Non-linear Optics and Exciton Condensation in Low-dimensional Materials

Strong Coulomb interactions in low-dimensional materials result in the formation of bound and resonant electron-hole pairs, or excitons, which can dramatically change the optical response in these materials. In this talk, I will largely focus on the effect of excitons on the non-linear optical response. We develop and implement an approach to compute the non-linear optical response within the framework of the GW-Bethe Salpeter equation. Examples of different non-linear phenomena will be given, including second harmonic generation, spontaneous parametric down-conversion [1], and shift currents. I will describe how excitons and the coupling between excitons changes the origins of these non-linear phenomena, in comparison with the independent particle picture. On a separate note, I will also discuss our predictions of high temperature exciton condensation in heterostructures of organic phthalocyanine molecules on two-dimensional materials [2].

[1] F. Xuan, M. Lai, Y. Wu, S.Y. Quek, arXiv:2305.08345

[2] K. Ulman, S.Y. Quek, Nano Letters, 21, 8888 (2021)