Researchers at Duke University have significantly increased the capabilities of light-manipulating metasurfaces while simultaneously improving their resistance to the environment by flipping a conventional lab-based production procedure on its head.
A new upside-down fabrication method for light-manipulating metamaterials envelops nanoparticles with a transparent spacing layer followed by a coating of metal. The way the metal coating envelops part of the nanoparticle while maintaining tight, nanometer tolerances allow for a much larger design space than was previously possible. Image Credits: Duke Pratt School of Engineering
The combination might enable the employment of these rapidly developing technologies in a variety of real-world applications, such as cameras that can swiftly take images in a range of lighting conditions.
The journal Nano Letters published the findings online on July 1st, 2022.
In essence, plasmonics is a technique that traps light energy in collections of vibrating electrons on a metal surface. As a result, a weak yet potent electromagnetic field is produced, which interacts with incoming light.
These electron groups, known as plasmons, are often stimulated on the surfaces of metal nanocubes. The system can be modified to absorb certain light wavelengths by adjusting the metal base underneath, the size of the nanocubes, and their distance from one another.
Three layers make up these so-called plasmonic metasurfaces: a metal base covered in a transparent substrate that is only a few nanometers thick, followed by silver nanocubes. This arrangement has been effective for laboratory tests, but it limits creative freedom.
Researchers were limited in the shapes they could employ since a portion of the nanoparticle had to be only a few nanometers away from the metal surface below.
The James N. and Elizabeth H. Barton Associate Professor of Electrical and Computer Engineering at Duke, Maiken Mikkelsen, and her colleagues decided to try to place each nanoparticle in its own dimple or well to get past this requirement for flatness.
This would completely enclose the lower half of the nanoparticles in metal, enabling the sides as well as the bottoms to host plasmons. However, this is much easier said than done due to the extremely small tolerances.
We need to control certain dimensions with single-nanometer precision over the surface of a centimeter-sized wafer. That is like trying to control the thickness of the blades of grass on a football field.
Maiken Mikkelsen. The James N. and Elizabeth H. Barton Associate Professor, Electrical and Computer Engineering, Duke Pratt School of Engineering
Mikkelsen and her group effectively turned the conventional manufacturing process upside down to address this difficulty. They begin with the nanocubes, cover them with an extremely thin spacer layer that closely matches the underlying geometry, and then finish with a metal coating rather than starting with a metal surface and adding a thin transparent substrate on top.
The nanocubes are the pineapples that are coated in caramelized sugar and baked onto a thin bottom, making it nearly like a pineapple upside-down cake.
Mikkelsen and her colleagues were able to conduct 3D experiments with novel nanoparticle forms since several surfaces of the nanocubes could now capture plasmons within gaps. The scientists tested solid spheres and cuboctahedra, which is a structure with eight triangle sides and six square faces, as well as metal spheres with quartz cores in the study.
Mikkelsen added, “Synthesizing nanoparticles can be tricky and there are limitations for each shape. 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 indicated that the new production technique can not only match or even surpass the capabilities of earlier techniques that employed silver nanocubes, but it can also increase the spectrum of frequencies that are tapped by utilizing these various metals and geometries.
The study also showed that these differences alter the locations of the nanoparticles’ energy-capture sites on their surfaces. The new method may allow the technology to be used to drive chemical processes or thermal detectors, in addition to the added benefit of effectively weatherizing the entire gadget by encasing the nanoparticles.
The $7.5 million Department of Defense project Mikkelsen is working on to develop a “super camera” that can record and process a variety of characteristics of light, including polarization, depth, phase, coherence, and incidence angle takes precedence over all other projects for her.
“What is really significant here, is that large, macroscopic areas can be covered by the metasurfaces very inexpensively, as we use entirely lithography-free fabrication techniques. This means the metasurfaces can be integrated with other existing technologies and also create inspiration for new plasmonic metasurface applications,” stated Mikkelsen.
The Army Research Office (W911NF1610471), the Office of Naval Research (N00014-17-1-2589), and the Air Force Office of Scientific Research (FA9550-18-383 1-0326, FA9550-21-1-0312) provided funding for this study.
Journal Reference:
Stewart, J. W., et al. (2022) Control of Nanoscale Heat Generation with Lithography-Free Metasurface Absorbers. Nano Letters. doi:10.1021/acs.nanolett.2c00761