Optical contrast and refractive index of natural van der Waals heterostructure nanosheets of franckeite

Abstract

We study mechanically exfoliated nanosheets of franckeite by quantitative optical microscopy. The analysis of transmission mode and epi-illumination mode optical microscopy images provides a rapid method to estimate the thickness of the exfoliated flakes at first glance. A quantitative analysis of the optical contrast spectra by means of micro-reflectance allows one to determine the refractive index of franckeite in a broad range of the visible spectrum through a fit of the acquired spectra to a Fresnel law based model.

Main text (6 figures) + Supp. Info. (6 figures)

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• Symmetry breakdown in franckeite: spontaneous strain, rippling and interlayer moiré
• Optical-based thickness measurement of MoO${}_3$ nanosheets
• Investigation of the Nonlinear Optical Frequency Conversion in Ultrathin Franckeite Heterostructures