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Optical metasurfaces, typically referred to as two-dimensional metamaterials, are arrays of engineered subwavelength inclusions suitably designed to tailor the light properties, including amplitude, phase and polarization state, over deeply subwavelength scales. By exploiting anomalous localized interactions of surface elements with optical waves, metasurfaces can go beyond the functionalities offered by conventional diffractive optical gratings. The innate simplicity of implementation and the distinct underlying physics of their wave-matter interaction distinguish metasurfaces from three-dimensional metamaterials and provide a valuable means of moulding optical waves in the desired manner. Here, we introduce a general approach based on the electromagnetic equivalence principle to develop and synthesize graded, non-periodic metasurfaces to generate arbitrarily prescribed distributions of electromagnetic waves. Graded metasurfaces are realized with a single layer of spatially modulated, electrically polarizable nanoparticles, tailoring the scattering response of the surface with nanoscale resolutions. We discuss promising applications based on the proposed local wave management technique, including the design of ultrathin optical carpet cloaks, alignment-free polarization beam splitters and a novel approach to enable broadband light absorption enhancement in thin-film solar cells. This concept opens up a practical route towards efficient planarized optical structures with potential impact on the integrated nanophotonic technology. [ABSTRACT FROM AUTHOR]

Electrically tunable mid‐infrared metasurfaces with nanosecond response time and broad tuning range are reported. Electrical tuning is achieved by employing strong polaritonic coupling of electromagnetic modes in metallic nanoresonators with voltage‐tunable inter‐subband transitions in semiconductor heterostructures, tailored for a giant quantum‐confined Stark effect. Experimentally, a 220‐nm‐thick multi‐quantum‐well semiconductor layer is sandwiched between a ground plane and a metal layer patterned with plasmonic nanoresonators. Approximately 300 nm absorption peak tuning and over 30% absorption change are demonstrated at around 7 μm wavelength at normal incidence by changing the DC bias voltage from 0 to +5 V. Fast reflectivity modulation of the metasurface is shown with a response time less than ∼10 ns using bias voltage modulation between +2 V and +4 V. Since the bias affects the optical response at the individual nanoresonator level, this approach may be used to create metasurfaces for ultrafast electrical wavefront tuning and beam steering. [ABSTRACT FROM AUTHOR]

Metamaterial perfect absorbers (MPAs) are artificial materials composed of an array of subwavelength structures that manipulate electromagnetic waves to achieve extraordinary light absorption properties. Driven by the advent of the Internet of Things, MPAs are employed in microelectromechanical systems for the development of efficient and miniaturized IR detectors, imagers, and spectrometers, thanks to their lithographically tunable peak absorption, spectral selectivity, and ultrathin thickness. MPAs characterized by high absorptance in narrow spectral bands are particularly desirable for the implementation of high‐resolution IR spectroscopic sensors. Yet, no accurate analytical model is currently available to guide the design of an MPA with ultra‐narrow absorption bandwidth, while meeting all the stringent requirements for spectroscopic sensors. Here, a circuit model capable of accurately predicting spectral responses of metal–insulator–metal (MIM) IR absorbers is reported. The model is experimentally validated in the mid‐wavelength IR spectral range and exploited for the first demonstration of an MIM IR absorber that exhibits performance approaching the predicted physical limits: full‐width at half‐maximum ≈3% and near‐unity absorption (η > 99.7%) at 5.83 µm wavelength, while independent of incident angle and polarization of the impinging IR radiation. These unprecedented absorption properties are key enablers for the development of miniaturized, low‐cost, and high‐resolution spectrometers. Ultra‐narrowband metamaterial infrared absorbers and the modified lumped circuit model are presented. The accurate spectral responses obtained by the circuit model are utilized to optimize the design of metal–insulator–metal (MIM) infrared absorbers for high spectral selectivity. The model is experimentally validated and exploited for the first demonstration of a MIM infrared absorber that exhibits performance approaching the predicted physical limits. [ABSTRACT FROM AUTHOR]