Scalable and efficient grating couplers on low-index photonic platforms enabled by cryogenic deep silicon etching

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Abstract

Efficient fiber-to-chip couplers for multi-port access to photonic integrated circuits are paramount for a broad class of applications, ranging, e.g., from telecommunication to photonic computing and quantum technologies. Grating-based approaches are often desirable for providing out-of-plane access to the photonic circuits. However, on photonic platforms characterized by a refractive index ≃ 2 at telecom wavelength, such as silicon nitride or thin-film lithium niobate, the limited scattering strength has thus far hindered the achievement of coupling efficiencies comparable to the ones attainable in silicon photonics. Here we present a flexible strategy for the realization of highly efficient grating couplers on such low-index photonic platforms. To simultaneously reach a high scattering efficiency and a near-unitary modal overlap with optical fibers, we make use of self-imaging gratings designed with a negative diffraction angle. To ensure high directionality of the diffracted light, we take advantage of a metal back-reflector patterned underneath the grating structure by cryogenic deep reactive ion etching of the silicon handle. Using silicon nitride as a testbed material, we experimentally demonstrate coupling efficiency up to − 0.55 dB in the telecom C-band with high chip-scale device yield.

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Lomonte, E., Stappers, M., Krämer, L., Pernice, W. H. P., & Lenzini, F. (2024). Scalable and efficient grating couplers on low-index photonic platforms enabled by cryogenic deep silicon etching. Scientific Reports, 14(1). https://doi.org/10.1038/s41598-024-53975-4

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