Optical forces between metallic particles

Abstract

We calculate the optical attractive forces that occur between 30-nm Au or Ag nanocrystals when irradiated at visible wavelengths. These forces show resonances at dipolar plasmon wavelengths, similar to resonances in the near-field electromagnetic intensities. At MW/cm 2 intensities, optical forces can be stronger than van der Waals forces and could be used to organize metallic particles. We also suggest that photonucleation of organic crystals from supersaturated liquid solutions may be caused by optical forces.nanoparticles ͉ photonucleation ͉ bispherical coordinates T his article investigates the possibility that optical forces might be used to organize metallic nanocrystals. It also considers whether optical forces might modify the precise geometry of junctions between metallic nanocrystals. Optical forces derive from the electromagnetic-field intensification that occurs near Ag and Au particles at certain resonance wavelengths. This effect is a local mode of the electromagnetic field: the dipolar surface plasmon. The particle acts as a microscopic antenna, increasing absorption and re-emission of light by nearby molecules and causing the surface-enhanced Raman effect (SERS) (1-5). This enhancement is especially interesting for two particles separated by just a few nanometers. The metallic electron excited-state polarization concentrates at the junction (6), producing a ''hot spot'' of enormous electromagnetic enhancement (7), sufficient to allow the observation of single-molecule Raman spectra (8-10).This article considers the optical forces present between two such closely spaced r ϭ 30-nm metallic particles. The internal electronic polarization in the metal that creates the enhanced local field in the junction also creates force between the particles. This force can be simply understood. Consider two identical particles (or molecules) separated by a distance z. In an electromagnetic field E ៝ , each particle will develop an ac dipole p ៝ ϭ ␣⅐E ៝ oscillating coherently with respect to E ៝ . (In general, such electronic polarization optical dipoles P are the source of Rayleigh scattering, the optical Kerr effect, and trapping of single glass beads in focused laser beams.) Neglecting retardation, the two dipoles are in phase with each other. If the light polarization is along z, then the dipoles are head to tail and the interaction is attractive. With perpendicular polarization, the interaction is repulsive. The dipole-dipole-interaction force scales linearly with the light intensity E 2 and falls off as z Ϫ4 . In the dark, the normal van der Waals force falls off as z Ϫ7 . When the particles are far apart, only the dipole-dipole optical interaction need be considered. When they are separated by a distance of the same magnitude as their size, their mutual polarization is strong, and higher-order moments are important. In this article we consider all separations z for two classical spheres of wavelength-dependent, arbitrary complex dielectric coefficient.A single polarizable particle in a uniform plane-wave fi...

Electrodynamics study of plasmonic bonding and antibonding forces in a bisphere

“…Following Ref. 12, we assume an incident intensity of I 0 = 0.05 W / m 2 , which can be achieved, for instance, with a focused beam. We note that the optical forces linearly scale with the incident intensity.…”

“…It was pointed out in Ref. 12 that there are three distinct regions in the optical force versus separation curve: they are interparticle separations being much smaller, comparable to, and much larger than the particle size. In the near field region, Ref.…”

“…Such forces are important because they may affect the signal of surface enhanced Raman spectroscopy 10,11 or promote controllable aggregation of nanoparticles. 12,13 Two recent papers also studied such forces by using the quasistatic approximation ͑QSA͒, 12,14 but in this paper, we go beyond QSA and employ electrodynamics. The electrodynamics calculation incorporates the retardation effect and is thus more accurate.…”

Bow-tie optical antenna probes for single-emitter scanning near-field optical microscopy

“…Resonant optical antennas in the field of nano-photonics, with their architecture stimulated by their radio frequency counterparts, synergistically combine (i) electromagnetic field confinement and enhancement defined by the size of their feed gap width and (ii) impedance matching of optical waves mediated by the effective length of their antenna arms [1]. The control of sub-wavelength confined and enhanced optical fields pushes the limit in optical characterization [2][3][4] manipulation [5][6][7], and optimization of single nanoscale light sources for information processing [8][9][10][11] on the nanometre scale. In particular, the impedance matching of optical waves opens an efficient pathway to transfer near-field information into the optical far-field, and vice versa [12].…”

“…manipulation [5][6][7], and optimization of single nanoscale light sources for information processing [8][9][10][11] on the nanometre scale. In particular, the impedance matching of optical waves opens an efficient pathway to transfer near-field information into the optical far-field, and vice versa [12].…”