Characterization of Nanomaterials by Locally Determining Their Complex Permittivity with Scattering-Type Scanning Near-Field Optical Microscopy

Abstract

Scattering-type scanning near-field optical microscopy (s-SNOM) is currently regarded as a powerful tool for exploring important optical properties at nanoscale resolutions depending only on the size of a sharp tip that is scanned across the sample surface while being excited with a focused laser beam. Recently, it was shown that, among others, s-SNOM can quantitatively map the complex permittivity of materials and biological samples and hence other intrinsic related optical properties, such as the refractive index. In this work we apply this capability, previously demonstrated only at proof-of-concept level, in an experiment dealing with three distinct types of nanostructured materials: microcapsules for drug delivery assembled with layer-by-layer strategies, ultrathin optical coatings with controllable color properties, and plasmonic ceramic nanoparticles. We show that complex permittivity mapping with s-SNOM can contribute to the better understanding of such materials, providing information that is difficult or even impossible to assess with other techniques.

SSNOMBACTER: A collection of scattering-type scanning near-field optical microscopy and atomic force microscopy images of bacterial cells

“…At least 3 different 10 × 10 μm fields of view (FOVs) were acquired including both glass substrate regions (used as reference, required for potential quantitative s-SNOM image analyses [ 26 , 40 , 60 ]) and bacterial cells. In addition, ≥1 FOV was imaged at higher magnification (i.e., by scanning a region of lower dimension, namely, of 2 × 2 or 4 × 4 μm).…”

“…This process depends on the local dielectric properties of the sample [ 25 ], given the mutual perturbations occurring between the polarizabilities of the sample and the probe. Interferometric detection of the backscattered light yields thus nanoscale-resolved amplitude and phase images, which can reveal various important properties of nanostructured materials [ 12 , 26 ]. Measuring the amplitude and the phase changes separately is of interest given that these two different signals contain complementary information about the sample.…”

“…The sample properties dictate the weight of each of these terms in the recorded signals, but for an s-SNOM configuration based on detection at higher harmonics of the tip's oscillation frequency, the incident field scattered by the sample should be discarded [ 29 ]. From the s-SNOM amplitude and phase images one can typically extract material contrasts instead of absolute values, but in the case of samples where a reference material is available next to unknown ones, absolute values of optical parameters (e.g., refractive index, reflectance) are also available [ 26 ].…”

“…In the previous work of Tranca et al [ 40 ], a proof-of-concept experiment focused on nanoscale RI mapping of erythrocytes showed that such properties (that are much easier to interpret compared with raw s-SNOM data) are easily available with s-SNOM, supposing that a ground-truth reference (a region of known RI) is available on the sample [ 40 ]. The utility of s-SNOM imaging for quantitative imaging of the dielectric function and connected optical parameters has been also showcased in a recent experiment in connection to various types of nanomaterials [ 26 ].…”

“…The SSNOMBACTER dataset consists of 4,400 images collected with s-SNOM and AFM modalities on 15 bacterial species. The dimensions of this dataset can be further expanded by processing the available images to extract other types of data representations (e.g., by assembling 3D representations from the available AFM topographic information or by calculating dielectric function maps from the amplitude and phase s-SNOM images [ 26 ]). A different way to expand our dataset can rely on data augmentation strategies that apply various transformations to an initial image in order to render new representations that simulate other potential acquisition conditions.…”