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

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 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 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.

High-efficiency self-focusing metamaterial grating coupler in silicon nitride with amorphous silicon overlay

“…These gratings leverage bi-level etching topologies or multi-layer gratings formed by SiN 27 , Si 28,29 , or SOI 30,31 material stacks. To reach the sub-dB loss level, both near-field apodization and the selfimagining effect have been utilized to improve the fiber-to-grating field overlap 25,26,32 . These demand precise alignment between multiple patterning layers 25,26 or a customized fabrication flow not typically available in public photonic foundries 32 .…”

“…To reach the sub-dB loss level, both near-field apodization and the selfimagining effect have been utilized to improve the fiber-to-grating field overlap 25,26,32 . These demand precise alignment between multiple patterning layers 25,26 or a customized fabrication flow not typically available in public photonic foundries 32 . Recently, the use of high-index overlays has been shown to improve the radiation characteristics of SiN couplers and enhance the overall fiber-chip coupling efficiency 28,[33][34][35][36][37] .…”