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Alexis Carlotti - One of the best experts on this subject based on the ideXlab platform.
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apodized pupil lyot coronagraphs development of designs with reduced iwa and robustness to low order aberrations
Proceedings of SPIE, 2014Co-Authors: Mamadou Ndiaye, Laurent Pueyo, Remi Soummer, Alexis CarlottiAbstract:The Apodized Pupil Lyot Coronagraph (APLC) is a di_raction suppression system adopted in the baseline of several new and recent high-contrast imaging instruments (Palomar P1640, Gemini Planet Imager, VLT/SPHERE) to enable direct imaging of exoplanets at small angular separations (> 0.2 arcsec) from their host star. This coronagraph combines an entrance pupil apodizer, a hard-edge focal plane mask (FPM) and a Lyot stop in a relayed pupil plane to form the coronagraphic image of an observed star onto a camera located in the image plane. The APLC designs underlying these instruments take advantage of the eigen-properties between entrance pupil and Lyot stop, and rely on prolate Apodization to reach a contrast performance of 10 7 over a 20% spectral bandwidth and with a moderate inner working angle (IWA, ~ 5 λ/=D) in the presence of central obstruction and support structures. In this communication we propose novel designs relying on the linearity between the coronagraphic electric field at the science camera and the Apodization function. We use this relationship to devise a numerical optimization scheme that extends the APLC performance (contrast, IWA, apodizer through- put, chromaticity) beyond the intrinsic properties of prolate Apodizations. We explore the parameter space by considering different aperture geometries, contrast levels, dark-zone size, spectral bandwidth and FPM size. We present the application of these new solutions to the case of the High-Contrast Imager for Complex Aperture Telescopes (HiCAT).
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apodized phase mask coronagraphs for arbitrary apertures ii comprehensive review of solutions for the vortex coronagraph
Astronomy and Astrophysics, 2014Co-Authors: Alexis Carlotti, Dimitri MawetAbstract:Context. With a clear circular aperture, the vortex coronagraph perfectly cancels an on-axis point source and offers a 0.9 or 1.75λ/D inner working angle for topological charge 2 or 4, respectively. Current and near-future large telescopes are on-axis, however, and the diffraction effects of the central obscuration, and the secondary supports are strong enough to prevent the detection of companions 10^(-3)–10^(-5) as bright as, or fainter than, their host star. Aims. Recent advances show that a ring apodizer can restore the performance of this coronagraph by compensating for the diffraction effects of a circular central obscuration in a 1D modeling of the pupil. Our aim is to extend this work and design optimal apodizers for arbitrary apertures in 2D in order to tackle the diffraction effects of the spiders and other noncircular artifacts in the pupil. Methods. We fold this analytical result into a numerical optimization scheme that yields hybrid coronagraph designs that combine the advantages of the vortex coronagraph (small in IWA) and of shaped pupils coronagraphs (robustness to central obscuration and pupil asymmetric structures). The transmission of the apodizer is maximized, while constraints are set on the extremum values of the electric field that is computed in chosen regions of the Lyot plane through closed form expressions derived even for topological charges. Optimal apodizers are computed for topological charges 2 and 4 vortex coronagraphs and for telescope apertures with 10–30% central obscurations and 0%, 0.5%, and 1% thick spiders. Results. We put the results of our numerical optimizations in perspective with the analytical solutions and show that our Apodizations converge to the ring Apodizations. We then characterize the impacts of the obscuration ratio and the thickness of the spiders on the throughput and the IWA. For the apodized charge-2 vortex coronagraph the throughputs are slightly below those of the ring apodized vortex coronagraph, and the inner working angle is mostly unaffected by the Apodization. The throughputs of the apodizers for the charge-4 vortex coronagraph are higher than those of the ring apodized vortex coronagraph. This effect increases with the obscuration ratio, though the inner working angle does, too, and it ranges between 2 and 3λ/D. Conclusions. The results presented in this paper show that high contrast at small inner working angles can be obtained with a vortex coronagraph for on-axis telescopes, in spite of the presence of a secondary mirror and its secondary support structures.
