Towards higher electro-optic response in AlScN

From integrated photonics to quantum information science, controlling light with electric fields is essential for applications such as light modulation and frequency transduction. These functionalities rely on nonlinear optic phenomena such as electro-optic (EO) effect and sum- or difference-frequency generation. The EO response of a material in the linear regime comprises three additive components: electronic, ionic and piezoelectric. We employ the density functional perturbation theory approach of Veithen, Gonze and Ghosez [1] to evaluate different contributions (Raman susceptibility, mode polarity and phonon properties) to the EO response. Among these contributions, the purely electronic component is directly related to the second-order nonlinear susceptibility which enables second-harmonic generation. Following the theoretical framework of Sipe and Ghahramani [2], we calculated χ⁽²⁾ using its analytic expressions derived from perturbation theory in the independent-particle approximation. We apply these techniques to novel nonlinear optical materials. Conventional materials such as lithium niobate have large electro-optic responses but are hard to integrate with silicon devices. In the search for silicon-compatible materials, aluminum scandium nitride (AlScN) has come to the fore. We perform first-principles calculations to investigate the electro-optic effect in AlScN alloys [3]. At elevated Sc concentrations in alloys, the EO coefficients increase where we find that cation ordering along the c axis leads to enhanced EO response. Strain engineering can be used to further manipulate the EO coefficients of AlScN films. With applied in-plane strains, the piezoelectric contributions to the EO coefficients increase dramatically, even exceeding 250 pm/V. We also explore the possibility of EO enhancement through superlattice engineering, finding that nonpolar a-plane superlattices increase EO coefficients beyond 40 pm/V. Our findings provide microscopic insights into the origins of enhanced EO response in AlScN and provide design principles to enhance the electro-optic effect through alloy engineering and heterostructure architecture.

Work performed in collaboration with Sai Mu, Yunfan Liang, and Chris G. Van de Walle, and supported by ARO, SRC, and DARPA.