Plasmonic gold nanojets fabricated by a femtosecond laser irradiation.

Gold nanojets with various morphologies, from nanopillar to nanotip with up to 800 nm height, and finally to nanotip with droplet, are fabricated on gold thin film by a femtosecond laser irradiation. The near-field localized surface plasmon resonance (LSPR) and photothermal effects of gold nanojets are studied through finite element electromagnetic (EM) analysis, supporting in nanojets design for potential applications of high-resolution imaging, nanomanipulation and sensing. For an individual nanotip, the confined electron oscillations in LSPR lead to an intense local EM field up to three orders of magnitude stronger than the incident field strength at the end of gold tip, where the vertical resolution for the field enhancement was improved down to nanoscale due to the small size of the sharp gold tip (5-nm-radius). At specific wavelength, nanopillar can serve as an effective light-to-heat converter and its heating can be fine-tuned by external irradiation, and its dimension. The long-range periodic nanojet arrays (periods from 1.5 µm to 2.5 µm) with different geometry were printed using several pulse energy levels. By confining more light into the tip (two orders of magnitude stronger than single tip), nanotip array shows more pronounced potential to serve as a refractometric sensor due to their high sensitivity and reproducibility. These results promote fs laser printing as a high-precision tool for nanoarchitecture in optical imaging, nanomanipulation and sensing application.