Bain and Mellor began by overlapping two laser beams on the surface of a silica prism. The beams were made to strike the surface above the critical angle, so that only the evanescent -- or surface -- fields penetrate out into the space beyond the prism. Next, the researchers placed a drop of water containing a dilute solution of polystyrene beads about 300 to 600 nm in diameter on the surface of the prism. The spheres are attracted by the evanescent field and spontaneously assemble into 2D arrays (see figure 1).

"For most physicists, the idea of materials held together by light is still foreign," says Bain. "The most surprising result in this new work is the formation of a square array of 390 nm particles with orthogonally polarized laser beams. Although the electric field is quite uniform in the plane of the surface, a large regular array is observed."

The new optical matter arrays are distinct from optical tweezers, in which spatially varying electric fields are used to control the positions of particles. According to Bain and Mellor, the 2D ordering in optical arrays comes from the scattering of the evanescent light field by the particles themselves and not from an imposed field gradient.

"The arrays show many of the dynamical features of molecular crystals, such as surface diffusion, migration of defects, nucleation of phase transformations and 'Ostwald ripening' – where two arrays coalesce into one," says Bain. "As well as being a new way to assemble matter on the nanoscale, such arrays may also provide a way of visually studying, in real time, the processes that occur invisibly in crystals on sub-nanoscales."

Bain now plans to develop a quantitative model to explain the optical binding in these arrays, and to study how particles with different shapes and sizes assemble. He also hopes to extend the optical matter arrays into 3D.