Researchers at imec have
developed an innovative strategy to tune plasmon resonances. They do so by breaking
the symmetric geometry of the nanostructures, using a combination of bottom-up
and top-down fabrication processes. Such broken symmetry can lead to strongly
enhanced local electric fields. A potential application is the detection of
biomolecules via surface-enhanced Raman scattering (SERS).
Metal-based nanophotonics (plasmonics) is a field concerned with manipulating
and focusing light on nanoscale structures that are much smaller than conventional
optic components. Plasmonic technology, today still in an experimental stage,
has the potential to be used in future applications such as nanoscale optical
interconnects for high performance computer chips, highly efficient thin-film
solar cells, and extremely sensitive (bio)molecular sensors.
(top) Schematic illustration of various shapes of plasmonic nanostructures and (bottom) the corresponding electron microscopy images.
Plasmonic applications can be made from nanostructured (noble) metals. When
such nanostructures are illuminated with visible to near-infrared light, the
excitation of collective oscillations of conduction electrons - called
surface plasmons - generates strong optical resonances, focusing electromagnetic
energy in deep-sub-wavelength-scales. The resonance spectra of the metallic
nanostructures strongly depend on their geometry. Imec has extensive experience
in synthesizing various shapes of nanostructures to tune the resonances from
the ultraviolet to the near-infrared region. Examples of such shapes are nanospheres,
nanocubes, nanorods, nanoshells, and nanorings.
Recently, researchers at imec have developed an innovative strategy to precisely
tune the plasmon resonances. They do so by breaking the symmetric geometry of
the nanostructures, using a combination of bottom-up and top-down fabrication
processes. This allows making a geometrical transition from nanocubes to nanoplates
(see Jian Ye, et al. Nanotechnology, 2008, 19, 325702), from nanoshells to semishells
and nanobowls (see Jian Ye, et al. the Journal of Physical Chemistry C, 2009,
113, 3110; Jian Ye, et al. Langmuir, 2009, 25, 1822; Jian Ye, et al. ACS Nano,
2010, 4, 1457), from nanocages to open-nanocages (see Jian Ye, et al. Optics
Express, 2009, 17, 23765).
Combining bottom-up and top-down fabrication turns out to be a cost-effective
method to obtain large areas covered with engineered metal nanostructures. The
nano-dimensions are still set by the bottom-up fabrication procedures, and the
geometrical tweaking occurs through well-characterized top-down fabrication
techniques such as metal evaporation and ion milling.
Imec has gained a substantial insight in the optical properties of these nanostructures
using a combination of electromagnetic simulations and advanced optical spectroscopy.
This allows explaining the optical properties using the so-called plasmon hybridization
model, where the resonances of complex nanostructures can be described as bonding
and anti-bonding arrangements of the parent plasmon resonances of the individual
constituents. This paves the way to tweaking the optical properties of metal
nanostructures for various applications. More specifically, the broken symmetry
can lead to strongly enhanced local electric fields, which show a potential
application in surface-enhanced Raman scattering-based bio-molecular detection.