Manipulating phonon polaritons in low loss B enriched hexagonal boron nitride with polarization control.

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 polariton 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 B isotope-enriched boron nitride (99.2% B hBN). The intrinsic HPPs excited in 99.2% B hBN exhibit a lower damping rate and longer propagation length than that in natural hBN. We experimentally realize a tuning from tip-dominated to edge-dominated excited HPPs by rotating the polarization of incident light. The near-field electric field intensity (NEFI) of edge-excited HPPs E and the angle β (between the hBN edge and the projective direction of the incident electric field on the hBN plane) present a sine function relationship as E∝|sin β| under an s-polarized incident light. The NEFI of edge-excited HPPs in 99.2% B hBN shows a 10% enhancement compared to natural hBN under the same measurement conditions. Our findings demonstrate an effective approach to reducing phonon polariton damping and manipulating phonon polariton excitation in hBN, which are beneficial for developing HPPs-based nanophotonic applications.