Single metalens focuses all colors of the rainbow in one point; opens new possibilities in virtual, augmented reality

January 1, 2018, Harvard John A. Paulson School of Engineering and Applied Sciences
This flat metalens is the first single lens that can focus the entire visible spectrum of light -- including white light -- in the same spot and in high resolution. It uses arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. Credit: Jared Sisler/Harvard SEAS

Metalenses—flat surfaces that use nanostructures to focus light—promise to revolutionize optics by replacing the bulky, curved lenses currently used in optical devices with a simple, flat surface. But, these metalenses have remained limited in the spectrum of light they can focus well. Now a team of researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has developed the first single lens that can focus the entire visible spectrum of light—including white light—in the same spot and in high resolution. This has only ever been achieved in conventional lenses by stacking multiple lenses.

The research is published in Nature Nanotechnology.

Focusing the entire visible spectrum and white - combination of all the colors of the spectrum—is so challenging because each moves through materials at different speeds. Red wavelengths, for example, will move through glass faster than the blue, so the two colors will reach the same location at different times resulting in different foci. This creates image distortions known as chromatic aberrations.

Cameras and optical instruments use multiple curved lenses of different thicknesses and materials to correct these aberrations, which, of course, adds to the bulk of the device.

"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 and 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."

The Harvard Office of Technology Development (OTD) has protected the intellectual property relating to this project and is exploring commercialization opportunities.

The metalenses developed by Capasso and his team use arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. Previous research demonstrated that different wavelengths of light could be focused but at different distances by optimizing the shape, width, distance, and height of the nanofins. In this latest design, the researchers created units of paired nanofins that control the speed of different wavelengths of light simultaneously. The paired nanofins control the refractive index on the metasurface and are tuned to result in different time delays for the light passing through different fins, ensuring that all wavelengths reach the focal spot at the same time.

"One of the biggest challenges in designing an achromatic broadband 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. "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."

"Using our achromatic lens, we are able to perform high quality, imaging. This brings us one step closer to the goal of incorporating them into common such as cameras," said Alexander Zhu, co-author of the study.

Next, the researchers aim to scale up the lens, to about 1 cm in diameter. This would open a whole host of new possibilities, such as applications in virtual and augmented reality.

Explore further: Flat lens to work across a continuous bandwidth allows new control of light

More information: A broadband achromatic metalens for focusing and imaging in the visible, Nature Nanotechnology (2018).

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1 / 5 (3) Jan 01, 2018
Simulation of metamaterial lens. The problem of metalens is, they're quite dispersive as they focus the light wave by passing through mixture of more refractive "blobs" and less refractive "bubbles". The blobs diffract light of shorter wavelength more than this longwavelenght ones. The bubbles are doing it oppositely and they diffract light of shorter wavelength less. Therefore the material with carefully balanced mixture of blobs and bubbles can disperse light independently to its wavelength, thus making "achromatic lens" without color artifacts. But this advantage is balanced by worser optical characteristics of such lens.
1 / 5 (3) Jan 01, 2018
The metalens can be observed in nature too, if you know where to look at it. For example during rain the air behaves like system or refractive blobs and it focuses the short-wavelenth light outside of its path in a rainbow. But when this rain gets sufficiently heavy and dense, than the atmosphere behaves like mixture of blobs and bubble spaces between them and another secondary rainbow emerges with order of its colors inverted. Between both bows the sky gets darker (so-called Alexander's dark band), which is just the area, when the light gets deflected and the atmosphere filled by water droplets behaves like nondispersive metalens for infrared light.
1.8 / 5 (5) Jan 01, 2018
In some models the vacuum between event horizons of black holes also behaves like dark transparent area deflecting the light from its path. It's not so surprising, because quantum fluctuations give vacuum character of metamaterial foam - a dynamic mixture of blobs (virtual photons) and bubbles (virtual neutrinos and scalar waves). At the proximity of massive bodies the relative excess of virtual photons results into gravitatonal lensing, at more distant areas above their surface the relative excess of scalar waves results into dark matter lensing. So we are living inside Alexander's dark band of vacuum, where the light scatters the least in achromatic way.
4.6 / 5 (9) Jan 01, 2018
Hmmm, is my dodgy rule of "several posts in a rule = a nutter" applicable to the above? It sorta smells that way.
1 / 5 (2) Jan 01, 2018
BTW The animation presented here is quite instructive, as it fits the actual metalens geometry used in above article (after all, it results from the same research group of Wei Ting Chen). But it doesn't illustrate principle of metamaterial achromatic behavior so well like system of blobs and bubbles.
2 / 5 (2) Jan 02, 2018
Hmmm, is my dodgy rule of "several posts in a rule = a nutter" applicable to the above? It sorta smells that way.

It is one of the EU nutters, spouting technobabble and collecting 5-votes from sockpuppets.
5 / 5 (2) Jan 02, 2018
Mackita is a sockpuppet of Zephir, a proponent of his own aether wave theory, not EU. https://www.reddi..._zephir/
not rated yet Jan 03, 2018
The design of metalens is complex. The question is, how transmissive hologram of normal glass lens would work?
not rated yet Jan 04, 2018
Imagine a completely flat 70mm Zoom Telephoto EF 20-200mm f/2.8 lens on the back of your phone
not rated yet Jan 07, 2018
Actually as I can see, someone already constructed it - together with mount object and whole the microscope around the lens...
Thorium Boy
not rated yet Jan 07, 2018
Imagine a completely flat 70mm Zoom Telephoto EF 20-200mm f/2.8 lens on the back of your phone

200/2.8 = 75mm. Can't change the laws of physics, you NEED a 3" diameter lens front. Now, try to squeeze 8 inches of back focus into your phone.
Thorium Boy
not rated yet Jan 07, 2018
There is a lot more to consider than chromatic correction. Spherical aberration, distortion, coma, astigmatism. Correct them, you might have a lens.

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