Monolayer-to-mesoscale modulation of the optical properties in 2D CrI3 mapped by hyperspectral microscopy

Magnetic 2D materials hold promise to change the miniaturization paradigm of unidirectional photonic components. However, the integration of these materials in devices hinges on the accurate determination of the optical properties down to the monolayer limit, which is still missing. By using hyperspectral wide-field imaging at room temperature, we reveal a nonmonotonic thickness dependence of the complex optical dielectric function in the archetypal magnetic 2D material CrI3 extending across different length scales: onsetting at the mesoscale, peaking at the nanoscale, and decreasing again down to the single layer. These results portray a modification of the electronic properties of the material and align with the layer-dependent magnetism in CrI3, shedding light on the long-standing structural conundrum in this material. The unique modulation of the complex dielectric function from the monolayer up to more than 100 layers will be instrumental for understanding mesoscopic effects in layered materials and tuning light-matter interactions in magnetic 2D materials. The project that gave rise to these results received the financial support of a fellowship from the “la Caixa” Foundation (ID No. 100010434, fellowship codes LCF/BQ/PR21/11840011 and LCF/BQ/DI22/11940022) and Grant No. PID2020-118938GA-100 from the Spanish Ministerio de Ciencia e Innovación (MICINN). E. N. M. acknowledges the European Research Council (ERC) under the Horizon 2020 research and innovation program (ERC StG, Grant Agreement No. 803092). E. N. M. and M. G. acknowledge the MICINN and the European Union NextGenerationEU for financial support from the Ramon y Cajal program (Grants No. RYC2018-024736-I and No. RYC2021-034609-I). F. C. P. also acknowledges the MICINN for the FPU program (Grant No. FPU17/01587). F. R. M. acknowledges the MICINN for financial support (Grant No. PID2021-123163OB-I00). This work was also supported by the Spanish Unidad de Excelencia “María de Maeztu” (Grant No. CEX2019-000919-M) and is part of the Advanced Materials program supported by MICINN with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Generalitat Valenciana. D. S. and A.C. acknowledge Generalitat Valenciana for financial support (from the CIDEGENT program Grant No. CIDEGENT/2021/052 and PROMETEO2020-016 respectively) and computational resources at University of Pisa.