Impact of scandium substitution on the second-order nonlinearity of epitaxial aluminum nitride thin films

Given its strong electro-optic and optical nonlinearities as well as CMOS compatibility, scandium-alloyed aluminum nitride (ScAlN) stands out as a material with transformative potential for next-generation integrated photonics. A comprehensive understanding of its nonlinear characteristics is critical for developing practical nonlinear optical devices and fully integrated photonic systems. In this work, we experimentally and computationally investigate the second-order nonlinear properties of epitaxial ScAlN thin films grown by molecular beam epitaxy across a range of scandium concentrations (0%–30%). We report a 17-fold increase in the d31 coefficient compared to aluminum nitride, reaching 2.59 pm/V at 30% Sc at 1266 nm while shorter wavelengths and larger Sc concentrations are needed to increase d33. Our two independent first-principles calculations verify these qualitative trends. With its enhanced second-order nonlinearities, the ScAlN material class holds promise for enabling efficient type-I/II nonlinear interactions in modulators, frequency converters, and entangled photon sources.