The optical interaction of circularly polarized light with structurally chiral optical antennas results in chiral nanophotonics and antenna-enhanced chiral molecular sensing. Here we develop a simple analytical framework, connecting the far-field chiroptical transmission (circular dichroism) of feasible 2D and 3D nanoantennas with the electromagnetic polarizability of their elements. We report that the real and imaginary parts of the polarizability determine the line shape of the circular dichroism spectra, while the optical coupling of the comprising elements controls the spectral width and the absolute amplitude. On the example of the coupled-nanorods antennas, as a fundamental nanoplasmonic structure, we visualize the effect of the structural parameter variation on properly addressing and controlling the observed chiroptical effects.

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As a significant characteristic of many biomolecules, chemical substances, and artificial nanostructures, chirality conduce different types of optical interactions with the spin angular momentum of the impinging light field. Although, chiral arrangement and spatial phase retardation are the key factors for obtaining chirality in three-dimensional (3D) structures, the origin of chirality in the feasible planar structures has not been thoroughly addressed. Here using an intuitive and simple analytical approach, called cross-hybridization model, the essence and properties of the optical chirality of individual planar nanostructures are unveiled. In order to fundamentally address this chirality in terms of circular dichroism (CD), the chiroptical response of a simple dimer composed of the lossy nanoblocks in L-shape arrangement are investigated based on the provided optical interaction and loss effects. The theoretical findings, adequately supported by the numerical calculations, reveal that chiroptical activity occurs predominantly due to handedness-dependent absorption or heating loss in a nanostructured metasurface.

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.