Journal
NANO LETTERS
Volume 18, Issue 3, Pages 2124-2132Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.nanolett.8b00368
Keywords
Metasurface; flat optics; flat lenses; dielectric nanoantennas; high numerical aperture; confocal imaging
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Funding
- Data Storage Institute core funds (Singapore)
- A*STAR SERC Pharos program (Singapore) [152 73 00025]
- National Research Foundation (Singapore) [NRF-CRP14-2014-04]
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The numerical aperture (NA) of a lens determines its ability to focus light and its resolving capability. Having a large NA is a very desirable quality for applications requiring small light matter interaction volumes or large angular collections. Traditionally, a large NA lens based on light refraction requires precision bulk optics that ends up being expensive and is thus also a specialty item. In contrast, metasurfaces allow the lens designer to circumvent those issues producing high-NA lenses in an ultraflat fashion. However, so far, these have been limited to numerical apertures on the same order of magnitude as traditional optical components, with experimentally reported NA values of <0.9. Here we demonstrate, both numerically and experimentally, a new approach that results in a diffraction-limited flat lens with a near-unity numerical aperture (NA > 0.99) and subwavelength thickness (similar to lambda/3), operating with unpolarized light at 715 nm. To demonstrate its imaging capability, the designed lens is applied in a confocal configuration to map color centers in subdiffractive diamond nanocrystals. This work, based on diffractive elements that can efficiently bend light at angles as large as 82 degrees, represents a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop associated with the standard, phase mapping approach.
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