Gallium nanoparticles grow where light is

The study of metallic nanoparticles has a long tradition in linear and nonlinear optics 1 , with current emphasis on the ultrafast dynamics, size, shape and collective effects in their optical response 2 3 4 5 6 . Nanoparticles also represent the ultimate confined geometry: high surface-to-volume ratios lead to local field enhancements and a range of dramatic modifications of the material’s properties and phase diagram 7 8 9 . Confined gallium has become a subject of special interest as the light-induced structural phase transition recently observed in gallium films 10 11 has allowed for the demonstration of all-optical switching devices that operate at low laser power 12 . Spontaneous self-assembly has been the main approach to the preparation of nanoparticles (for a review see 13). Here we report that light can dramatically influence the nanoparticle self-assembly process: illumination of a substrate exposed to a beam of gallium atoms results in the formation of nanoparticles with a relatively narrow size distribution. Very low light intensities, below the threshold for thermally-induced evaporation, exert considerable control over nanoparticle formation through non-thermal atomic desorption induced by electronic excitation. We studied nanoparticle formation at the end of an optical fibre exposed to a beam of gallium atoms in vacuum (see fig. 1). This arrangement allowed us to accurately control the deposition conditions and simultaneously to probe the optical properties of the developing structures. Our experiments were conducted in a vacuum chamber evacuated to ~10 -6 mbar