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.

通过扫描探针所携带的纳米级热点进行高效、宽带照明和收集，将为光与物质相互作用的研究赋予纳米级空间信息。然而，近场扫描光学显微镜探针，尤其是高分辨率的探针，需要笨重的光学器件，且只能会聚不到10⁻³的入射光，这限制了它的应用。在此，我们报道了一种两步连续宽带纳米聚焦技术，在光纤耦合纳米线扫描探针上，在几乎整个可见光范围内，外部纳米聚焦效率接近50%，该探针能够以纳米级空间分辨率进行光传输和光谱收集。通过将其与基本的便携式扫描隧道显微镜集成，我们展示了无透镜针尖增强拉曼光谱，并实现了1纳米的空间分辨率。这种基于光纤的纳米聚焦技术所提供的高性能和广泛的通用性，使得纳米光学显微镜能够轻松地融入各种现有的测量平台。