Abstract
AbstractThe design of far‐field radiation diagrams from combined electric and magnetic dipolar sources has recently found applications in nanophotonic metasurfaces that realize tailored reflection and refraction. Such dipolar sources also exhibit important near‐field evanescent coupling properties with applications in polarimetry and quantum optics. Here, a rigorous theoretical framework is introduced for engineering the angular spectra encompassing both far‐ and near‐fields of electric and magnetic sources and a unified description of both free space and guided mode directional radiation is developed. The approach uses the full parametric space of six complex‐valued components of magnetic and electric dipoles in order to engineer constructive or destructive near‐field interference. Such dipolar sources can be realized with dielectric or plasmonic nanoparticles. It is shown how a single dipolar source can be designed to achieve the selective coupling to multiple waveguide modes and far‐field simultaneously with a desired amplitude, phase, and direction.
Highlights
Amplitude and Phase Control of Guided Modes Excitation from a Single Dipole Source: Engineering Far- and Near-Field Directionality
The approach uses the full parametric space of six complex-valued components of magnetic and electric dipoles in order to engineer constructive or destructive near-field interference. Such dipolar sources can be realized with dielectric or plasmonic nanoparticles. It is shown how a single dipolar lar sources, remarkable physics lies in the near-field[18] where dipolar sources can be used for directional excitation of waveguided modes, relevant to integrated photonics
Which can be modeled as dipolar sources, has been initially studied based on circularly polarized dipoles,[19,20,21,22,23,24,25,26,27] and can be related to the spin-momentum locking characteristic of guided modes.[28,29,30]
Summary
The coupling of a dipole to a waveguide can be understood via Fermi’s Golden Rule, where directional excitation of modes is interpreted as destructive interference between different dipolar components. The design of the far-field of a source, that is, its radiation diagram, is a common practice in antenna engineering,[36] and it can be interpreted as the design of the source’s angular spectrum (Equation (1)) in the region of transverse momenta inside the light cone (kt < k ⇒ kz ∈ R), where k2t = k2x + k2y is the transverse wavevector, and each angular component E(kx, ky) corresponds to far-field radiation in a different direction in space Such radiation diagram engineering is applied in nanophotonics for the design of asymmetric far-field scattering, reflection and transmission of photonic metasurfaces.[3,4,5,6,7,8,9,10] In this work, we apply the engineering of the source angular spectrum beyond the light cone (kt > k) in a region of complex wave-vectors (kz ∈ I), to encompass the whole transverse momentum space.[15,34,37,38] In this way, we include near field evanescent waves, responsible for the evanescent coupling of the source to guided modes in a nearby waveguide or surface modes on an adjacent surface
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