In the past decades, many-body approaches based on the GW approximation and the Bethe-Salpeter equation have become state-of-the-art for calculating optical absorption in solids and nanostructures. In this talk, I’ll first present a real-time approach derived from the non-equilibrium Green’s function, that allows extending the GW+BSE approach beyond the linear regime.[1,2] Using this approach, I’ll address the importance of many-body effects and in particular of excitonic effects for nonlinear optical properties.[3] For example, I’ll look at the case of single-layer monochalcogenide whose strong Second Harmonic Generation cannot be reproduced within the independent-particle approximation.[4] In the second part of the talk, I’ll then discuss the possibility of a real-time approach based on time-dependent density-functional theory, that can describe excitonic effects.[5]

[1] Attaccalite, C., Grüning, M. & Marini, A., (2011) Physical Review B 84, 24, 245110
[2] Attaccalite, C. & Grüning, M., (2013) Physical Review B 88, 235113
[3] Grüning, M. & Attaccalite, C., (2014) Physical Review B 89,081102
[4] Claudio Attaccalite, Maurizia Palummo, Elena Cannuccia, Myrta Grüning, (2019) Phys Rev Mat 3, 074003
[5] Grüning, M., Sangalli, D. & Attaccalite, C., (2016) Physical Review B 94, 035149