Sources of attosecond light pulses
Beyond this fundamental interest, plasma mirrors open very promising application perspectives in research. They are considered as the best candidates for the next generation of attosecond light sources, potentially providing the more energetic pulses that are now needed for the development of attosecond science [Tsa06,Dro06,Whe12,Ma15]. Exploiting such beamlines however still requires solving major challenges. While simulations predict the generation of harmonic orders of several thousands for laser intensities exceeding 1020 W/cm2 [Gor04,Tsa06], this has so far only been observed with rather long (500 fs) laser pulses from a PW laser facility [Dro06], while harmonic orders of only a few tens have been produced with ultrashort (20-30 fs) table-top lasers [Dol13,Kah13], despite seemingly comparable intensities [Dol13]. The origin of this huge discrepancy is at present a major puzzle. Moreover, plasma mirrors will be useful for attosecond science only if they can provide isolated attosecond pulses. But relativistic laser intensities are almost exclusively achieved with lasers pulses of 10 optical periods or more, which rather produce attosecond pulse trains. We now need to learn how to generate isolated attosecond pulses with such lasers. This should be possible in the near future using the recently discovered attosecond lighthouse effect [Vin12,Whe12].
Synchronized light and electrons bunches from a single source
Our recent results [The15] on electron beams produced from plasma mirrors demonstrate that, with the advent of PW-class lasers, plasma mirrors could become very competitive sources of relativistic electrons beams. According to theory, the gain in energy provided by the interaction of ejected electrons with the reflected laser field in vacuum is such that GeV energies might be expected at intensities of a few 1021 W/cm2, that are now becoming achievable with PW lasers. Even at lower intensities, attosecond bunches of MeV electrons could be obtained, synchronized with the attosecond light pulses generated in the reflected beam (Fig.1). This would provide unique sources for attosecond science, e.g. to probe systems excited by attosecond light pulses by time-resolved electron diffraction.