Scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have built the first single lens that can focus the entire visible spectrum of light. The breakthrough means bulky stacked lenses could soon be replaced with simpler, flat metalenses — without loss of resolution. It also paves the way for more immersive virtual reality (VR) and augmented reality (AR) applications.
“Metalenses have advantages over traditional lenses,” says Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, as well as senior author of the research. “Metalenses are thin, easy to fabricate, and cost effective. This breakthrough extends those advantages across the whole visible range of light. This is the next big step.”
A key hurdle in miniaturizing optical devices is that these often rely on multiple curved lenses of different thicknesses to correct chromatic aberration, which results in color fringing and blurry images due to a failure of a lens to focus all colors to a single convergence point. While increasing the focal length, in-camera adjustments, and auto-correct imaging software all have been used to reduce chromatic aberration, scientists have been focusing on flat metalenses to effectively eliminate these image distortions.
Most metalenses remain quite limited in their ability to focus light covering the entire visible spectrum. However, last year, Harvard researchers unveiled the first achromatic metalens (AML). It operates over a continual bandwidth of colors, from blue to green, through a technique called dispersion engineering and via an ultra-thin array of nanopillars — custom-shaped and spaced, depending on what wavelengths need to be focused.
Expanding this research, the same group says they now have built the first single lens that can focus the entire visible spectrum of light — including white light — in the same spot and in high resolution. In their latest design, featuring paired titanium dioxide nanofins, "it is possible to simultaneously control the phase, group delay and group delay dispersion of light, thereby achieving a transmissive achromatic metalens with large bandwidth."
“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,” said Wei Ting Chen, a postdoctoral fellow at SEAS and first author of the paper published in Nature Nanotechnology. “By combining two nanofins into one element, we can tune the speed of light in the nanostructured material, to ensure that all wavelengths in the visible are focused in the same spot, using a single metalens. This dramatically reduces thickness and design complexity compared to composite standard achromatic lenses.”
The researchers aim to scale up their metalens to about 1 cm in diameter, reports the Daily Mail.
“Using our achromatic lens, we are able to perform high quality, white light imaging. This brings us one step closer to the goal of incorporating them into common optical devices such as cameras,” said Alexander Zhu, co-author of the study.
As flat metalenses potentially replace bulky optical elements in a device, "more can be done with the image processing, quality and definition which will make virtual reality (VR) experiences more realistic and engrossing," according to International Business Times.