Metalenses at visible wavelengths: Diffraction limited focusing and subwavelength resolution imaging
Multifunction planar optics Specially designed two dimensional (2D) arrays of nanometer scale metallic antennas, or metasurfaces, may allow bulky optical components to be shrunk down to a planar device structure. Khorasaninejad et al. show that arrays of nanoscale fins of TiO can function as high end optical lenses. At just a fraction of the size of optical objectives, such planar devices could turn your phone camera or your contact lens into a compound microscope. Maguid et al. interleaved sparse 2D arrays of metal antennas to get multifunctional behavior from the one planar device structure (see the Perspective by Litchinitser) The enhanced functionality of such designed metasurfaces could be used in sensing applications or to increase the communication capacity of nanophotonic networks. Science, this issue pp. 1190 and 1202; see also p. 1177 A planar metasurface can function as a high end optical lens. Subwavelength resolution imaging requires high numerical aperture (NA) lenses, which are bulky and expensive. Metasurfaces allow the miniaturization of conventional refractive optics into planar structures. We show that high aspect ratio titanium dioxide metasurfaces can be fabricated and designed as metalenses with NA 0.8. Diffraction limited focusing is demonstrated at wavelengths of 405, 532, and 660 nm with corresponding efficiencies of 86, 73, and 66% The metalenses can resolve nanoscale features separated by subwavelength distances and provide magnification as high as 170x, with image qualities comparable to a state of the art commercial objective. Our results firmly establish that metalenses can have widespread applications in laser based microscopy, imaging, and spectroscopy.
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