New metasurface design can control optical fields in three dimensions


A team led by scientists at the University of Washington has designed and tested 3D-printed metamaterial that can manipulate light with nanoscale precision. As they report in a paper published Oct. 4 in the journal Science Advances, their optical elements 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 meta devices that can precisely manipulate optical fields with high spatial resolution in three dimensions. The helicopter – a spiral helix – a light helix – a spiral helix – a light blue

Devices with this precision of control over light could be used only for 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.

An image of the surface of an optical element.

A scanning electron micrograph image of the surface of the optical element. James Whitehead / University of Washington

"This reported device really has no classical analog in refractive optics – the optics that we encounter in our day-to-day life," said corresponding author Arka Majumdar, to UW assistant professor of electrical engineering and physics, and member of the UW Institute for Nano-Engineered Systems and the Institute for Molecular & Engineering Sciences. "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, took a lesser-than-used approach in the optical metamaterials field to design the optical element: inverse design. Using the inverse pattern designed to create a pattern that would create that pattern.

Said Majumdar. "We are not intuitively aware of the appropriate structure of an optical element given to specific functionality." "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 isnot widely used for optically active large-area metamaterials because it requires a large number of simulations, computationally intensive making inverse design.

Here, the team avoided this pitfall thanks to an insight by Alan Zhan, lead author on the paper, who recently graduated with a doctoral degree in physics. Zhan realized that the team could use My scattering theory to design the optical element. My scattering describes how light waves of a particular wavelength are scattered by spheres or cylinders that are similar in size to the optical wavelength. My 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.

"Zhan said," Our implementation of Mie scattering theory is specific to certain shapes – spheres- which we had to incorporate those shapes into the design of the optical element. "But, relying on My scattering theory made the design simulation process much easier, we could make very specific calculations about the properties of light when it interacts with the optical element."

Their approach could be used to include different geometries such as cylinders and ellipsoids.

An image showing how the optical element focuses on a specific point in 3D space above the element's surface.

These images show the performance of the 1,550-nanometer optical element. The images are light-intensity profiles of the optical field as it appears approximately 185 micrometers above the surface of the optical element. To the left is a simulated light-intensity profile that predicts how the optical element should perform. Notes the focal point of light near the center of the image. The device does produce at the focal point of a light at the predicted location. The researchers designed the element to focus light on eight points at different distances above the element's surface. Scale bar is 10 micrometers.Alan Zhan / University of Washington

The optical element of a small square of different sizes, arranged in a periodic square latex. 3D printer to fabricate two prototype optical elements – the one with two 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 on 1,550 nanometers, the other at 3,000 nanometers.

1,550 or 3,000 nanometers at eight specific points along a 3D helical pattern. The researchers visualized the optical elements under a microscope to see how well they performed. 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.

The team would be able to improve the design of the process, to incorporate other design elements compatible with Mie scattering theory.

"Now that we've got the basic design principles work, there are lots of directions with this level of precision in fabrication," said Majumdar.

One particularly promising direction to progress beyond a single-surface to create true-volume, 3D metamaterial.

"3D-printing allows us to create a stack of these surfaces, which was not possible before," said Majumdar.

Co-authors are Ricky Gibson with the Air Force Research Laboratory and the University of Dayton Research Institute; Evan Smith and Joshua Hendrickson with the Air Force Research Laboratory; and James Whitehead, a UW doctoral student in the Department of Electrical and Computer Engineering. The research was funded by the National Science Foundation, the Air Force Office of Scientific Research, Samsung, the UW Reality Lab, Facebook, Google and Huawei.

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