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. 2018 Dec 10;8:17746. doi: 10.1038/s41598-018-36082-z

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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(a) Forward scattering design of a grating coupler. The grating is excited with a frequency above the photonic band gap. Thus, the coupler scatters into a positive scattering angle φ. The adjustment of the first slot in the grating cannot prevent a reflection at the waveguide-to-grating-boundary. (b) Backscattering design investigated in this work. Light from the waveguide is coupled at a frequency below the photonic band gap of the grating and is therefore scattered under a negative angle φ. The first slot of the grating is modified to avoid a reflection in this case. Half of the nominal slot width δ is filled with silicon.