Simplified aluminum nitride processing for low-loss integrated photonics and nonlinear optics

Aluminum nitride (AlN) is an extremely promising material for integrated photonics because of the combination of strong χ⁽²⁾ and χ⁽³⁾ nonlinearities. However, the intrinsic hardness of the material and charging effects during electron beam lithography make AlN nanofabrication a challenging process. Conventional approaches often require multiple hard masks and a metal mask to fabricate nanostructures. In this letter, we report a novel, simple method to fabricate AlN microresonators by using a single layer of silicon nitride mask combined with a thin conductive polymer layer. The conductive layer can be conveniently removed during developing without requiring an additional etching step. We achieve a high etching selectivity of 4:1 between AlN and the mask, enabling mid-infrared photonic device fabrication, as well as high intrinsic quality (Q) factors of up to 1.0 × 10⁶ in AlN microresonators. Furthermore, we demonstrate several nonlinear phenomena within these devices, including frequency comb generation, Raman lasing, third-harmonic generation, and supercontinuum generation.