Upside down design expands the possibilities of wide spectrum super cameras

By turning a traditional lab-based fabrication process on its head, Duke University researchers have vastly expanded the capabilities of metasurfaces for manipulating light, while also making them much more robust against the elements.

The combination allows these rapidly maturing devices to be used in a wide variety of practical applications, such as cameras capturing images in a wide spectrum of light in a single shutter click.

The results will appear online in the magazine on 1 July Nano letters†

Plasmonics is a technology that essentially traps the energy of light in groups of electrons that oscillate together in a metal surface† This makes for a small but powerful electromagnetic field that interacts with incoming light.

Traditionally, these groups of electrons – called plasmons – have been excited on the surfaces of metal nanocubes. By controlling the size and spacing of the nanocubes, as well as the metal base beneath them, the system can be tuned to absorb specific wavelengths of light.

These so-called plasmonic meta-surfaces consist of three layers: a metal base covered with a nanometer-thin transparent substrate topped with silver nanocubes. While this configuration has worked well for lab demonstrations, it leaves little room for creativity. Because an area of ​​the nanoparticle must be within a few nanometers of the metal surface beneath it, researchers were unable to use a wide variety of shapes.

To get around this need for flatness, Maiken Mikkelsen, the James N. and Elizabeth H. Barton Associate Professor of Electrical and Computer Engineering at Duke, and her team decided to try and place each nanoparticle in its own pit or pit. This would surround the entire bottom halves of the nanoparticles with metal, allowing the sides to contain plasmons as well as the bottoms. But due to incredibly tight tolerances, this is easier said than done.

“We need to control certain dimensions with a precision of one nanometer over the surface of a centimeter-sized wafer,” Mikkelsen said. “That’s like trying to control the thickness of the blades of grass on a football field.”

To meet this challenge, Mikkelsen and her lab have essentially turned the traditional manufacturing process upside down. Instead of starting with a metal surface and overlaying a thin transparent substrate followed by nanocubes, they start with the nanocubes, which they cover with a precisely thin spacer coating that follows the underlying shape, and end with a metal coating. It’s almost like an inverted pineapple cake, where the nano-cubes are the pineapples covered in caramelized sugar and baked to a thin base.

Since more than one surface of the nanocubes could now trap plasmons between holes, Mikkelsen and her colleagues were able to experiment with new nanoparticle shapes in 3D. In the paper, the team tried solid spheres and cuboctahedrons — a shape made up of eight triangular faces and six square faces — as well as metallic spheres with a quartz core.

“Synthesizing nanoparticles can be tricky and there are limitations to any shape,” Mikkelsen said. “By being able to use almost all shapes, we really open up a lot of new possibilities, including exploring a variety of metals.”

Test results showed that the new fabrication method can not only match or exceed the capabilities of previous methods with silver nanocubes, but also extend the frequency range exploited by using these different shapes and metals. The study also found that these variations change where the nanoparticles capture energy on their surface. Combined with the added bonus of essentially making the entire device weatherproof by encasing the nanoparticles, the new technique could potentially expand the use of the technology to power chemical reactions or thermal detectors.

Mikkelsen’s first priority, however, is to apply the fabrication technique to her project to create a “super camera” that can capture and process a wide range of light properties, such as polarization, depth, phase, coherence and angle of view.

“What’s really important here is that large, macroscopic areas can be covered by the metasurfaces very cheaply because we use completely lithography-free fabrication techniques,” Mikkelsen said. “This means that the metasurfaces can be integrated with other existing technologies and also create inspiration for new plasmonic metasurface applications.”