Polarization-independent Fano resonances in arrays of core-shell nanoparticles

We study the scattering properties of arrays of core-shell nanoparticles and reveal the existence of polarization-independent Fano resonances. Such Fano resonances occur through the interaction of a pair of degenerate orthogonal electric and artificial magnetic modes of the same strength with the spectrally narrow geometric resonance due to the diffractive coupling in the array. Furthermore, we show that for different polarizations of the incident plane waves, the electric and magnetic modes supported by each nanoparticle can be selectively controlled, providing extra freedom for near-field manipulation, with applications to nonlinear and lasing devices.

Figure 1

Extinction efficiency spectra for (a) silver sphere of radius 50 nm and (b) CSNP (silver core and dielectric shell of $n=3.5$) with inner radius 38 nm and outer radius 150 nm. The silver sphere can be viewed as an ED [top of panel (a)] and the CSNP can be approximated as a pair of orthogonal ED and MD, which coincide spectrally with the same strength [top of panel (b)]. (c) The schematic geometry of the one-dimensional (1D) structure under consideration. The unit cell particle could be either a silver sphere or CSNP. The incident plane wave is propagating along $z$ and the polarization angle (electric field with respect to $x$ direction) is $\theta$.

Figure 2

CDA and FDTD results of the extinction efficiency spectra for a 1D array of (a) and (b) silver spheres with $d=0.5\mu$m; (c) and (d) CSNP with $d=1.15\mu$m for five polarization angles of $\theta =0^{\circ },{25}^{\circ },{45}^{\circ },{75}^{\circ },\text{and}{90}^{\circ }$. For both cases the Fano dips are located at $\lambda =d$. Parameters for single particles are the same as those in Fig. 1.

Figure 3

(a) A dielectric ($n=3.5$) shell structure with hollow core that support a MD resonance. The inner radius is 38 nm, the outer radius is 150 nm, and the central MD resonant wavelength is $\lambda =1095$ nm. (b) Corresponding near-field (normalized electric and magnetic field) distributions. (c)–(h) Normalized near-field distributions of a unit cell in a 1D infinite array of CSNPs for three polarization angles of $\theta =0^{\circ },{45}^{\circ },\text{and}{90}^{\circ }$ at the Fano peak position $\lambda =1159.5$ nm. Other parameters are the same as in Figs. 2 and 1.

Figure 4

(a) Schematic geometry of the 2D rectangular lattice with lattice constants $d_x$ and $d_y$ along vertical and horizontal directions respectively. Extinction efficiency spectra of (b)–(d) the Ag sphere lattice with $d_x=0.45\mu$m and $d_y=0.43\mu$m and (e)–(g) the CSNP lattice with $d_x=1.17\mu$m and $d_y=1.11\mu$m. For both lattices the results of three polarization angles are shown with $\theta =0^{\circ },{45}^{\circ },\text{and}{90}^{\circ }$. Parameters of single particles are the same as in Fig. 1. $F_x$ and $F_y$ mark the peaks of the Fano resonances arising from coupling between particles along $x$ and $y$ directions, respectively.