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We report that low-intensity light can dramatically influence and regulate the nanoparticle self-assembly process: Illumination of a substrate exposed to a beam of gallium atoms results in the formation of gallium nanoparticles with a relatively narrow size distribution. Very low light intensities, below the threshold for thermally induced evaporation, exert considerable control over nanoparticle formation.

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We have developed an effective medium theory for the optical properties of nanoparticle films by considering the exact local fields of spheroidal nanoparticles on a substrate. The model allows the calculation of reflectivity, transmission and absorption of nanoparticle films for a wide range of filling factors, nanoparticle aspect ratios and substrate dielectric characteristics. It is suitable for many applications as it can treat films of homogeneous or binary core-shell nanoparticles.

Planar structures containing oriented and ordered metallic nanoparticles with shapes lacking an inversion centre can act as a nonlinear medium for generation of second harmonic optical radiation by a process whose directional features resemble those of phase matched second harmonic generation (SHG) in bulk media. The nonlinearity of the metallic patterns stems from the asymmetric modulation of the local field inside nanoparticle by electron oscillations and is deeply rooted in the nanostructured nature of the system. The SHG efficiency is inversely proportional to the second power of the nanoparticle size.Keywords: nanoparticles, second harmonic generation, metamaterials Microstructured and nanostructured metallic surfaces offer a range of unusual and useful optical properties that give them a well-deserved place in meta-material research. For instance, chiral structuring brings about strong polarization effects in light transmitted, reflected and scattered from a planar structure [1], while plasmon excitations provide unexpectedly high levels of light transmission through microperforated metallic screens [2] and are behind the complex polarization properties and efficiency of metallic gratings [3,4]. Nanostructured planar structures containing non-centrosymmetric metallic inclusion show second order nonlinear properties and may be used to generate second harmonic optical radiation [5][6][7]. The main advantage of these nonlinear media is their simplicity and the fact that they can be fabricated using well-established microfabrication/nanofabrication techniques developed for the semiconductor industry. However, there has so far been no discussion of the significance of the inclusion size or the phase matching conditions specific to ordered arrays of nanoparticles. This paper will demonstrate that the nonlinearity of metallic non-centrosymmetric structures is deeply rooted in the nanostructured nature of the system. A simple scaling formula is introduced which shows that the second harmonic efficiency decreases as the second power of the nanoparticle size.The optical second harmonic generation (SHG) processthe conversion of radiation at a fundamental pump frequency into radiation at a harmonic with twice the frequency-is a symmetry breaking effect. In the dipole approximation it can only be seen in systems lacking an inversion centre. SHG was first seen in bulk non-centrosymmetric crystal and powders, and later in geometries of reflections from solid surfaces, where symmetry is broken by the presence of the interface [8]. Here we show that a regular planar structure containing metallic nanoparticles lacking inversion symmetry provides a nonlinear medium capable of converting the pump energy into the second harmonic in a single-layer structure. The relative efficiency of this process increases rapidly with decreasing nanoparticle size, and the arrangement of nanoparticles in regular 2D arrays provides coherent addition of second harmonic fields in a manner analogous to the non-collinear phase matching proc...