High-efficiency, cryogenic-compatible grating couplers on an AlN-on-sapphire platform through bottom-side coupling

TL;DR

This paper introduces a bottom-side coupling method with a metal reflector to significantly improve grating coupler efficiency on AlN-on-sapphire platforms, achieving high transmission and cryogenic robustness for photonic applications.

Contribution

The study demonstrates a novel bottom-side coupling approach with metal reflectors that enhances efficiency and cryogenic performance of sapphire-based grating couplers.

Findings

Achieved 42% transmission efficiency per coupler.

Maintained performance at cryogenic temperatures as low as 3 K.

Applicable to wide-bandgap III-nitride photonic devices.

Abstract

Sapphire is a commonly used substrate for wide-bandgap III-nitride photonic materials. However, its relatively high refractive index results in low transmission efficiency in grating couplers. Here, we propose and demonstrate that the transmission efficiency can be significantly enhanced by bottom-side coupling. A metal reflector is deposited on the top side of the chip, and the fiber array is glued to the bottom side of the substrate. We experimentally achieve a transmission efficiency as high as 42% per coupler on an aluminum nitride (AlN) on sapphire platform at the telecom wavelength. In addition, the grating couplers show a robust performance at a cryogenic temperature as low as 3~K for both transverse-electric (TE) and transverse-magnetic (TM) modes. Our results can be useful to a wide range of sapphire-based applications that require low coupling loss and cryogenic operation.