Guided modes in nanometer thick anisotropic van der Waals materials are experimentally investigated and their refractive indices in visible wavelengths are extracted. Our method involves near-field scanning optical microscopy of waveguide (transverse electric) and surface plasmon polariton (transverse magnetic) modes in h-BN/SiO2/Si and Ag/h-BN stacks, respectively. We determine the dispersion of these modes and use this relationship to extract anisotropic refractive indices of h-BN flakes. In the wavelength interval 550-700 nm, the in-plane and out-of-plane refractive indices are in the range 1.98-2.12 and 1.45-2.12, respectively. Our approach of using near-field scanning optical microscopy allows for the direct study of the interaction between light and two-dimensional van der Waals materials and heterostructures.
Bibliographical noteFunding Information:
This work was supported by the NSF EFRI, Award No. 1542807. This work was performed in part at the Harvard University Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF ECCS Award No. 1541959. M.T. acknowledges the support of the Swiss National Science Foundation (SNSF) Grant No. 168545. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and JSPS KAKENHI Grant No. JP15K217.