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. 2021 Jan 4;12:96. doi: 10.1038/s41467-020-20365-z

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© The Author(s) 2021

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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Fig. 2 — a Scanning electron microscope (SEM) image of the complete device and the measurement and control schematics. The complete PMMC device consists of an encapsulated GST phase gradient metasurface (red box) and a mode selector (yellow box). The white box appears because of the edge of the 218-nm-thick Al₂O₃ encapsulating layer. b Zoomed-in SEM image of the phase-gradient metasurface on the waveguide before depositing the Al₂O₃ encapsulation layer for better imaging. c Zoomed-in SEM image of the TE₀/TE₁ mode selector. d The transmission coefficient (insertion loss) of the devices for TE₀ and TE₁ modes and aGST and cGST phases. The transmission of the TE₀ mode is switched with a high extinction ratio of >16 dB or 4000%. e The mode purity is controlled by the mode converter to >80% for both modes. f The programable mode converter controls the mode contrast Γ at 64 distinct levels, corresponding to 6-bit programming resolution. Upper inset: zoomed-in view of the contrast levels. Lower inset: histograms of 20 programming operations to set the contrast at two adjacent levels (level 30 and 31). The well-separated histograms demonstrate the programming repeatability and accuracy.