Metasurface with Helical Patterns

A team led by scientists at the Uni­versity of Washington has designed and tested a 3D-printed metamaterial that can mani­pulate light with nanoscale precision. Their designed optical element focuses light to discrete points in a 3D helical pattern. The team’s design principles and experi­mental findings demonstrate that it is possible to model and construct meta­material devices that can precisely mani­pulate optical fields with high spatial resolution in three dimensions. Though the team chose a helical pattern for their optical element to focus light, their approach could be used to design optical elements that control and focus light in other patterns.

A scanning electron micrograph image of the surface of the optical element. (Source: J. Whitehead, U. Washington)

Devices with this level of precision control over light could be used not only to minia­turize today’s optical elements, such as lenses or retroreflectors, but also to realize new varieties. In addition, designing optical fields in three dimensions could enable creation of ultra-compact depth sensors for autonomous trans­portation, as well as optical elements for displays and sensors in virtual- or aug­mented-reality headsets. “This reported device really has no classical analog in refractive optics – the optics that we encounter in our day-to-day life,” said Arka Majumdar, a UW assistant professor of electrical and computer engineering and physics. “No one has really made a device like this before with this set of capa­bilities.”

The team, which includes researchers at the Air Force Research Labora­tory and the Univer­sity of Dayton Research Institute, took a lesser-used approach in the optical metamaterials field to design the optical element: inverse design. Using inverse design, they started with the type of optical field profile they wanted to generate – eight focused points of light in a helical pattern – and designed a meta­material surface that would create that pattern. “We do not always intui­tively know the appro­priate structure of an optical element given a specific func­tionality,” said Majumdar. “This is where the inverse design comes in: You let the algorithm design the optics.”

While this approach seems straightforward and avoids the drawbacks of trial-and-error design methods, inverse design isn’t widely used for optically active large-area metamaterials because it requires a large number of simu­lations, making inverse design compu­tationally intensive. Here, the team avoided this pitfall thanks to an insight by Alan Zhan, who recently graduated the UW with a doctoral degree in physics. Zhan realized that the team could use Mie scattering theory to design the optical element. Mie scat­tering describes how light waves of a particular wavelength are scattered by spheres or cylinders that are similar in size to the optical wave­length. Mie scattering theory explains how metallic nanoparticles in stained glass can give certain church windows their bold colors, and how other stained glass artifacts change color in different wavelengths of light, according to Zhan.

“Our implementation of Mie scattering theory is specific to certain shapes – spheres – which meant we had to incorporate those shapes into the design of the optical element,” said Zhan. “But, relying on Mie scattering theory significantly simplified the design and simu­lation process because we could make very specific, very precise calcu­lations about the properties of light when it interacts with the optical element.” Their approach could be employed to include different geometries such as cylinders and ellipsoids. The optical element the team designed is essentially a surface covered in thousands of tiny spheres of different sizes, arranged in a periodic square lattice. Using spheres simplified the design, and the team used a commer­cially available 3D printer to fabricate two proto­type optical elements – the larger of the two with sides just 0.02 centimeters long – at the Washington Nano­fabrication Facility on the UW campus. The optical elements were 3D-printed out of an ultraviolet epoxy on glass surfaces. One element was designed to focus light at 1,550 nanometers, the other at 3,000 nano­meters.

The researchers visualized the optical elements under a microscope to see how well they performed as designed – focusing light of either 1,550 or 3,000 nanometers at eight specific points along a 3D helical pattern. Under the microscope, most focused points of light were at the positions predicted by the team’s theoretical simu­lations. For example, for the 1,550-nan­ometer wavelength device, six of eight focal points were in the predicted position. The remaining two showed only minor deviations. With the high performance of their proto­types, the team would like to improve the design process to reduce background levels of light and improve the accuracy of the placement of the focal points, and to incor­porate other design elements compatible with Mie scattering theory. “Now that we’ve shown the basic design principles work, there are lots of direc­tions we can go with this level of precision in fabri­cation,” said Majumdar. One particularly promising direction is to progress beyond a single-surface to create a true-volume, 3D metamaterial. “3D-printing allows us to create a stack of these surfaces, which was not possible before,” said Majumdar. (Source: U. Washington)

Reference: A. Zhan et al.: Controlling three-dimensional optical fields via inverse Mie scattering, Sci. Adv. 5, eaax4769 (2019); DOI: 10.1126/sciadv.aax4769

Link: Nano Optoelectronic Integrated System Engineering, Dept. of Physics, University of Washington, Seattle, USA

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