Beam pointing of relativistic high-order harmonics generated on a nonuniform pre-plasma

The use of tunable pre-pulse is a common technique to enhance the high-order harmonic generation from surface plasma. The shape and dynamic of the electron density, the degree of ionization and its rate, and the plasma heating are influenced by the pre-pulse properties. A non-uniform pre-pulse can generate a spatially varying pre-plasma density, which offers the surface for spectral up-conversion and reflection. The geometrical characteristics and plasma dynamics under intense laser fields subsequently affect the properties of the emitted harmonics. In this study, the variation in harmonic beam pointing due to the electron density shape was investigated. Particle-in-cell simulations demonstrated that both plasma hydrodynamics and geometrical optical effect induce the deviation of harmonic beam from specular reflection. This research contributes to the understanding of the surface plasma dynamics during high harmonic generation process.