An extended analytical method based on the dipole-quadrupole electromagnetic interaction is proposed to investigate the optical properties of strongly interacting plasmonic nanoparticles for which the known coupled dipoles approximation (CDA) is inaccurate. The introduced simple and novel method used here, namely coupled dipole-quadrupole approximation (CDQA), is used to elaborate on the optical interactions of individual modes including dipole-dipole, dipole-quadrupole and quadrupole-quadrupole. A simple and versatile formula is presented for the modified dipole-polarizability by considering an adjacent quadrupole effect, leading to accurate prediction of remarkable features in the optical properties of nanoparticle clusters in simple or complex forms. Interestingly, in a nanodimer configuration, it is shown that the quadrupole strongly affects the dipolar resonance energy, though the dipole impact on quadrupole properties are negligible. The findings are verified by the approximated methods, numerical computations and generalized Mie theory.
Wave scattering from a human cornea illuminated with a submillimeter-wave Gaussian beam is explored with Fourier analysis. This new approach enabled us to investigate the cornea as a coated sphere rather than a homogenized one. The cornea was modeled as an aqueous spherical shell using effective medium theory, with 60 percent water, enclosing a sphere of pure water. The corneal model was illuminated at 220 GHz -330 GHz. The interaction of the incident and back-reflected beam, backscattered field, and back-scattering from one usual beam-cornea alignment scheme were evaluated; beam waist collocated with the surface apex. The result indicates the amount of difference between the reflection from planar stratified and back-scattering from the cornea in the case of focusing the beam waist at the corneal apex.
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