Bulk and interface second harmonic generation in the Si3N4 thin films deposited via ion beam sputtering

The nonlinear optical second harmonic generation (SHG) in Si3N4 has attracted considerable attention due to a variety of promising applications in optoelectronics. However, reports on SHG in Si3N4 thin films and microstructures lead to diverse conclusions about the SHG origin, pointing towards the Si3N4 bulk, as well as to the Si3N4-Si interface. Here we report on the measurement of polarization-resolved angle-dependent SHG in Si3N4 thin films in the reflective mode. This mode allowed us to measure the nonlinear response of Si3N4 thin films on the Si single crystal substrate. By measuring three samples deposited via ion beam sputtering, we were able to analyze the bulk and interface contributions. We have demonstrated that apart from the bulk SHG, the Si3N4-Si interface contributes with a significant amount of SHG for the thin sample (600 nm). Our result provides a link between the previous measurements in the Si3N4 thin films and on the microstructures.