Copper Plasmonics Explored for Nanophotonics Applications
MOSCOW, March 22, 2016 — Experimental demonstration of copper components has expanded the list of potential materials suited to nanophotonic devices beyond gold and silver.
According to researchers from the Moscow Institute of Physics and Technology (MIPT), copper components are not only just as good as components based on noble metals, such as gold and silver, they can be easily implemented in integrated circuits using industry-standard fabrication processes. Gold and silver, as noble metals, may not enter into the requisite chemical reactions to create nanostructures readily and require expensive, difficult processing steps.
Nanoscale copper plasmonic waveguides on a silicon chip in a scanning near-field optical microscope (left) and their image obtained using electron microscopy (right). Courtesy of MIPT.
In nanophotonics, the diffraction limit of light is overcome by using metal-dielectric structures. Light may be converted into surface plasmon polaritons, surface waves propagating along the surface of a metal, which make it possible to switch from conventional 3D photonics to 2D surface plasmon photonics, also known as plasmonics. This allows control of light at the 100-nm scale, far beyond the diffraction limit.
Now researchers from MIPT’s Laboratory of Nanooptics and Plasmonics have found a solution to the problems posed by noble metals. Based on a generalization of the theory for so-called plasmonic metals, in 2012 they found that copper as an optical material is not only able to compete with gold, but it can also be a better alternative. Unlike gold, copper can be easily structured using wet or dry etching. This gives a possibility to make nanoscale components that are easily integrated into silicon photonic or electronic integrated circuits.
Silicon chip with nanoscale copper plasmonic components. Courtesy of MIPT.
It took more than two years for the researchers to purchase the required equipment, develop the fabrication process, produce samples, conduct several independent measurements and confirm their hypothesis experimentally.
“As a result, we succeeded in fabricating copper chips with optical properties that are in no way inferior to gold-based chips,” says the research leader Dmitry Fedyanin. “Furthermore, we managed to do this in a fabrication process compatible with the CMOS technology, which is the basis for all modern integrated circuits, including microprocessors. It’s a kind of revolution in nanophotonics.”
The researchers said that the optical properties of thin polycrystalline copper films were determined by their internal structure, and that controlling this structure to achieve and consistently reproduce the required parameters in technological cycles was the most difficult task.
Having demonstrated copper’s suitable material characteristics, as well as nanoscale manufacturing capability, the researchers believe the devices could be integrated with both silicon nanoelectronics and silicon nanophotonics. Such technologies could enable LEDs, nanolasers, highly sensitive sensors and transducers for mobile devices, and high-performance optoelectronic processors with several tens of thousands of cores for graphics cards, personal computers and supercomputers.
“We conducted ellipsometry of the copper films and then confirmed these results using near-field scanning optical microscopy of the nanostructures. This proves that the properties of copper are not impaired during the whole process of manufacturing nanoscale plasmonic components,” says Dmitry Fedyanin.
The research was published in Nano Letters (doi: 10.1021/acs.nanolett.5b03942).
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