High harmonic generation in Mott insulatos: Effects of intersite interactions

With the development of laser technology, research on nonlinear optical response has been actively carried out. In particular, high harmonic generation (HHG), in which the frequency is an integer multiple of the incident photon, is an essential phenomenon from the view point of applications such as attosecond pulse generation. HHG has been studied mainly in atomic and molecular gases, but since the observation of HHG in solids was reported, research on HHG in solids has been actively conducted. The generation mechanism in atomic and molecular gases
is explained by the semiclassical three-step model. However, the conventional three-step model cannot simply be applied to solids consisting of many atoms. The generation mechanism is expected to be more complicated because the band structure is related to the harmonics. In low dimensional materials, electron correlation and excitonic effect may contribute to HHG. From the perspective of strongly correlated electron systems, HHG in the Hubbard model has been studied theoretically.
In our study, we investigate HHG in strongly correlated systems using an extended Hubbard model that incorporates nearest-neighbor site repulsive interactions. In the photoexcited state of this model, the bound states of doublon and holon are formed by the nearest-neighbor site repulsion. Numerical calculations are performed using the time-dependent Lanczos method and iTEBD method, and the pump light is introduced via the Peierls phase. The harmonic spectra in the spin-density-wave phase, where the onsite repulsion is dominant, show the effect of excitons formed by the nearest-neighbor site repulsion (see Fig. 1). In the presentation, we will show the parameter dependence of the results and the subcycle analysis excluding the finite-size effect by iTEBD, so that we will discuss the effect of excitons on HHG.