Research On Near-Field Optical Properties And Manipulation Of Phonon Polaritons In ¹¹B Enriched Hexagonal Boron Nitride

Phonon polaritons(PP)are novel quasiparticles formed by coupled collective excitations between photons and phonons in crystals.Phonon polaritons have many excellent properties,such as strong field confinement,long propagation distance and low loss.Therefore,phonon polaritons have wide applications in fields of sub-diffractive focusing and biosensors.Hexagonal boron nitride(hBN)supports two types of hyperbolic phonon polaritons(HPPs),whose strong electromagnetic field confinement and low propagation loss properties have been proposed for various applications in nanophotonics.Scattering-type scanning near-field optical microscopy(s-SNOM)can overcome the diffraction limit and achieve spatial resolution on the order of nanometers,which has been widely used to achieve real space images of phonon polaritons.Hexagonal boron nitride(hBN)is a representative van der Waals(vdW)material.The real part of the in-plane and out-of-plane dielectric constant along orthogonal crystal axes have opposite signs in two frequency bands,which are called Reststrahlen bands.In these two bands,the dispersion relation of hBN are two different types of hyperboloid,which makes it possible to support hyperbolic phonon polaritons.Conventionally,real-space imaging of HPPs by s-SNOM with vertical polarized laser excitation contains both tip and edge launched polariton modes.But edge-excited HPPs is too low to be observed,therefore,it is not the main research object.Compared with the plasmon in graphene,HPPs in hBN has a longer lifetime and a longer propagation distance,which is beneficial for polaritons-based nanophotonic devices.However,hBN is an insulator material,which restricts manipulation methods of HPPs.In this work,we symmetrically studied the tip and edge excited HPPs in both the natural distribution of boron isotopes(Natural hBN)and 11B isotope-enriched boron nitride(99.2%11B hBN).Using s-SNOM with polarization control of the incident light and numerical simulation methods,the near-field optical properties of phonon polariton in two kinds of hBN were systematically studied.The intrinsic HPPs in the 99.2%11B hBN exhibits a smaller damping rate and longer propagation length than that in the natural hBN.We experimentally realize a tuning from a tip-lanching to an edge-launching HPPs by rotating the polarization of the incident light and directly observed edge lanuched HPPs.The intrinsic HPPs excited by the hBN edge is a propagation wave,which is excited by the edge of hBN and propagates inward.The near-field electric field intensity(NEFI)of edge-excited HPPs depends on the β,which is the angle between the hBN edge and the projective direction of the incident electric field on hBN plane under an s-polarized incident light.Combined with numerical simulation,we revealed that HPPs can be launched only by the component of the electric field that is vertical to the hBN edge,therefore,the NEFI of edge-excited HPPs and the angle β present a sine function relationship as Eedge ∝ |sin β|.The NEFI of edge-excited HPPs in 99.2%11B hBN shows a 10%enhancement compared to natural hBN under same experimental conditions.In summary,our findings demonstrate an effective approach to reduce phonon polaritons damping and manipulate phonon polaritons excitation in the hBN,which are beneficial for developing HPPs-based nanophotonic applications.