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In this work, a momentum-space geometrical structure in helical evanescent electromagnetic waves is revealed. It is shown that for every helical evanescent wave on a helicity-dependent half tangent line in momentum space, the orientation of each of its field, spin, and Poynting vectors is the same. This geometric structure reveals itself as a remarkable relation between the far-field and near-field components of the angular spectrum. Any general evanescent wavevector is linked to two points on the $k_{rho}=k_0$ circle of propagating wavevectors via two helicity-dependent tangent lines. Knowing the field on the $k_{rho}=k_0$ circle of a general dipolar source is sufficient to determine its entire evanescent angular spectrum. Applying this concept, we gain insights into near-field directionality by showing that every zero in the angular spectrum is a helicity singularity where two half-tangent lines of opposite helicity intersect. A powerful method for synthetic design of near-field directional sources is also devised, using structured helical illumination to gain full control of the near-field directionality. The results provide a fundamental insight of helical evanescent waves and have implications in areas where chiral light-matter interaction plays a central role.
Directional excitation of guidance modes is central to many applications ranging from light harvesting, optical information processing to quantum optical technology. Of paramount interest, especially, the active control of near-field directionality p
Intense electromagnetic evanescent fields are thermally excited in near fields on material surfaces (at distances smaller than the wavelength of peak thermal radiation). The property of the fields is of strong interest for it is material-specific and
There has been significant interest in imaging and focusing schemes that use evanescent waves to beat the diffraction limit, such as those employing negative refractive index materials or hyperbolic metamaterials. The fundamental issue with all such
Abbes resolution limit, one of the best-known physical limitations, poses a great challenge for any wave systems in imaging, wave transport, and dynamics. Originally formulated in linear optics, this Abbes limit can be broken using nonlinear optical
Far-field directional scattering and near-field directional coupling from simple sources have recently received great attention in photonics: beyond circularly-polarized dipoles, whose directional coupling to evanescent waves was recently applied to