Quantitative imaging of anisotropic material properties with vectorial ptychography

Following the recent establishment of the formalism of vectorial ptychography [Ferrand et al., Opt. Lett. 40, 5144 (2015)], first measurements are reported in the optical range, demonstrating the capability of the proposed method to map the four parameters of the Jones matrix of an anisotropic specimen, and therefore to quantify a wide range of optical material properties, including power transmittance, optical path difference, diattenuation, retardance, and fast-axis orientation.Comment: 5 figures, accepted for publication in Optics Letter

Ion beam sputtered surface dynamics investigated with two-time correlation functions : a model study

Ion beam sputtering is a widely used technique to obtain patterned surfaces. Despite the large use of this approach on different materials to create surface nanostructures, the theoretical model to explain the time evolution of the erosion process is still debated. We show with the help of simulations, that two-time correlation functions can serve to assess the validity of different models. These functions can be measured experimentally with the X-ray photon correlation spectroscopy technique

Ageing dynamics of ion bombardment induced self-organization processes

Instabilities caused during the erosion of a surface by an ion beam can lead to the formation of self-organized patterns of nanostructures. Understanding the self-organization process requires not only the in-situ characterization of ensemble averaged properties but also probing the dynamics. This can be done with the use of coherent X-rays and analyzing the temporal correlations of the scattered intensity. Here, we show that the dynamics of a semiconductor surface nanopatterned by normal incidence ion beam sputtering are age-dependent and slow down with sputtering time. This work provides a novel insight into the erosion dynamics and opens new perspectives for the understanding of self-organization mechanisms

Ptychography in anisotropic media

International audiencePtychography is described in the context of polarized light probing anisotropic specimen, i.e., showing properties of birefringence and/or diattenuation. We establish an optimization strategy using a vectorial formalism. A measurement scheme using a set of linearly polarized probes and linear polarization analyzers is proposed, allowing to retrieve the full anisotropy map of the specimen

Imaging of highly inhomogeneous strain field in nanocrystals using x-ray Bragg ptychography: A numerical study

International audienceX-ray ptychography is a lensless microscopy method able to provide extended field of view with spatial resolution above the diffraction limit. A series of intensity coherent diffraction patterns measured in the far field is used to obtain the numerical deconvolution between the sample scattering contrast and the illumination function. The measurements are performed with a finite-size beam spot scanned across the sample. The scan step, smaller than the beam size, ensures a high redundancy in the data set, which allows for the convergence of the iterative inversion algorithm. This work explores the possibility to use ptychography for the investigation of strained crystals by means of coherent x-ray Bragg diffraction, taking advantage of the high sensitivity to the atomic displacement fields. The Bragg diffraction scattering contrast is described by an effective complex-valued electron density, where the phase holds the information on the displacement field. The detailed two-dimensional numerical study of Bragg ptychography is presented, both for the known and unknown illumination cases. It demonstrates the high robustness of the ptychographical iterative engine for highly nonhomogeneous strain fields. In particular, the local information is extracted from the individual diffraction patterns to calculate the modulus and phase estimates of the electron density, which are further used to constrain the newly derived algorithm. From this work, it is foreseen that Bragg ptychography when experimentally feasible, will open the way to the nondestructive imaging of strain fields at the nanoscale

Noise models for low counting rate coherent diffraction imaging

International audienceCoherent diffraction imaging (CDI) is a lens-less microscopy method that extracts the complex-valued exit field from intensity measurements alone. It is of particular importance for microscopy imaging with diffraction set-ups where high quality lenses are not available. The inversion scheme allowing the phase retrieval is based on the use of an iterative algorithm. In this work, we address the question of the choice of the iterative process in the case of data corrupted by photon or electron shot noise. Several noise models are presented and further used within two inversion strategies, the ordered subset and the scaled gradient. Based on analytical and numerical analysis together with Monte-Carlo studies, we show that any physical interpretations drawn from a CDI iterative technique require a detailed understanding of the relationship between the noise model and the used inversion method. We observe that iterative algorithms often assume implicitly a noise model. For low counting rates, each noise model behaves differently. Moreover, the used optimization strategy introduces its own artefacts. Based on this analysis, we develop a hybrid strategy which works efficiently in the absence of an informed initial guess. Our work emphasises issues which should be considered carefully when inverting experimental data

Strain in a silicon-on-insulator nanostructure revealed by 3D x-ray Bragg ptychography

International audienceProgresses in the design of well-defined electronic band structure and dedicated functionalities rely on the high control of complex architectural device nano-scaled structures. This includes the challenging accurate description of strain fields in crystalline structures, which requires non invasive and three-dimensional (3D) imaging methods. Here, we demonstrate in details how x-ray Bragg ptychography can be used to quantify in 3D a displacement field in a lithographically patterned silicon-on-insulator structure. The image of the crystalline properties, which results from the phase retrieval of a coherent intensity data set, is obtained from a well-controlled optimized process, for which all steps are detailed. These results confirm the promising perspectives of 3D Bragg ptychography for the investigation of complex nano-structured crystals in material science

Detector tilt considerations in high-energy Bragg coherent diffraction imaging: a simulation study

This paper addresses three-dimensional signal distortion and image reconstruction issues in x-ray Bragg coherent diffraction imaging (BCDI) in the event of a general non-orthogonal orientation of the area detector with respect to the diffracted beam. Growing interest in novel BCDI adaptations at fourth-generation synchrotron light sources has necessitated improvisations in the experimental configuration and the subsequent data analysis. One such possibly unavoidable improvisation that is envisioned in this paper is a photon-counting area detector whose face is tilted away from the perpendicular to the Bragg-diffracted beam during acquisition of the coherent diffraction signal. We describe a likely circumstance in which one would require such a detector configuration, along with experimental precedent at third generation synchrotrons. Using physically accurate diffraction simulations from synthetic scatterers in the presence of such tilted detectors, we analyze the general nature of the observed signal distortion qualitatively and quantitatively, and provide a prescription to correct for it during image reconstruction. Our simulations and reconstructions are based on an adaptation of the known theory of BCDI sampling geometry as well as recently developed projection-based methods of wavefield propagation. Such configurational modifications and their numerical remedies are potentially valuable in realizing unconventional coherent diffraction measurement geometries and eventually paving the way for the integration of BCDI into new materials characterization experiments at next-generation light sources.Comment: 13 pages, 5 figure