Near-field refractometry of van der Waals crystals

TL;DR

This paper demonstrates a near-field optical microscopy technique to accurately measure the anisotropic refractive indices of van der Waals crystals like MoS₂, overcoming limitations of traditional methods.

Contribution

It introduces a novel near-field refractometry method capable of resolving in-plane and out-of-plane permittivity in van der Waals crystals with high precision.

Findings

Measured in-plane permittivity of MoS₂ as 16.11

Measured out-of-plane permittivity of MoS₂ as 6.25

Achieved sub-1% uncertainty in permittivity measurements

Abstract

Common techniques for measuring refractive indices, such as ellipsometry and goniometry, are ineffective for van der Waals crystal flakes because of their high anisotropy and small, micron-scale, lateral size. To address this, we employ near-field optical microscopy to analyze the guided optical modes within these crystals. By probing these modes in MoS$_2$ flakes with subwavelength spatial resolution at a wavelength of $1570\,\mathrm{nm}$, we determine both the in-plane and out-of-plane permittivity components of MoS$_2$ as $16.11$ and $6.25$, respectively, with a relative uncertainty below $1\%$, while overcoming the limitations of traditional methods.