with elliptical spots of dimensions \(\sim\)4.5 mm \(\times\) 6.3 mm on the PM closest to the focusing OAP and \(\sim\)3.5 mm \(\times\) 5 mm on the second PM. The peak intensity on the PMs reached \(\sim\)5.3 \(\times\) 10\({}^{13}\) W cm\({}^{-2}\), while the prepulse plateau intensity was \(\sim\)2.6 \(\times\) 10\({}^{10}\) W cm\({}^{-2}\). These values are, respectively, well above and below the ionization threshold (\(\sim\)2.5 \(\times\) 10\({}^{12}\) W cm\({}^{-2}\), considering the \(\sim\)750 fs prepulse duration) of the fused silica material of the PM [49]. The on-target intensity contrast between the prepulse plateau and pulse maximum was then increased by the square of the ratio between the reflection factor of the ionized PM (\(\sim\)0.52) and the antireflection coating factor of each PM (\(\sim\)10\({}^{-3}\)), i.e., by a factor of \(\sim\)2.7 \(\times\) 10\({}^{5}\). This would bring the prepulse plateau intensity down to \(\sim\)4 \(\times\) 10\({}^{12}\) W cm\({}^{-2}\), causing minimal, if any, disturbance to the foil targets. This prediction is supported by the observed smooth spatial profiles of the protons accelerated from targets as thin as 20 nm (see below). These profiles exhibit no modulations, in contrast to previous findings [34; 40; 41]. This difference may be linked to the fact that the latter studies employed longer laser pulses than ours, leaving more time for the target surface to deform during ion acceleration. Figure 3(b1) shows a typical image of the specularly reflected beam, as obtained on the Spectralon plate without a DPM. The beam profile appears to be diffuse and of low intensity, meaning that the laser beam was reflected off an extended preplasma, as a result of the prepulse-induced target expansion. By contrast, the reflected beam profiles recorded on DPM shots (see Figure 3(b2)) were much more intense and collimated, suggesting that the laser interacted with a sharp target surface. This behavior is consistent with previous experiments using DPMs [39; 43]. Figure 3(c1) displays the transmitted laser beam profile measured with a DPM in the absence of a target. A very similar image (Figure 3(c2)), associated with a laser transmission rate of \(\sim\)88%, was recorded when shooting at the thinnest (10 nm) Si foils, in which case no proton acceleration was detected. This indicates that, despite the enhanced contrast achieved by the DPM, such ultrathin targets remain vulnerable to the ASE. However, when the target thickness was increased to 50 nm, fast protons were measured up to \(\sim\)20 MeV while the laser transmission rate plummeted to \(\sim\)0.5% (see Spectralon image in Figure 3(c3)).