Fig. 1. Scattering cross sections and their spherical multipole decomposition calculated for (a) Au heptamer, (b) passive Au−SiO2 heptamer with gain coefficient κ=0, and (c) active Au−SiO2 heptamer with gain coefficient κ=0.28. The structure schematics are shown in the insets.

Fig. 2. Contributions of the spherical ED and the Cartesian ED and TD into the scattering cross sections of an Au−SiO2 heptamer with the insets of near-field distributions E/E0 on top of Au disks (at the plane of z=50  nm) at 900 nm (a) for a passive nanosystem κ=0 and (c) for an active nanosystem κ=0.28. (e) Silicon nanosphere. The E-field directions and the polarized directions of Au metamolecules are indicated by the white arrows and the yellow arrows, respectively. (b), (d), and (f) Corresponding normalized phase of p and ikT.

Fig. 3. (a) FEM calculation (solid curves) and ECO model fit (black dot curves) of the total scattering cross sections of an Au−SiO2 heptamer with the gain coefficients varying from 0 to 0.37. (b) First oscillator (Cartesian ED moment) damping coefficient γ1 and second oscillator (Cartesian TD moment) damping coefficient γ2. (c) Coupling strength υ12.

Fig. 4. Scattering cross sections and their spherical multipole decomposition calculated for Au-SiO2 thirteen polymer with the near-field distribution E/E0 on top of Au disks (at the plane of z=50  nm) for an active nanosystem (a) κ=0 and (b) κ=0.32.