434 research outputs found
Asymptotics of surface-plasmon redshift saturation at sub-nanometric separations
Many promising nanophotonics endeavours hinge upon the unique plasmonic
properties of nanometallic structures with narrow non-metallic gaps, which
support super-concentrated bonding modes that singularly redshift with
decreasing separations. In this letter, we present a descriptive physical
picture, complemented by elementary asymptotic formulae, of a nonlocal
mechanism for plasmon-redshift saturation at subnanometric gap widths. Thus, by
considering the electron-charge and field distributions in the close vicinity
of the metal-vacuum interface, we show that nonlocality is asymptotically
manifested as an effective potential discontinuity. For bonding modes in the
near-contact limit, the latter discontinuity is shown to be effectively
equivalent to a widening of the gap. As a consequence, the resonance-frequency
near-contact asymptotics are a renormalisation of the corresponding local ones.
Specifically, the renormalisation furnishes an asymptotic plasmon-frequency
lower bound that scales with the -power of the Fermi wavelength. We
demonstrate these remarkable features in the prototypical cases of nanowire and
nanosphere dimers, showing agreement between our elementary expressions and
previously reported numerical computations
Robustness of the Rabi splitting under nonlocal corrections in plexcitonics
We explore theoretically how nonlocal corrections in the description of the
metal affect the strong coupling between excitons and plasmons in typical
examples where nonlocal effects are anticipated to be strong, namely small
metallic nanoparticles, thin metallic nanoshells or dimers with narrow
separations, either coated with or encapsulating an excitonic layer. Through
detailed simulations based on the generalised nonlocal optical response theory,
which simultaneously accounts both for modal shifts due to screening and for
surface-enhanced Landau damping, we show that, contrary to expectations, the
influence of nonlocality is rather limited, as in most occasions the width of
the Rabi splitting remains largely unaffected and the two hybrid modes are well
distinguishable. We discuss how this behaviour can be understood in view of the
popular coupled-harmonic-oscillator model, while we also provide analytic
solutions based on Mie theory to describe the hybrid modes in the case of
matryoshka-like single nanoparticles. Our analysis provides an answer to a so
far open question, that of the influence of nonlocality on strong coupling, and
is expected to facilitate the design and study of plexcitonic architectures
with ultrafine geometrical details
Nonlocal optical response in metallic nanostructures
This review provides a broad overview of the studies and effects of nonlocal
response in metallic nanostructures. In particular, we thoroughly present the
nonlocal hydrodynamic model and the recently introduced generalized nonlocal
optical response (GNOR) model. The influence of nonlocal response on plasmonic
excitations is studied in key metallic geometries, such as spheres and dimers,
and we derive new consequences due to the GNOR model. Finally, we propose
several trajectories for future work on nonlocal response, including
experimental setups that may unveil further effects of nonlocal response
The impact of nonlocal response on metallo-dielectric multilayers and optical patch antennas
We analyze the impact of nonlocality on the waveguide modes of
metallo-dielectric multilayers and optical patch antennas, the latter formed
from metal strips closely spaced above a metallic plane. We model both the
nonlocal effects associated with the conduction electrons of the metal, as well
as the previously overlooked response of bound electrons. We show that the
fundamental mode of a metal-dielectric-metal waveguide, sometimes called the
gap-plasmon, is very sensitive to nonlocality when the insulating, dielectric
layers are thinner than 5 nm. We suggest that optical patch antennas, which can
easily be fabricated with controlled dielectric spacer layers and can be
interrogated using far-field scattering, can enable the measurement of
nonlocality in metals with good accuracy
How nonlocal damping reduces plasmon-enhanced fluorescence in ultranarrow gaps
The nonclassical modification of plasmon-assisted fluorescence enhancement is
theoretically explored by placing two-level dipole emitters at the narrow gaps
encountered in canonical plasmonic architectures, namely dimers and trimers of
different metallic nanoparticles. Through detailed simulations, in comparison
with appropriate analytical modelling, it is shown that within classical
electrodynamics, and for the reduced separations explored here, fluorescence
enhancement factors of the order of can be achieved, with a divergent
behaviour as the particle touching regime is approached. This remarkable
prediction is mainly governed by the dramatic increase in excitation rate
triggered by the corresponding field enhancement inside the gaps. Nevertheless,
once nonclassical corrections are included, the amplification factors decrease
by up to two orders of magnitude and a saturation regime for narrower gaps is
reached. These nonclassical limitations are demonstrated by simulations based
on the generalised nonlocal optical response theory, which accounts in an
efficient way not only for nonlocal screening, but also for the enhanced Landau
damping near the metal surface. A simple strategy to introduce nonlocal
corrections to the analytic solutions is also proposed. It is therefore shown
that the nonlocal optical response of the metal imposes more realistic, finite
upper bounds to the enhancement feasible with ultrasmall plasmonic cavities,
thus providing a theoretical description closer to state of the art
experiments
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