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Probing the electromagnetic field of a 15-nanometre hotspot by single molecule imaging

Nature volume 469, pages 385–388 (2011)Cite this article

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Abstract

When light illuminates a rough metallic surface, hotspots can appear, where the light is concentrated on the nanometre scale, producing an intense electromagnetic field. This phenomenon, called the surface enhancement effect1,2, has a broad range of potential applications, such as the detection of weak chemical signals. Hotspots are believed to be associated with localized electromagnetic modes3,4, caused by the randomness of the surface texture. Probing the electromagnetic field of the hotspots would offer much insight towards uncovering the mechanism generating the enhancement; however, it requires a spatial resolution of 1–2 nm, which has been a long-standing challenge in optics. The resolution of an optical microscope is limited to about half the wavelength of the incident light, approximately 200–300 nm. Although current state-of-the-art techniques, including near-field scanning optical microscopy5, electron energy-loss spectroscopy6, cathode luminescence imaging7 and two-photon photoemission imaging8 have subwavelength resolution, they either introduce a non-negligible amount of perturbation, complicating interpretation of the data, or operate only in a vacuum. As a result, after more than 30 years since the discovery of the surface enhancement effect9,10,11, how the local field is distributed remains unknown. Here we present a technique that uses Brownian motion of single molecules to probe the local field. It enables two-dimensional imaging of the fluorescence enhancement profile of single hotspots on the surfaces of aluminium thin films and silver nanoparticle clusters, with accuracy down to 1.2 nm. Strong fluorescence enhancements, up to 54 and 136 times respectively, are observed in those two systems. This strong enhancement indicates that the local field, which decays exponentially from the peak of a hotspot, dominates the fluorescence enhancement profile.

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Figure 1: The principle of Brownian motion single molecule super-resolution imaging.
Figure 2: Hotspots on an aluminium film.
Figure 3: A hotspot on silver nanoparticle clusters.

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Acknowledgements

We thank G. Bartal and A. Niv for discussions. This research was supported by the US Department of Energy Office of Science, Basic Energy Sciences and Lawrence Berkeley National Laboratory under contract no. DE-AC02-05CH11231.

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Authors and Affiliations

  1. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, 94720, California, USA

    Hu Cang, Xiaobo Yin & Xiang Zhang

  2. NSF Nano Scale Science and Engineering Center (NSEC), 3112 Etcheverry Hall, University of California, Berkeley, 94720, California, USA

    Hu Cang, Anna Labno, Changgui Lu, Xiaobo Yin, Ming Liu, Christopher Gladden, Yongmin Liu & Xiang Zhang

  3. Biophysics Program, University of California Berkeley, Berkeley, 94720, California, USA

    Anna Labno

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Contributions

H.C., A.L., X.Y. and X.Z. designed the experiments; H.C., A.L., C.G. and M.L. conducted experiments; C.L. and Y.L. conducted computer simulations and theoretical analysis; H.C., A.L., X.Y. and X.Z. wrote the paper.

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Correspondence to Xiang Zhang.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains details of Supplementary Movie 1, Supplementary Information sections 1-8 (see Table of Contents for details) and additional references. (PDF 1043 kb)

Supplementary Movie 1

This movie illustrates the principle of our Brownian motion single molecule super-resolution technique (see Supplementary Information file for full details). (MOV 5380 kb)

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Cang, H., Labno, A., Lu, C. et al. Probing the electromagnetic field of a 15-nanometre hotspot by single molecule imaging. Nature 469, 385–388 (2011). https://doi.org/10.1038/nature09698

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