(a) Illustration of charge distributions of different orders of surface plasmon resonant modes. (b) Corresponding electric field patterns of a silver nanosphere (Drude model, , ω_p = 9.149 eV, τ = 3.1 × 10⁻¹⁴ s) in air calculated by Mie theory (from left to right, R = 20 nm, λ = 335 nm; R = 30 nm, λ = 315 nm; R = 55 nm, λ = 310 nm; R = 100 nm, λ = 315 nm; R = 300 nm, λ = 335 nm). (c) Illustration of the assumption of a surface plasmon resonant mode of a spherical nanoparticle and surface plasmons on the interface between plane metal and dielectric medium. The surface charges of an m = 4 order surface plasmon resonance can be viewed as a standing wave of an SPP wavelength λ_θ, which is formed as two waves (clockwise and counterclockwise). It is equivalent to the surface charge distribution of a surface plasmon wave on a plane interface between the metal and dielectric medium.