ИСТИНА

The family of 2D transition-metal chalcogenides (TMCs) exhibits 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 keep inversion symmetry braking of the stacked crystal, which results in non-zero off-diagonal elements of the c(n) tensor, where n is even integer number. The Janus structures made of TMCs break additionally more symmetries of the crystal, thereby giving rise to strong enhancement of linear and nonlinear optical response. To study the linear and nonlinear response of the semiconductor materials under optical excitation, we use the Bethe-Salpeter and Kadanoff-Baym approaches. 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 intrinsically non-perturbative Kadanoff-Baym approach is suitable for going out of equilibrium and determine the nonlinear response under strong electro-magnetic field, which provides the possibility to calculate not only the second-order and third-order nonlinear response, but also higher-order nonlinearities. The combination of these methods with the many-body GW calculation of the self-energy, taking band gap renormalization into account, results in good agreement of linear and nonlinear excitonic spectra with experiments. This work was supported by the Air Force Office of Scientific Research (AFOSR) under awards no. FA9550-23-1-0455 and FA9550-23-1-0472. Ab-initio calculations were performed at the Stokes high performance computer (HPC) of the University of Central Florida. We acknowledge support by the NSF ACCESS program under allocation no. PHY230182 for access to their HPCs. We also acknowledge support by the AFOSR under award no. FA9550-23-1-0455 for access to DoD HPCs.