Highly efficient plasmonic tip design for plasmon nanofocusing in near-field optical microscopy

Near-field scanning optical microscopy (NSOM) combined with plasmon nanofocusing is a powerful nano-analytical tool due to its attractive feature of efficient background suppression as well as light energy compression to the nanoscale. In plasmon nanofocusing-based NSOM, the metallic tip plays an important role in inducing plasmon nanofocusing. It is, however, very challenging to control plasmonic properties of tips for plasmon nanofocusing with existing tip fabrication methods, even though the plasmonic properties need to be adjusted to experimental environments such as the sample or excitation wavelength. In this study, we propose an efficient tip design and fabrication which enable one to actively control plasmonic properties for efficient plasmon nanofocusing. Because our method offers flexibility in the material and structure of tips, one can easily modify the plasmonic properties depending on the requirements. Importantly, through optimization of the plasmonic properties, we achieve almost 100% reproducibility in plasmon nanofocusing in our experiments. This new approach of tip fabrication makes plasmon nanofocusing-based NSOM practical and reliable, and opens doors for many scientists working in related fields.