The multipole expansion is a key tool in the study of light-matter interactions. All the information about the radiation of and coupling to electromagnetic fields of a given charge-density distribution is condensed into few numbers: The multipole moments of the source. These numbers are frequently computed with expressions obtained after the long-wavelength approximation. Here, we derive exact expressions for the multipole moments of dynamic sources that resemble in their simplicity their approximate counterparts. We validate our new expressions against analytical results for a spherical source, and then use them to calculate the induced moments for some selected sources with a nontrivial shape. The comparison of the results to those obtained with approximate expressions shows a considerable disagreement even for sources of subwavelength size. Our expressions are relevant for any scientific area dealing with the interaction between the electromagnetic field and material systems.PACS numbers: 78.67. Pt, 13.40.Em,78.67.Bf, 03.50.De The multipolar decomposition of a given chargecurrent distribution is taught in every undergraduate course in physics. The resulting set of numbers are called the multipolar moments. They are classified according to their order, i.e. dipoles, quadrupoles etc... For each order, there are electric and magnetic multipolar moments. Each multipolar moment is uniquely connected to a corresponding multipolar field. Their importance stems from the fact that the multipolar moments of a charge-current distribution completely characterize both the radiation of electromagnetic fields by the source, and the coupling of external fields onto it. The multipolar decomposition is important in any scientific area dealing with the interaction between the electromagnetic field and material systems. In particle physics, the multipole moments of the nuclei provide information on the distribution of charges inside the nucleus. In chemistry, the dipole and quadrupolar polarizabilities of a molecule determine most of its properties. In electrical engineering, the multipole expansion is used to quantify the radiation from antennas. And the list goes on.In this Letter, we present new exact expressions for the multipolar decomposition of an electric charge-current distribution. They provide a straightforward path for upgrading analytical and numerical models currently using the long-wavelength approximation. After the upgrade, the models become exact. The expressions that we provide are directly applicable to the many areas where the multipole decomposition of electrical current density distributions is used. For the sake of concreteness, in this article we apply them to a specific field: Nanophotonics.In nanophotonics, one purpose is to control and manipulate light on the nanoscale. Plasmonic or highindex dielectric nanoparticles are frequently used for this purpose 1,2 . The multipole expansion provides insight into several optical phenomena, such as Fano resonances 3,4 , electromagnetically-induced-transparen...

Metamaterial-based perfect absorbers promise many applications. Perfect absorption is characterized by the complete suppression of transmission and reflection and complete dissipation of the incident energy by the absorptive meta-atoms. A certain absorption spectrum is usually assigned to a bulk medium and serves as a signature of the respective material. Here we show how to use graphene flakes as building blocks for perfect absorbers. Then, an absorbing meta-atom only consists of a molecular monolayer placed at an appropriate distance from a metallic ground plate. We show that the functionality of such device is intuitively and correctly explained by a Fabry-Perot model.

A nanoantenna with balanced electric and magnetic dipole moments, known as the first Kerker condition, exhibits a directive radiation pattern with zero backscattering. In principle, a nanoantenna can provide even better directionality if higher order moments are properly balanced. Here, we study a generalized Kerker condition at the example of a nanoring nanontenna supporting electric dipole and electric quadrupole moments. Nanoring antennas are well suited since both multipole moments can be almost independently tuned to meet the generalized Kerker condition.

Based on an analytic approach, we present a theoretical review on the absorption, scattering, and extinction of both dipole scatterers and regular arrays composed of such scatterers i.e., metasurfaces. Besides offering a tutorial by outlining the maximum absorption limit for electrically/magnetically resonant dipole particles/metasurfaces, we give an educative analytical approach to their analysis. Moreover, we put forward the analysis of two known alternatives in providing perfect absorbers out of electrically and or magnetically resonant metasurfaces; one is based on the simultaneous presence of both electric and magnetic responses in so called Huygens metasurfaces while the other is established upon the presence of a back reflector in so called Salisbury absorbers. Our work is supported by several numerical examples to clarify the discussions in each stage.