We study the optical forces on particles, either dielectric or metallic, in or out their Mie resonances, near a subwavelength slit in extraordinary transmission regime. Calculations are two-dimensional, so that those particles are infinite cylinders. Illumination is with p-polarization. We show that the presence of the slit enhances by two orders of magnitude the transversal forces of optical tweezers from a beam alone. In addition, a drastically different effect of these particle resonances on the optical forces that they experience; namely, we demonstrate an enhancement of these forces, also of binding nature, at plasmon resonance wavelengths on metallic nanocylinders, whereas dielectric cylinders experience optical forces that decrease at wavelengths exciting their whispering gallery modes. Particles located at the entrance of the slit are easily bound to apertures due to the coincidence in the forward direction of scattering and gradient forces, but those particles at the exit of the slit suffer a competition between forward scattering force components and backward gradient forces which make more complex the bonding or antibonding nature of the resulting mechanical action.

We report stable and reproducible trapping of luminescent dielectric YAG:Ce(3+) nanoparticles with sizes down to 60 nm using far-field dual fiber tip optical tweezers. The particles are synthesized by a specific glycothermal route followed by an original protected annealing step, resulting in significantly enhanced photostability. The tweezers properties are analyzed by studying the trapped particles residual Brownian motion using video or reflected signal records. The trapping potential is harmonic in the transverse direction to the fiber axis, but reveals interference fringes in the axial direction. Large trapping stiffness of 35 and 2 pN μm(-1) W(-1) is measured for a fiber tip-to-tip distance of 3 μm and 300 nm and 60 nm particles, respectively. The forces acting on the nanoparticles are discussed within the dipolar approximation (gradient and scattering force contributions) or exact calculations using the Maxwell Stress Tensor formalism. Prospects for trapping even smaller particles are discussed.

We report numerical experiments of optical wave propagation in composites of high refractive index dielectric rods at frequencies where their first electric and magnetic Mie resonances are excited. The arrays of these particles have been extensively studied and proposed as non-absorbing and isotropic metamaterials. We show that negative refraction, observed in ordered particle arrays, is due to diffraction and that an effective medium theory yields constitutive parameters that do not reproduce the observations in these composites, whose transmission also depends on the sample shape. This is further confirmed by disordering the arrays, a case in which large transmission losses appear due to extinction by resonant scattering from the particles. Therefore, these composites although little absorbing have large extinction due to scattering.