The design of 2D metasurface integrated with strip waveguide in SOI to achieve high coupling efficiency for 3.8 µm wavelength is presented. The metasurface design has been achieved using a commercial FDTD and the high coupling efficiency has been achieved by systematically optimizing the radius of circularly shaped unit cells and period between them by performing 3D FDTD simulations. The effective coupling efficiency is the performance metric in our case and it is defined as the ratio of the light coupled into strip waveguide to the difference of the light illuminated on the surface and the light reflected back due to interface. The effective coupling efficiency achieved for the designed 2D metasurface integrated with waveguide is ~98% in the in-plane waveguide for the out-of-plane surface illumination. The achieved bandwidth of the structure is 1µm. We believe our design is a good alternative for conventionally employed grating coupler and inverse taper. The integrated design also helps mitigate inefficient coupling using mid-IR fibers currently available and is consistent with the available lithography using 400 nm thick SOI for mid-IR applications. The monolithic integration can also be achieved using standard multi-project wafer (MPW) run.

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Asif Bilal and Usman Younis, "High coupling efficiency surface illuminated 2D metasurface waveguide coupler for mid-IR wavelengths," Proc. SPIE 11031, Integrated Optics: Design, Devices, Systems, and Applications V, 1103106 (Presented at SPIE Optics + Optoelectronics: April 01, 2019; Published: 26 April 2019); https://doi.org/10.1117/12.2520405.