Figure 29.

Schematic illustrating processes that could limit the spatial confinement of the nanocavity plasmon field. Middle: Illustration of the NCP field in the junction between the tip and substrate. Left: In the subnanometer gaps, the AC quantum tunneling current J_ω across the junction significantly changes the optical response and quenches the electric field enhancement. Upper right: Considering the nonlocality of the surface response, the equilibrium electron density n₀(r) exponentially decays across the metal surface and the induced charges n₁(r) are smeared both inside and outside the metal–air interface, a phenomenon known as the Thomas-Fermi screening effect. The vertical dashed line represents the ideal metal surface disregarding the electron spill-out effect. Lower right: The NCP field is quenched by directly exciting electron–hole pairs in the metal via Landau damping. The momentum mismatch between the NCP and electrons can be compensated by scattering with phonons, defects, or surfaces.