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apodized phase mask coronagraphs for arbitrary apertures
Astronomy and Astrophysics, 2013Co-Authors: Alexis CarlottiAbstract:Context. With a clear circular aperture, the vortex coronagraph perfectly cancels an on-axis point source and offers a 0.9 or 1.75λ/D inner working angle for topological charge 2 or 4, respectively. Current and near-future large telescopes are on-axis, however, and the diffraction effects of the central obscuration, and the secondary supports are strong enough to prevent the detection of companions 10−3–10−5 as bright as, or fainter than, their host star. Aims. Recent advances show that a ring apodizer can restore the performance of this coronagraph by compensating for the diffraction effects of a circular central obscuration in a 1D modeling of the pupil. Our aim is to extend this work and design optimal apodizers for arbitrary apertures in 2D in order to tackle the diffraction effects of the spiders and other noncircular artifacts in the pupil. Methods. We fold this analytical result into a numerical optimization scheme that yields hybrid coronagraph designs that combine the advantages of the vortex coronagraph (small in IWA) and of shaped pupils coronagraphs (robustness to central obscuration and pupil asymmetric structures). The transmission of the apodizer is maximized, while constraints are set on the extremum values of the electric field that is computed in chosen regions of the Lyot plane through closed form expressions derived even for topological charges. Optimal apodizers are computed for topological charges 2 and 4 vortex coronagraphs and for telescope apertures with 10–30% central obscurations and 0%, 0.5%, and 1% thick spiders. Results. We put the results of our numerical optimizations in perspective with the analytical solutions and show that our Apodizations converge to the ring Apodizations. We then characterize the impacts of the obscuration ratio and the thickness of the spiders on the throughput and the IWA. For the apodized charge-2 vortex coronagraph the throughputs are slightly below those of the ring apodized vortex coronagraph, and the inner working angle is mostly unaffected by the Apodization. The throughputs of the apodizers for the charge-4 vortex coronagraph are higher than those of the ring apodized vortex coronagraph. This effect increases with the obscuration ratio, though the inner working angle does, too, and it ranges between 2 and 3λ/D. Conclusions. The results presented in this paper show that high contrast at small inner working angles can be obtained with a vortex coronagraph for on-axis telescopes, in spite of the presence of a secondary mirror and its secondary support structures.
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optimal pupil Apodizations of arbitrary apertures for high contrast imaging
Optics Express, 2011Co-Authors: Alexis Carlotti, Robert J Vanderbei, N J KasdinAbstract:In the context of exoplanet direct detection and characterization, where high-contrast imaging is mandatory, we present fully optimized two-dimensional pupil Apodizations for which no specific geometric constraints are put on the pupil plane Apodization, apart from the shape of the aperture itself. Masks for circular and segmented apertures are displayed, with and without a central obstruction and spiders. We can now optimize apodizers for any aperture shape, and examples of optimal masks are shown for the Subaru telescope, the Space Infrared telescope for Cosmology and Astrophysics (SPICA) and the James Webb Space Telescope (JWST). Several high-contrast regions are considered with different sizes, positions, shapes and contrasts. It is interesting to note that all the masks that result from these optimizations tend to have a binary transmission profile.
Dusan Losic - One of the best experts on this subject based on the ideXlab platform.
