Abstract
Optical near field enhancements in the vicinity of particles illuminated by laser light are increasingly recognized as a powerful tool for nanopatterning applications, but achieving sub-wavelength details from the near-field distribution remains a challenge. Here we present a quantitative analysis of the spatial modulation of the near optical fields generated using single 8 ps, 355 nm (and 532 nm) laser pulses around individual colloidal particles and small close packed arrays of such particles on silicon substrates. The analysis is presented for particles in air and, for the first time, when immersed in a range of liquid media. Immersion in a liquid allows detailed exploration of the effects on the near field of changing not just the magnitude but also the sign of the refractive index difference between the particle and the host medium. The level of agreement between the results of ray tracing and Mie scattering simulations, and the experimentally observed patterns on solid surfaces, should encourage further modelling, predictions and demonstrations of the rich palette of sub-wavelength surface profiles that can be achieved using colloidal particles immersed in liquids.
Highlights
The quest to fabricate ever more complex nanopatterns and extended periodic arrays on solid surfaces using low cost yet effective lithographic methods has stimulated huge interest in near field (NF) optical effects, i.e. in phenomena associated with non-propagating and highly localized electromagnetic fields and their interaction with matter
Optical near field enhancements in the vicinity of particles illuminated by laser light are increasingly recognized as a powerful tool for nanopatterning applications, but achieving sub-wavelength details from the near-field distribution remains a challenge
We present a quantitative analysis of the spatial modulation of the near optical fields generated using single 8 ps, 355 nm laser pulses around individual colloidal particles and small close packed arrays of such particles on silicon substrates
Summary
The quest to fabricate ever more complex nanopatterns and extended periodic arrays on solid surfaces using low cost yet effective lithographic methods has stimulated huge interest in near field (NF) optical effects, i.e. in phenomena associated with non-propagating and highly localized electromagnetic fields and their interaction with matter. Previous studies have shown the emergence of sub-wavelength photonic beams (“nanojets”) from the reverse of a dielectric microsphere illuminated by light of wavelength λ [4] These non-evanescent and non-resonant beams, which can be generated using a wide range of sphere sizes, are characterized by a narrow lateral size (of order ~λ/3) and can propagate over distances d > λ provided that the ratio of the refractive indices of the sphere and the background medium is < 2:1. The study provides the first quantitative analysis of the spatial modulation of the NF as a result of localized focusing/defocusing of the propagating light and interference between the incident and scattered light around individual colloidal particles, on silicon substrates immersed in different liquid media, following irradiation with a single picosecond (ps) UV laser pulse.
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