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