Image polaritons in boron nitride for extreme polariton confinement with low losses

In-Ho Lee,Phaedon Avouris,Xi Zhang,Tony Low,Sang-Hyun Oh,James H Edgar,Ke Wang,Song Liu,Mingze He,Yujie Luo,Joshua D Caldwell

doi:10.1038/s41467-020-17424-w

Abstract

Polaritons in two-dimensional materials provide extreme light confinement that is difficult to achieve with metal plasmonics. However, such tight confinement inevitably increases optical losses through various damping channels. Here we demonstrate that hyperbolic phonon polaritons in hexagonal boron nitride can overcome this fundamental trade-off. Among two observed polariton modes, featuring a symmetric and antisymmetric charge distribution, the latter exhibits lower optical losses and tighter polariton confinement. Far-field excitation and detection of this high-momenta mode become possible with our resonator design that can boost the coupling efficiency via virtual polariton modes with image charges that we dub ‘image polaritons’. Using these image polaritons, we experimentally observe a record-high effective index of up to 132 and quality factors as high as 501. Further, our phenomenological theory suggests an important role of hyperbolic surface scattering in the damping process of hyperbolic phonon polaritons.

Highlights

Polaritons in two-dimensional materials provide extreme light confinement that is difficult to achieve with metal plasmonics
In the case of hexagonal boron nitride (hBN), hyperbolic phonon polaritons (HPhPs) modes supported in a slab of thickness t are quantized with the out-of-plane wavevector of ky = lπ/t, where l denotes the mode number
Far-field observations of such ultra-confined modes become possible with our image polariton resonator that utilizes a pristine unpatterned hBN flake coupled with an array of ultraflat metallic ribbons, separated by a thin dielectric layer

Summary

Introduction

Polaritons in two-dimensional materials provide extreme light confinement that is difficult to achieve with metal plasmonics. 1234567890():,; Surface phonon polaritons (SPhPs)[1,2,3] are collective oscillations of atomic lattice vibrations coupled with electromagnetic waves, supported by polar materials such as hexagonal boron nitride (hBN)[4,5], silicon carbide (SiC)[6], and molybdenum trioxide (MoO3)[7] These modes are supported within spectral regions bounded by the frequencies of the longitudinal (LO) ωLO and transverse optic (TO) ωTO phonon modes, which is referred to as the Reststrahlen band. Based on the strong dependence of the Q upon the dielectric gap thickness, we suggest surface scattering effects as one of the primary damping pathways and develop the phenomenological theory for describing the loss mechanisms of the HiPPs. The high Q’s achieved for such large neff are unprecedented compared with other electromagnetic modes such as SPPs in metals and graphene, where the increase in the confinement is compensated by increases in the optical loss

Methods

Results

Conclusion