Non-uniform metasurfaces (electrically thin composite layers) can be used for shaping refracted and reflected electromagnetic waves. However, known design approaches based on the generalized refraction and reflection laws do not allow realization of perfectly performing devices: there are always some parasitic reflections into undesired directions. In this paper we introduce and discuss a general approach to the synthesis of metasurfaces for full control of transmitted and reflected plane waves and show that perfect performance can be realized. The method is based on the use of an equivalent impedance matrix model which connects the tangential field components at the two sides on the metasurface. With this approach we are able to understand what physical properties of the metasurface are needed in order to perfectly realize the desired response. Furthermore, we determine the required polarizabilities of the metasurface unit cells and discuss suitable cell structures. It appears that only spatially dispersive metasurfaces allow realization of perfect refraction and reflection of incident plane waves into arbitrary directions. In particular, ideal refraction is possible only if the metasurface is bianisotropic (weak spatial dispersion), and ideal reflection without polarization transformation requires spatial dispersion with a specific, strongly non-local response to the fields.

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.

Recent advances in engineered gradient metasurfaces have enabled unprecedented opportunities for light manipulation using optically thin sheets, such as anomalous refraction, reflection, or focusing of an incident beam. Here we introduce a concept of multi-channel functional metasurfaces, which are able to control incoming and outgoing waves in a number of propagation directions simultaneously. In particular, we reveal a possibility to engineer multi-channel reflectors. Under the assumption of reciprocity and energy conservation, we find that there exist three basic functionalities of such reflectors: Specular, anomalous, and retro reflections. Multi-channel response of a general flat reflector can be described by a combination of these functionalities. To demonstrate the potential of the introduced concept, we design and experimentally test three different multi-channel reflectors: Three-and five-channel retro-reflectors and a three-channel power splitter. Furthermore, by extending the concept to reflectors supporting higher-order Floquet harmonics, we forecast the emergence of other multiple-channel flat devices, such as isolating mirrors, complex splitters, and multi-functional gratings.arXiv:1610.04780v2 [physics.optics]

Azimuthally polarized beams are gaining fundamental importance for near-field force microscopy systems to inspect photoinduced magnetism in special molecules or nanostructures, due to their strong axial magnetic field and vanishing electric field. The magnetic dominant region represents a unique trait of such a beam as a potentially ideal structured light to probe photoinduced magnetism at the nanoscale. Therefore, we present a near-field characterization of an optical, sharply focused azimuthally polarized beam using photoinduced force microscopy, a technique with simultaneous near-field excitation and detection, achieving nanoscale resolution well beyond the diffraction limit. Such a method exploits the photoinduced gradient force on a nanotip, mechanically detected as forced oscillations of the cantilever in an atomic force microscopy system upon external light illumination. The photoinduced force is strongly localized, which that depends only on the near-field signal free from background scattering photons, granting photoinduced force microscopy a superior performance over its precedent near-field scanning optical microscopy. We develop an analytical model to correct the tip-induced measurement anisotropy, suppress the background noise, and reveal the local electric field distribution of the azimuthally polarized beam. These measurements are used to retrieve its strong longitudinal axial magnetic field at the center of the polarization vortex where the electric field vanishes. This study can lead to a plethora of possibilities in optomechanical, chemical, or biomedical applications. We also propose and discuss how to use such beams with polarization azimuthal symmetry as a way to calibrate microscope nanotips.