Non-contact surface grating coupling for arrays of atom–photonic devices

For future atom–photonic devices, the incoupling method is expected to become the main bottleneck in the scaling of the number of devices, as current devices use side-coupling, which is restricted to the sample edges. This paper describes how light from above is coupled directly from free space into a freestanding membrane of 220 nm thin silicon nitrite (Si₃N₄) with surface grating couplers. The design for these couplers is modeled, captured in an elegant design formula, and experimentally tested. A coupling efficiency of 12% makes experimental coupling to an array of atom–photonic devices possible. The elegant design formula makes grating design straightforward for all-dielectric materials and wavelengths. For this formula, determination of the correct effective index of the specific waveguide mode inside the grating layer is essential.