Tunable optical sorting and manipulation of nanoparticles via plasmon excitation

Abstract

We numerically investigate the optical forces exerted by an incident light beam on Rayleigh metallic particles over a dielectric substrate. In analogy with atom manipulation, we identify two different trapping regimes depending on whether the illumination is performed within the plasmon band or out of it. By adjusting the incident wavelength, the particles can be selectively guided, or immobilized, at the substrate interface. © 2006 Optical Society of America OCIS codes: 140.7010, 240.5420, 260.3910, 240.0240. Due to their potential confinement down to subwavelength volumes, evanescent fields bound at interfaces open novel opportunities for efficient manipulation of nano-objects. 1-4 Lately, we have shown both theoretically and experimentally that plasmon fields at a metal/dielectric interface can be used to dramatically enhance the optical forces on a dielectric object. We consider a metal sphere immersed in water ͑n water = 1.33͒, floating at 15 nm from the surface of a heavy flint glass prism ͑n flint = 1.8͒. The illumination is performed from the glass under total internal reflection (TIR) by either a plane wave or a tightly focused three-dimensional Gaussian beam [see 9 This formalism accounts for multipolar contributions, which cannot be neglected for metal particles of diameter larger than a few tens of nanometers. Due to their dispersion, the optical forces that experience metal nanoparticles under illumination are susceptible to dramatic variations with the incident wavelength. For a small dielectric particle, using a dipolar approximation, the total force combines a gradient contribution that attracts the particle towards the highest field value and a scattering component that tends to push it along the incident wave vector. In the case of absorbing particles, an absorption force is added to the scattering component. The evolution with the incident wavelength of the X and Z components of the total force exerted on an 80 nm diameter gold sphere is shown i