  The spectra produced by high-harmonic generation (HHG) typically exhibit
well-defined peaks at odd integers times the laser frequency. However, in
recent investigations of HHG from correlated materials, spectra exhibit signals
at noninteger harmonics which do not conform to the well-known symmetry-based
selection rules for HHG-spectra. Here, we use the Fermi-Hubbard model to study
HHG from a linear chain of atoms. This model allows us to study both the
correlated and uncorrelated phases through a specification of the amount of
onsite electron-electron repulsion. The presence of signal at noninteger
harmonics can be interpreted as originating from the population of multiple
Floquet states. We show how this coupling to different Floquet states depends
on the characteristics of the driving pulse and the strength of the
electron-electron interaction in the system.
