Alloys of coinage metals work better for plasmonics than gold
Mixing in copper, silver, and potentially aluminum adds tunability as well as better optical properties.
While the optical properties of noble metals such as gold and silver work well for photonic devices at optical frequencies (especially those based on plasmonics), there is always room for improvement. Researchers at the University of Maryland (UMD; College Park, MD) have discovered that particular alloys of gold, silver, and copper have a higher surface-plasmon-polariton quality factor than the pure metals themselves.1 In addition, varying the proportions of different metals allows the optical properties to be tuned.
The result could be metal-based optical thin films (1D) and nanostructures (2D) having what the researchers call "on-demand" optical responses. Such structures can lead to spectrally and angularly wide all-metal optical absorbers for photovoltaic cells, along with other uses for matamaterials such as invisibility cloaks.
"This work is a perfect example of the power of materials science and engineering: we discovered a way to control and change metals' optical properties by mixing them," says Marina Leite, assistant professor of materials science and engineering at UMD. "These alloys obtain a unique functionality that is not achievable using their pure counterparts—making them a better, more powerful tool for tunable optical response than gold, silver, or copper alone. Our results are relevant to my colleagues working on photonic devices -- components for creating, manipulating, or detecting light -- as these devices are highly dependent on the tunability of the optical response of their building blocks."
The results also lead to the possibility of replacing high-cost metals with low-cost and earth-abundant ones. Although gold is immediately recognizable as a precious and expensive metal, copper and aluminum are much more readily available. Leite and her colleagues are now looking into how they can incorporate alloys using these metals into high-performance optical devices.
The work was supported by the National Science Foundation grant no. HRD1008117, the University of Maryland ADVANCE program, the Minta Martin Award at the A. James Clark School of Engineering at the University of Maryland, and the University of Maryland 2015 Graduate School Summer Research Fellowship program.
1. Chen Gong and Marina S. Leite, ACS Photonics (2016); doi: 10.1021/acsphotonics.5b00586