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Flat optics with designer metasurfaces

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Abstract

Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

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Figure 1: Generalized laws of refraction and reflection.
Figure 2: Planar devices based on metasurfaces.
Figure 3: Complex wavefront shaping based on metasurfaces.
Figure 4: Variation of the polarization state of light is associated with a geometrical phase change (that is, the Pancharatnam–Berry phase).
Figure 5: Hologram-based flat optics.
Figure 6: Thin-film flat optics.

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Acknowledgements

The authors acknowledge the contributions of Z. Gaburro, P. Genevet, M. A. Kats, F. Aieta, R. Blanchard, J. Lin, J.-P. Tetienne, G. Aoust, D. Sharma, Z. Yang, S. Ramanathan, M. Mumtaz Qazilbash and D. N. Basov to the research reviewed in this article. Quantum cascade laser materials were provided by Hamamatsu Photonics KK. The authors acknowledge support from the Harvard Nanoscale Science and Engineering Center (NSEC) under contract NSF/PHY 06-46094, and the Center for Nanoscale Systems (CNS) at Harvard University, which is a member of the National Nanotechnology Infrastructure Network (NNIN). This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) N/MEMS S&T Fundamentals program under Grant N66001-10-1-4008 issued by the Space and Naval Warfare Systems Center Pacific (SPAWAR) and by the Air Force Office of Scientific Research under grant number FA9550-12-1-0289. N.Y. acknowledges funding provided by the Fu Foundation School of Engineering and Applied Science, and the Department of Applied Physics and Applied Mathematics, Columbia University.

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Yu, N., Capasso, F. Flat optics with designer metasurfaces. Nature Mater 13, 139–150 (2014). https://doi.org/10.1038/nmat3839

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