Researchers demonstrate practical metal nanostructures

Researchers at the University of Ottawa have debunked the decade-old myth that metals are useless in photonics – the science and technology of light – with their findings, recently published in Nature communication, is expected to lead to many applications in the field of nanophotonics.

“We have broken the record for the resonance quality factor (Q factor) of a periodic array of metal nanoparticles of an order of magnitude compared to previous reports,” said senior author Dr. Ksenia Dolgaleva, Canada Research Chair in Integrated Photonics (Tier 2) and Associate Professor in the School of Electrical Engineering and Computer Science (EECS) at the University of Ottawa.

“It is a known fact that metals are very loss-making when interacting with light, which means that they cause the dissipation of electrical energy. The high losses jeopardize their use in optics and photonics. We have ultra-high Q- resonances demonstrated in a metasurface (an artificially textured surface) consisting of a series of metal nanoparticles embedded in a flat glass substrate. These resonances can be used for efficient light manipulation and enhanced light-matter interaction, showing that metals are useful in photonics. “

“In previous works, researchers tried to mitigate the adverse effect of losses to gain access to beneficial properties of metal nanoparticle arrays,” said study co-lead author Md Saad Bin-Alam, a uOttawa doctoral student in EECS.

“Their efforts, however, did not significantly improve the quality factors of the resonances of the arrays. We implemented a combination of techniques rather than a single approach and achieved an order of magnitude improvement demonstrating a metal nanoparticle array (metasurface). a record high quality factor. “

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

Metasurfaces made of precious metal nanoparticles – gold or silver, for example – have some unique advantages over non-metallic nanoparticles. They can confine 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), ”explains Md Saad Bin-Alam.

“Interestingly, unlike non-metallic nanoparticles, the light is not trapped or trapped in the metallic nanoparticles, but is concentrated close to their surface. This phenomenon is scientifically called ‘localized surface plasma resonances (LSPRs)’. great superiority with metal nanoparticles compared to their dielectric counterparts, because one could use such surface resonances to detect bio-organisms or molecules in medicine or chemistry. Such surface resonances could also be used as the feedback mechanism needed for laser amplification. one can realize a tiny nanoscale laser that can be used in many future nanophotonic applications, such as light detection and range finding (LiDAR) for the detection of distant objects. “

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

“Unfortunately, due to the high ‘absorptive’ and ‘radiation loss’ in metal nanoparticles, the Q factors of LSPRs are very low,” said study co-lead author Dr. Orad Reshef, a postdoctoral researcher in the University of Ottawa Department of Physics.

“More than a decade ago, researchers found a way to reduce the dissipative loss by carefully arranging the nanoparticles in a grid. Such surface lattice manipulation creates a new surface lattice resonance (SLR) with suppressed losses. , the maximum Q factor reported in DSLRs was about a few hundred. While such early reported DSLRs were better than the low Q LSPRs, they were still not very impressive for efficient applications, leading to the myth that metals are not usable. are for practical applications. “

A myth that the group managed to deconstruct between 2017 and 2020 during her work at the Advanced Research Complex at the University of Ottawa.

“Initially, we made numerical models of a gold nanoparticle meta-surface and were surprised to obtain quality factors of several thousand,” said Md Saad Bin-Alam, who primarily designed the meta-surface structure.

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

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

“Our research proved that we are still far from knowing all the hidden mysteries of metallic (plasmonic) nanostructures,” concluded Dr. Orad Reshef, who manufactured the metasurface sample. “Our work has debunked a ten-year myth that such structures are not suitable for real-life optical applications due to the high losses. feed. “

The article “Ultra-high-Q resonances in plasmonic metasurfaces” was published in Nature Communications