Figure 1. Schematics of a surface-enhanced light scattering process.

(a) Inelastic scattering process (ω₁≠ω₂) from an object (O) in the presence of a metal nanostructure that acts as an optical antenna (A). (b) Elastic scattering process (ω₁=ω₂=ω). E_inc(ω) denotes the incident field, E_A(ω) is the field directly radiated by the antenna, and E_AOA(ω) is the field radiated by the object via the antenna. To select E_AOA, the antenna–object distance d is modulated at frequency Ω, and the detector signal is demodulated at the higher harmonic frequency nΩ.