Broadband Optical Metasurfaces
Introduction to Metasurfaces
Optical metasurfaces represent a new class of two-dimensional metamaterials comprising a single layer of metallic nanostructures that possess exceptional capabilities for manipulating light in an ultrathin, planar platform [39, 54, 101]. Metasurfaces also exhibit reduced loss and less fabrication complexity as compared to threedimensional metamaterial devices, making them attractive for integration into practical optical systems . The spectral and spatial dispersion of the metasurface optical response can also be tailored to generate a specific abrupt interfacial phase changes and cross-polarized responses on a subwavelength scale by engineering the nanostructured geometry. Because of these unique capabilities, metasurfaces have been exploited to demonstrate a variety of new physical phenomena and associated optical devices over the past few years, including anomalous reflection and refraction [40, 72, 87, 99], frequency-selective near-perfect absorption [15, 45, 65], optical wavefront manipulation [1, 73, 78], spin-hall effect of light [83, 98], spin-controlled photonics , polarization-dependent unidirectional SPP excitation [41, 63], and metasurface holograms [42, 74].
A potential highly desirable application for metasurfaces is optical waveplates with broadband polarization conversion and a wide FOV. Such devices could be used in systems that perform optical characterization, sensing, and communication functions . Simultaneously achieving broadband and wide-angle performance is difficult using conventional multilayer stacks of birefringent materials because such structures rely on dispersive birefringence properties. By contrast, metasurfaces could provide a pathway toward broadband and wide-angle polarization conversion in an ultrathin, submicron-thick layer.
Several optically thin, metasurface-based polarization-control components have been theoretically proposed and demonstrated, including various polarizers [30, 106], near-field polarization shapers , and ultrathin waveplates [8, 34, 51, 53, 77, 79, 88, 100, 107]. These examples have employed homogenous arrays of weakly coupled anisotropic resonant elements, including crossed nanodipoles [30, 77] and nanoslits [8, 53, 79], L-shaped  or V-shaped  nanoantennas, and elliptical nanoholes . Because the anisotropic optical responses of these elements rely on the resonance of each isolated element, where strong dispersion and impedance mismatches may exist, these structures typically suffer from a narrow FOV, limited bandwidth, and/or low efficiency.
More recently, a near-IR quarter-wave plate composed of an array of orthogonally coupled nanodipole elements was reported that achieved broadband circular-to-linear polarized light conversion. However, this example provided an average power efficiency of less than 50% . In the next section, we will demonstrate that broadband and wide-angle optical waveplates can be achieved by carefully tailoring the anisotropic properties of the metasurface and the interference of light on a subwavelength scale. A superoctave bandwidth plasmonic half-wave plate as well as a quarter- wave plate are experimentally demonstrated in the near-IR regime, exhibiting polarization conversion ratios and energy efficiencies of 90% or greater.