Fano Profiles Induced by Near-Field Coupling in Heterogeneous Dimers of Gold and Silver Nanoparticles

The near-field coupling between a gold and a silver spherical nanoparticle is investigated theoretically. Fano profiles are observed in the absorption cross section of the gold nanoparticle due to the coupling between the spectrally localized surface plasmon resonance of the silver nanoparticle and the continuum of interband transitions of the gold one. The effect of dimer internal characteristics (particle sizes and distance), surrounding medium (through the refractive index), and external excitation (polarization and propagation directions) are addressed. In particular, it is shown that the near-field coupling can be tuned from the weak to the strong regime by rotating the polarization direction, and that the Fano profiles are sensitive to the shadowing effect even for small particle sizes.

Figure 1

Normalized absorption cross sections of the silver and gold nanoparticles. The dashed and full lines correspond to the case of isolated particles or particles within the dimer, respectively. The overall absorption cross section of the coupled particles is given by the bold line. The radius of both particles is 5 nm and the interparticle distance 3 nm. The incoming light is polarized parallel to the dimer axis. The gold and the silver particles are represented by the light gray and the gray disks, respectively.

Figure 2

Normalized absorption cross section of a $R_{\mathrm{Au}}=5\mathrm{nm}$ gold nanoparticle (a) as a function of the interparticle distance $d$ (from top to bottom $d$ ranges from 1 to 10 nm by steps of 1 nm) for a fixed silver size ($R_{\mathrm{Ag}}=5\mathrm{nm}$) and (b) as a function of silver particle size (from the bottom to the top $R_{\mathrm{Ag}}=2.5$, 5, 10, 15, 20, and 30 nm) for a fixed interparticle distance $d=3\mathrm{nm}$. The dash-dotted line is used for $R_{\mathrm{Ag}}=30\mathrm{nm}$. The dashed curves correspond to the absorption cross section of an isolated gold particle. The field configuration is the same as in Fig. 1.

Figure 3

Normalized absorption cross section of a $R=5\mathrm{nm}$ gold nanoparticle as a function of the angle $\gamma$ between the incoming electric field and the dimer axis (the wave vector is perpendicular to the dimer axis). The bold lines from top to bottom (as indicated by the arrow) correspond to $\gamma =0$, $\pi /3$, and $\pi /2$, respectively. For the thin lines, $\gamma$ increases by steps of 10 degrees. The dashed curves correspond to the absorption cross section of an isolated gold particle. The radius of the silver nanoparticle is 5 nm and the interparticle distance 3 nm.

Figure 4

Normalized absorption cross section of a $R=5\mathrm{nm}$ gold nanoparticle as a function of the orientation of the dimer with respect to the incoming wave vector. The light gray and gray disks correspond to the gold and silver particles, respectively. The incoming electric field is always perpendicular to the dimer axis ($\gamma =\pi /2$) and the dashed curves correspond to the absorption cross section of an isolated gold particle.