Mapping of gold nanostructure-enhanced near fields via laser scanning second-harmonic generation and ablation

Abstract

The optical near-field of metal films can be modified in a straightforward manner by incorporating nanostructures on the surface. The corresponding field enhancement, which may be due to the lightning rod effect as well as the excitation of plasmon modes, results in a local change of the optical surface response. A transparent thin film on top of the nanostructures can be partially ablated via illumination with near-infrared light. Local variations of the ablation rate due to field enhancement are readily mapped with subdiffractional resolution, as confirmed by a direct comparison to theoretical calculations. Variation of the thickness of the transparent film enables discrimination between localized enhancements at the sharp corners of the structures and collective enhancements at locations between the structures due to surface plasmon polariton modes. In addition, applying the same method to study the effect of nanostructure morphology on localized second-harmonic generation using arrays of rectangular as well as triangular structures, we observed a second-harmonic (SH) signal from both centrosymmetric and noncentrosymmetric nanostructure arrays, indicating that the SH excitation is not due to a collective phenomenon but originates locally from the individual structures.