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

Abstract

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 ~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.

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The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

M.L. and R.Y. acknowledge start-up support from the Bourns College of Engineering, University of California–Riverside. The probe development and Raman measurements were supported by NSF-CHE-1654794. Theory development and far-field characterization were supported by NSF-DMR-1654746 and NSF-ECCS-1810453.

Author information

M.L. and R.Y. initiated the project, designed the experiments and supervised the research. S.K. fabricated and characterized the probe, performed the TERS measurements and analysed the data. S.K., N.Y. and Y.Z. synthesized the sharp-tip AgNW. X.M. and Q.L. performed the k-space measurements. M.L. developed the theoretical modelling. All authors contributed to discussion of the results. M.L., R.Y. and S.K. wrote the manuscript.

Correspondence to Ming Liu or Ruoxue Yan.

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The authors declare no competing interests.

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Supplementary Information

This file contains more information about the work, Supplementary Figs. 1–18 and Supplementary Table 1.

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Fig. 1: Two-step sequential nanofocusing concept.
Fig. 2: Experimental demonstration of selective and effective TM0 mode excitation and nanofocusing.
Fig. 3: Lens-free STM-based FIFO-TERS.
Fig. 4: High-resolution FIFO-TERS mapping.
Fig. 5: Mode evolution in the separate-region resonant mode coupling process.