The acceleration of ions from ultra-thin foils has been investigated using 250 TW, sub-ps laser pulses, focused on target at intensities up to 3×10 20 W cm −2 . The ion spectra show the appearance of narrow band features for proton and Carbon peaked at higher energy (in the 5-10 MeV/nucleon range) and with significantly higher flux than previously reported. The spectral features, and their scaling with laser and target parameters, provide evidence of a multispecies scenario of Radiation Pressure Acceleration in the Light Sail mode, as confirmed by analytical estimates and 2D Particle In Cell simulations. The scaling indicates that monoenergetic peaks with more than 100 MeV/nucleon energies are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.

We report on the generation of a narrow divergence (θ ≈ 2.5 mrad), multi-MeV (EMAX = 18 MeV) and ultra-high brilliance (≈ 2 × 10 19 photons s −1 mm −2 mrad −2 0.1% BW) γ-ray beam from the scattering of an ultra-relativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude a0 ≈ 2). The spectrum of the generated γ-ray beam is measured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear evidence of the onset of non-linear Thomson scattering. The photon source has the highest brilliance in the multi-MeV regime ever reported in the literature.The generation of high-quality Multi-MeV γ-ray beams is an active field of research due to the central role of these beams not only in fundamental research [1], but also in extremely important practical applications, which include cancer radiotherapy [2,3], active interrogation of materials [4], and radiography of dense objects [5]. As an example, Giant Dipole Resonances of most heavy nuclei occur in an energy range of [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], exciting photofission of the nucleus.Different mechanisms have been proposed to generate γ-ray beams with these properties, including bremsstrahlung emission, synchrotron emission, and Compton scattering. Bremsstrahlung sources are routinely used for medical applications, and exploit electron beams accelerated by linear accelerators (LINAC) [7]. Laser-driven bremsstrahlung sources, whereby the electron beam is generated via laser-wakefield acceleration (LWFA) [8] have also been recently reported [5,9,10]. However, the relatively broad divergence and source size limit the maximum brightness achievable with this technique and a more promising physical mechanism in this respect has been identified in Compton scattering. Laserdriven electron beams with energy per particle of the order of the GeV are now routinely available in the laboratory [8], allowing for the possibility of all-optical and compact Compton-scattering sources [11,12].Previous investigations of laser-driven Compton scattering have mostly focused on the linear regime, i.e. whenever the dimensionless intensity of the laser pulse is less than 1 (a 0 < 1, whereby a 0 = eE L /(m e ω L c), with E L and ω L being the laser electric field and central frequency, respectively, and m e being the electron rest mass) [13,14] and report on γ-ray energies ranging from a few hundreds of keV [13] up to 3-4 MeV [14]. Three main factors can in principle be modified in order to increase the energy of the generated photons: the electron Lorentz factor (γ e ), the laser photon energy ω L , or the laser intensity a 0 . The mean energy of the generated photons can in fact be estimated as:with f (a 0 ) ≈ 1 for a 0 1 andLiu and collaborators recently reported on an increase in photon energy (up to 8-9 MeV) by frequency converting the scattering laser up to its second harmonic (thus increasing ω L in Eq. 1) [15]. However, using a higher laser frequency for scattering significantly ...

Laser-driven coherent extreme-ultraviolet (XUV) sources provide pulses lasting a few hundred attoseconds 1,2 , enabling real-time access to dynamic changes of the electronic structure of matter 3,4 , the fastest processes outside the atomic nucleus. These pulses, however, are typically rather weak. Exploiting the ultrahigh brilliance of accelerator-based XUV sources 5 and the unique time structure of their laser-based counterparts would open intriguing opportunities in ultrafast X-ray and high-field science, extending powerful nonlinear optical and pump-probe techniques towards X-ray frequencies, and paving the way towards unequalled radiation intensities. Relativistic laser-plasma interactions have been identified as a promising approach to achieve this goal 6-13 . Recent experiments confirmed that relativistically driven overdense plasmas are able to convert infrared laser light into harmonic XUV radiation with unparalleled efficiency, and demonstrated the scalability of the generation technique towards hard X-rays 14-19 . Here we show that the phases of the XUV harmonics emanating from the interaction processes are synchronized, and therefore enable attosecond temporal bunching. Along with the previous findings concerning energy conversion and recent advances in high-power laser technology, our experiment demonstrates the feasibility of confining unprecedented amounts of light energy to within less than one femtosecond.The nonlinear response of matter exposed to intense femtosecond laser pulses gives rise to the emission of highfrequency radiation at harmonics of the laser oscillation frequency. If the harmonics are phase-locked, their superposition results in a train of attosecond bursts 20 . The concept has been so far successfully implemented in atomic gases 21 , and culminated in isolated attosecond pulses by using few-cycle laser drivers 1,2 . The low generation efficiency of harmonic radiation from atoms has motivated research into alternative concepts. Dense, femtosecond-laser-produced plasmas hold promise of converting laser light into coherent harmonics with much higher efficiency and of exploiting much higher laser intensities, because the plasma medium-in contrast to the atomic emitters-imposes no restriction on the strength of the laser field driving the harmonics [6][7][8][9][10][11][12][13] . Recent experimental studies of harmonics produced from overdense plasmas impressively corroborate several theoretical predictions: the high conversion efficiency 19 , the favourable scalability of the generation technique towards high photon energies 14,16,19 and excellent divergence due to the spatial coherence of the generated harmonics 19,22 . Whether the high-order harmonics that are produced in overdense plasmas • gold-coated off-axis parabolic mirror with the same focal length as the laser focusing parabola. The recollimating mirror is mounted on a flipper stage for easy withdrawal, thus enabling the spectral characterization of the emitted XUV light. Thin metal filters (typically 150 nm Al, In or Sn)...