An
artist’s view of a metasurface consisting of a rectangular array of rectangular
gold nanostructures generating plasmonic surface lattice resonances. Credit:
Yaryna Mamchur, co-author and Mitacs Summer Student from the National Technical
University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute,” who worked in
Professor Ksenia Dolgaleva’s lab in the summer of 2019 at uOttawa.
Researchers
at the University of Ottawa have debunked the decade-old myth of metals being
useless in photonics – the science and technology of light – with their
findings, recently published in Nature Communications, expected to lead to many
applications in the field of nanophotonics.
“We broke
the record for the resonance quality factor (Q-factor) of a periodic array of
metal nanoparticles by one 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
well-known fact that metals are very lossy when they interact with light, which
means they cause the dissipation of electrical energy. The high losses
compromise their use in optics and photonics. We demonstrated ultra-high-Q
resonances in a metasurface (an artificially structured surface) comprised of
an array of metal nanoparticles embedded inside a flat glass substrate. These
resonances can be used for efficient light manipulating and enhanced
light-matter interaction, showing metals are useful in photonics.”
“In
previous works, researchers attempted to mitigate the adverse effect of losses
to access favorable properties of metal nanoparticle arrays,” observed the
co-lead author of the study Md Saad Bin-Alam, a uOttawa doctoral student in
EECS.
“However,
their attempts did not provide 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-of-magnitude improvement
demonstrating a metal nanoparticle array (metasurface) 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
beam shaping and steering are just a few examples of the many applications.
“Metasurfaces
made of noble metal nanoparticles – gold or silver for instance – possess some
unique benefits over non-metallic nanoparticles. They can confine and control
light in a nanoscale volume that is less than one quarter of the wavelength of
light (less than 100 nm, while the width of a hair is over 10 000 nm),”
explained Md Saad Bin-Alam.
“Interestingly,
unlike in non-metallic nanoparticles, the light is not confined or trapped
inside the metal nanoparticles but is concentrated close to their surface. This
phenomenon is scientifically called ‘localized surface plasmon resonances
(LSPRs)’. This feature gives a great superiority to metal nanoparticles
compared to their dielectric counterparts, because one could exploit such
surface resonances to detect bio-organisms or molecules in medicine or
chemistry. Also, such surface resonances could be used as the feedback
mechanism necessary for laser gain. In such a way, one can realize a nanoscale
tiny laser that can be adopted in many future nanophotonic applications, like
light detection and ranging (LiDAR) for the far-field object detection.”
According
to the researchers, the efficiency of these applications depends on the
resonant Q-factors.
“Unfortunately,
due to the high ‘absorptive’ and ‘radiative’ loss in 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 of Ottawa.
“More than
a decade ago, researchers found a way to mitigate the dissipative loss by
carefully arranging the nanoparticles in a lattice. From such ‘surface lattice’
manipulation, a new ‘surface lattice resonance (SLR)’ emerges with suppressed
losses. Until our work, the maximum Q-factors reported in SLRs was around a few
hundred. Although such early reported SLRs were better than the low-Q LSPRs,
they were still not very impressive for efficient applications. It led to the
myth that metals are not useful for practical applications.”
A myth
that the group was able to deconstruct during its work at the University of
Ottawa’s Advanced Research Complex between 2017 and 2020.
“At first,
we performed numerical modeling of a gold nanoparticle metasurface and were
surprised to obtain quality factors of several thousand,” said Md Saad
Bin-Alam, who primarily designed the metasurface structure.
“This
value has never been reported experimentally, and we decided to analyze why and
to attempt an experimental demonstration of such a high Q. We observed a very
high-Q SLR of value nearly 2400, that is at least 10 times larger than the
largest SLRs Q reported earlier.”
A
discovery that made them realize that there’s still a lot to learn about
metals.
“Our
research proved that we are still far from knowing all the hidden mysteries of
metal (plasmonic) nanostructures,” concluded Dr. Orad Reshef, who fabricated
the metasurface sample. “Our work has debunked a decade-long myth that such
structures are not suitable for real-life optical applications due to the high
losses. We demonstrated that, by properly engineering the nanostructure and
carefully conducting an experiment, one can improve the result significantly.”