polarizability tensor surface plasmon resonance поверхневий плазмонний резонанс тензор поляризовностi dielectric function plasmon resonance polarizability relaxation rate scattering cross-section дiелектрична функцiя плазмонний резонанс поверхневий плазмон швидкiсть релаксацiї - Fermi energy Q-factor absorption absorption and scattering cross-sections absorption cross-section aspect ratio bicone bimetallic nanoparticle bipyramid dipole polarizability dumbbell-shaped nanoparticle effective aspect ratio equivalent prolate spheroid equivalent spheroid equivalent-spheroid approach extinction cross-section