Piezoelectric AlScN based modulator for Si3N4 waveguides

Photonic integrated circuits (PICs) have become key components in several emerging technologies ranging from quantum computing to low Earth orbit satellite communications. This work aims to develop a scalable, fast and efficient tunable waveguide platform, suitable for quantum computing applications. For this purpose, Si₃N₄ waveguides are combined with SiO₂ as cladding material, providing sufficient refractive index contrast and transparency over a broad wavelength range from visible to mid-infrared. This work primarily investigates light modulation based on the acousto-optical principle. Piezoelectrically driven surface acoustic waves (SAWs) periodically modulate the refractive index of the waveguide material. The SAWs are generated by applying a voltage to the interdigital transducers (IDTs), for which the lead-free, CMOS-compatible piezoelectric Al_0.68Sc_0.32N (AlScN) is used. Due to its high piezoelectric coefficient, AlScN generates strong mechanical strain and exhibits high electromechanical coupling, thereby inducing changes in the refractive index of the Si₃N₄ waveguide. The IDT geometry and the thicknesses of the SiO₂ cladding layers are optimized using finite element method simulations to maximize modulation-induced strain in the waveguides, thereby establishing the foundation of the manufacturing flow. To characterize the piezoelectric AlScN layer, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements were conducted. The influence of the interdigital electrode periodicity on the SAWs was evaluated by measuring the scattering parameters. The design, fabrication efforts and the experimental results are reported in this paper.