Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties. These properties have been employed, for example, to create innovative wireless receivers and transmitters. In addition, metasurfaces have recently been proposed to confine electromagnetic waves, thereby avoiding undesired leakage of energy and increasing the overall efficiency of electromagnetic instruments and devices. The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Due to these advantages, they are promising candidates for real-world solutions to overcome the challenges posed by the next generation of transmitters and receivers of future high-rate communication systems that require highly precise and efficient antennas, sensors, active components, filters, and integrated technologies. This Roadmap is aimed at binding together the experiences of prominent researchers in the field of metasurfaces, from which explanations for the physics behind the extraordinary properties of these structures shall be provided from viewpoints of diverse theoretical backgrounds. Other goals of this endeavour are to underline the advantages and limitations of metasurfaces, as well as to lay out guidelines for their use in present and future electromagnetic devices. This Roadmap is divided into five sections: 1. Metasurface based antennas. In the last few years, metasurfaces have shown possibilities for advanced manipulations of electromagnetic waves, opening new frontiers in the design of antennas. In this section, the authors explain how metasurfaces can be employed to tailor the radiation properties of antennas, their remarkable advantages in comparison with conventional antennas, and the future challenges to be solved. 2. Optical metasurfaces. Although many of the present demonstrators operate in the microwave regime, due either to the reduced cost of manufacturing and testing or to satisfy the interest of the communications or aerospace industries, part of the potential use of metasurfaces is found in the optical regime. In this section, the authors summarize the classical applications and explain new possibilities for optical metasurfaces, such as the generation of superoscillatory fields and energy harvesters. 3. Reconfigurable and active metasurfaces. Dynamic metasurfaces are promising new platforms for 5G communications, remote sensing and radar applications. By the insertion of active elements, metasurfaces can break the fundamental limitations of passive and static systems. In this section, we have contributions that describe the challenges and potential uses of active components in metasurfaces, including new studies on non-Foster, parity-time symmetric, and non-reciprocal metasurfaces. 4. Metasurfaces with higher symmetries. Recent studies have demonstrated that the properties of metasurfaces are influenced by the symmetries of their constituent elements. Therefore, by controlling the properties of these constitutive elements and their arrangement, one can control the way in which the waves interact with the metasurface. In this section, the authors analyze the possibilities of combining more than one layer of metasurface, creating a higher symmetry, increasing the operational bandwidth of flat lenses, or producing cost-effective electromagnetic bandgaps. 5. Numerical and analytical modelling of metasurfaces. In most occasions, metasurfaces are electrically large objects, which cannot be simulated with conventional software. Modelling tools that allow the engineering of the metasurface properties to get the desired response are essential in the design of practical electromagnetic devices. This section includes the recent advances and future challenges in three groups of techniques that are broadly used to analyze and synthesize metasurfaces: circuit models, analytical solutions and computational methods.

Since ancient times, plasmonic structural coloring has inspired humanity; glassmakers achieved vibrant colors by doping glass with metal nanoparticles to craft beautiful objects such as the Roman Lycurgus cup and stained glass. These lovely color filtering effects are a consequence of the resonant coupling of light and free electrons in metal nanoparticles, known as surface plasmons. Thanks to the continuing improvement of nanofabrication technology, the dimensions of nanoparticles and structures can now be precisely engineered to form “optical nanoantennas,” allowing for control of optical response at an unprecedented level. Recently, the field of plasmonic structural coloring has seen extensive growth. In this review, we provide an up-to-date overview of various plasmonic color filtering approaches and highlight their uses in a broad palette of applications. Various surface plasmon resonance modes employed in the plasmonic color filtering effect are discussed. We first review the development of the pioneering static plasmonic colors achieved with invariant optical nanoantennas and ambient environment, then we address a variety of emerging approaches that enable dynamic color tuning, erasing, and restoring. These dynamic color filters are capable of actively changing the filtered colors and carrying more color information states than the static systems. Thus, they open an avenue to high-density data storage, information encryption, and plasmonic information processing. Finally, we discuss the challenges and future perspectives in this exciting research area.

