Short Communication
Nanoscale mapping of refractive index by using scattering-type scanning near-field optical microscopy

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Abstract

We present a novel method for nanoscale reconstruction of complex refractive index by using scattering-type Scanning Near-field Optical Microscopy (s-SNOM). Our method relies on correlating s-SNOM experimental image data with computational data obtained through simulation of the classical oscillating point-dipole model. This results in assigning a certain dielectric function for every pixel of the s-SNOM images, which further results in nanoscale mapping of the refractive index. This method is employed on human erythrocytes to demonstrate the approach in a biologically relevant manner. The presented results advance the current knowledge on the capabilities of s-SNOM to extract quantitative information with nanoscale resolution from optical data sets with biological application.

Graphical Abstract

The experimental s-SNOM results (Amplitude and Phase images) are correlated to the theoretical model of oscillating point-dipole. Together they lead to a 4D visual representation with nanoscale resolution of the sample (red blood cells). The topography is represented in three spatial dimensions, while the refractive index is coded using a colormap.

Image 1
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Section snippets

Method

Control erythrocytes (sample A) were prepared by smearing human peripheral blood drops on glass slides. Sample A was used for testing the s-SNOM measurements of erythrocytes' RI by comparing the results to those reported in the literature.10, 11, 12, 13, 14 Sample B was prepared by mixing equal volumes of peripheral blood and isotonic saline solution and smearing the resulting mixture on glass slide. Both samples were imaged immediately after drying at room temperature.

The cover-slips (Zeiss,

Results

The algorithm described in the Methods is run on s-SNOM images acquired from both samples. Figure 1 shows the AFM images (Figure 1, A and D) and the resulting real (Figure 1, B and E) and imaginary (Figure 1, C and F) parts of the RI maps computed using the s-SNOM images corresponding to the two samples. To clearly visualize the results, in Figure 1 RI data corresponding to the glass substrate was removed and only the data for the erythrocytes is displayed. The rectangle area delimited in

Conclusions

Optical data collected with s-SNOM has an optical resolution lying well beyond the diffraction barrier (~35 nm) and is collected in tandem with topography information. The RI mapping based on s-SNOM allows for precise optical investigations at nanoscale which permits local detailed representation of this optical property. Limitations of this method are same as in any scanning‐probe imaging approach and mainly refer to the probe's tip (which limits the resolution to its size) and the sample's

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    Funding Sources: This work has received support from the European Community's Seventh Framework Programme (FP7/2012-2015) under grant agreement no. 280804 (LANIR, www.lanir.eu) and no. 212533 (BioElectricSurface, www.bioelectricsurface.eu). This communication reflects the views only of the authors, and the Commission cannot be held responsible for any use which may be made of the information contained therein. The presented work has been supported as well by the Romanian Executive Agency for Higher Education, Research, Development and Innovation Funding through the research grants PN-II-PT-PCCA-2011-3.2-1162 (NANOLASCAN) and PN-II-RU-TE-2014-4-1803 (MICRONANO).

    No competing interests are present.

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