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Efficient fiber-to-chip couplers for multi-port access to photonic integratedcircuits are paramount for a broad class of applications, ranging, e.g., fromtelecommunication to photonic computing and quantum technologies. Whilegrating-based approaches are convenient for out-of-plane access and oftendesirable from a packaging point of view, on low-index photonic platforms, suchas silicon nitride or thin-film lithium niobate, the limited grating strengthhas thus far hindered the achievement of coupling efficiencies comparable tothe ones attainable in silicon photonics. Here we present a flexible strategyfor the realization of highly efficient grating couplers on low-index photonicplatforms. To simultaneously reach a high scattering efficiency and anear-unitary modal overlap with optical fibers, we make use of self-imaginggratings designed with a negative diffraction angle. To ensure highdirectionality of the diffracted light, we take advantage of a metalback-reflector patterned underneath the grating structure by cryogenic deepreactive ion etching of the silicon handle. Using silicon nitride as a testbedmaterial, we experimentally demonstrate coupling efficiency up to -0.55 dB inthe telecom C-band with near unity chip-scale device yield.

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