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

TL;DR

This paper introduces a novel method for creating highly efficient grating couplers on low-index photonic platforms using cryogenic deep silicon etching, achieving near-unity coupling efficiency suitable for various photonic applications.

Contribution

The authors develop a flexible strategy employing self-imaging gratings with negative diffraction angles and cryogenic etching to significantly improve coupling efficiency on low-index photonic platforms.

Findings

Achieved up to -0.55 dB coupling efficiency in the telecom C-band.

Demonstrated high device yield with near-unity chip-scale fabrication.

Validated the approach on silicon nitride platforms.

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. 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.