ИСТИНА

Abstract: Structure of the near field (intensity and polarization distribution, the latter is described with the generalized three-dimensional Stokes parameters) of a spherical Si subwavelength nanoparticle in the non-magnetic and non-absorbing media near a dielectric substrate has been studied in detail with the help of Mie theory and an extension of Weyl's method for the calculation of the reflection of dipole radiation by a flat surface. It is shown that for the nanoparticle near the substrate the interference effects due to the scattering by the nanoparticle and interaction with the substrate play an essential role. We also demonstrate how these effects depend on the dielectric properties of the nanoparticle, its size, distance to the substrate and polarization, wavelength and angle of the incident light field. Control of the near-field in the proximity of nanostructures is a key to shaping the spatial intensity of light and its polarization distribution at the nanoscale [1]. Near-field being formed by the interference of the incident electromagnetic field with the local field of the nanoparticle strongly depends both on the polarization of the incident field and the shape and materials of nanoparticles. The orientation of the field vector of local polarization is a key quantity in many theoretical studies on nanooptics, nanophotonic devices and optical sciences in general. Recently, we studied theoretically how the near-field polarization distribution of a plasmonic prolate nanospheroid interacting with a plane electromagnetic wave depends on the polarization and frequency of the incident electromagnetic field [2,3]. Аlong with plasmonic nanoparticles the nanoparticles made of high refractive dielectric or semiconductor materials have recently received considerable attention in the nanophotonics for control and manipulation of light in the near-field [4,5]. They allow direct engineering a magnetic field response at optical frequencies in addition to the electric field response in plasmonic structures. As a fundamental building block, the dielectric spherical nanoparticle presents both strong magnetic dipole and electric dipolar responses corresponding to the basic Mie resonances. In many real applications the nanoparticles are often located near a substrate and the influence of the substrate may be crucial. In our work, we have studied the structure of the near field of a spherical Si nanoparticle in a non-magnetic and non-absorbing media near a dielectric substrate with the help of Mie theory for scattering by a sphere in a homogeneous medium and an extension of Weyl's method for calculation of the reflection of dipole radiation by a flat surface [5,6]. Тhe distribution of the intensity and polarization has been obtained by this method. We consider two types of substrates with the different absorption coefficients: graphite (Im n>1) and fused quartz (Im n<1). Figure shows the distribution of the near-field polarization under the linearly polarized incident field for Si nanoparticles without a substrate (left plot), near a quartz substrate (middle plot) and near a graphite substrate (right plot). Significant differences for the distributions near a substrate are due to the Fresnel reflection coefficients, which introduce a phase shift in each plane wave in the expansion of the field. Fig. The distribution of the degree of polarization of the near-field under the linearly polarized incident field for Si nanoparticles without substrate (left plot), quartz substrate (middle plot) and graphite substrate (right plot). In terms of the generalized three-dimensional Stokes parameters we analyzed the influence of the substrate on the structure of the near-field versus such parameters of the system as the radius and height of the particles over the surface, polarization, wavelength, the angle of the incident wave, and the dielectric constant of the all elements of the system. It is shown that for the nanoparticle near the substrate the interference effects due to the scattering by the nanoparticle and interaction with the substrate play an essential role leading specifically to the much more complicated polarization distribution of the near-field by contrast with the case of a nanoparticle in free space filled with a homogeneous medium. This work has been supported by the Russian Foundation for Basic Research, grant No. 16-02-00816.