Design of efficient high-order immersed metagratings using an evolutionary algorithm

Abstract

Immersed reflection gratings improve spectral resolving power by enabling diffraction within a high refractive index medium. This principle has been widely adopted to make grating spectrometers more compact. Conventional immersed gratings have blazed profiles which typically show the highest efficiency for one main design wavelength. In addition, the blazed profiles tend to cause significant polarization sensitivity. In this work, we propose an alternative approach for designing an immersed grating composed of sub-wavelength structures, designed to increase diffraction efficiency and reduce polarization dependence. For a theoretical demonstration, a reflective metagrating immersed in silicon is optimized over the short-wave infrared band-3 (SWIR-3, here 2.304 𝜇m –2.405 𝜇m), targeting the same diffraction angles as the immersion grating used in the Sentinel-5 Earth observation mission. The structure is optimized using a modified Covariance Matrix Adaptation Evolution Strategy (CMA-ES). The optimized immersed metagrating achieves an average efficiency of (over the SWIR-3 band) ∼ 78%, compared to ∼62% for the conventional immersed blazed grating, and reduces polarization sensitivity from roughly ∼15% to ∼5%. A manufacturing tolerance analysis is also conducted to evaluate the design’s performance under systematic manufacturing errors, which revealed a degradation of ∼ 10% efficiency at feature size errors of ±25 nm and almost negligible effect on the efficiency at −10 nm and of ∼5% at +10 nm.

Authors 4

1. Ralf Kohlhaas Aachen SRON Netherlands Institute for Space Research

   Space Research Organisation Netherlands · Delft University of Technology

   Affiliation as printed

   Department of Precision and Microsystems Engineering , Delft University of Technology , 2628 CD Delft , The Netherlands

   SRON Netherlands Institute for Space Research , Leiden , The Netherlands

2. Bernhard Brandl Aachen Leiden Observatory

   Leiden University · Leiden Observatory · Delft University of Technology

   Affiliation as printed

   Faculty of Aerospace Engineering , Delft University of Technology , Kluyverweg 1 , 2629 HS Delft , The Netherlands

   Leiden Observatory , Leiden University , Leiden , The Netherlands

3. Jacob de Nobel Aachen

   Leiden University

   Affiliation as printed

   LIACS , Leiden University , The Netherlands

4. Anna V. Kononova Aachen

   Leiden University

   Affiliation as printed

   LIACS , Leiden University , The Netherlands