Description
The reflection of ultra-intense laser pulses from a plasma mirror is a highly promising approach for reaching unprecedented field intensities experimentally, and could become a central tool for the next generation of fundamental physics experiments. During this reflection, the relativistic oscillation of the plasma mirror surface Doppler-shifts the reflected pulse down to the extreme ultra-violet (XUV) range, corresponding to a train of attosecond pulses in the temporal domain. In parallel, the pulse radiation pressure imprints a curvature on the surface, which focuses these XUV pulses down to a sub-µm spot. These combined effects act as an intensity boost for the laser pulse up to two orders of magnitude, potentially enabling intensities exceeding $10^{25}~\mathrm{W/cm}^2$ with multi-PetaWatt lasers [1]. Such Doppler-boosted fields colliding with matter could pave the way to probing strong-field QED in the near future.
In this contribution, we report on the first harmonic generation experiment with plasma mirrors at laser intensities substantially exceeding $1\times 10^{21}~\mathrm{W/cm}^2$ (up to $7\times 10^{21}~\mathrm{W/cm}^2$), produced at the BELLA 1-PW laser facility of the Lawrence Berkeley National Laboratory [2]. While, according to theory, increasing laser intensity on target should yield a gradually stronger harmonic signal, we observed the opposite behavior: a severe drop in the harmonic generation with increasing intensity. Using numerical simulations, we demonstrate that this drop is caused by the few-100 fs pedestal inherent to such laser pulses. This limitation has not been encountered before, as such sub-picosecond contrast only plays a critical role beyond laser intensities of $10^{21}~\mathrm{W/cm}^2$.
This study shows that increasing laser intensity on target raises new key challenges for efficient harmonic generation, and discusses possible strategies to adapt contrast-cleaning systems for a controlled reflection of the laser on the plasma mirror.
The experimental campaign was supported by the U.S. Department of Energy’s Office of Science Fusion Energy Sciences program, and by the LaserNetUS initiative, under Contract No. DE-AC02-05CH11231.
[1] H. Vincenti, Phys. Rev. Lett. 123, 105001 (2019).
[2] B. Groussin, P. Sikorski et al., arXiv:2602.10709 (2026) (submitted to Phys.Rev.Lett., under review).