Electron-electron interactions and high-order harmonics in solids

The generation of high-order harmonics in bulk systems is a young and fast-growing area of research. Beginning in 2011 with the pioneering studies of high-order harmonic generation in bulk dielectrics, significant progress has already been made in understanding the details of the microscopic mechanisms behind this phenomenon, such as the role of intra- and interband polarization, the contribution of the electronic band structure and dipole moments to the harmonic spectrum, and the effects of structural and electronic symmetry. However, the role of electron-electron correlations in the excited system is much less understood. In this work, we study the role of these effects in the high-order harmonic spectrum by using the Time-Dependent Density Functional Theory (TDDFT) approach with the Dynamical Mean-Field Theory (DMFT) exchange-correlation kernel. In this approximation, one takes into account the time-resolved on-site electron-electron interactions. As we demonstrate, the correlation effects significantly affect the high-harmonic spectrum, most importantly through ultrafast modification of the interband polarization of the system.