Frenkel Exciton Physics in Breathing Kagome Lattice Materials

Higher harmonic generation (HHG) lies at the core of nonlinear optics, serving as a cornerstone for applications such as light source generation, imaging, and sensing, and, more importantly, forming the foundation of attosecond science. Recently, HHG in layered materials has garnered significant interest due to the fascinating properties exhibited by these 2D materials, including room-temperature excitons and large, tunable nonlinear susceptibilities. However, their nanometer-scale thickness inherently results in low harmonic generation efficiency, posing a major challenge to their practical integration into functional devices. Here, by using many-body GW-Bethe-Salpeter and Kadanoff-Baym approaches, we demonstrate that single-layer Nb₃Cl₈ exhibits an exceptionally large nonlinear optical response—approximately three orders of magnitude higher than that of conventional 2D semiconductors. This remarkable response arises from the well-isolated flat bands, which are decoupled from the continuum of states, a unique feature induced by the breathing Kagome lattice structure of Nb₃Cl₈.

In this talk we will discuss the ab-initio calculations of the linear and nonlinear optical properties of Nb₃Cl₈ and compare our results with experiments.