Ultra-smooth silver nanoneedles as nanofocusing waveguides

Tapered metallic nanostructures can channel and focus surface plasmons to the sharp apex to generate enormous near field enhancement. This distinct feature enables light guiding and manipulating at nanoscale confinement, which have been widely applied on high-resolution near-field optical imaging, high-order harmonic generation enhancement and ultrasensitive detection, etc. In this paper, we shown that silver nanoneedles with high aspect rate and ultra-smooth surface can be utilized as on-chip plasmonic nanofocusing waveguides operated at visible region. The nanoneedles were synthesized through a focused optical field induced chemical process. Verified by the observation of non-uniform plasmon beats, we shown that these nanoneedles are excellent nanofocusing waveguides with divergent effective refractive index for plasmon propagating modes. These findings would be beneficial for the development of remote-excitation detection/sensing and plasmonic circuits constructing, etc.