Abstract
Abstract Scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared nanospectroscopy (nano-FTIR) are emerging tools for physical and chemical nanocharacterization of organic and inorganic composite materials. Being based on (i) diffraction-limited illumination of a scanning probe tip for nanofocusing of light and (ii) recording of the tip-scattered radiation, the efficient suppression of background scattering has been critical for their success. Here, we show that indirect tip illumination via far-field reflection and scattering at the sample can produce s-SNOM and nano-FTIR signals of materials that are not present at the tip position – despite full background suppression. Although these artefacts occur primarily on or near large sample structures, their understanding and recognition are of utmost importance to ensure correct interpretation of images and spectra. Detailed experimental and theoretical results show how such artefacts can be identified and eliminated by a simple signal normalization step, thus critically strengthening the analytical capabilities of s-SNOM and nano-FTIR spectroscopy.
Highlights
In scattering-type scanning near-field optical microscopy (s-SNOM) [1,2,3,4] and Fourier transform infraredIn order to obtain pure and reliable and quantitative near-field signals, it is key to suppress various background signals
The local electric field at the tip apex can be modified by far-field reflection and scattering of the incident field at a sample surface [58, 59], which is often not considered in Scattering-type scanning near-field optical microscopy (s-SNOM) mapping and nano-FTIR spectroscopy of dielectric samples
We provide a detailed analysis of s-SNOM and nano-FTIR data obtained on various representative samples, highlighting and documenting that a careful assessment of far-field reflection effects is of critical importance for the correct interpretation of near-field signals
Summary
In scattering-type scanning near-field optical microscopy (s-SNOM) [1,2,3,4] and Fourier transform infraredIn order to obtain pure and reliable and quantitative near-field signals, it is key to suppress various background signals. Additive background signals (for example caused by light scattering at the tip-shaft or the sample) are suppressed by oscillating the tip vertically at frequency Ω and recording the detector signal at a higher harmonic n of the oscillation frequency, nΩ [38,39,40]. L. Mester et al.: High-fidelity nano-FTIR spectroscopy up to several tens of micrometers away from the probing tip [45], showing clearly that s-SNOM and nano-FTIR signals originating from local material properties can be masked by electromagnetic fields that are generated far away from the tip – even when additive and multiplicate background signals are fully suppressed. It is unclear to what extent the farfield effects can be suppressed and how the signal ratios have to be interpreted
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