  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 [2-6]. 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 [7-9]. 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.
