Alloy Engineering of a Polar (Si,Ge)₂N₂O System for Controllable Second Harmonic Performance

Although silicon oxynitrides are important semiconductors for many practical applications, their second-order nonlinear optical (NLO) properties have never been systemically explored. Using the first-principles calculations, in this article, we discover that the sinoite (e.g., silicon oxynitride Si₂N₂O) can simultaneously exhibit wide optical band gap, strong second-harmonic generation (SHG) effect, and large birefringence, which are further confirmed by our preliminary experimental measurement. Importantly, we propose that alloying engineering can be further applied to control the balanced NLO properties in the Si₂N₂O system. Based on first-principles cluster expansion theory, we demonstrate that alloying Ge into Si₂N₂O can form low formation-energy Si_2(1-Ge₂N₂O alloys, which can in turn achieve controllable phase-matching harmonic output with high SHG efficiency at different energy ranges. Therefore, alloy engineering could provide a unique approach to effectively control the NLO performance of Si_2(1-Ge₂N₂O, making this polar alloy system hold potential applications in tunable laser conversion and controllable optical devices.