92 research outputs found
Simultaneous measurements of electrophoretic and dielectrophoretic forces using optical tweezers
Herein, charged microbeads handled with optical tweezers are used as a
sensitive probe for simultaneous measurements of electrophoretic and
dielectrophoretic forces. We first determine the electric charge carried by a
single bead by keeping it in a predictable uniform electric field produced by
two parallel planar electrodes, then, we examine same bead's response in
proximity to a tip electrode. In this case, besides electric forces, the bead
simultaneously experiences non-negligible dielectrophoretic forces produced by
the strong electric field gradient. The stochastic and deterministic motions of
the trapped bead are theoretically and experimentally analysed in terms of the
autocorrelation function. By fitting the experimental data, we are able to
extract simultaneously the spatial distribution of electrophoretic and
dielectrophoretic forces around the tip. Our approach can be used for
determining actual, total force components in the presence of high-curvature
electrodes or metal scanning probe tips.Comment: 9 pages, 3 figure
Enhancement factor statistics of surface enhanced Raman scattering in multiscale heterostructures of nanoparticles
Suitable metal nanostructures may induce surface-enhanced Raman scattering (SERS) enhancement factors (EFs) large-enough to reach single-molecule sensitivity. However, the gap hot-spot EF probability density function (PDF) has the character of a long-tail distribution, which dramatically mines the reproducibility of SERS experiments. Herein, we carry out electrodynamic calculations based on a 3D finite element method of two plasmonic nanostructures, combined with Monte Carlo simulations of the EF statistics under different external conditions. We compare the PDF produced by a homodimer of nanoparticles with that provided by a self-similar trimer. We show that the PDF is sensitive to the spatial distribution of near-field enhancement specifically supported by the nanostructure geometry. Breaking the symmetry of the plasmonic system is responsible for inducing particular modulations of the PDF tail resembling a multiple Poisson distribution. We also study the influence that molecular diffusion towards the hottest hot-spot, or selective hot-spot targeting, might have on the EF PDF. Our results quantitatively assess the possibility of designing the response of a SERS substrate so as to contain the intrinsic EF PDF variance and significantly improving, in principle, the reproducibility of SERS experiments
Dark spots along slowly scaling chains of plasmonic nanoparticles
We numerically investigate the optical response of slowly scaling linear
chains of mismatched silver nanoparticles. Hybridized plasmon chain resonances
manifest unusual local field distributions around the nanoparticles that result
from symmetry breaking of the geometry. Importantly, we find localization
patterns characterized by bright hot-spots alternated by what we term
\textit{dark} spots. A dark spot is associated to dark plasmons that have
collinear and antiparallel dipole moments along the chain. As a result, the
field amplification in the dark interjunction gap is extinguished for incident
polarization parallel to the chain axis. Despite the strong plasmonic coupling,
the nanoparticles on the sides of this dark gap experience a dramatic
asymmetric field amplification with amplitude gain contrast .
Remarkably, also for polarization orthogonal to the axis, gap hot-spots form on
resonance.Comment: 6 pages, 2 figure
Spatially resolved refractive index profiles of electrically switchable computer-generated holographic gratings
We describe a spatially resolved interferometric technique combined with a phase reconstruction method that provides a quantitative two-dimensional profile of the refractive index and spatial distribution of the optical contrast between the on-off states of electrically switchable diffraction gratings as a function of the external electric field. The studied structures are holographic gratings optically written into polymer/liquid crystal composites through single-beam spatial light modulation by means of computer-generated holograms. The electro-optical response of the gratings is also discussed. The diffraction efficiency results to be dependent on the incident light polarization suggesting the possibility to develop polarization dependent switching devices
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