Controlling spatial distribution of plasmonic near-fields on Si excited with a femtosecond laser pulse using a SiO2 film

Surface plasmon polaritons (SPPs) are collective oscillations of free electrons coupled with electromagnetic waves at the dielectric/metal interfaces. Recently, the fundamental parameters of SPPs excited on non-metallic surfaces transiently metallized with intense femtosecond (fs) laser pulses, such as wavelength, propagation length, and coupling efficiency, have been clarified by measuring the reflectivity of silicon (Si) surfaces. It is essential for plasmon-based applications to not only investigate these fundamental parameters but also control the spatial distribution of the plasmonic near-fields, as the reflectivity-based measurement does not provide information on the spatial distribution of plasmonic near-fields. This paper reports on controlling the spatial distribution of plasmonic near-fields generated on dielectric surfaces by depositing a thin SiO2 film, resulting in the localization of the near-fields and nanoscale surface ablation. In our experiment, intense fs laser pulses generated high-density free electrons in Si grating surfaces coated with 40 nm and 65 nm thick SiO2 films, and a subsequent time-delayed fs pulse excited and probed SPPs on the metallized surface. Surface reflectivity measurements as a function of incident angle showed that the SiO2 films shifted the incident angle at which the reflectivity decreased drastically from 24° to 32°. Moreover, the modulation depth reached a maximum at the incident angle, while the morphologically changed area decreased. The experimental and theoretical results demonstrate that the plasmonic near-field localized by a SiO2 film induces nanoscale surface ablation, resulting in an effective approach for high-spatial-resolution laser material processing and for nanophotonic technologies based on non-metallic materials.