Surface delivery of a single nanoparticle under moving evanescent standing-wave illumination

We study the delivery of a submicrometre-sized spherical dielectric particle suspended in water and confined in an evanescent field in the proximity of a glass–water interface. When illuminated by a single evanescent wave, the particle is propelled along the glass surface by the radiation pressure. Illumination by two counter-propagating and coherent evanescent waves leads to the formation of a surface-bound evanescent standing wave serving as a onedimensional array of optical traps for the stable confinement of the particle. These traps can be translated simultaneously along the surface by shifting the phase of one of the two interfering evanescent waves, carrying the confined particle along in an optical conveyor belt (OCB). However, due to the thermal activation, the particle jumps between neighboring optical traps, and its delivery conditions in the OCB are thus more complex than in the case of the single evanescent wave propulsion. We analyze the delivery speed of a single particle confined in the OCB moving with different speeds and formed by optical traps of different depths. We present a theoretical description of the particle delivery speed in the OCB and compare it with the delivery speed in the single evanescent wave. We support our theoretical conclusions by experimental observations and demonstrate that especially particles having diameters smaller than∼220 nm are delivered faster in the OCB using the same total optical power. 1 Authors to whom any correspondence should be addressed. 2 Present address: School of Physics and Astronomy, University of St Andrews, North Haugh, Fife KY16 9SS, UK. New Journal of Physics 10 (2008) 11301