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 $> 2 \times 10^2$. 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