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A set-up for organizing nanoparticles into a two-dimensional lattice with a laser

A. Slablab, G. Rizza and G. Coddens

An experimental facility has been built to organize dielectric nano-objects (e.g. polystyrene nano-spheres) into a two-dimensional lattice within a liquid by means of a laser beam. The idea has been adopted from the seminal papers of Burns et al. [1] and Mellor et al. [2]. We use a 532 nm continuous laser beam. The liquid is a very thin layer sandwiched between the top of a glass prism and a microscope glass. The laser beam is sent into the prism and focalized at the glass-liquid interface in such a way that it reaches the interface at the angle corresponding to the onset of total reflection. This gives rise to an evanescent wave within the liquid at the interface with the prism. The reflected part of the laser beam is reflected by a concave mirror focalizing it onto the same spot as the incident beam, also at the onset of total reflection. The interference pattern created by the incident and reflected evanescent laser beams functions as a laser trap, wherein the nano-spheres organize themselves into a two-dimensional lattice. By cross-polarizing the incident and reflected laser beams one can extinguish the interference. The particles then still organize themselves into a lattice through a mutual interaction based on the interference between the Mie scattering from the particles and the homogeneous laser field. The nano-particles can be observed through an optical microscope and imaged on a computer screen using a CCD.

Experimental set-up. The red arrows indicate the trajectory of the incident (AB) and (twice) reflected (BCB) laser beams

[1] M.M. Burns, J.-M. Fournier and J.A. Golovchenko, Science 249 (1990) 749-754.
[2] C.D. Mellor and C.D. Bain, ChemPhysChem 7 (2006) 329-332; C.D. Mellor, T.A. Fennerty, and C.D. Bain, Optics Express 14 (2006) 10079-10088.