Thin, flat lenses focus light as sharply as their curved counterparts
12 May 2015
Using standard micro-fabrication techniques, Caltech engineers have created flat micro-lenses with performance on a par with conventional, curved lenses.
"The lenses we use today are bulky," says Amir Arbabi, a senior Caltech researcher. "The structure we have chosen for these flat lenses can open up new areas of application that were not available before."
The research, led by Andrei Faraon, an assistant professor at Caltech, appears in the May 7 issue of Nature Communications.
The new lens type is known as a high-contrast 'transmitarray'. Made of silicon, it is just a millionth of a metre thick, and is studded with silicon 'posts' of varying sizes. Depending on their heights and thicknesses, the posts focus different wavelengths of light.
A lens focuses light or forms an image by delaying for varying amounts of time the passage of light through different parts of the lens. In curved glass lenses, light takes longer to travel through the thicker parts of the lens than through the thinner parts.
On the flat lens, these delays are achieved by the silicon posts, which trap and delay the light for an amount of time that depends on the diameter of the posts. With careful placement of these differently sized posts on the lens, the researchers can guide incident light as it passes through the lens to form a curved wavefront, resulting in a tightly focused spot.
The Caltech researchers found that their flat lenses focus as much as 82 percent of infrared light passing through them. By comparison, previous studies have found that metallic flat lenses have efficiencies of only around a few percent, in part because their materials absorb some incident light.
Although curved glass lenses can focus nearly 100 percent of the light that reaches them, they usually require sophisticated designs with nonspherical surfaces that can be difficult to polish. On the other hand, the design of the flat lenses can be modified depending upon the exact application for which the lenses are needed, simply by changing the pattern of the silicon nanoposts.
A limitation of flat lenses is that each lens can only focus a narrow set of wavelengths. These monochromatic lenses could find application in devices such as a night-vision camera, which 'sees' in infrared over a narrow wavelength range. More broadly, they could be used in any optical device involving lasers, as lasers emit only a single colour of light.
Multiple monochromatic lenses could be used to deliver multi-colour images, much as television and computer displays employ combinations of the colours red, green, and blue to produce a rainbow of hues. Because the micro-lenses are so small, integrating them in optical systems would take up little space compared with the curved lenses commonly found in cameras or microscopes.
Although the lenses currently are expensive to manufacture, the researchers believe it should be possible to produce thousands at once using photolithography or nano-imprint lithography techniques. In these common, high-throughput manufacturing techniques, a stamp presses into a polymer, leaving behind a desired pattern that is then transferred into silicon through dry etching of silicon in a plasma.
Key to illustration
(a) Schematic of the aperiodic high contrast transmitarray lens used to realize a high-NA micro-lens. (b) Optical microscope image of a fabricated high contrast transmitarray lens with large NA. Scale bar, 100µm. (c,d) Scanning electron microscope images of the silicon posts forming the high contrast transmitarray lens micro-lens. Scale bars, 1µm.