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

Yiyu Zhou, Mohan Shen, Chunzhen Li

|January 31, 2025

Summary

We enhanced grating coupler transmission efficiency using bottom-side coupling on sapphire substrates. This method achieves 42% efficiency for aluminum nitride photonic devices, even at cryogenic temperatures.

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Area of Science:

Photonics
Materials Science
Optoelectronics

Background:

Sapphire substrates are widely used for wide-bandgap III-nitride photonic devices.
High refractive index of sapphire leads to low transmission efficiency in conventional grating couplers.
Efficient light coupling is crucial for integrated photonic applications.

Purpose of the Study:

To significantly enhance transmission efficiency in grating couplers on sapphire substrates.
To investigate a novel bottom-side coupling technique.
To assess the performance of grating couplers at cryogenic temperatures.

Main Methods:

Proposed and demonstrated a bottom-side coupling method.
Utilized a metal reflector on the top side and fiber array on the bottom side of the substrate.
Experimentally tested aluminum nitride (AlN) on sapphire grating couplers at telecom wavelengths.

Main Results:

Achieved a maximum transmission efficiency of 42% per coupler.
Demonstrated robust performance at cryogenic temperatures as low as 3 K.
Verified performance for both transverse-electric (TE) and transverse-magnetic (TM) modes.

Conclusions:

Bottom-side coupling effectively enhances transmission efficiency for grating couplers on sapphire.
The developed grating couplers exhibit excellent performance under cryogenic conditions.
This technique is beneficial for sapphire-based photonic applications requiring low coupling loss and low-temperature operation.