Orbital perspective of high harmonic generation in ReS₂

High harmonic generation has allowed unparalleled monitoring and control of coherent electron dynamics. In atoms and molecules, this has been made possible thanks to a theoretical framework based on a time-domain and real-space perspective of the highly non-perturbative light-matter interaction, linking the characteristics of the emitted harmonic radiation to sub-laser-cycle dynamics of atomic and molecular orbitals. In solids, high harmonic generation is understood using a similar framework, albeit exchanging the real-space perspective for one in reciprocal-space. This band structure perspective has proven very insightful, for example, to reconstruct the dispersion in ZnO, or to explain multiple plateaus observed in the HHG spectrum of rare-gas solids.

Linking features in the harmonic spectrum to the electron-hole motion along specific bands requires that the bands change appreciably with crystal momenta and that they have a sizeable separation between them. Yet, many materials do not meet these conditions. One example is ReS₂ whose dense and flat band structure makes associating a specific harmonic with the electron motion along a particular laser-dressed band hardly straightforward.

On the other hand, an orbital-based framework is ideally suited for these materials since the small bandwidth indicates that the electrons are very localized in the individual atoms of the lattice. Several works have discussed the importance of using a real-space perspective to understand high harmonic generation from solids; yet, there has been no evidence linking features in the harmonic spectrum to dynamics of orbitals in the lattice.

Here, we measure a strong intensity-dependent anisotropy in the high harmonic spectrum of ReS₂, despite the seemingly isotropic band structure. We use numerical theory to reveal how this is a manifestation of interferences between localized orbitals in the lattice and show how the intensity and polarization of the laser field activates or suppresses specific atoms from the laser-driven dynamics that leads to the emission of a particular harmonic order, offering an orbital-perspective of high harmonic generation in solids.