Optical metasurfaces are judicously engineered electromagnetic interfaces that can control and manipulate many of light’s quintessential properties, such as amplitude, phase, and polarization. These artificial surfaces are composed of subwavelength arrays of optical antennas that experience resonant light-matter interaction with incoming electromagnetic radiation. Their ability to arbitrarily engineer optical interactions has generated considerable excitement and interest in recent years and is a promising methodology for miniaturizing optical components for applications in optical communication systems, imaging, sensing, and optical manipulation. However, development of optical metasurfaces requires progress and solutions to inherent challenges, namely large losses often associated with the resonant structures; large-scale, complementary metal-oxide-semiconductor-compatible nanofabrication techniques; and incorporation of active control elements. Furthermore, practical metasurface devices require robust operation in high-temperature environments, caustic chemicals, and intense electromagnetic fields. Although these challenges are substantial, optical metasurfaces remain in their infancy, and novel material platforms that offer resilient, low-loss, and tunable metasurface designs are driving new and promising routes for overcoming these hurdles. In this review, we discuss the different material platforms in the literature for various applications of metasurfaces, including refractory plasmonic materials, epitaxial noble metal, silicon, graphene, phase change materials, and metal oxides. We identify the key advantages of each material platform and review the breakthrough devices that were made possible with each material. Finally, we provide an outlook for emerging metasurface devices and the new material platforms that are enabling such devices.

construction. Alternative materials such as transition metal nitrides and complex oxide such as transparent conducting oxides [16] have been also introduced for various plasmonic applications. For alternative material platforms, the primary focus is on low loss, tunability, fabrication compatibility for integrated platforms and high temperature applications. [17] Most reported alternative materials are dielectric or very weakly plasmonic in the blue region of the visible spectrum. [18] For visible frequencies metasurfaces, plasmonic noble metals, such as aluminum [19,20] and silver, [21] have been utilized so far. Noble metals, such as gold and silver, have inherent loss limitations for fully realizing the potentials of plasmonics. [22] While making thin films or nanostructures with silver, the grain-boundaries introduce additional loss in the material [23] and therefore, the full potential of the plasmonic noble metals cannot be utilized. Aluminum is lossy compared to silver in the visible spectrum. On the other hand, dielectric metasurfaces have been introduced with increased efficiency for optical metasurface applications, [24] but with thicknesses much larger than the optical wavelength in the corresponding material.To prepare subwavelength optical elements for visible wavelengths operation, silver is the most suitable elemental material. Silver films grown on transparent substrates are typically polycrystalline. The grain boundaries in the film give rise to additional optical losses, which prohibits the full potential of silver as a plasmonic material from being realized. Silver films, free from grain boundaries, can significantly reduce the losses incurred in polycrystalline films and can therefore reduce the losses. In this paper, we use low loss ultrasmooth epitaxial silver films as the material platform to generate hologram. It has been demonstrated previously that silver (001) can be grown epitaxially on MgO (001) substrate with the help of epitaxial TiN layer. [25] Demonstrated silver films show lowest loss for silver grown on transparent substrate. Monochromatic holograms have been created at subwavelength scale using extreme light confinement of resonant plasmonic structures, such as V-antennas, [26] nanorods, [27] nanopillars, [28] and subwavelength gratings. [29] Principally, the combination of the three basic element colors, red, green, and blue, can be utilized to make a color image. Most plasmonic metals are dielectric or less plasmonic in the blue region. It is therefore difficult to produce a blue color component for multicolor holograms using a plasmonic metasurface. Color holograms have been generated This study demonstrates visible color hologram using a plasmonic metasurface. The metasurface is fabricated by perforating nanoslits in a 50 nm thick monocrystalline silver film that is ultrasmooth and has ultralow loss compared to conventional polycrystalline silver films commonly used in plasmonics. The designed plasmonic hologram is the thinnest metasurface hologram operating in transmission...

Conventional plasmonic materials, namely noble metals, hamper the realization of practical plasmonic devices due to their intrinsic limitations, such as lack of capabilities to tune in real-time their optical properties, failure to assimilate with CMOS-standards, and severe degradation at elevated temperatures. Transparent conducting oxides (TCOs) is a promising alternative as plasmonic material throughout the near- and mid-infrared wavelengths. In addition to compatibility with established silicon-based fabrication procedures, TCOs provide great flexibility in the design and optimization of plasmonic devices since their intrinsic optical properties can be tailored and dynamically tuned. In this work, we experimentally demonstrate metal-oxide metasurfaces operating as quarter-wave plates (QWP) over a broad near infrared (NIR) range from 1.75 to 2.50 µm. We employ zinc oxide highly doped with gallium (Ga:ZnO) as the plasmonic constituent material of the metasurfaces, and fabricate arrays of orthogonal nanorod pairs. Our Ga:ZnO metasurfaces provide a high degree of circular polarization across a broad range of two distinct optical bands in the NIR. Flexible broadband tunability of the QWP metasurfaces is achieved by the significant shifts of their optical bands, and without any degradation in their performance, after a post annealing process up to 450 °C.