Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2. Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies1,2,3,4.
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Q.G., S.J.P. and A.T.S.W. acknowledge financial support from the Ministry of Education Tier 2 grant MOE2017-T2-2-139. Q.G. sincerely thanks Q. Zhang, K. Zheng and G. Xu for help with chemicals and furnace facilities for crystal synthesis and X-ray diffraction measurements. Q.G. sincerely thanks M. Li for valuable discussions. Q.G. thanks M. Wu, Y. Yu and J. Dan for help with STEM. X.-F.R. and G.-C.G. acknowledge financial support of the Innovation Program for Quantum Science and Technology (grant number 2021ZD0303200), the National Natural Science Foundation of China (grant numbers 61590932, 11774333, 62061160487 and 12004373), the Anhui Initiative in Quantum Information Technologies (grant number AHY130300) and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant number XDB24030601). This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication. The work at University of Chinese Academy of Sciences was financially supported by the National Key R&D Program of China (2018YFA0305800) and the Beijing Outstanding Young Scientist Program (BJJWZYJH01201914430039). C.-W.Q. acknowledges the financial support of the National Research Foundation, Prime Minister’s Office, Singapore under Competitive Research Program Award NRF-CRP26-2021-0004. H.G. acknowledges the financial support of the National Science Fund for Distinguished Young Scholars (grant number T2225027). Z.X. acknowledges the financial support of the National National Science Foundation of China (grant number 51972130). X.Z. acknowledges the financial support of the Peking University startup funding and the National Natural Science Foundation of China (grant number 52273279).
The authors declare no competing interests.
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This file includes Supplementary Sections 1–7, Figs. 1–29, Tables 1–4 and References. Further information and data on structural characterization, electronic structure and optical transition analysis, interlayer properties, lattice vibrational properties, SHG and SPDC are listed.
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Guo, Q., Qi, XZ., Zhang, L. et al. Ultrathin quantum light source with van der Waals NbOCl2 crystal. Nature 613, 53–59 (2023). https://doi.org/10.1038/s41586-022-05393-7