Metasurfaces – from science to applications

Metasurfaces are artificial photonic structures that allow exceptional control of the optical wavefront over deeply subwavelength thicknesses, realizing several optical functionalities of interest, including beam steering, focusing, and holography, in planarized devices that are extremely thin.In order to enable this unusual response, it is imperative to strongly enhance the interaction between light and matter.Multiple techniques have been explored in this context, which range from dielectric arrayed resonators, to plasmonic antennas or waveguide-like nanostructures.This special issue includes contributions of leading experts in the field of metasurfaces, surveying the latest approaches to manipulate light in extreme ways over a surface, reviewing, and detailing both metasurface-based scientific advances and technological opportunities in this field.This collection of articles is dedicated to a pioneering scientist -Dr.Federico Capasso -one of the leading experts in this field.We open the issue with a personal review article from Dr. Capasso himself on "The future and promise of flat optics: a personal perspective".A question still being debated and actively explored in the field is the choice of materials for metasurfaces, because their resulting properties determine the response and thus the application functionality.The review article "Materials platforms, metasurface, plasmonics, dielectric metasurface" by Choudhury et al. discusses a wide variety of metasurface materials to include dielectric, metallic, meta-oxides, 2-dimensional materials, and semiconductors. Dielectric metasurfacesThe article "Metasurfaces and their applications" by Li et al. provides a general overview of metasurface research, reviewing a variety of physical concepts that enable various functionalities, including beam shaping, cloaking, polarization and phase control, amplitude or phase modulation, and imaging.A goal in metasurface technology is to enable efficient harmonic generation using nonlinear phenomena.The paper "Controlling the phase of optical nonlinearity with plasmonic metasurfaces" by Chen et al. shows how tailoring the local polarizability allows controlling both the amplitude and phase of nonlinear phenomena by using the rotational symmetry of the meta-atom.The key here is that the inversion of the incoming light being circularly polarized is accompanied by a geometric phase, called the Pancharatnam-Berry phase.This leads to an antenna-orientation controlled phase that does not depend on the specific antenna design or wavelength of light.Such phase-control can be used in nonlinear holography, vortex beam generation for OAM or two-dimensional flat optical lenses.Many metasurface applications and demonstrations aim to control a free-space light beam reflecting off, or transmitting through the metasurface.The contribution "Molding light with metasurfaces: from far-field to near-field interactions" by Gangaraj and Monticone, instead, discusses how metasurfaces enable near-field effects to control the generation and propagation of surface waves.For instance, anisotropic and hyperbolic metasurfaces show some of the most exciting and extreme examples of anomalous surface-wave propagation on planarized artificial structures, with important implications for light focusing, confinement, and subwavelength imaging.The authors also connect these ideas with the emerging area of two-dimensional materials, and discuss how to implement hyperbolic metasurfaces with graphene and black phosphorus.The contribution "A review of dielectric optical metasurfaces for wavefront control" by Kamali et al. provides an overview of recent developments of metasurfaces made of dielectric structures.In particular, they show how to achieve steep phase gradients with high efficiency, simultaneous polarization and phase control, control of the chromatic dispersion, or control of the angular response.The low-optical absorption of dielectric-based metasurfaces enables applications in imaging, conformal optics, tunable devices, and optical systems.The two articles "Bianisotropic metasurfaces: physics and applications" by Asadchy et al. and "Design, concepts and applications of electromagnetic metasurfaces" by Achouri and Caloz discuss, amongst others, bianisotropy in metasurfaces.These are specific metasurfaces providing strong magnetoelectric coupling at the surface, which enables