Sustained poling-induced second-order optical nonlinearity in sodium-doped amorphous niobium oxide waveguides.

Nonlinear optical devices have attracted increasing interest, with lithium niobate being the benchmark material due to its strong second-order optical nonlinearity (SONL). However, its hardness, chemical inertness, and anisotropy present significant fabrication challenges, motivating the search for alternative platforms. Amorphous materials with poling-induced SONL offer promising opportunities, as their isotropic nature enables flexible optical design. In this work, we investigate amorphous sodium niobate thin films and demonstrate waveguide structures with active SONL through thermally controlled micrometer-scale poling combined with standard lithography. Using polarized second-harmonic generation (SHG) microscopy, we show precise spatial and geometrical control of the induced SONL, and its retention after waveguide processing steps, including UV lithography and dry-etching. The persistence of strong SHG signals, particularly near poling electrode edges, highlights the potential of aligning lithography mask with SONL patterns to enhance device performance, providing a foundation for developing efficient nonlinear photonic devices based on poled amorphous thin films.