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Optimizing the subwavelength grating of L-band annular groove phase masks for high coronagraphic performance
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
Université de Liège, Belgium.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
Université de Liège, Belgium.
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2016 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 595, no A127, p. 1-8Article in journal (Refereed) Published
Abstract [en]

Context. The annular groove phase mask (AGPM) is one possible implementation of the vector vortex coronagraph, where the helical phase ramp is produced by a concentric subwavelength grating. For several years, we have been manufacturing AGPMs by etching gratings into synthetic diamond substrates using inductively coupled plasma etching. Aims. We aim to design, fabricate, optimize, and evaluate new L-band AGPMs that reach the highest possible coronagraphic performance, for applications in current and forthcoming infrared high-contrast imagers. Methods. Rigorous coupled wave analysis (RCWA) is used for designing the subwavelength grating of the phase mask. Coronagraphic performance evaluation is performed on a dedicated optical test bench. The experimental results of the performance evaluation are then used to accurately determine the actual profile of the fabricated gratings, based on RCWA modeling. Results. The AGPM coronagraphic performance is very sensitive to small errors in etch depth and grating profile. Most of the fabricated components therefore show moderate performance in terms of starlight rejection (a few 100: 1 in the best cases). Here we present new processes for re-etching the fabricated components in order to optimize the parameters of the grating and hence significantly increase their coronagraphic performance. Starlight rejection up to 1000: 1 is demonstrated in a broadband L filter on the coronagraphic test bench, which corresponds to a raw contrast of about 10 5 at two resolution elements from the star for a perfect input wave front on a circular, unobstructed aperture. Conclusions. Thanks to their exquisite performance, our latest L-band AGPMs are good candidates for installation in state of the art and future high-contrast thermal infrared imagers, such as METIS for the E-ELT.

Place, publisher, year, edition, pages
2016. Vol. 595, no A127, p. 1-8
Keyword [en]
instrumentation: high angular resolution, planetary systems, planets and satellites: detection
National Category
Astronomy, Astrophysics and Cosmology Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-284857DOI: 10.1051/0004-6361/201628739ISI: 000388573500066OAI: oai:DiVA.org:uu-284857DiVA, id: diva2:920906
Available from: 2016-04-19 Created: 2016-04-19 Last updated: 2018-03-23
In thesis
1. Microfabrication of Optical Components in Synthetic Diamond: Infrared Optics for Applications  in Astronomy and Spectroscopy
Open this publication in new window or tab >>Microfabrication of Optical Components in Synthetic Diamond: Infrared Optics for Applications  in Astronomy and Spectroscopy
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Infrared optics is a broad general term, relevant to a range of fields. The manufactured diamond optical components utilized within this thesis were applied to both astronomy, in which direct imaging of star system using large ground-based telescopes and diamond coronagraphs was performed, and in absorption spectroscopy probing solvents and proteins using a tuneable quantum cascade laser and diamond waveguides.

The optical components presented in this thesis are all made from diamond, as it is one of few materials that is transparent in the infrared regime. Furthermore, diamond has other unique properties that include high thermal conductivity, low thermal expansion and chemically inertness. In this thesis synthetic diamond grown by chemical vapor deposition has been used, using commercially available components and equipment.

The focus of this thesis was to produce optical gratings for different applications using plasma etching. The first steps involved understanding the etch process and optimizing the plasma etch parameters to enable the fabrication of new types of nano/micro meter sized structures in diamond. Optimization of the etch masks is also included in the work. With this newfound knowledge, deeper and narrower optical gratings than before could be realized.

Optical evaluation of the gratings in special designed optical test benches was used to determine the coronagraphic performance of the manufactured diamond coronagraphs. Most often the designed etch depth could not be reached in the first attempt and therefore a post-fabrication method for tuning the etch depth was developed.  This showed to be vital to realising high performing diamond coronagraphs. Diamond coronagraphs were also installed in several ground based telescopes and discovery of new astronomical objects are presented. With deeper understanding of the etch process more complex coronagraphs in diamond were manufactured opening up for use in the next generation of giant telescopes.

In the second part of this thesis, fabrication of diamond waveguides for sensitive analysis of solvents and proteins using infrared spectroscopy is presented.  Different designs of diamond waveguides are demonstrated and initial analysis of organic compounds and glucose using a quantum cascade laser as the light source is presented. This type of biosensor will be used to study the secondary structure of proteins relevant for different diseases.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 71
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1653
Keyword
Diamond, microfabrication, microoptics, astronomy, IR-spectroscopy, corongraph, waveguide
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-346531 (URN)978-91-513-0293-5 (ISBN)
Public defence
2018-05-18, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2018-04-27 Created: 2018-03-23 Last updated: 2018-05-22

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Vargas Catalan, ErnestoForsberg, PontusKarlsson, Mikael

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