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Journal of Materials Science and Nanotechnology | Volume: 3
March 20-21, 2019 | London, UK
Materials Science and Materials Chemistry
2nd International Conference on
Nanophotonics localizes an optical phenomenon with
small metallic particles. The effect is largest at a plasmon
resonance. Plasmonics use resonances of the density of surface
electrons with an incoming field to locally enhance the electric
field strength. This increases the optical interaction in that small
volume of space where the resonance is taking place. These
plasmon resonances can be tuned by particle size and shape, or
by gold coating thickness. A keymanner inwhichnanophotonics
can control an optical interaction is that the metal increases the
local photon density of states (LDOS), so photon transition rates
are sped up while phonon (non-radiative) rates remain fixed.
Rare earth ion doped upconverting nanoparticles are excited in
the near infrared (NIR) and fluoresce via anti-Stokes emission in
the visible energy range (400-650 nm). The NIR light provides
large penetration depth of excitation, while the particles
exhibit no blinking, and high signal-to-noise ratio due to zero
tissue autofluorescence. In addition, since upconversion is
a two-photon fluorescence process, it has the same ability
as other 2-photon fluorescence microscopies to resolve the
3-dimensional structure of objects. In the co-doped rare
earth ion upconverter system studied here, the Ytterbium and
Erbium dopant couple, the upconversion occurs through an
energy transfer upconversion (ETU) process, where the Yb3+ ion
transfers its energy to the Er3+ ion. Despite using a real rather
thanvirtual intermediate state, thebrightness andupconversion
efficiency of these nanoparticles is not comparable to that of
semiconductor nanoparticles and dyes. The down-scaling of
particle size also leads to a rapid loss of brightness. This has
been attributed to the low absorption cross-section of the
rare earth ion dopants. That is because transitions to the inner
4f-shell levels in rare earth ions are only very weakly allowed;
hence their absorption coefficients are very small, limiting their
maximum emission intensities. Although that shortcoming is
partially compensated by its zero background fluorescence and
its non-blinking and non-bleaching properties, we show that
plasmonics lead to 1) local field enhancements that increase the
absorption and emission efficiencies, and 2) a large anisotropy
in the fluorescence yield if illuminated with polarized light. The
emission is dependent on the particle orientation and is also
polarized and directional.
Upconversion nanostructures are optimized with predictive
finite elementmodelling (derivationof the LDOS) and correlated
structural and optical single nanoparticle spectroscopy is
performed to explore the influence of the nanostructure
orientation, and geometry on the time scale of the optical
transitions. Isolation at the single particle level allow for
establishment of quantitative relationships between the crystal
architecture and orientation that control emission properties, to
enable direct comparisons with other lasing systems and allow
for rational engineering. The single particle results are alsomore
consistent with finite element calculations, without having to
correct for anomalies generated by ensemble measurements.
The optimized nanostructures can potentially be applied in
an array format in a display, quantum computing or in solar
harvesting devices.
Speaker Biography
Following completion of a Ph.D. at the University of Cambridge, UK, 2004, Shuang Fang Lim
served in a postdoctoral Research Position at Princeton University from 2004-2008. Her
worktherefocusedonupconvertingnanoparticlels(UCNPs)andthesynthesis,photophysics
and bio-applications of nanoparicles. Following this assignment, she then served as in a
postdoctoral position for one year at NC State University and then in a Research Assistant
professor position for three years before accepting a Professor appointment staring in the
fall of 2012.
e:
Shuang Fang Lim
North Carolina State University, USA
Upconversion nanophotonics : Photophysics, simulations, and applications