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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 figures

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 figures

In this work we report on the simultaneous measurement of the hydrodynamic coefficient and the electric charge of single {\it Bacillus subtilis} spores. The latter has great importance in protein binding to spores and in the adhesion of spores onto surfaces. The charge and the hydrodynamic coefficient were measured by an accurate procedure based on the analysis of the motion of single spores confined by an optical trap. The technique has been validated using charged spherical polystyrene beads. The excellent agreement of our results with the expected values demonstrates the quality of our procedure. We measured the charge of spores of {\it B. subtilis} purified from a wild type strain and from two isogenic mutants characterized by an altered spore surface. Our technique is able to discriminate the three spore types used, by their charge and by their hydrodynamic coefficient which is related to the hydrophobic properties of the spore surface., Comment: 21 pages 5 figures

For a Brownian system the evolution of thermodynamic quantities is a stochastic process. In particular, the work performed on a driven colloidal particle held in an optical trap changes for each realization of the experimental manipulation, even though the manipulation protocol remains unchanged. Nevertheless, the work distribution is governed by established laws. Here, we show how the measurement of the work distribution is influenced by the presence of rotational, i.e. nonconservative, radiation forces. Experiments on particles of different materials show that the rotational radiation forces, and therefore their effect on the work distributions, increase with the particle refractive index., Comment: 12 pages, 4 figures

The forces acting on an optically trapped particle are usually assumed to be conservative. However, the presence of a non-conservative component has recently been demonstrated. Here we propose a technique that permits one to quantify the contribution of such non-conservative component. This is an extension of a standard optical tweezers calibration technique and, therefore, can easily become a standard test to verify the conservative optical force assumption. Using this technique we have analyzed different-sized optically trapped particles under different trapping conditions. We conclude that the non-conservative effects are effectively negligible and do not affect the standard calibration procedure, unless for extremely low-power trapping, far away from the trapping regimes usually used in experiments., Comment: 8 pages, 4 figures; Europhys. Lett. (EPL) (2009), accepted