## 1 Introduction  
Since their inception in the early 2000s, laser-driven ion beams have opened novel pathways compared to those powered by traditional accelerators [1; 2; 3]. Owing to their very compact size (\(\sim\)100 \(\upmu\)m), ultrafast timescales (\(\sim\)ps), and very high currents (\(\sim\)kA) at the source, these beams enable a wide range of applications, spanning flash radiography [4], radiobiology [5], nuclear physics [6], and even ion therapy [7]. They can also be leveraged to generate bright neutron sources [8; 9; 10; 11], which already outperform traditional neutron sources in terms of their intensity [12] and hold promise for material probing [13] and astrophysics [14]. Over the past 20 years, research has shown that laser pulses focused at relativistic intensities (\(>\)10\({}^{18}\) W cm\({}^{-2}\)) onto thin solid foils (e.g., a few \(\upmu\)m thick) can generate fast (i.e., up to \(\sim\)150 MeV nowadays [15]) and highly directional ion beams. This process, named target normal sheath acceleration (TNSA) [16], relies on the strong (\(\sim\)10\({}^{12}\) V m\({}^{-1}\)) electrostatic sheath fields induced by the laser-generated hot electrons [17] at the target surfaces. Typically, TNSA is most efficient at the nonirradiated target backside [18; 19; 20], where it produces ion beams with a high particle flux (e.g., \(\sim\)10\({}^{10}\)- 10\({}^{12}\) particles exceeding a few MeV) and a relatively small angular divergence (\(\sim\)15-30\({}^{\circ}\) depending on the energy [21]) [22; 23]. Moreover, different kinds of ions can be accelerated simply by changing the target material [24]. Now that we are at the dawn of the multi-petawatt (PW) laser era [25], the vast body of theoretical studies [1; 2; 26] tells us that further progress should be expected via little-explored, yet likely more efficient ion acceleration processes, i.e., radiation pressure acceleration (RPA) [27; 28; 29], breakout-afterburner (BOA) [30], and relativistically induced transparency (RIT) [31; 32; 33]. Numerical simulations predict better scaling of the maximum proton energy with laser intensity when utilizing ultrathin (nanometer-range) targets. In experiments, however, triggering the above mechanisms in a controlled way remains very challenging because the amplified spontaneous emission (ASE) pedestal, extending over picosecond or sometimes nanosecond scales ahead of the peak of the laser pulse, can easily preheat or even fully expand the thin target (less than a few microns) [22], resulting in weak or no acceleration at all. Plasma mirrors (PMs) are a well-known solution to eliminate the ASE and hence to improve the temporal laser contrast, defined as the ratio of the peak intensity to the ASE intensity [34; 35]. PMs are commonly made of polished glass slabs or liquid crystals [36] through which the low-intensity ASE light is transmitted. The subsequent rise in laser intensity eventually causes the irradiated PM surface to be ionized and, once the local electron density becomes overcritical, to reflect the main pulse. The resulting higher laser contrast mitigates the formation of a preplasma at the target front, in which the laser can filament [37], and thus allows ultrathin foils to survive until the laser peak, leading to interaction conditions suitable for efficient proton acceleration. For example, using a PM to achieve a high laser contrast of 10\({}^{10}\), laser pulses of 40 fs duration and 1 J energy have been reported to drive protons up to 10 MeV energies from 20-30 nm thick foils [38; 39]. Yet, such ultrathin targets tend to result in filamentary proton beam profiles, as compared to the smooth profiles obtained with micrometer-range targets [34; 40; 41]. Concerning energy conversion efficiency, standard TNSA with \(\sim\)\(\upmu\)m thick foils is typically limited to \(\sim\)1% efficiency for joule-class lasers [3; 40], which can be boosted up to \(\sim\)3% if a hybrid TNSA/RPA regime is realized using thinner foils [42].