Metasurfaces: Subwavelength nanostructure arrays for ultrathin flat optics and photonics
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Introduction Light is composed of three-dimensional (3D) electromagnetic waves that satisfy the Helmholtz equation derived from the Maxwell equations. The propagation of light can be described by optical properties such as amplitude, phase, and wavelength. By controlling the phase of light, the optical wavefront can be tuned. Optical wavefront engineering techniques are indispensable in modern technology, for instance, in microscopy, photolithography, and laser processing. However, conventional techniques to control the phase of light have fundamental limitations. There are two types of optical lenses, one is typical convex or concave lenses, and the other is echelette lenses, which are compact lenses made by dividing a lens into several concentric annular rings. Convex lenses have been developed for a long time, and their design and manufacturing processes are well optimized. However, shaping 3D spherical surfaces still requires a series of time-consuming processes. Aspherical lenses require more sophisticated processes. They are also bulky and heavy. For instance, typical lenses of digital singlelens reflex cameras weigh approximately 500 g. High-end lenses consisting of more than 20 unit lenses are more than 30 cm long and weigh up to 4 kg.1 In contrast, echelette lenses, which are also called Fresnel lenses, are thin and compact. They can be manufactured by printing methods at low cost, and high focusing efficiency can be achieved in the case of low numerical aperture. However, their focusing efficiency
and controllability are drastically degraded as the numerical aperture increases.2 The drastic height variation at their surfaces disturbs the propagation of transmitted light, therefore, echelette lenses are not enough to replace conventional refractive optical elements. Metasurfaces that consist of subwavelength optical nanoantennas have provided unprecedented opportunities to overcome the limitations of conventional lenses, and have demonstrated many promising applications such as high-numerical aperture ultrathin lenses,3 high-resolution multicolor holograms,4 and optical skin cloaks.5 Various optical properties such as amplitude, phase, and even frequency can be tuned by adjusting the physical shape of individual antennas and their arrangement. Metasurfaces have been developed in terms of structuring materials and design methods to improve their functionality. Plasmonic metasurfaces composed of metallic components suffered from low efficiency in the visible range, but dielectric building blocks can drastically increase the efficiency. Many active materials in which optical properties can be controlled by external voltage bias or temperature modulation have been exploited to dynamically change the optical response of metasurfaces. Computer-assisted design algorithms now enable the design of sophisticated antennas, which can optimize the functionality of metasurfaces. The articles in this issue of MRS Bulletin review recent progress in metasurfaces, including:
Junsuk Rho, Department of Mechanical Engineerin
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