36,103 research outputs found
Reconstructing Detailed Line Profiles of Lamellar Gratings from GISAXS Patterns with a Maxwell Solver
Laterally periodic nanostructures were investigated with grazing incidence
small angle X-ray scattering (GISAXS) by using the diffraction patterns to
reconstruct the surface shape. To model visible light scattering, rigorous
calculations of the near and far field by numerically solving Maxwell's
equations with a finite-element method are well established. The application of
this technique to X-rays is still challenging, due to the discrepancy between
incident wavelength and finite-element size. This drawback vanishes for GISAXS
due to the small angles of incidence, the conical scattering geometry and the
periodicity of the surface structures, which allows a rigorous computation of
the diffraction efficiencies with sufficient numerical precision. To develop
dimensional metrology tools based on GISAXS, lamellar gratings with line widths
down to 55 nm were produced by state-of-the-art e-beam lithography and then
etched into silicon. The high surface sensitivity of GISAXS in conjunction with
a Maxwell solver allows a detailed reconstruction of the grating line shape
also for thick, non-homogeneous substrates. The reconstructed geometrical line
shape models are statistically validated by applying a Markov chain Monte Carlo
(MCMC) sampling technique which reveals that GISAXS is able to reconstruct
critical parameters like the widths of the lines with sub-nm uncertainty
Perfect Anomalous Reflection with a Binary Huygens' Metasurface
In this paper we propose a new metasurface that is able to reflect a known
incoming electromagnetic wave into an arbitrary direction, with perfect power
efficiency. This seemingly simple task, which we hereafter call perfect
anomalous reflection, is actually highly non-trivial due to the differing wave
impedances and complex interference between the incident and reflected waves.
Heretofore, proposed metasurfaces which achieve perfect anomalous reflection
require complicated, deeply subwavelength and/or multilayer element structures
which allow them to couple to and from leaky and/or evanescent waves. In
contrast, we demonstrate that using a Binary Huygens' Metasurface (BHM) --- a
passive and lossless metasurface with only two cells per period --- perfect
anomalous reflection can be achieved over a wide angular and frequency range.
Through simulations and experiments at 24 GHz, we show that a properly designed
BHM can anomalously reflect an incident electromagnetic wave from to , with perfect power efficiency to within
experimental precision
Correlated Diffuse X-ray Scattering from Periodically Nano-Structured Surfaces
Laterally periodic nanostructures were investigated with grazing incidence
small angle X-ray scattering. To support an improved reconstruction of
nanostructured surface geometries, we investigated the origin of the
contributions to the diffuse scattering pattern which is correlated to the
surface roughness. Resonant diffuse scattering leads to a palm-like structure
of intensity sheets. Dynamic scattering generates the so-called Yoneda band
caused by a resonant scatter enhancement at the critical angle of total
reflection and higher-order Yoneda bands originating from a subsequent
diffraction of the Yoneda enhanced scattering at the grating. Our explanations
are supported by modelling using a solver for the time-harmonic Maxwell's
equations based on the finite-element method
Structural dynamics of surfaces by ultrafast electron crystallography: Experimental and multiple scattering theory
Recent studies in ultrafast electron crystallography (UEC) using a reflection diffraction geometry have enabled the investigation of a wide range of phenomena on the femtosecond and picosecond time scales. In all these studies, the analysis of the diffraction patterns and their temporal change after excitation was performed within the kinematical scattering theory. In this contribution, we address the question, to what extent dynamical scattering effects have to be included in order to obtain quantitative information about structural dynamics. We discuss different scattering regimes and provide diffraction maps that describe all essential features of scatterings and observables. The effects are quantified by dynamical scattering simulations and examined by direct comparison to the results of ultrafast electron diffraction experiments on an in situ prepared Ni(100) surface, for which structural dynamics can be well described by a two-temperature model. We also report calculations for graphite surfaces. The theoretical framework provided here allows for further UEC studies of surfaces especially at larger penetration depths and for those of heavy-atom materials
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