Extremely confined gap plasmon modes: when nonlocality matters

Abstract

Historically, the field of plasmonics has been relying on the framework of classical electrodynamics, with the local-response approximation of material response being applied even when dealing with nanoscale metallic structures. However, when the confinement of electromagnetic radiation approaches atomic scales, mesoscopic effects are anticipated to become observable, e.g., those associated with the nonlocal electrodynamic surface response of the electron gas. Here, we investigate nonlocal effects in propagating gap surface plasmon modes in ultrathin metal–dielectric–metal planar waveguides, exploiting monocrystalline gold flakes separated by atomic-layer-deposited aluminum oxide. We use scanning near-field optical microscopy to directly access the near-field of such confined gap plasmon modes and measure their dispersion relation via their complex-valued propagation constants. We compare our experimental findings with the predictions of the generalized nonlocal optical response theory to unveil signatures of nonlocal damping, which becomes appreciable for few-nanometer-sized dielectric gaps.

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

“…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.…”

Optical reflective metasurfaces based on mirror-coupled slot antennas

“…Due to increased ohmic losses in metallic layers (as compared to the tabulated values) especially at shorter wavelengths, the total reflectivity in the simulation and experiment exhibits different dependences on the optical wavelength. As a final comment, we emphasize that the efficiencies of MCSA metasurfaces could be further improved using single-crystalline metallic materials in the fabrication process to reduce the ohmic loss 39 , 40 …”

“…As a final comment, we emphasize that the efficiencies of MCSA metasurfaces could be further improved using single-crystalline metallic materials in the fabrication process to reduce the ohmic loss. 39,40…”

Nonlocal Cable‐Network Metamaterials

“…Nonlocality in electromagnetism [13][14][15][16][17][18][19] is distinct from Bragg resonances as, e.g., used in photonic crystals. [20] Our present builds on networks of waveguides exhibiting spatial dispersion in electromagnetism [21,22] and acoustics.…”