Optical resonances in dimer of spherical metallic nanoparticles

The paper studies resonant optical phenomena in the dimer, which consists of spherical metallic nanoparticles. The relations for the diagonal components of the polarizability tensor, spectral figure of merit tensor, extinction cross-section, as well as for the frequencies of the transverse and longitudinal optical resonance are obtained. The qualitative similarity of the frequency dependences of the real and imaginary parts, as well as the modulus of the transverse and longitudinal polarizability, has been established, and the spectral shift of the extrema for the transverse component of the polarizability with respect to the longitudinal one has been determined. It has been shown that decreasing the gap between the nanoparticles in the dimer leads to increased scattering and decreased energy absorption associated with longitudinal electric fields, as well as increased splitting of the frequencies of the transverse and longitudinal optical resonances. The quantitative agreement between the theoretical and experimental results for dimers of spherical Au nanoparticles with respect to optical resonance splitting is established. In addition to the convergence of nanoparticles, the increase in the permittivity of the environment, and the use of metallic nanoparticles with a higher plasma frequency and a lower contribution of interband transitions to the permittivity to create the dimer also lead to the increase in resonance splitting. The size dependences of the diagonal components of the spectral figure of merit tensor were investigated. It is demonstrated that the behavior of the transverse and longitudinal spectral figure of merit is significantly different when the distance between the nanoparticles of moderate radius in the dimer is decreased. It was established that the longitudinal spectral figure of merit of the dimer of small nanoparticles, located at a sufficiently large distance from each other, has a sharp maximum. The reason for this maximum is the large change in the longitudinal optical resonance frequency with a small change in the permittivity of the surrounding dielectric and the decrease in scattering of the longitudinal electric fields. The feasibility of using the dimer of small and sufficiently distant nanoparticles as the sensitive element of a sensor based on longitudinal optical resonance is demonstrated.