Planar Photonics with Metasurfaces

Background Metamaterials (MMs) are smartly engineered structures with rationally designed, nanostructured building blocks that allow us to build devices with distinct responses to light, acoustic waves, and heat flows that are not attainable with naturally available materials. Plasmonic metasurfaces at work. (A) A nanoantenna-array plasmonic metasurface used to experimentally demonstrate negative refraction (inset) and reflection angles. The unit cell of a representative metasurface (blue) consists of eight gold V-shaped antennas repeated periodically. (B) A hyperbolic metasurface enhancing the radiation rate of quantum emitters. The metasurface is arranged of a thin subwavelength metallic grating deposited on a dielectric substrate. Advances The latest developments have shown that optical metasurfaces comprising a class of optical MMs with a reduced dimensionality can exhibit exceptional abilities for controlling the flow of light; achieve the anomalously large photonic density of states; and, similar to their bulk analog, provide superresolution imaging. Such a planar photonics technology is expected to facilitate new physics and enhanced functionality for devices that are distinctly different from those observed in their three-dimensional MM counterparts. As a result, this technology will enable new applications in imaging, sensing, data storage, quantum information processing, and light harvesting. Outlook The recent progress in optical metasurfaces can address the major issues hampering the full-scale development of MM technology, such as high loss, cost-ineffective fabrication, and challenging integration. The studies of new, low-loss, tunable plasmonic materials—such as transparent conducting oxides and intermetallics—that can be used as building blocks for metasurfaces will complement the exploration of smart designs and advanced switching capabilities. This progress in metasurface design and realization will lead to novel functionalities and improved performance and may result in the development of new types of ultrathin metasurface designs with unparalleled properties, including increased operational bandwidths and reduced losses. These new designs would also be compatible with planar, low-cost manufacturing. In turn, these advances will lead to ultrathin devices with unprecedented functionalities, ranging from dynamic spatial light modulation to pulse shaping and from subwavelength imaging or sensing to novel quantum optics devices. A Move to Planar Optics Metamaterials allow light to be manipulated in ways that cannot be done with naturally available materials. Subwavelength metallic nanoantenna arrays patterned onto a surface can provide the basis for planar optical devices, in which bulk optical elements that are typically thousands of wavelengths in size can be "flattened" into a two-dimensional sheet less than a wavelength thick. Kildishev et al. (10.1126/science.1232009) review progress in the optics of metasurfaces and discuss promising applications for surface-confined planar photonics components. Metamaterials, or engineered materials with rationally designed, subwavelength-scale building blocks, allow us to control the behavior of physical fields in optical, microwave, radio, acoustic, heat transfer, and other applications with flexibility and performance that are unattainable with naturally available materials. In turn, metasurfaces—planar, ultrathin metamaterials—extend these capabilities even further. Optical metasurfaces offer the fascinating possibility of controlling light with surface-confined, flat components. In the planar photonics concept, it is the reduced dimensionality of the optical metasurfaces that enables new physics and, therefore, leads to functionalities and applications that are distinctly different from those achievable with bulk, multilayer metamaterials. Here, we review the progress in developing optical metasurfaces that has occurred over the past few years with an eye toward the promising future directions in the field.