Impact of Excitons on the Non-Linear Optical Properties of 2D Materials

Non-linear optical phenomena are at the core of many important technologies and spectroscopic techniques. The second-order optical susceptibility is responsible for spontaneous parametric down conversion (SPDC) which generates entangled photon pairs, shift currents which give rise to bulk photovoltaic effects, and second harmonic generation that is useful for spectroscopy. In recent years, there has been increasing interest in studying non-linear optical phenomena in the 2D limit, where phase-matching conditions are eliminated while allowing for more facile integration with hybrid quantum photonic platforms. Reduced electronic screening in 2D leads to enhanced excitonic effects, and it is therefore critical to elucidate the impact of excitons on non-linear optical properties. We develop 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. We apply the theory to niobium oxydihalides (NbOX₂, X = Cl, Br, I) [1], a material of significant current interest in non-linear optics [2]. We predict that excitonic effects greatly enhance the probability for SPDC [3] and increase the magnitude of the shift currents in this material. We further benchmark our approach by comparison with experiment for the shift current in bulk BaTiO₃ and the SHG spectra for layered NbOI₂, where we clearly demonstrate the important role of excitons in non-linear optics. Our work provides a fundamental understanding of non-linear optical phenomena in excitonic materials and paves the way for 2D non-linear quantum photonics.<br/><br/>[1] Y. Wu, I. Abdelwahab, K. C. Kwon, I. Berzhbitskiy, L. Wang, W. H. Liew, K. Yao, G. Eda, K. P. Loh, L. Shen and S. Y. Quek, “Data-driven discovery of high performance layered van der Waals piezoelectric NbOI₂”, Nature Communications, 13, 1884 (2022)<br/><br/>[2] Nature 613, 53 (2023), Nature Photonics 16, 644 (2022), Advanced Optical Materials 2202833 (2023), Advanced Materials 33, 2101505 (2021)<br/><br/>[3] F. Xuan, M. Lai, Y. Wu, S. Y. Quek, arXiv:2305.08345