Vortex phase masks have been shown to be an efficient means to reduce the blinding stellar light in high-contrast imaging instruments. Once placed at the focal plane of the telescope, the helical phase ramp of a vortex phase mask diffracts the light of a bright on-axis source outside the re-imaged telescope pupil, while transmitting the light of a faint off-axis companion nearly unaffected. The Annular Groove Phase Mask (AGPM) is a broadband metasurface implementation of a vector vortex phase mask using the artificial birefringence of a circular subwavelength grating etched onto a diamond substrate. To date, the AGPM design has been optimized using rigorous coupled-wave analysis (RCWA), which is a valid tool to simulate periodic straight gratings. However, we have now reached a performance level where the curvature of the grating lines at the center becomes a limiting factor. Here, we use a finite-difference time-domain (FDTD) method to correctly describe the AGPM performance, including the effect of the curved grating close to its center. We confirm the validity of this simulation framework by comparing its predictions with experimental results obtained on our infrared coronagraphic test bench, and we show that RCWA fails at reproducing correctly the central AGPM performance, confirming the need for a full 3d simulation tool such as FDTD. Finally, we use FDTD to optimize the grating parameters at the AGPM center, and conclude with a new optimal design.

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Complete absorption of electromagnetic waves is paramount in today's applications, ranging from photovoltaics to cross-talk prevention into sensitive devices. In this context, we use a genetic algorithm (GA) strategy to optimize absorption properties of periodic arrays of truncated square-based pyramids made of alternating stacks of metal/dielectric layers. We target ultra-broadband quasi-perfect absorption of normally incident electromagnetic radiations in the visible and near-infrared ranges (wavelength comprised between 420 and 1600 nm). We compare the results one can obtain by considering one, two or three stacks of either Ni, Ti, Al, Cr, Ag, Cu, Au orWfor the metal, and poly(methyl methacrylate) (PMMA) for the dielectric. More than 1017 configurations of geometrical parameters are explored and reduced to a few optimal ones. This extensive study shows that Ni/PMMA, Ti/PMMA, Cr/PMMA and W/PMMA provide high-quality solutions with an integrated absorptance higher than 99% over the considered wavelength range, when considering realistic implementation of these ultra-broadband perfect electromagnetic absorbers. Robustness of optimal solutions with respect to geometrical parameters is investigated and local absorption maps are provided. Moreover, we confirm that these optimal solutions maintain quasi-perfect broadband absorption properties over a broad angular range when changing the inclination of the incident radiation. The study also reveals that noble metals (Au, Ag, Cu) do not provide the highest performance for the present application.

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Light management strategies using photonic crystals have been proven to efficiently improve light harvesting and subsequently conversion efficiency of various optoelectronic devices. This study focuses on 3D inverse opal CH3NH3PbI3 photoanodes in perovskite solar cells from a combined numerical and experimental approach. Varying the pore size and the layer thickness in numerical simulations, we first determined theoretical optimum from a purely optical point of view. Corresponding 3D inverse opal photonic nanostructures were then fabricated through spin-coating protocols using polystyrene nanospheres of various diameters as hard templating sacrificial agents. It demonstrates how the photonic nanostructuration of the perovskite layer impacts both optical and electronic properties of experimental samples. Regarding the individual 3D inverse opal perovskite layers, an optimum of light absorption is reached for an ∼500 nm diameter pore photonic nanostructure, with a photonic absorption enhancement as high as 16.1% compared to an unstructured compact benchmark. However, in addition to electronic-related countereffects, local light absorption in the hole transporting material is observed in assembled solar cells, weakening the light management benefits of the perovskite layer nanostructuration to only ∼3% photonic enhancement.@en