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biomimetic nanoporous anodic alumina distributed bragg reflectors in the form of films and microsized particles for sensing applications
ACS Applied Materials & Interfaces, 2015Co-Authors: Yuting Chen, Abel Santos, Ye Wang, Tushar Kumeria, Changhai Wang, Dusan LosicAbstract:In this study, we produce for the first time biomimetic films and microsized particles based on nanoporous anodic alumina distributed Bragg reflectors (NAA-DBRs) by a rational galvanostatic pulse-anodization approach. These biomimetic photonic structures can feature a broad range of vivid bright colors, which can be tuned across the UV–visible spectrum by engineering their nanoporous structure through different anodization parameters. The effective medium of NAA-DBRs films is systematically assessed as a function of the anodization period, the anodization temperature, and the current density ratio by reflectometric interference spectroscopy (RIfS). This analysis makes it possible to establish the most sensitive structure toward changes in its effective medium. Subsequently, specific detection of vitamin C molecules is demonstrated. The obtained results reveal that NAA-DBRs with optimized structure can achieve a low limit of detection for vitamin C molecules as low as 20 nM, a sensitivity of 227 ± 4 nm μM–...
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optimizing anodization conditions for the growth of titania nanotubes on curved surfaces
Journal of Physical Chemistry C, 2015Co-Authors: Karan Gulati, Abel Santos, David M Findlay, Dusan LosicAbstract:Titania nanotubes (TNTs), fabricated by electrochemical anodization due to their outstanding properties, have been widely explored for solar cells, catalysis, electronics, drug delivery, biosensing, and medical implants. Rational design of the anodization conditions is the key to obtaining high quality TNTs that are well aligned and strongly adherent onto the underlying titanium substrate. With the development of many anodization procedures on a substrate with various shapes and sizes, catering to various applications, the mechanical stability of anodic layers is often neglected. Here we consider the factors that lead to unstable and poorly adherent nanotube arrays produced upon anodization of curved titanium surfaces. The role of electrolyte aging, water content, voltage/time of anodization, and the substrate dimensions were investigated for optimization of the fabrication of nanotubes on curved surfaces such as Ti wires. Finally, the most optimal fabrication procedure and anodization parameters are pres...
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nanopore gradients on porous aluminum oxide generated by nonuniform anodization of aluminum
ACS Applied Materials & Interfaces, 2010Co-Authors: Krishna Kant, Suet P Low, Asif Marshal, Joseph G Shapter, Dusan LosicAbstract:A method for surface engineering of structural gradients with nanopore topography using the self-ordering process based on electrochemical anodization of aluminum is described. A distinct anodization condition with an asymmetrically distributed electric field at the electrolyte/aluminum interface is created by nonparallel arrangement between electrodes (tilted by 45°) in an electrochemical cell. The anodic aluminum oxide (AAO) porous surfaces with ordered nanopore structures with gradual and continuous change of pore diameters from 80 to 300 nm across an area of 0.5−1 cm were fabricated by this anodization using two common electrolytes, oxalic acid (0.3 M) and phosphoric acid (0.3 M). The formation of pore gradients of AAO is explained by asymmetric and gradual distribution of the current density and temperature variation generated on the surface of Al during the anodization process. Optical and wetting gradients of prepared pore structures were confirmed by reflective interferometric spectroscopy and con...
Ulrich Gösele - One of the best experts on this subject based on the ideXlab platform.
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self ordered anodic aluminum oxide formed by h2so4 hard anodization
ACS Nano, 2008Co-Authors: Kathrin Schwirn, Kornelius Nielsch, Martin Steinhart, R Hillebrand, Ulrich GöseleAbstract:The self-ordering of nanoporous anodic aluminum oxide (AAO) in the course of the hard anodization (HA) of aluminum in sulfuric acid (H2SO4) solutions at anodization voltages ranging from 27 to 80 V was investigated. Direct H2SO4-HA yielded AAOs with hexagonal pore arrays having interpore distances Dint ranging from 72 to 145 nm. However, the AAOs were mechanically unstable and cracks formed along the cell boundaries. Therefore, we modified the anodization procedure previously employed for oxalic acid HA (H2C2O4-HA) to suppress the development of cracks and to fabricate mechanically robust AAO films with Dint values ranging from 78 to 114 nm. Image analyses based on scanning electron micrographs revealed that at a given anodization voltage the self-ordering of nanopores as well as Dint depend on the current density (i.e., the electric field strength at the bottoms of the pores). Moreover, periodic oscillations of the pore diameter formed at anodization voltages in the range from 27 to 32 V, which are remin...
