Linear and nonlinear optical response based on many-body Bethe-Salpeter and Kadanoff-Baym approaches for two-dimensional semiconductors

The Bethe-Salpeter and Kadanoff-Baym approaches are powerful first-principles methods to predict linear and nonlinear response from materials under optical excitations. The Bethe-Salpeter approach allows to include the many-body interaction between electron and hole in excitons in the calculation of the linear optical response in equilibrium. The combination of this method with the many-body GW calculation of self-energy, which takes the band gap renormalization into account, results in good agreement of linear excitonic spectra with experiment. The Kadanoff-Baym approach is intrinsically non-perturbative and allows to calculate the nonlinear response of materials going out of equilibrium under excitation from strong electromagnetic fields, accounting for not only second-order and third-order nonlinear response, but also higher-order nonlinearities.

The family of 2D transition-metal chalcogenides (TMCs) exhibit exceptional nonlinear optical properties. Experimentally, the non-resonant second harmonic response of GaSe is the strongest among all the 2D layered semiconductors. AB layer stacking of these materials allows to preserve the inversion symmetry breaking of the stacked crystal, which results in non-zero off-diagonal elements of the chi⁽²⁾ tensor.

In this talk we will discuss the theoretical results for linear and nonlinear response in 2D and bulk semiconductors made of TMCs and compare them with experiments.