Heterogeneous Integration of Performant Lithium Niobate-On-Silicon Micro-Ring Modulator by HighPrecision Micro-Transfer Printing

This paper presents the first demonstration of a monolithic 3D integrated lithium niobate-on-silicon micro-ring modulator (LN-on-Si MRM), which is achieved using a high-precision micro-transfer printing (MTP) method. The innovative approach integrates fully patterned, low-loss lithium niobate devices on a silicon photonic chip, providing a scalable and cost-effective solution for photonic interconnects in next-generation optical communication systems.

Key Findings:

1. MTP Integration Method:

• The MTP method enables the transfer of lithium niobate (LN) micro-ring modulators (MRMs) onto silicon photonic platforms with sub-150 nm alignment accuracy, resulting in ultra-low insertion loss (<0.6 dB).

• This method allows for both in-plane (2D) and vertical stacking (3D) integration of LN with silicon, offering flexibility and compactness for high-density photonic integration.

• Compared to hybrid bonding techniques, MTP significantly reduces chip area and parasitic interconnect delays, overcoming issues like thermal mismatch and alignment challenges.

2. Device Performance:

• The hybrid LN-on-Si MRM demonstrates high performance with a high-Q factor (1.8×10⁵) and high extinction ratio (24.8 dB).

• The half-wave voltage-length product (VπL) for the MRM is optimized to 1.5 V·cm, which is crucial for efficient electro-optic modulation.

• The optical coupling efficiency is evaluated with vertical adiabatic couplers (VAC), showing low-loss coupling with a transmission loss of 0.58 dB and effective VAC insertion loss of <0.3 dB.

3. Fabrication and Structural Optimization:

• The LN waveguides are optimized using ICP etching and wet polishing techniques to achieve ultra-low propagation loss (0.28 dB/cm).

• The VAC design is fine-tuned for effective optical mode transfer, with a misalignment tolerance of up to 200 nm, ensuring stable performance.

• Electrode design balances modulation efficiency with optical loss, achieving a gap of 1.5 µm between the electrode and waveguide.

4. Thermal and EO Performance:

• Thermal tests confirm the low thermal impact of the LN-on-Si MRM, with minimal resonance wavelength shift (below 1 pm) even under heating conditions.

• The electro-optic modulation of the LN MRM exhibits good linearity, with the resonance wavelength shifting proportionally to the applied voltage.

5. Benchmarking and Future Potential:

• The benchmark shows that this hybrid LN-on-Si MRM outperforms previous Si-based modulators, offering higher EO efficiency and lower insertion loss.

• The integration method paves the way for low-cost, high-performance photonic modulators for future high-speed optical interconnects in systems like AI and 5G/6G networks.

Conclusion:
This work introduces a scalable and efficient approach to integrating lithium niobate modulators with silicon photonics, leveraging micro-transfer printing to achieve high precision and low loss. The demonstrated hybrid integration offers significant improvements over traditional methods, with potential applications in next-generation high-speed, energy-efficient optical data transmission systems.