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Hyperbolic phonon polaritons in hexagonal boron nitride


Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with artificially fabricated nanostructures, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. In this dissertation, we studied polaritonic phenomena in hBN using scattering-type scanning near-field optical microscopy (s-SNOM). We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons (HP2s) in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers. We have also carried out the tunable hyperbolic response in metastructures comprised of a monolayer graphene deposited on hBN. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of HP2s in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the “hyperlens” for subdiffractional focusing and imaging using a slab of hBN with a record high resolution due to the natural lattice structure in hBN. Finally, we have systematically studied the relative efficiency of polariton emission in two-dimensional materials in the case of HP2S in hBN. We have observed polariton waves launched by various kinds of emitters and compared their relative efficiency and analyzed the origins of the efficiency difference.

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