Flat metasurface doublet lenses. (Image courtesy of Caltech.)
Metasurfaces and Silicon Nanoposts
The new lenses are made of special sheets of material called metasurfaces. Each metasurface lens is dotted with millions of small cylinders called silicon nanoposts, which are 600 nm tall and have varying diameters in the range of a few hundred nanometers.
The silicon nanoposts allow the metasurface to act as a lens by mimicking the behaviour of classical curved lenses. Traditionally, a lens made of glass is curved in such a way that light travels faster through the thin edges than the thick center, focusing the light.
This same effect is achieved by varying the diameters of the silicon nanoposts. Light travels faster though nanoposts with smaller diameters than it does through nanoposts with larger diameters, meaning the arrangement of nanoposts can replicate the effect of a curved lens, even though all the nanoposts are on a flat surface.
The final trick used by the engineers was to stack two metasurface lenses on top of each other, nanopost-side-out, to create what’s called a metasurface doublet lens. This technique is used to correct monochromatic aberrations that occur when using only a single metasurface.
The final lens system captures and focuses light from an angle-of-view greater than 60° x 60°, and operates at a wavelength of 850 nm with 70 percent focusing efficiency.
A New Outlook on Lenses
The metasurface doublet lenses have a number of advantages that could see them replace traditional optical lenses. For one, the lenses can be produced easily and in large quantities.
"Metasurfaces like these can be easily mass produced, much the way computer chips are," said researcher Amir Arbabi. "That means this could be a cheap and easily scalable way to create tiny lenses just a few millimeters in diameter."
Furthermore, the lenses can be easily integrated with CMOS (complementary metal-oxide semiconductor) image sensors, as they’re fabricated using the same materials and techniques.
Flat, lightweight, and cheap lenses are in demand for various consumer electronics equipped with cameras, or medical devices such as endoscopes, according to researcher Andrei Faraon. The new metasurface lenses may be just the tool to meet this demand.
To learn more about the metasurface lenses, you can read the team’s paper here. For more from the world of optics, read How Robots Can Feel with Optical Waveguides.