Low-$\mathrm{V}_{\pi} \cdot \mathrm{L}$ Acousto-Optic Modulators in Silicon Loaded Scandium Aluminum Nitride on Sapphire

This work presents the implementation and characterization of resonant surface acoustic wave (SAW)based acousto-optic modulators in silicon (Si)-loaded 30% scandium aluminum nitride (ScAlN) on sapphire (SoS), advancing an integrated photonics platform for low-$\mathrm{V}_{\pi}$modulation. Exploiting the index-of-refraction contrast between silicon and scandium aluminum nitride with the low-index sapphire yields robust mode guidance within the heterostructure, ensuring efficient optical mode confinement. Together with the high electromechanical coupling$\left(k_{t}{}^{2}\right)$of SoS, SAW resonators generate high strain to modulate the effective refractive index, enabling resonant acousto-optic modulation. Fabricated modulators in this platform achieve a measured$\mathrm{V}_{\pi} \cdot \mathrm{L}$of$3.60 \mathrm{V} \cdot \text{cm}$, highlighting its potential for integrable, high-performance optomechanical devices.