Non-Linear Light-Matter Interactions in Quantum Materials

Non-linear optical phenomena are at the heart of many important technologies and spectroscopic techniques. In order to fundamentally understand, and predict, non-linear optical properties in quantum materials, it is necessary to incorporate the physics of excitons, or bound electron-hole pairs. In this work, we develop and implement a theory for the second-order optical susceptibility that takes into account excitonic effects with a first principles GW-Bethe-Salpeter-Equation (GW-BSE) approach. Using this theory, we elucidate the impact of excitons on sum-frequency generation and spontaneous parametric down-conversion in niobium oxydihalides (NbOX₂, X = Cl, Br, I) [1], a material of significant current interest in non-linear optics [2]. We also predict difference-frequency generation effects in quantum materials. In addition to shift current effects (the zero-frequency response), we consider the generation of terahertz pulses due to difference frequency mixing from incoming laser pulses. Our work provides a framework to predict and understand the non-linear effects arising from continuous-wave and ultrafast laser light incident on quantum materials.<br/><br/>[1] F. Xuan, M. Lai, Y. Wu, S. Y. Quek, arXiv:2305.08345<br/><br/>[2] Nature 613, 53 (2023), Nature Photonics 16, 644 (2022), Advanced Optical Materials 2202833 (2023), Advanced Materials 33, 2101505 (2021)