Scattering-type scanning near-field optical microscopy (s-SNOM) combines an atomic force microscope (AFM) with external infrared laser illumination to detect scattered light from probe-sample region. The scattered light from the near-field contains molecular fingerprints with nanoscale spatial resolution. Based on near-field probe-sample interactions, s-SNOM has been utilized to study many physical, chemical and biological systems including triggering and detecting surface plasmons and phonon polaritons in polar dielectrics, delivering nanoscale polymer recognition, and imaging proteins in lipid bilayers and single virus. Despite wide applications, s-SNOM can only map two-dimensional optical nearfields along a sample surface, and near-field responses in the vertical direction remain missing. Here, we present a method to enable s-SNOM to map three-dimensional near-field responses by sectioning background-free near-field interaction at different probe-sample distances. This improvement is done by accurately controlling probe-sample distance with the resolution better than 1 nm when collecting scattering light through an infrared detector. Additionally, compared with traditional s-SNOM, our new technique has a less complex setup and is adaptive to other AFM modalities to give out more information on properties of samples. In Figure 1, the spatial distribution of phonon polaritons in a boron nitride nanotube can be revealed at different probe-sample distances by this method. Moreover, heterogeneities of vertical near-field decay range along the nanotube are discovered through calculation from the results. The new technique opens the door to the tomographic characterization of nearfields, and we expect it to improve s-SNOM to a more complete platform to perform three-dimensional near-field analysis on a broad range of samples.

Three-dimensional mapping of optical near-field responses by controlling probe-sample distance

256th National Meeting (2018)

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