Mid-infrared high-Q germanium microring resonator

Area of Science:

• Integrated photonics
• Materials science
• Optoelectronics

Background:

• Germanium is a CMOS-compatible material suitable for mid-infrared (MIR) integrated photonics.
• Existing germanium integration platforms face challenges in achieving high-quality, structurally configured on-chip resonators.
• Realizing high-Q germanium resonators in the MIR fingerprint region (2-15 µm) remains a significant hurdle.

Purpose of the Study:

• To experimentally demonstrate a novel air-cladding MIR germanium microring resonator.
• To achieve the highest reported loaded Q-factor for germanium-based integration platforms in the MIR spectral range.
• To address the limitations of current germanium integration platforms for high-performance on-chip devices.

Main Methods:

• Development of high-quality germanium-on-insulator (GOI) wafers using smart-cut methods.
• Fabrication of a suspended-membrane air-cladding microring resonator structure.
• Experimental characterization of the resonator's performance, including Q-factor and propagation loss.

Main Results:

• Demonstration of an air-cladding MIR germanium microring resonator with a loaded Q-factor of ~57,000, the highest to date for germanium platforms.
• Achieved a propagation loss of 5.4 dB/cm.
• Realized a high extinction ratio of 22 dB, approaching critical coupling conditions.

Conclusions:

• The developed high-Q germanium microring resonator represents a significant advancement for MIR integrated photonics.
• The fabrication methods enable high-quality germanium integration, overcoming previous limitations.
• This work is a crucial step towards enabling various on-chip applications in the MIR spectral range using germanium.