A computationally efficient method for identifying the parameters of the Jiles-Atherton hysteresis model is presented. Adjoint analysis is used in conjecture with an accelerated gradient descent optimization algorithm. The proposed method is used to estimate the Jiles-Atherton model parameters of two different materials. The obtained results are found to be in good agreement with the reported values. By comparing with existing methods of model parameter estimation, the proposed method is found to be computationally efficient and fast converging.
Nanostructured dielectric waveguides are of high interest for biosensing applications, light emitting devices as well as solar cells. Multiperiodic and aperiodic nanostructures allow for custom-designed spectral properties as well as near-field characteristics with localized modes. Here, a comparison of experimental results and simulation results obtained with three different simulation methods is presented. We fabricated and characterized multiperiodic nanostructured dielectric waveguides with two and three compound periods as well as deterministic aperiodic nanostructured waveguides based on Rudin-Shapiro, Fibonacci, and Thue-Morse binary sequences. The near-field and far-field properties are computed employing the finite-element method (FEM), the finite-difference time-domain (FDTD) method as well as a rigorous coupled wave algorithm (RCWA). The results show that all three methods are suitable for the simulation of the above mentioned structures. Only small computational differences are obtained in the near fields and transmission characteristics. For the compound multiperiodic structures the simulations correctly predict the general shape of the experimental transmission spectra with number and magnitude of transmission dips. For the aperiodic nanostructures the agreement between simulations and measurements decreases, which we attribute to imperfect fabrication at smaller feature sizes.
This Letter covers the design and implementation of a generalizable system for the precise alignment of X-ray gratings. Next-generation high-energy grating-based Differential Phase Contrast (gDPC) X-ray imaging systems require precise alignment of the X-ray gratings as low as 1 mrad in rotation and 0.5 mm in translation. In this work, we designed holographic fiducial marks, consisting of four reflective Fresnel zone plates, each placed in a separate quadrant of the mark. When illuminated with a collimated laser beam, each mark creates a predefined pattern of four points, which changes quantitatively for any misalignment in each of the three translational and three rotational degrees of freedom. We fabricated the designed fiducial marks using photolithography and etching processes. The experimental system is implemented using a HeNe laser and an optical imaging system, which includes a beam expander, a plate beam splitter, and a CMOS camera, suitable for aligning practical gratings in gDPC X-ray imaging systems. Our experimental results demonstrate the rotational precision capabilities of the reported alignment system down to 0.42 mrad around the optical axis and 0.03 mrad around the axes perpendicular to the optical axis. The translational precision of 83.64 μm along the optical axis and 1.22 μm along the axes perpendicular to the optical axis is also demonstrated.
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