‘Metalens’ can Focus the Entire Rainbow on a Single Point

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Expected to revolutionize imaging and optical technology, so-called “metalenses” are lenses with sub-wavelength features custom designed to manipulate light in extremely precise ways.

According to a new study published in in Nature Nanotechnology, Harvard University researchers have developed a metalens capable of focusing the entire spectrum of visible light on a singular point – a feat that has never been achieved before with a single lens.

Focusing the complete visible spectrum and white light, which includes all the colors of the rainbow, is difficult due to the fact that every single wavelength passes through a conventional lens at different speeds. For instance, blue wavelengths will pass through a lens more slowly than red wavelengths, resulting in the colors reaching the same location at different times. This effect creates different focal points and image distortions referred to as chromatic aberrations.

Conventional cameras and optical instruments use multiple curved lenses to correct these distortions, but this fix results in added bulk for the device that uses them.

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.

Federico Capasso, Study Author and Professor of Applied Physics, Harvard

The study team created their metalens with arrays of titanium dioxide nanofins to evenly focus wavelengths of light and get rid of chromatic aberrations. An earlier study had showed various wavelengths of light might be focused but at different distances by refining the contour, thickness, distance and height of the nanofins. In the study team’s design, paired nanofins were designed to control the pace of passing wavelengths of light at the same time. The paired nanofins change the refractive index on the metasurface and were crafted to cause various time delays, causing all wavelengths to reach the focal spot at precisely the same time.

Study co-author Wei Ting Chen, a postdoctoral fellow at Harvard, said one of the greatest challenges in designing the new ‘achromatic’ metalens was ensuring that all the wavelengths of light passing through it hit the intended focal point at the same time.

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.

Wei Ting Chen, Study Co-author and Postdoctoral Fellow, Harvard

“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,” added co-author Alexander Zhu, a PhD researcher at Harvard.

The study team said their next research objective is to scale up the lens to about 1 centimeter across. Current metalenses are tens of millimeters in diameter. Making a metalens on the centimeter scale would open up a whole new realm of optical possibilities, such as improving virtual and augmented reality technology.

Experts also say that metalenses could revolutionize optical processing. Currently, processing chips use diffraction gratings to break white light down into its separate, more useful parts. Replacing diffraction gratings with metalenses would allow for complicated optical systems to be crafted onto chips for optical processing.

Another exciting possibility is using meta lenses on the end of optical fibers to create tiny endoscopes for medical imaging purposes. Astronomers could also use metalenses to replace the bulky optical systems we currently use to view the cosmos, such as the massive lensing system onboard the Hubble telescope.

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