A
pair of cylindrical gold nanoparticles, where the plasmonic excitations are
represented by the clouds surrounding the nanoparticles. Credit: University of
Exeter.
A team of
scientists have solved the longstanding problem of how electrons move together
as a group inside cylindrical nanoparticles.
The new
research provides an unexpected theoretical breakthrough in the field of
electromagnetism, with perspectives for metamaterials research.
The team
of theoretical physicists, from the University of Exeter and the University of
Strasbourg, created an elegant theory explaining how electrons move collectively
in tiny metal nanoparticles shaped like cylinders.
The work
has led to new understanding of how light and matter interact at the nanoscale,
aland has implications for the realization of future nanoscale devices
exploiting nanoparticle-based metamaterials with spectacular optical
properties.
Metallic
nanoparticles have a positively charged ionic core, with a cloud of negatively
charged electrons swirling around it. When light is shone on such a metallic
object, the electronic cloud is displaced.
This displacement
causes the whole group of electrons to be set into oscillation about the
positive core. The group of electrons sloshing back and forth behaves like a
single particle (a so-called quasiparticle), known as a "plasmon."
The
plasmon is primarily characterized by the frequency at which it oscillates,
which is known as the plasmon resonance frequency.
Exploring
how the resonance frequency of the plasmon changes depending on the geometry of
its hosting nanoparticle is a fundamental task in modern electromagnetism. It
is commonly thought that only some particular nanoparticle geometries can be
described with analytical theory—that is, without recourse to heavy,
time-consuming numerical computations.
The list
of geometries permitting an analytical description is widely believed to be
very short, being composed of only spherical and ellipsoidal nanoparticles.
This fact
is highly inconvenient due to the experimental ubiquity of cylindrical
nanoparticles, which arise in a variety of aspect ratios from long, needle-like
nanowires to thin, pancake-like nanodisks.
In the
research, the researchers addressed how plasmons in cylindrical nanoparticles
oscillate. By using a theoretical technique inspired by nuclear physics, the
researchers built an elegant analytic theory describing the behavior of
plasmons in cylinders with an arbitrary aspect ratio.
The theory
has enabled a complete description of cylindrical plasmonic nanoparticles,
describing simply the plasmonic resonance in metallic nanoparticles from
nanowires to circular nanodisks.
The two
condensed matter theorists also considered the plasmonic response of a pair of
coupled cylindrical nanoparticles and found quantum mechanical corrections to
their classical theory, which is relevant due to the small, nanometric dimensions
of the nanoparticles.
Dr. Charles Downing from the University of Exeter's Physics and Astronomy department explains: "Quite unexpectedly, our theoretical work provides deep, analytic insight into plasmonic excitations in cylindrical nanoparticles, which can help to guide our experimental colleagues fabricating metallic nanorods in their laboratories."
Guillaume
Weick from the University of Strasbourg adds: "There is a trend for
increasing reliance on heavy duty computations in order to describe plasmonic
systems. In our throwback work, we reveal humble pen-and-paper calculations can
still explain intriguing phenomena at the forefront of metamaterials
research."
The
theoretical breakthrough is of immediate utility to a swathe of scientists
working with nano-objects in the cutting edge science of plasmonics. Longer
term, it is hoped that plasmonic excitations can be exploited in the next
generation of ultra-compact circuitry, solar energy conversion and data storage
as our technology becomes increasingly miniaturized.
Plasmonic
modes in cylindrical nanoparticles and dimers is published in Proceedings of
the Royal Society A.