  Using the Hubbard model we study how the process of high-order harmonic
generation (HHG) is modified by beyond mean-field electron-electron correlation
for both finite and bulk systems. A finite-size enhancement of the HHG signal
is found and attributed to electrons backscattering off the lattice edges.
Additionally, with increasing strength of the electron-electron correlation an
enhancement of the high-frequency regime of the HHG spectrum is found. This is
attributed to the on-site Coulomb repulsion between electrons giving rise to a
localized quiver motion of the electrons. The finite-lattice enhancement
dominates the HHG spectra from a few harmonic orders until a threshold from
which the correlational enhancement dominates. This threshold is determined by
the degree of correlation and decreases into the low-frequency regime for
increasing electron-electron correlation. This infers that as the Mott
insulator limit of high electron-electron correlation is approached, the
finite-size effect on the electron dynamics becomes negligible.
