@article {doi:10.1021/acsphotonics.7b01518, title = {Guided Modes of Anisotropic van der Waals Materials Investigated by near-Field Scanning Optical Microscopy}, journal = {ACS Photonics}, volume = {5}, number = {4}, year = {2018}, pages = {1196-1201}, abstract = {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{\textendash}700 nm, the in-plane and out-of-plane refractive indices are in the range 1.98{\textendash}2.12 and 1.45{\textendash}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.}, doi = {10.1021/acsphotonics.7b01518}, url = {https://doi.org/10.1021/acsphotonics.7b01518}, author = {Daniel Wintz and Kundan Chaudhary and Ke Wang and Jauregui, Luis A. and Ambrosio, Antonio and Tamagnone, Michele and Zhu, Alexander Y. and Robert C. Devlin and Crossno, Jesse D. and Pistunova, Kateryna and Kenji Watanabe and Takashi Taniguchi and Philip Kim and Capasso, Federico} }