Excitons in Linear/Nonlinear and Time-Resolved Photo Responses of 2D Materials and Heterostructures

Recent studies revealed spectroscopic signatures of novel photo-excited states and intriguing pump-probe responses in 2D van der Waals structures. The nature of many of these phenomena remains to be fully understood. Here, we present some recent theoretical results in their understanding and predictions. We show that there is a rich diversity of excitons in transition metal dichalcogenide (TMD) moiré superlattices, including unforeseen novel intralayer charge-transfer moiré excitons.[1, 2] In pump-probe calculations, we discovered a self-driven exciton-Floquet effect in the time-resolved ARPES of 2D materials, wherein prominent satellite bands and renormalization of the quasiparticle bands are induced by excitons, analogously to the optical Floquet effect driven by photons.[3] We demonstrated a new exciton mechanism (direct coupling of intralayer with interlayer excitons) in the ultrafast optical response of TMD heterobilayers.[4] Moreover, we showed that strong excitonic physics in 2D materials can greatly enhances their nonlinear optical responses (e.g., shift currents and second harmonic generation).[5,6] This has led to the discovery of a striking phenomenon of formation of light-induced shift current vortex crystals in TMD moiré systems – <i>i.e</i>., 2D periodic arrays of moiré-scale current vortices and associated magnetic fields with remarkable intensity under laboratory laser setup.[7] Our studies are made possible with the development of new methods (based on the GW-BSE and time-dependent GW approaches) that allow for the <i>ab initio</i> calculations of excitonic physics and photo responses of systems with thousands of atoms in the unit cell and in the time domain.<br/><br/><b>Acknowledgment:</b><br/>This work was supported by the U.S. Department of Energy and by the National Science Foundation, and was done in collaboration with members of the Louie group. Computational resources have been provided by NERSC and XSEDE.<br/><br/>[1] M. H. Naik, <i>et al</i>., <i>Nature</i> <b>609</b>, 52 (2022).<br/>[2] H.-Y. Li, <i>et al.</i>, <i>Nat. Mat.</i> <b>23</b>, 633 (2024).<br/>[3] Y.-H. Chan, D. Y. Qiu, F. H. da Jornada, and S. G. Louie, <i>Proc. Natl. Acad. Sci. U.S.A.</i> <b>120</b>, e2301957120 (2023).<br/>[4] C. Hu, M. H. Naik, Y.-H. Chan, and S. G. Louie, <i>Phys. Rev. Lett.</i> <b>131</b>, 236904 (2023)<br/>[5] Y.-H. Chan, D. Y. Qiu, F. H. da Jornada, and S. G. Louie, <i>Proc. Natl. Acad. Sci. U.S.A.</i> <b>118</b>, e1906938118 (2021).<br/>[6] J.-W. Ruan, Y.-H. Chan, and S. G. Louie, submitted. https://doi.org/10.48550/arXiv.2310.09674<br/>[7] C. Hu, M. H. Naik, Y.-H. Chan, J.-W. Ruan and S. G. Louie, <i>Proc. Natl. Acad. Sci. U.S.A.</i> <b>120</b>, e2314775120 (2023).