  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. While
grating-based approaches are convenient for out-of-plane access and often
desirable from a packaging point of view, on low-index photonic platforms, such
as silicon nitride or thin-film lithium niobate, the limited grating 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 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 near unity chip-scale device yield.
