Many-body effects on high-harmonic generation in Hubbard ladders

We show how many-body effects associated with background spin dynamics control the high-harmonic generation (HHG) in Mott insulators by analyzing the two-leg ladder Hubbard model. Spin dynamics activated by the interchain hopping ty drastically modifies the HHG features. When two chains are decoupled (ty=0), HHG originates from the dynamics of coherent doublon-holon pairs because of spin-charge separation. With increasing ty, the doublon-holon pairs lose their coherence due to their interchain hopping and resultant spin-strings. Furthermore, the HHG signal from spin-polarons -- charges dressed by spin clouds -- leads to an additional plateau in the HHG spectrum. For large ty, we identify unconventional HHG processes involving $three$ elementary excitations -- two polarons and one magnon. Our results demonstrate the nontrivial nature of HHG in strongly correlated systems, and its qualitative differences to conventional semiconductors.