Phonon Polaritons in Hexagonal Boron Nitride Heterostructures

Using scanning probe infrared nano-imaging, we investigated phonon polaritons in hBN heterostructures assembled from monoisotopic crystals. Our combined experimental and theoretical study reveals multiple branches of hyperbolic phonon polaritons in hBN heterostructures. We also show that wave dispersions for hyperbolic phonon polaritons can be delicately altered by engineering the heterostructures. These phonon polaritons can be interpreted by the geometric optics model of the hyperbolic rays in the heterostructures and are promising for nanophotonic and thermal applications. The method of building new forms of van der Waals heterostructures may be extended toward a range of materials and properties.