Linear Electro-optic Effect in Silicon Nitride Waveguides Enabled by Electric-Field Poling

Stoichiometric silicon nitride (Si₃N₄) is one of the most mature integrated photonic platforms for linear and nonlinear optical applications on-chip. However, because it is a centrosymmetric material, second-order nonlinear processes are inherently not available in Si₃N₄, limiting its use for multiple classical and quantum applications. In this work, we implement thermally assisted electric-field poling, which allows charge carrier separation in the waveguide core, leading to a depletion zone formation and the inscription of a strong electric field reaching 20 V/μm. The latter results in an effective second-order susceptibility (χ⁽²⁾) inside the Si₃N₄ waveguide, making linear electro-optic modulation accessible on the platform for the first time. We develop a numerical model for simulating the poling process inside the waveguide and use it to calculate the diffusion coefficient and the concentration of the charge carriers responsible for the field formation. The charge carrier concentration, as well as the waveguide core size, is found to play a significant role in determining the achievable effective nonlinearity experienced by the optical mode inside the waveguide. Current findings establish a strong groundwork for further advancement of χ⁽²⁾-based devices on Si₃N₄.