Introduction
Lithium niobate (LN) has long been a preferred material for electro-optic modulators due to its excellent Pockels effect and low optical loss. However, integrating LN with silicon photonics has been challenging due to the difficulty in heterogeneous integration. A promising approach is the use of thin-film lithium niobate (TFLN) integrated onto a silicon photonic integrated circuit (PIC) through micro-transfer printing. This tutorial will explain how researchers at Ghent University have demonstrated a very compact photonic crystal modulator by micro-transfer printing TFLN onto a silicon photonic crystal cavity.
Heterogeneous Integration Approach
The researchers started with a silicon photonic circuit fabricated on a 300mm silicon-on-insulator (SOI) wafer using standard 193nm immersion lithography. This circuit contained a one-dimensional (1D) photonic crystal cavity designed for resonance at ~1550nm wavelengths.
In parallel, they prepared a source chip with fully suspended TFLN coupons (small rectangular pieces). This involved patterning and etching a commercially available lithium niobate-on-insulator (LNOI) wafer, followed by a release etch to fully suspend the TFLN coupons.
The key innovation was the use of a micro-transfer printer to pick up the suspended TFLN coupons from the source chip and print them directly onto the silicon photonic circuit in a very compact region, as shown in Figure 1.
After printing the TFLN coupons, a final e-beam lithography step patterned metal electrodes on top to allow application of an electric field to the TFLN for modulation.
Device Design
The 1D photonic crystal cavity consisted of a periodic array of holes with modulated radii to engineer the resonance wavelength and linewidth. The overall cavity size was just 420nm x 11μm. By covering this cavity with the integrated TFLN layer, the applied electric field could modulate the resonance wavelength via the Pockels effect in the LN.
Characterization Results
The fabricated device exhibited a sharp resonance at 1581nm with over 30dB extinction ratio and just 2.6dB insertion loss, as shown in Figure 2a. This demonstrates successful realization of the photonic crystal cavity resonance.
Applying a voltage across the TFLN layer electrodes shifted the resonance wavelength, allowing electro-optic modulation. The measured tuning efficiency was an impressive 6.23pm/V, as plotted in Figures 2b and 2c.
To evaluate the modulation bandwidth, the researchers performed frequency domain measurements using a vector network analyzer. The 3dB electro-optic modulation bandwidth was measured to be 3.5GHz, as shown in Figure 2d.
Advantages and Applications
This work demonstrates several key advantages of the micro-transfer printing approach for heterogeneous Si-LN integration:
High integration density - The TFLN coupons are just 50µm x 230µm in size and spaced ~50µm apart, enabling very dense integration impossible with traditional bondingapproaches.
Scalability - Micro-transfer printing separates the TFLN fabrication from the silicon PIC processing, enabling independent optimization and leveraging standard CMOS tooling.
Compact modulator size - The 1D photonic crystal cavity is only 420nm x 11µm yet provides over 30dB extinction ratio modulation, making it highly compact compared to Mach-Zehnder approaches.
High modulation efficiency - The combination of resonant enhancement and optimal LN orientation yields a tuning efficiency over 6pm/V for low power operation.
Broadband operation - Outside the narrow resonance, the device shows broad suppression of transmission from 1540-1600nm, enabling wide band operation.
This compact, efficient Si-LN modulator enabled by micro-transfer printing could find applications in telecommunications, datacom, LiDAR, quantum photonics and other fields benefiting from integrated LN functionality. Further improvements in bandwidth, uniformity and operating wavelength are areas of ongoing research in this field.
Conclusion
This work highlights the promise of micro-transfer printed TFLN as a powerful platform for next-generation integrated photonic modulators and other linear and nonlinear photonic devices combining lithium niobate with advanced silicon photonic circuits.
Reference
[1] Y. Tan et al., "Compact Photonic Crystal Si-LN Modulator Realized by Micro-Transfer Printing," Photonics Research Group, Department of Information Technology (INTEC), Ghent University - imec, Ghent, Belgium; IDLab, Department of Information Technology (INTEC), Ghent University–imec, Ghent, Belgium; OPERA-Photonique CP 194/5, Université Libre de Bruxelles, Brussels, Belgium, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.
Comentários