  In this letter, cutting-edge 3D Particle-In-Cell simulations are used to
demonstrate that so-called relativistic plasma mirrors irradiated by PetaWatt
(PW) lasers and naturally curved by laser radiation pressure can be used to
tightly focus Doppler-generated harmonics to extreme intensities between
$10^{25}-10^{26}W.cm^{-2}$. Those simulations are then employed to develop and
validate a general 3D model of harmonic focusing by a curved relativistic
plasma mirror. Finally, the insight gained from this model is used to propose
novel all-optical techniques that would further increase the plasma mirror
curvature with the ultimate goal of approaching the Schwinger limit.
