Denis E. Tranca

High-resolution quantitative determination of dielectric function by using scattering scanning near-field optical microscopy

A new method for high-resolution quantitative measurement of the dielectric function by using scattering scanning near-field optical microscopy (s-SNOM) is presented. The method is based on a calibration procedure that uses the s-SNOM oscillating dipole model of the probe-sample interaction and quantitative s-SNOM measurements. The nanoscale capabilities of the method have the potential to enable novel applications in various fields such as nano-electronics, nano-photonics, biology or medicine.

A study on the image contrast of pseudo-heterodyned scattering scanning near-field optical microscopy

The dependence of the near-field signal on the dielectric function of a specific material proposes scattering-type near-field optical microscopy (s-SNOM) as a viable tool for material characterization studies. Our experiment shows that specific material identification by s-SNOM is not a straightforward task as parameters involved in the detection scheme can also influence material contrast measurements. More precisely, we demonstrate that s-SNOM contrast in a pseudo-heterodyne detection configuration depends on the oscillation amplitude of the reference mirror and that for reliable measurements of the contrast between different materials this aspect needs to be taken into consideration.

Surface Charge and Carbon Contamination on an Electron-Beam-Irradiated Hydroxyapatite Thin Film Investigated by Photoluminescence and Phase Imaging in Atomic Force Microscopy

The surface properties of hydroxyapatite, including electric charge, can influence the biological response, tissue compatibility, and adhesion of biological cells and biomolecules. Results reported here help in understanding this influence by creating charged domains on hydroxyapatite thin films deposited on silicon using electron beam irradiation and investigating their shape, properties, and carbon contamination for different doses of incident injected charge by two methods. Photoluminescence laser scanning microscopy was used to image electrostatic charge trapped at pre-existing and irradiation-induced defects within these domains, while phase imaging in atomic force microscopy was used to image the carbon contamination. Scanning Auger electron spectroscopy and Kelvin probe force microscopy were used as a reference for the atomic force microscopy phase contrast and photoluminescence laser scanning microscopy measurements. Our experiment shows that by combining the two imaging techniques the effects of trapped charge and carbon contamination can be separated. Such separation yields new possibilities for advancing the current understanding of how surface charge influences mediation of cellular and protein interactions in biomaterials.

Improved quantification of collagen anisotropy with polarization‐resolved second harmonic generation microscopy

Imaging tissue samples by polarization-resolved second harmonic generation microscopy provides both qualitative and quantitative insights into collagen organization in a label-free manner. Polarization-resolved second harmonic generation microscopy goes beyond simple intensity-based imaging by adding the laser beam polarization component and applying different quantitative metrics such as the anisotropy factor. It thus provides valuable information on collagen arrangement not available with intensity measurements alone. Current established approaches are limited to calculating the anisotropy factor for only a particular laser beam polarization and no general guidelines on how to select the best laser beam polarization have yet been defined. Here, we introduce a novel methodology for selecting the optimal laser beam polarization for characterizing tissues using the anisotropy in the purpose of identifying cancer signatures. We show that the anisotropy factor exhibits a similar laser beam polarization dependence to the second harmonic intensity and we combine it with the collagen orientation index computed by Fast Fourier Transform analysis of the recorded images to establish a framework for choosing the laser beam polarization that is optimal for an accurate interpretation of polarization-resolved second harmonic generation microscopy images and anisotropy maps, and hence a better differentiation between healthy and dysplastic areas. SHG image of skin tissue (a) and a selected area of interest for which we compute the SHG intensity (b) and anisotropy factor (c) dependence on the laser beam polarization and also the FFT spectrum (d) to evaluate the collagen orientation index.

Contrast enhancement influences the detection of gradient based local invariant features and the matching of their descriptors

The detection and matching of gradient based local invariant features is influenced by image contrast enhancement.Contrast enhancement increases the amount of detectable SIFT & SURF keypoints.Contrast enhancement negatively affects the matching of SIFT & SURF descriptor sets. Contrast enhancement (CE) plays an important role in digital photography, medical imaging or scientific visualization, compensating for deficient dynamic range aspects. Our experiments show that CE via histogram modification influences the detection of gradient based local invariant features (LIF) and the matching of their descriptors. We bring evidence that the number of keypoints that can be automatically extracted by gradient based detectors increases with CE, and that matching gradient based keypoint descriptors extracted from image sets processed by CE is negatively affected in terms of Precision-Recall. We observed the effects of several classical and state-of-the-art CE methods on two widely used LIF detection/description techniques: Scale Invariant Feature Transform (SIFT) and Speeded-Up Robust Features (SURF).

Automatic moiré pattern removal in microscopic images

This paper deals with the filtering of microscopic images with moiré like noise patterns. The proposed method analyzes the magnitude spectrum of the Fast Fourier Transform of the noisy image, determines and removes the undesired components. Comparisons with a classical method are provided. The experimental results obtained so far are promising.

Nonlinear optical effects used for investigations on biological samples at micro and nanoscale

In our work we present some investigations on biological samples at micro and nanoscale by using nonlinear effects in laser scanning microscopy techniques. We developed an integrated multimodal system that allows for the simultaneous image acquisition from multiple optical imaging modalities. Our system includes several microscopy techniques working in far field and in near field. The main advantage of the system is connected with the possibility to image the same area of the sample at micro and nanoscale so that a correlation between the images could be made and an explanation of the images at nanoscale can be done. By overlaying the structural images obtained from different techniques, a more comprehensive view of different biological specimens can be obtained. In addition the possibility to see the same sample area at micro and nanoscales offers the possibility to make a connection between the different elements of the biological structures.

Mapping electron-beam-injected trapped charge with scattering scanning near-field optical microscopy.

Scattering scanning near-field optical microscopy (s-SNOM) has been demonstrated as a valuable tool for mapping the optical and optoelectronic properties of materials with nanoscale resolution. Here we report experimental evidence that trapped electric charges injected by an electron beam at the surface of dielectric samples affect the sample-dipole interaction, which has direct impact on the s-SNOM image content. Nanoscale mapping of the surface trapped charge holds significant potential for the precise tailoring of the electrostatic properties of dielectric and semiconductive samples, such as hydroxyapatite, which has particular importance with respect to biomedical applications. The methodology developed here is highly relevant to semiconductor device fabrication as well.

Bags of features for classification of Laser Scanning Microscopy data

The Bag-of-Features (BoF) paradigm has been introduced to the field of computer vision about a decade ago. Since then, its potential for image classification and retrieval tasks has been demonstrated in numerous experiments, which contributed to BoF approaches becoming well-established in the field. The BoF methods reported to date use mainly spatial intensity information but data collected by Laser Scanning Microscopy (LSM) techniques many times embed additional information that could be exploited in parallel in sophisticated BoF scenarios. In this contribution we discuss complementary LSM information categories that BoF frameworks could take advantage of when addressing the classification of LSM datasets.

Metallic samples investigated by using a scattering near field optical microscope

Scanning near-field microscopy can overcome diffraction limitation by exploiting the evanescent near fields existing close to any illuminated object. For investigations we used a scattering-type near-field optical microscope (s-SNOM). As the main problem of this kind of microscopy is connected with near field detection we focused on the metallic samples investigations based on the pseudoheterodyne detection. The influence of the oscillating mirror amplitude on the contrast image was studied. Lately the s-SNOM was successfully used for different investigations on the metallic nanoparticles.