  Using the one-dimensional Hubbard model, which is commonly used for
describing, e.g., high-$T_c$ superconducting cuprates, we study high-harmonic
generation (HHG) from doped, correlated materials. Doping is modeled by
changing the number of electrons in the lattice from the conventional
half-filling case. For relatively small Hubbard $U$, i.e., small
electron-electron correlation, we find little to no effect of doping on the
dynamics and the HHG spectra. For increasing $U$ the degree of doping has a
marked effect on the dynamics and spectra. We explain these findings through
the quasiparticle-based doublon-holon picture. The dynamics are separated into
two types, firstly doublon and holon movement, and, secondly, doublon-holon
pair creation and annihilation. Doping results in all configurations containing
doublons or holons. Those quasiparticles can move at no extra cost in energy
regardless of the correlation level. This motion at no energy cost increases
the high-harmonic gain for low and medium harmonic orders. We discuss that in
the high-$U$ limit, antiferromagnetic ordering becomes increasingly unlikely
with increasing doping rates and explain an associated drop in the high-order
harmonics relative to the case of half filling.
