Optical Forces at the Nanoscale: Size and Electrostatic Effects

Abstract

The reduced magnitude of the optical trapping forces exerted over sub-200 nm dielectric nanoparticles complicates their optical manipulation, hindering the development of techniques and studies based on it. Improvement of trapping capabilities for such tiny objects requires a deep understanding of the mechanisms beneath them. Traditionally, the optical forces acting on dielectric nanoparticles have been only correlated with their volume, and the size has been traditionally identified as a key parameter. However, the most recently published research results have shown that the electrostatic characteristics of a sub-100 nm dielectric particle could also play a significant role. Indeed, at present it is not clear what optical forces depend. In this work, we designed a set of experiments in order to elucidate the different mechanism and properties (i.e., size and/or electrostatic properties) that governs the magnitude of optical forces. The comparison between experimental data and numerical simulations have shown that the double layer induced at nanoparticle's surface, not considered in the classical description of nanoparticle's polarizability, plays a relevant role determining the magnitude of the optical forces. Here, the presented results constitute the first step toward the development of the dielectric nanoparticle over which enhanced optical forces could be exerted, enabling their optical manipulation for multiples purposes ranging from fundamental to applied studies.

Light Induced Inverse-Square Law Interactions between Nanoparticles: “Mock Gravity” at the Nanoscale

“…For a ¼ 20 nm and r∼6a ¼ 120 nm, F vdW ∼1 fN we obtain similar F MG at 1 mW=μm 2 while, for 1 W=μm 2 , Mock gravity forces would reach values of the order of pN. In a liquid environment, electrostatic double layer forces, coming from the interaction with the ions of the solvent, could also compete with optical forces [34] but can be cancelled by setting an isoelectric point at which the socalled zeta potential is zero (see e.g., [35] for Ag nanopartiples in aqueous environments). Moreover, Fröhlich resonances can be significantly enhanced and shifted when the particles are immersed in lossless solvents [26].…”

All-dielectric nanotweezers for trapping and observation of a single quantum dot

“…The use of femtosecond laser pulses with all-dielectric nanoantenna tweezers for the trapping of QDs and other nanomaterials might be an interesting topic for the future study. We also suggest that it may be fruitful to investigate the effect of the electric double layer coating on nanoparticles on trapping with optical nanotweezers, as recent work has shown it to be important with conventional optical tweezers [46].…”

Opto-thermoelectric pulling of light-absorbing particles

“…Hence, when the CTAC concentration increases from 1 to 4 mM, the trapping force successively decreases. It should be noted that the influence of the zeta potential on the trapping stiffness was also observed in the optical trapping of dielectric nanoparticles when the particle size was much smaller than the trapping wavelength and the particles could be approximated as point dipoles 32,33 . The out-of-plane force at a CTAC concentration of 1 mM is also analysed (Fig.…”