Self-Similar Chain of Metal Nanospheres as an Efficient Nanolens

As an efficient nanolens, we propose a self-similar linear chain of several metal nanospheres with progressively decreasing sizes and separations. To describe such systems, we develop the multipole spectral expansion method. Optically excited, such a nanolens develops the nanofocus (“hottest spot”) in the gap between the smallest nanospheres, where the local fields are enhanced by orders of magnitude due to the multiplicative, cascade effect of its geometry and high $Q$ factor of the surface plasmon resonance. The spectral maximum of the enhancement is in the near-ultraviolet region, shifting toward the red region as the separation between the spheres decreases. The proposed system can be used for nanooptical detection, Raman characterization, nonlinear spectroscopy, nanomanipulation of single molecules or nanoparticles, and other applications.

Figure 1

Local fields (absolute value relative to that of the excitation field) in the equatorial plane of symmetry for the linear self-similar chain of three silver nanospheres. The ratio of the consecutive radii is $\kappa =R_{i+1}/R_i=1/3$; the distance between the surfaces of the consecutive nanospheres $d_{i,i+1}=0.6R_{i+1}$. Inset: the geometry of the system in the cross section through the equatorial plane of symmetry.

Figure 2

Same as in Fig. 1, but for the distance between the sphere surfaces $d_{i,i+1}=0.3R_{i+1}$.

Figure 3

Local fields (absolute value relative to that of the excitation field) in the cross section through the equatorial plane of symmetry for the linear center-symmetric self-similar chain of five silver nanospheres. The ratio of the consecutive radii is $\kappa =R_{i+1}/R_i=1/3$; the distance between the surfaces of the consecutive nanospheres $d_{i,i+1}=0.6R_{i+1}$. Inset: the geometry of the system in the cross section through the equatorial plane of symmetry.

Figure 4

Same as in Fig. 3, but for the symmetric chain of six nanospheres.