Near-field refractometry of van der Waals crystals

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 MoS₂ flakes with subwavelength spatial resolution at a wavelength of 1,570 nm, we determine both the in-plane and out-of-plane permittivity components of MoS₂ as 16.11 and 6.25, respectively, with a relative uncertainty below 1 %, while overcoming the limitations of traditional methods.