The announcement was hailed as a breakthrough in optics and was termed among Science Magazine’s top discoveries of 2016. But the lens had a limitation that it could just focus single color at the time. Now, the same group has introduced the first flat lens that works within a regular bandwidth of colors, from green to blue. Such bandwidth, close to that of an LED< paves the novel applications in spectroscopy, imaging and sensing.
One of the major limitations in developing a flat, broadband lens has been rectifying for chromatic dispersion, the procedure where different wavelengths of light are focused at distinct distances from the lens. “Conventional lenses for cameras and microscopes – comprising those in laptops and cell phones, need multiple curved lenses to correct chromatic aberrations, which adds weight, thickness and complexity,” says Federico Capasso, Vinton Hayes Senior Research Fellow in Electrical Engineering and Robert L. Wallace, Lecturer of Applied Physics. “Our novel breakthrough flat metalens has built-in chromatic aberrations corrections so that a singular lens is needed.”
Rectifying for chromatic dispersion, known as dispersion engineering is a critical topic in optics and a vital design requirement in any optical system that deals with light of distinct colors. The ability to regulate the chromatic dispersion of flat lenses broadens their applications and introduced novel applications that have not yet been feasible.
“By harnessing chromatic aspects, we can have much more regulation over the light,” says Reza Khorasaninejad, a research associate in the Capasso Lab and lead author of the study. “Here we illustrate achromatic flat lenses and also invent a novel sort of flat lend with reverse chromatic dispersion. We revealed that one can break away from the limitations of traditional optics, offering novel possibilities only bound by the imagination of the designer.”
To craft an achromatic lens – a lens without chromatic dispersion – the group optimizied the shape, distance, height, and width of the nanopillars that make up the heart of the metalens. As in previous study, the scientists employed abundant titanium dioxide to prepare the nanoscale array. Such structure enables metalens to focus wavelengths from 490nm to 550nm, basically from green to blue, without any chromatic dispersion.
“Such method for dispersion engineering can be employed to design numerous ultra-slim components with a desired performance,” says Zhujun Shi, a Ph. D student in the Capasso lab and co-author of the study. “Such platform is based on singular step lithography and is compatible with high throughout manufacturing method like nano-imprinting.”
The Office of Technology Development, Harvard has filed patent applications on a portfolio of flat lens technologies and is working closely with Capasso and team members of his study group to catalyze the commercialization of such technology through a startup company
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