No abstract

Plasma mirrors are devices capable of switching very high laser powers on subpicosecond time scales with a dynamic range of 20–30 dB. A detailed study of their performance in the near-field of the laser beam is presented, a setup relevant to improving the pulse contrast of modern ultrahigh power lasers (TW–PW). The conditions under which high reflectivity can be achieved and focusability of the reflected beam retained are identified. At higher intensities a region of high specular reflectivity with rapidly decreasing focusability was observed, suggesting that specular reflectivity alone is not an adequate guide to the ideal range of plasma mirror operation. It was found that to achieve high reflectivity with negligible phasefront distortion of the reflected beam the inequality csΔt<λLaser must be met (cs: sound speed, Δt: time from plasma formation to the peak of the pulse). The achievable contrast enhancement is given by the ratio of plasma mirror reflectivity to cold reflectivity.

Overdense plasmas are usually opaque to laser light. However, when the light is of sufficient intensity to drive electrons in the plasma to near light speeds, the plasma becomes transparent. This process-known as relativistic transparency-takes just a tenth of a picosecond. Yet all studies of relativistic transparency so far have been restricted to measurements collected over timescales much longer than this, limiting our understanding of the dynamics of this process. Here we present time-resolved electric field measurements (with a temporal resolution of ∼50 fs) of the light, initially reflected from, and subsequently transmitted through, an expanding overdense plasma. Our result provides insight into the dynamics of the transparent-overdense regime of relativistic plasmas, which should be useful in the development of laser-driven particle accelerators, X-ray sources and techniques for controlling the shape and contrast of intense laser pulses.

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)...

High-order harmonics and attosecond pulses of light can be generated when ultraintense, ultrashort laser pulses reflect off a solid-density plasma with a sharp vacuum interface, i.e., a plasma mirror. We demonstrate experimentally the key influence of the steepness of the plasma-vacuum interface on the interaction, by measuring the spectral and spatial properties of harmonics generated on a plasma mirror whose initial density gradient scale length L is continuously varied. Time-resolved interferometry is used to separately measure this scale length.

Ultraintense laser pulses with a few-cycle rising edge are ideally suited to accelerating ions from ultrathin foils, and achieving such pulses in practice represents a formidable challenge. We show that such pulses can be obtained using sufficiently strong and well-controlled relativistic nonlinearities in spatially well-defined near-critical-density plasmas. The resulting ultraintense pulses with an extremely steep rising edge give rise to significantly enhanced carbon ion energies consistent with a transition to radiation pressure acceleration.

When a pulse of light reflects from a mirror that is travelling close to the speed of light, Einstein's theory of relativity predicts that it will be up-shifted to a substantially higher frequency and compressed to a much shorter duration. This scenario is realized by the relativistically oscillating plasma surface generated by an ultraintense laser focused onto a solid target. Until now, it has been unclear whether the conditions necessary to exploit such phenomena can survive such an extreme interaction with increasing laser intensity. Here, we provide the first quantitative evidence to suggest that they can. We show that the occurrence of surface smoothing on the scale of the wavelength of the generated harmonics, and plasma denting of the irradiated surface, enables the production of high-quality X-ray beams focused down to the diffraction limit. These results improve the outlook for generating extreme X-ray fields, which could in principle extend to the Schwinger limit

The generation of ultra-relativistic positron beams with short duration (τ e + ≤ 30 fs), small divergence (θ e + 3 mrad), and high density (n e + 10 14 − 10 15 cm −3 ) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and γ-rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laserdriven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.Creating and characterizing high-density beams of relativistic positrons in the laboratory is of paramount importance in experimental physics, due to their direct application to a wide range of physical subjects, including nuclear physics, particle physics, and laboratory astrophysics. Arguably, the most practical way to generate them is to exploit the electromagnetic cascade initiated by the propagation of an ultra-relativistic electron beam through a high-Z solid. This process is exploited to generate low-energy positrons in injector systems for conventional accelerators such as the Electron-Positron Collider (LEP) [1]. In this case, an ultra-relativistic electron beam (E e − ≈ 200 MeV) was pre-accelerated by a LINAC and then directed onto a tungsten target. The resulting positron population, after due accumulation in a storage ring, was further accelerated by a conventional, large-scale (R ≈ 27 km), synchrotron accelerator up to a peak energy of 209 GeV. The large cost and size of these machines have motivated the study of alternative particle accelerator schemes. A particularly compact and promising system is represented by plasma devices which can support much higher accelerating fields (of the order of 100s of GV/m, compared to MV/m in solid-state accelerators) and thus significantly shorten the overall size of the accelerator. Laser-driven generation of electron beams with energies per particle reaching [2][3][4][5], and exceeding [6], 1 GeV have been experimentally demonstrated and the production of electron beams with energies approaching 100 GeV is envisaged for the next generation of highpower lasers (1 -10 PW) [7]. Hybrid schemes have also been proposed and successfully tested in first proof-ofprinciple experiments [8,9]. On the other hand, laserdriven low energy positrons (E e + ≈ 1−5 MeV) have been first experimentally obtained by C. Gahn and coworkers [10] and recently generated during the interaction of a picosecond, kiloJoule class laser with thick gold targets [11][12][13][14]. Despite the intrinsic interest of these results, the low energy and broad divergence reported (E e + ≤ 20 MeV and θ e + ≥ 350 mrad , respectively) still represent clear limitations for future use in hybrid machines.The possibility of generating high density and high energy electron-positron beams is of central importance also for astrophysics, due to their similarity to jets of long gamma-ray bursts (GRBs), whic...