Divergence control of relativistic harmonics by an optically shaped plasma surface.

The unique spatial and temporal properties of relativistic high harmonics generated from a laser-driven plasma surface allow them to be coherently focused to an extremely high intensity reaching the Schwinger limit. The ultimately achievable intensity is limited by the harmonic wavefront distortions during the interactions. Here we demonstrate experimentally that the harmonic divergence can be controlled by an optically shaped plasma surface with a prepulse that has the same spatial and temporal distribution as the main laser pulse. Simulations are also performed to explain the experimental observation, and we find that the harmonic wavefront curvature from a dented surface can be precompensated by a convex plasma. Our work suggests an active approach to control the harmonic divergence and wavefront by an optically shaped target. This can be critical for further high harmonics applications.