We characterize the modes with complex wavenumber for both longitudinal and transverse polarization states (with respect to the mode traveling direction) in three dimensional (3D) periodic arrays of plasmonic nanospheres, including metal losses. The Ewald representation of the required dyadic periodic Green's function to represent the field in 3D periodic arrays is derived from the scalar case, which can be analytically continued into the complex wavenumber space. We observe the presence of one longitudinal mode and two transverse modes, one forward and one backward. Despite the presence of two modes for transverse polarization, we notice that the forward one is "dominant" (i.e., it contributes most to the field in the array). Therefore, in case of transverse polarization, we describe the composite material in terms of a homogenized effective refractive index, comparing results from (i) modal analysis, (ii) Maxwell Garnett theory, (iii) Nicolson-Ross-Weir retrieval method from scattering parameters for finite thickness structures (considering different thicknesses, showing consistency of results), and (iv) the fitting of the fields obtained through HFSS simulations. The agreement among the different methods justifies the performed homogenization procedure in case of transverse polarization.
Bound and leaky modes with complex wavenumber in chains (linear arrays) of plasmonic nanospheres are characterized for both longitudinal and transverse polarization states (with respect to the array axis). The proposed method allows for the description of each mode evolution when varying frequency. As a consequence, full characterization of the guided modes with complex wavenumber is provided in terms of propagation direction, guidance or radiance, proper or improper, and physical or nonphysical conditions. Each nanosphere is modeled according to the single dipole approximation, and the metal permittivity is described by the Drude model. Modal wavenumbers are obtained by computing the complex zeroes of the homogeneous equation characterizing the field in the one dimensional periodic array. The required periodic Green's function is analytically continued into the complex wavenumber space by using the Ewald method. Furthermore, a parametric analysis of the mode wavenumbers is performed with respect to the geometrical parameters of the array.
Effective model and investigation of the near-field enhancement and subwavelength imaging properties of multilayer arrays of plasmonic nanospheres When a small radiating or scattering object is placed near a multilayer array of plasmonic nanospheres, on the other side the optical near field is enhanced due to the excitation of resonant modes in the layers. For some particular frequencies, the field behind the array is concentrated in a subwavelength region, creating a super resolution effect. Resonating layers are able to reproduce (transport) part of the evanescent spectrum to the other side of these layers which otherwise would decay rapidly. We explore the mechanism of evanescent field transport and subwavelength field concentration on the other side of the layered material and show the relationship between near-field enhancement, field concentration, and modal dispersion characteristics. A detailed investigation of these phenomena is carried out by using an effective numerical model based on the array scanning method (ASM) combined with the Ewald method to accelerate the convergence of the dyadic Green function calculation. The subwavelength-sized spheres forming the arrays are represented as single dipole radiators, and the model of their interactions takes into account all the radiative and reactive field components.
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