January 5, 2018
Researchers at Harvard University’s John A. Paulson School of Engineering and Applied Sciences (SEAS) made a breakthrough in metalenses, flat surfaces that focus light via nanostructures. Metalenses, which would replace curved lenses, have thus far been able to focus only on a limited spectrum of light, but SEAS engineers created a metalens that can focus, in high resolution, on the entire visible spectrum of light in the same spot. Previously, that effect could only be achieved by stacking many conventional lenses.
According to SEAS, “focusing the entire visible spectrum and white light — combination of all the colors of the spectrum — is so challenging because each wavelength moves through materials at different speeds.” Red, for example, moves through glass faster than blue, meaning the two colors arrive at the same location at different times, thus creating image distortions called chromatic aberrations.
Cameras and other optical devices “use multiple curved lenses of different thicknesses and materials to correct these aberrations, which, of course, adds to the bulk of the device.”
SEAS research fellow and professor Federico Capasso, who is the senior author of the research, notes the advantages of metalenses. “Metalenses are thin, easy to fabricate and cost effective,” he said. “This breakthrough extends those advantages across the whole visible range of light.”
The SEAS metalenses “use arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration,” based on the discovery that “different wavelengths of light could be focused but at different distances by optimizing the shape, width, distance, and height of the nanofins.”
SEAS post doctoral fellow Wei Ting Chen, who is first author of the paper, notes that, “one of the biggest challenges in designing an achromatic broadband lens is making sure that the outgoing wavelengths from all the different points of the metalens arrive at the focal point at the same time.” Combining two nanofins into one element allows them to “tune the speed of light … to ensure that all wavelengths in the visible are focused in the same spot, using a single metalens.”
Research co-author Alexander Zhu reports that, by performing “high quality white light imaging,” the team is “one step closer to the goal of incorporating them into common optical devices such as cameras.” The next goal is to “scale up the lens, to about 1 cm in diameter,” which would “open a whole host of new possibilities, such as applications in virtual and augmented reality.”
Harvard has protected the project’s intellectual property and licensed it to a startup for commercial development. The paper was published in Nature Nanotechnology.