Researchers demonstrate practical metal nanostructures

Researchers at the University of Ottawa have uncovered the decades-old myth of metals that are useless in photonics – the science and technology of light – with their findings, which were recently Nature communication, will be expected to lead to many applications in the field of nanophotonics.

“We have the record for the resonance quality factor (Q-factor) of a periodic series of metal nanoparticles with one order of magnitude compared to previous reports,” said senior author, dr. Ksenia Dolgaleva, Canada’s Research Chair in Integrated Photonics (Level 2) said. ) and associate professor at the School of Electrical Engineering and Computer Science (EECS) at the University of Ottawa.

“It is a known fact that metals suffer a lot of loss when they interact with light, which means that it causes the dissipation of electrical energy. The high losses impair its use in optics and photonics. We have ultra-high Q- resonances shown in a meta-surface (an artificially structured surface) consisting of a series of metal nanoparticles embedded in a flat glass substrate.These resonances can be used for effective light manipulation and enhanced light-matter interaction , which shows that metals are useful in photonics. ‘

“In previous works, researchers have tried to mitigate the detrimental effect of losses on beneficial properties of metal nanoparticles,” said the co-lead author of the study, Ms. Saad Bin-Alam, a doctoral student at the UOttawa in EECS, observed.

“However, their efforts did not yield a significant improvement in the quality factors of the resonances of the arrays. We implemented a combination of techniques rather than a single approach and obtained an order-to-size improvement that metal nanoparticle array (metasurface) shows with a record high quality factor. “

According to the researchers, structured surfaces – also called metasurfaces – have very promising prospects in a variety of nanophotonic applications that can never be explored using traditional natural bulk materials. Sensors, nanolasers, light beamforming and steering are just a few examples of the many applications.

Meta-surfaces of precious metal nanoparticles – for example gold or silver – have some unique advantages over non-metallic nanoparticles. They can limit and control light in a nanoscale volume that is less than a quarter of the wavelength of light ( less than 100 nm), while the width of a hair is more than 10 000 nm), ‘Saad Bin-Alam explained.

“Interestingly, unlike in non-metallic nanoparticles, light is not confined or trapped in the metal nanoparticles, but is concentrated near the surface. This phenomenon is scientifically called ‘localized surface plasma resonances (LSPRs)’. This function gives a large superiority of metal nanoparticles compared to their dielectric counterparts, because such surface resonances can be used to detect bioorganisms or molecules in medicine or chemistry. Such surface resonances can also be used as the feedback mechanism required for laser augmentation. ‘ one can realize a small nanoscale laser that can be used in many future nanophotonic applications, such as light detection and range (LiDAR) for the detection of objects in the distant field. ‘

According to the researchers, the effectiveness of these applications depends on the resonant Q-factors.

“Unfortunately, due to the high ‘absorbing’ and ‘radiating’ loss of metal nanoparticles, the LSPRs Q factors are very low,” said co-lead author, dr. Orad Reshef, a postdoctoral fellow in the Department of Physics at the University, said. of Ottawa.

“More than a decade ago, researchers found a way to mitigate the scattering loss by carefully arranging the nanoparticles in a grid. From such ‘surface grid’ manipulation, a new ‘surface grid resonance (SLR)’ emerges with suppressed losses.Until our work, the maximum Q-factors reported in SLRs were about a few hundred, and although such early-reported SLRs were better than the low-Q LSPRs, they were still not very impressive for efficient applications. has led to the myth that metals are not useful for practical applications. “

A myth that the group was able to deconstruct between 2017 and 2020 while working on the Advanced Research Complex of the University of Ottawa.

“Initially, we performed numerical modeling of a gold nanoparticle meta-surface and were surprised that we obtained quality factors of a few thousand,” said Ms. Saad Bin-Alam, who mainly designed the meta-surface structure.

“This value was never reported experimentally, and we decided to analyze why and try to do an experimental demonstration of such a high Q. We observed a very high Q-SLR of almost 2400, which is at least 10 times larger than the largest SLRs Q reported earlier. ‘

A discovery that made them realize that there is still a lot to be learned about metals.

“Our research has proven that we are far from knowing all the hidden secrets of metal (plasmonic) nanostructures,” said Dr. Orad Reshef, who produced the meta-surface monster, shut down. “Our work has unleashed a ten-year myth that such structures are not suitable for real-life optical applications due to the large losses. We have shown that one can significantly improve the result by constructing the nanostructure properly and carefully to conduct an experiment. “

The paper “Ultra-high-Q resonances in plasmonic metasurfaces” is published in Nature communication.