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Structural engineering of nanoporous anodic aluminium oxide by pulse anodization of aluminium.
Nature nanotechnology, 2008Co-Authors: Woo K. Lee, Ulrich Goesele, Woo Lee, Eckhard Pippel, Kathrin Schwirn, Martin Steinhart, Roland Scholz, Ulrich GöseleAbstract:Nanoporous anodic aluminium oxide has traditionally been made in one of two ways: mild anodization or hard anodization. The first method produces self-ordered pore structures, but it is slow and only works for a narrow range of processing conditions; the second method, which is widely used in the aluminium industry, is faster, but it produces films with disordered pore structures. Here we report a novel approach termed "pulse anodization" that combines the advantages of the mild and hard anodization processes. By designing the pulse sequences it is possible to control both the composition and pore structure of the anodic aluminium oxide films while maintaining high throughput. We use pulse anodization to delaminate a single as-prepared anodic film into a stack of well-defined nanoporous alumina membrane sheets, and also to fabricate novel three-dimensional nanostructures.
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Fast fabrication of long-range ordered porous alumina membranes by hard anodization.
Nature materials, 2006Co-Authors: Woo K. Lee, Ulrich Goesele, Woo Lee, Ulrich Gösele, R Ji, Kornelius NielschAbstract:Nanoporous anodic aluminium oxide has been widely used for the development of various functional nanostructures. So far these self-organized pore structures could only be prepared within narrow processing conditions. Here we report a new oxalic-acid-based anodization process for long-range ordered alumina membranes. This process is a new generation of the so-called "hard anodization" approach that has been widely used in industry for high-speed fabrication of mechanically robust, very thick (>100 microm) and low-porosity alumina films since the 1960s. This hard anodization approach establishes a new self-ordering regime with interpore distances, (D(int))=200-300 nm, which have not been achieved by mild anodization processes so far. It offers substantial advantages over conventional anodization processes in terms of processing time, allowing 2,500-3,500% faster oxide growth with improved ordering of the nanopores. Perfectly ordered alumina membranes with high aspect ratios (>1,000) of uniform nanopores with periodically modulated diameters have been realized.
Craig A Grimes - One of the best experts on this subject based on the ideXlab platform.
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self assembled tio2 nanotube arrays by anodization of titanium in diethylene glycol approach to extended pore widening
Langmuir, 2010Co-Authors: Sorachon Yoriya, Craig A GrimesAbstract:We report on the formation of titanium dioxide nanotube arrays having the largest known pore size, approximately 350 nm diameter. The nanotube arrays are synthesized by Ti foil anodization in a diethylene glycol electrolyte containing low (0.5−2%) concentrations of hydrofluoric acid. The large pore size nanotube arrays are achieved with extended anodization durations of approximately 120 h, with the anodization duration showing a more significant effect on pore diameter than the anodization voltage. It appears that the combined effects of hydrofluoric acid content and anodization duration determine the lateral etching rate of the nanotubes, leading to the larger pore size nanotubes.
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photoelectrochemical and water photoelectrolysis properties of ordered tio2 nanotubes fabricated by ti anodization in fluoride free hcl electrolytes
Journal of Materials Chemistry, 2008Co-Authors: Nageh K Allam, Karthik Shankar, Craig A GrimesAbstract:Described is the synthesis of TiO2 nanotube array films by anodization of Ti foil in HCl electrolytes containing different H2O2 concentrations. Highly ordered nanotube arrays up to 860 nm in length, 15 nm inner pore diameter, and 10 nm wall thickness were obtained for one hour anodizations using a 0.5 M HCl aqueous electrolyte containing 0.1–0.5 M H2O2 concentrations for anodization potentials between 10–23 V. The use of ethylene glycol as the electrolyte medium significantly alters the anodization kinetics and resulting film morphologies; nanotube bundles several microns in length achieved for anodization potentials between 8 V and 18 V in only a few minutes. The nanotube arrays obtained from the ethylene glycol electrolytes show relatively higher photocurrents, ≈0.8 mA cm−2 under AM 1.5. Under 100 mW cm−2 AM 1.5 illumination a 500 °C annealed 1 cm2 nanotube array sample, obtained by anodization of a Ti foil sample in ethylene glycol + 0.5 M HCl + 0.4 M H2O2 electrolyte, demonstrates a hydrogen evolution rate of approximately 391 μL h−1 by water photoelectrolysis, time-power normalized evolution rate of 3.9 mL W−1 h−1, with water splitting confirmed by the 2 : 1 ratio of evolved hydrogen to oxygen.
