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Giant second-harmonic generation in ferroelectric NbOI2



Implementing nonlinear optical components in nanoscale photonic devices is challenged by phase-matching conditions requiring thicknesses in the order of hundreds of wavelengths, and is disadvantaged by the short optical interaction depth of nanometre-scale materials and weak photon–photon interactions. Here we report that ferroelectric NbOI2 nanosheets exhibit giant second-harmonic generation with conversion efficiencies that are orders of magnitude higher than commonly reported nonlinear crystals. The nonlinear response scales with layer thickness and is strain- and electrical-tunable; a record >0.2% absolute SHG conversion efficiency and an effective nonlinear susceptibility \(\chi _{\mathrm{eff}}^{(2)}\) in the order of 10−9 m V−1 are demonstrated at an average pump intensity of 8 kW cm2. Due to the interplay between anisotropic polarization and excitonic resonance in NbOI2, the spatial profile of the polarized SHG response can be tuned by the excitation wavelength. Our results represent a new paradigm for ultrathin, efficient nonlinear optical components.

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Fig. 1: Atomic structure of ferroelectric layered NbOI2.
Fig. 2: Anisotropic band structure and linear optical response of NbOI2.
Fig. 3: Characterization of SHG in NbOI2.
Fig. 4: Effects of strain, temperature and electric fields on SHG in NbOI2.

Data availability

The data supporting the findings of this study are available within the article and its Supplementary Information, or from the corresponding authors on reasonable request. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition nos. 2169918-2169919. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via


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I.A. acknowledges funding support through a Humboldt Research Fellowship from the Alexander von Humboldt Foundation. K.P.L. acknowledges support from the Singapore National Research Foundation (NRF), Competitive Research Program NRF-CRP22-2019-0006, Prime Minister’s Office, Singapore. T.C.S. acknowledges support from the Ministry of Education (MOE), Singapore, under AcRF Tier 2 grant (MOE2019-T2-1-006). G.E. acknowledges support from MOE, Singapore, under AcRF Tier 3 grant (MOE2018-T3-1-005) and the Singapore NRF for funding the research under medium-sized centre programme. The authors would like to acknowledge the Singapore Synchrotron Light Source (SSLS) for providing the facility necessary for conducting the Mueller matrix ellipsometry measurements. S.A.M. acknowledges LMUexcellent, the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy—EXC 2089/1—390776260, the EPSRC Reactive Plasmonics Programme EP/M013812/1, and the Lee Lucas Chair in Physics.

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Authors and Affiliations



I.A., S.A.M. and K.P.L. conceived the project and designed the experiments. I.A. performed micromechanical cleavage, two-dimensional dry transfer, device fabrication and material characterization under the supervision of S.A.M. and K.P.L. I.A., B.T., D.G. and R.B. performed all the optical characterizations under the guidance of L.d.S.M., T.C.S., S.A.M., and K.P.L. Y.W. and F.X. performed the calculations and theoretical analysis under supervision of S.Y.Q. I.V. synthesized the NbOI2 bulk crystals under the supervision of G.E. M.Z. performed the ARPES measurements. I.A. wrote the manuscript with input from all authors.

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Correspondence to Stefan A. Maier or Kian Ping Loh.

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Nature Photonics thanks Mikhail Glazov and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Tables S1–S3, Supplementary Figures S1–S16, Supplementary Note S1-S7

Supplementary Video 1

Polarization-dependent transmission measurements of NbOI2 nanosheet

Supplementary Data 1

Crystallographic Information File of NbOI2 at 298 K

Supplementary Data 2

Crystallographic Information File of NbOI2 at 100 K

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Abdelwahab, I., Tilmann, B., Wu, Y. et al. Giant second-harmonic generation in ferroelectric NbOI2. Nat. Photon. 16, 644–650 (2022).

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