High external-efficiency nanofocusing for lens-free near-field optical nanoscopy

Efficient, broadband illumination and collection through a nanometre-sized hotspot carried by a scanning probe will endow light-matter interaction research with nanoscale spatial information. However, near-field scanning optical microscopy probes, particularly the high-resolution ones, demand cumbersome optics but can only concentrate less than 10(-3) of the incident light, which has limited its applications. Here, we report a two-step sequential broadband nanofocusing technique with an external nanofocusing efficiency of similar to 50% over nearly all the visible range on a fibre-coupled nanowire scanning probe, which is capable of both light delivery and spectrum collection with nanoscale spatial resolution. By integrating this with a basic portable scanning tunnelling microscope, we have demonstrated lens-free tip-enhanced Raman spectroscopy and achieved 1 nm spatial resolution. The high performance and vast versatility offered by this fibre-based nanofocusing technique allow for the easy incorporation of nano-optical microscopy into various existing measurement platforms.