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fabrication of tapered conical shaped titania nanotubes
Journal of Materials Research, 2003Co-Authors: Gopal K Mor, Maggie Paulose, Niloy Mukherjee, Oomman K Varghese, Craig A GrimesAbstract:Using anodic oxidation with a time-dependent linearly varying anodization voltage, we have made films of tapered, conical-shaped titania nanotubes. The tapered, conical-shaped nanotubes were obtained by anodizing titanium foil in a 0.5% hydrofluoric acid electrolyte, with the anodization voltage linearly increased from 10-23 V at rates varying from 2.0-0.43 V/min. The linearly increasing anodization voltage results in a linearly increasing nanotube diameter, with the outcome being an array of conical-shaped nanotubes approximately 500 nm in length. Evidence provided by scanning electron-microscope images of the titanium substrate during the initial stages of the anodization process enabled us to propose a mechanism of nanotube formation.
Kornelius Nielsch - One of the best experts on this subject based on the ideXlab platform.
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self ordered anodic aluminum oxide formed by h2so4 hard anodization
ACS Nano, 2008Co-Authors: Kathrin Schwirn, Kornelius Nielsch, Martin Steinhart, R Hillebrand, Ulrich GöseleAbstract:The self-ordering of nanoporous anodic aluminum oxide (AAO) in the course of the hard anodization (HA) of aluminum in sulfuric acid (H2SO4) solutions at anodization voltages ranging from 27 to 80 V was investigated. Direct H2SO4-HA yielded AAOs with hexagonal pore arrays having interpore distances Dint ranging from 72 to 145 nm. However, the AAOs were mechanically unstable and cracks formed along the cell boundaries. Therefore, we modified the anodization procedure previously employed for oxalic acid HA (H2C2O4-HA) to suppress the development of cracks and to fabricate mechanically robust AAO films with Dint values ranging from 78 to 114 nm. Image analyses based on scanning electron micrographs revealed that at a given anodization voltage the self-ordering of nanopores as well as Dint depend on the current density (i.e., the electric field strength at the bottoms of the pores). Moreover, periodic oscillations of the pore diameter formed at anodization voltages in the range from 27 to 32 V, which are remin...
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Fast fabrication of long-range ordered porous alumina membranes by hard anodization.
Nature materials, 2006Co-Authors: Woo K. Lee, Ulrich Goesele, Woo Lee, Ulrich Gösele, R Ji, Kornelius NielschAbstract:Nanoporous anodic aluminium oxide has been widely used for the development of various functional nanostructures. So far these self-organized pore structures could only be prepared within narrow processing conditions. Here we report a new oxalic-acid-based anodization process for long-range ordered alumina membranes. This process is a new generation of the so-called "hard anodization" approach that has been widely used in industry for high-speed fabrication of mechanically robust, very thick (>100 microm) and low-porosity alumina films since the 1960s. This hard anodization approach establishes a new self-ordering regime with interpore distances, (D(int))=200-300 nm, which have not been achieved by mild anodization processes so far. It offers substantial advantages over conventional anodization processes in terms of processing time, allowing 2,500-3,500% faster oxide growth with improved ordering of the nanopores. Perfectly ordered alumina membranes with high aspect ratios (>1,000) of uniform nanopores with periodically modulated diameters have been realized.