Tuning of narrow geometric resonances in Ag/Au binary nanoparticle arrays

We extend the coupled dipole method to a semianalytical method that can be applied with high efficiency and accuracy to metallic heterogeneous binary particle arrays. The spectrum of the binary structure, which is composed of alternating silver and gold spherical nanoparticles, is characterized by additional geometric resonances near diffraction orders that originate from the real periodicity (i.e., twice the interparticle distance). The new diffraction orders can force the induced polarization of the heterogeneous particles out of phase, so that light scattered from them interferes destructively, which leads to geometric resonances imposed by destructive interference. By varying the constituent particle sizes, both the width and intensity of the additional geometric resonances can be effectively tuned. In particular, the Fano profiles of the geometric resonances can be tuned and are inverted when the contrast in resonance capabilities of the two types of constituent particles changes. The extensive tunability of the binary structure makes itself highly desirable for design of plasmon-based chemical and biological sensors.