Near-field Probing of Image Polaritons in van der Waals Crystals

Abstract

Polaritonic modes in van der Waals crystals provide an unexcelled degree of field confinement and a superior capability for light manipulation at nanoscale. Very recently, a new class of polaritonic modes has been spotlighted as a superior platform for low-dimensional nanophotonics: the image polaritons, supported by a polaritonic material in proximity to a metal when the polaritonic mode couples with its mirror image. We use scanning scattering-type near-field optical microscope (s-SNOM) to study the hyperbolic image phonon-polaritons (HIP) in hexagonal boron nitride (hBN) on gold. In our experiments, atomically flat gold crystals are used as a substrate, eliminating any roughness-mediated scattering of HIP. In such system, HIP dispersion properties can be accurately measured by the near-field mapping of their interference fringes. We use the polariton propagation length in optical cycles as a figure of merit (FOM). We observe the first-order HIP mode with FOM of 6.5, independently of hBN thickness, while its wavelength varies from 192 nm in a 30 nm-thick hBN slab to 565 nm in an 88 nm-thick slab, all measured at 1480 cm–1 in the upper reststrahlen band. In agreement with theory, measured FOM is 1.8 times larger than that of hyperbolic phonon-polaritons in the same hBN on a typically used SiO2 substrate. At the same time, HIP possess an impressively 3.9 times shorter wavelength. While the hBN used in our experiments has isotope composition of 10B/11B ≈ 82/18, we note that FOM of the HIP in the isotopically pure hBN with 10B = 98.7% is expected to be as high as 19. Our results are even more interesting in a context of the recent work by Menabde et al. reporting on the experimental observation of significantly improved FOM of the mid-infrared image (also called acoustic) graphene plasmons. Near-field probing has revealed image plasmon mode that is twice as compressed and has 1.4 times higher FOM compared to graphene surface plasmons under similar conditions. Importantly, these results have been obtained with a large-area, chemically grown graphene, suggesting that image graphene plasmons are less sensitive to loss in graphene. This can be explained by the field distribution of the image plasmons which is mostly confined within the dielectric spacer, in contrast to the surface graphene plasmons localized at the graphene plane. Summarizing, the direct near-field probing of image polaritons in van der Waals materials clearly demonstrates their superiority over pristine polaritonic modes in terms of both FOM and field confinement, promising the ultra-compact photonic devices where the wave phenomena and the exceptionally strong light-matter interaction can be put to work together.