Correlation-induced generation of superharmonics in the high-order harmonic spectrum of perovskite barium titanate

In this paper, we analyze the effects of electron-electron correlations in the high-order harmonic (HH) spectrum of perovskite BaTiO3 excited by an intense, few-cycle midinfrared laser pulse by using time-dependent density-functional theory with the exchange-correlation (XC) kernel obtained from the dynamical mean-field theory solution of the Hubbard model. As we show, the correlation effects considerably alter the HH spectrum, shifting the spectral weight to higher frequencies and leading to the generation of "superharmonics"-an energyperiodic enhancement and suppression of certain harmonic orders-when the correlation strength exceeds a critical value. The results are stable for variation of the peak field strength, duration, and central frequency of the driving laser pulse. Using a correlated two-level model, we find that the superharmonic emission results from transitions between correlation-induced Hubbard subband states and has frequency proportional to the effective electron-electron correlation energy.