High-Efficiency Metamaterial-Engineered Grating Couplers for Silicon Nitride Photonics

Introduction

Efficient optical coupling between optical fibers and integrated photonic chips is crucial for developing high-performance photonic integrated circuits (PICs). Grating couplers are widely used for this fiber-chip coupling as they are compact and enable flexible positioning of input/output ports on the chip surface. However, for silicon nitride (Si3N4) waveguide platforms, the moderate refractive index contrast makes it challenging to achieve high grating coupler efficiency. This tutorial presents a novel grating coupler design that utilizes a hybrid amorphous silicon (α-Si)/Si3N4 platform and subwavelength grating (SWG) metamaterials to realize highly efficient fiber-chip coupling for Si3N4 photonics.

Operating Principle

Grating couplers couple light between fibers and waveguides through diffraction. The fraction of power coupled depends on:

1. Reflections (R) and transmission (T) through the grating

2. Directionality (D) - the ratio of upward radiation towards the fiber vs total diffracted power

3. Overlap integral (OL) between the radiated beam and fiber mode fields

The overall coupling efficiency η = (1-R-T) x D x OL

The key is maximizing the directionality and mode overlap. For Si3N4, the lower index contrast yields weaker gratings, limiting directionality and mode overlap.

Proposed Hybrid α-Si/Si3N4 Platform

The proposed design uses a hybrid platform with a Si3N4 waveguide layer and an α-Si overlay where the grating is patterned, as shown in Figure 1. The high α-Si index increases the grating strength compared to pure Si3N4.

Figure 1: Schematics of the SWG-engineered α-Si/Si3N4 grating coupler structure

Subwavelength Grating Metamaterials

To further enhance mode overlap, SWG metamaterials are used to control the effective index in the grating trenches, enabling apodization. The SWG effective index depends on the duty cycle of unetched and etched regions, as shown in Figure 2.

Figure 2: Synthesized equivalent refractive index vs SWG duty cycle

By varying the SWG geometry along the grating, the radiated beam profile can be tailored to better match the fiber mode.

Simulation Results

Uniform Grating:

• Optimized for 1.31 μm wavelength, 40% duty cycle, 660 nm period

• Coupling efficiency = -2.3 dB, 1 dB bandwidth = 30 nm

Figure 3: Coupling efficiency spectrum for uniform grating

Apodized SWG Grating:

• 7 periods with SWG index decreasing from 3.0 to 2.2

• 23 uniform periods

• Coupling efficiency = -1.7 dB

• 1 dB bandwidth = 31 nm

• 96% mode overlap

Figure 4: (a) Intensity profiles of apodized grating and fiber mode (b) Coupling efficiency spectrum

The SWG apodization boosts the efficiency by 0.6 dB over the uniform case while maintaining a wide optical bandwidth.

Conclusion

This tutorial presented a high-efficiency grating coupler design for Si3N4 photonics using a hybrid α-Si/Si3N4 platform and SWG metamaterial engineering. The design achieves -1.7 dB coupling efficiency at 1.31 μm with 31 nm bandwidth, enabling efficient fiber-chip interfaces for applications like datacom and quantum photonics. The use of an α-Si overlay and SWG metamaterials overcomes limitations of pure Si3N4 gratings while maintaining a simple fabrication process compatible with standard semiconductor manufacturing.

Reference

[2] W. Fraser et al., "High-Efficiency Metamaterial-Engineered Grating Couplers for Silicon Nitride Photonics," Nanomaterials, vol. 14, no. 581, pp. 1-13, Mar. 2024. [Online]. Available: https://doi.org/10.3390/nano14070581