Although diffraction-limited, high-numerical-aperture, subwavelength-resolution imaging has benefited greatly from silicon- and plasmonic-based metamaterial lenses or "metalenses" that eliminate the bulky form factor of conventional refractive optics through miniaturization via a planar structure, these metalenses have high losses in the visible wavelength region from 400 to 700 nm. And while using dielectric materials with visible-light transparency including gallium phosphide, silicon nitride, and titanium dioxide (GaP, SiN, and TiO2) can have good efficiency in the visible, it is difficult to fabricate the high-aspect-ratio nanostructures needed to build the phase profiles required to create efficient metalenses.
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Using atomic layer deposition of TiO2 over a photoresist mask,however, researchers from Harvard University (Cambridge, MA) and the University of Waterloo (Waterloo, ON, Canada) have been able to prepare visible-light metalenses with mathematically modeled and optimized angled patterns of rectangular TiO2 pillars with height 600 nm and length/width of 85/410 nm, 95/250 nm, and 40/150 nm for wavelength operation at 660, 532, and 405 nm, respectively.
The 240 µm diameter and 90 µm focal-length metalenses have a high numerical aperture (NA) of 0.8 and are capable of focusing beams into diffraction-limited spots. Compared to a commercially available Nikon objective lens with 0.8 NA, the metalenses--operating at the three visible wavelengths for which they were optimized--provided 1.5X smaller and more symmetrical focal spots.
REFERENCE: M. Khorasaninejad, Science 352, 6290, 1190-1194 (June 3, 2016).
SOURCE: Science; http://science.sciencemag.org/content/352/6290/1190.abstract
Gail Overton | Senior Editor (2004-2020)
Gail has more than 30 years of engineering, marketing, product management, and editorial experience in the photonics and optical communications industry. Before joining the staff at Laser Focus World in 2004, she held many product management and product marketing roles in the fiber-optics industry, most notably at Hughes (El Segundo, CA), GTE Labs (Waltham, MA), Corning (Corning, NY), Photon Kinetics (Beaverton, OR), and Newport Corporation (Irvine, CA). During her marketing career, Gail published articles in WDM Solutions and Sensors magazine and traveled internationally to conduct product and sales training. Gail received her BS degree in physics, with an emphasis in optics, from San Diego State University in San Diego, CA in May 1986.