Near-field probing of image phonon-polaritons in hexagonal boron nitride on gold crystals

Abstract

Near-field mapping has been widely used to study hyperbolic phonon-polaritons in van der Waals crystals. However, an accurate measurement of the polaritonic loss remains challenging because of the inherent complexity of the near-field signal and the substrate-mediated loss. Here we demonstrate that large-area monocrystalline gold flakes, an atomically flat low-loss substrate for image polaritons, provide a platform for precise near-field measurement of the complex propagation constant of polaritons in van der Waals crystals. As a topical example, we measure propagation loss of the image phonon-polaritons in hexagonal boron nitride, revealing that their normalized propagation length exhibits a parabolic spectral dependency. Furthermore, we show that image phonon-polaritons exhibit up to a twice longer normalized propagation length, while being 2.4 times more compressed compared to the case of the dielectric substrate. We conclude that the monocrystalline gold flakes provide a unique nanophotonic platform for probing and exploitation of the image modes in low-dimensional materials.

Ultimate Limit for Optical Losses in Gold, Revealed by Quantitative Near-Field Microscopy

“…Due to the chemical growth, the flakes are expected to have low surface roughness. We measured ∼0.2 nm, which is in agreement with other reported values (<1, 0.1, and ∼0.2 nm). We further applied plasma cleaning for 1 h prior to our measurements to remove a miniscule organic layer that is residual from the crystal synthesis .…”

“…Furthermore, recent advances in quantum plasmonics proved the significance of nonlocal effects that put a fundamental limit on loss mitigation in plasmonic systems. Nonlocal losses are associated with the quantum surface-response of metals that manifest in effects such as Landau damping, electron spill-out, and charge screening. − Therefore, atomic-scale surface smoothness, as featured on chemically synthesized monocrystals, and a well-defined dielectric function are of crucial importance for experiments with extreme light confinement. − Yet, while the absence of electron scattering at grain boundaries is generally expected to lead to lower optical losses in a monocrystalline structure, , recent spectroscopic ellipsometry measurements of the relative permittivity of gold show no significant difference between monocrystalline and evaporated polycrystalline gold. − Likewise, in a study of plasmonic devices, the replacement of polycrystalline gold with monocrystalline gold shows no significant performance improvements . The superiority of monocrystalline gold flakes in plasmonic nanostructures is only observed in a few articles, − but without providing comprehensive measurements of the optical properties of gold.…”

Low‐Loss Anisotropic Image Polaritons in van der Waals Crystal α‐MoO3

“…The larger FOM stems from the faster reduction of the HIP wavelength compared to its group velocity (Figure 4G), which is a general feature of the image modes with positive group velocity. [24] Finally, we report the longest lifetime of the phonon-polaritons observed in α-MoO 3 so far. We have measured the HIP lifetime of 4.2 ps (9.7 ps) in RB2 (RB3) and the maximal FOM of 4.5 (3.2) in RB2 (RB3) in commercially available crystals (see Experimental Section).…”

“…The larger FOM stems from the faster reduction of the HIP wavelength compared to its group velocity (Figure 4G), which is a general feature of the image modes with positive group velocity. [ 24 ]…”

“…[4][5][6] The natural anisotropy of multilayer advantage: significantly stronger field confinement, yet similar lifetime compared to their counterparts in the same material on a low-loss dielectric substrate. [24] This unique dispersion property stems from the smaller group velocity while the material loss practically does not change, and manifests in a longer normalized propagation length in optical cycles. In case of the 2D polaritonic materials, image modes provide an unexcelled degree of field confinement into the nanometer-scale volumes due to the absence of the otherwise common geometry-driven cutoff.…”