L. Wang, R. Chen, M. Xue, S. Liu, J. Edgar and J. Chen
Nanoscale, Accepted Manuscript (2020)
Hexagonal boron nitride (hBN) supports two types of hyperbolic phonon polaritons (HPPs), whose properties of strong electromagnetic field confinement and low propagation loss have been proposed for various applications in nanophotonics. Conventionally, real-space imaging of HPPs by scattering-type scanning near-field optical microscopy (s-SNOM) with vertical polarization excitation contains both tip and edge launched polaritons modes, which leads to hybrid interference fringes. In this work, we symmetrically study the tip and edge excited HPPs in both boron nitride with the natural distribution of boron isotopes (Natural hBN) and 11B isotope-enriched boron nitride (99.2% 11B hBN). The intrinsic HPPs excited in 99.2% 11B hBN exhibits a lower damping rate and longer propagation length than that in Natural hBN. We experimentally realize a tuning from a tip-dominated to an edge-dominated excited HPPs by rotating the polarization of incident light. The near-field electric field intensity (NEFI) of edge-excited HPPs Eedge and the angle β (between the hBN edge and the projective direction of the incident electric field on hBN plane) present a sine function relationship as Eedge∝|sinβ| under an s-polarized incident light. The NEFI of edge-excited HPPs in 99.2% 11B hBN shows a 10% enhancement compared to Natural hBN under the same measurement conditons. Our findings demonstrate an effective approach to reducing phonon polaritons damping and manipulate phonon polaritons excitation in the hBN, which are beneficial for developing HPPs-based nanophotonic applications.