Surface acoustic waves for acousto-optic modulation in buried silicon nitride waveguides

Area of Science:

• Photonics
• Materials Science
• Acoustics

Background:

• Optical waveguides are crucial for photonic integrated circuits.
• Modulating the refractive index of waveguides is essential for optical modulation.
• Surface acoustic waves (SAWs) offer a potential mechanism for such modulation.

Purpose of the Study:

• To theoretically investigate the use of Rayleigh surface acoustic waves (SAWs) for refractive index modulation in silicon nitride (Si3N4) optical waveguides.
• To compare different geometries for SAW generation using lead zirconate titanate (PZT) films and interdigitated transducers (IDTs).
• To maximize the attainable amplitude of index modulation by optimizing device parameters.

Main Methods:

• Numerical solutions of coupled acoustic and optical wave equations were employed.
• Strain distribution of SAWs under resonant excitation was determined.
• Overlap between the acoustic strain field and the optical mode field was analyzed to calculate index modulation.

Main Results:

• A maximum relative effective refractive index shift of 0.7x10^-3 was achieved for TE polarized light at 840 nm.
• Optimized parameters included IDT period (30-35 μm), PZT film thickness (2.5-3.5 μm), and applied voltage (10 V), resulting in resonant frequencies of 70-85 MHz.
• Reducing cladding height to 3 μm increased the maximum shift to 1.2x10^-3 at 87 MHz, a significant improvement over prior non-resonant methods.

Conclusions:

• Rayleigh SAWs provide a highly effective method for refractive index modulation in Si3N4 waveguides.
• The achieved index modulation is orders of magnitude higher and at higher frequencies than previous approaches.
• This technique holds promise for developing compact, high-performance Mach-Zehnder modulators with short device lengths.