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.

Layers with gradient refractive-index profile are an attractive alternative to conventional homogeneous stack coatings. However, the optical characterization and monitoring of the graded refractive-index profile is a complex issue, which has been typically solved by using a simplified model of mixed materials. Although this approach provides a solution to the problem, the precision, which can be expected from optical characterization of the refractive index gradient, remains unclear. In this work, we study optical characterization of SiO x N y layers deposited via reactive dual ion beam sputtering. To characterize the deposited layers, we use several methods including reflectance, and transmittance spectra at a broad range of incident angles, together with spectral ellipsometry. All the data were simultaneously fitted with a general profile of refractive index. The expected profile used in our fit was based on characterization of SiO x N y layers with a varying stoichiometry. By altering of the profile, we discussed sensitivity of such alternation on fit quality and we studied ambiguity of merit-function minimization. We demonstrate that while the scanning of particular parameters of the profile can be seemingly very precise, we obtain a very good agreement between the experimental data and model for a broad range of gradient shapes, where the refractive-index value on major part of the profile can differ as much as 0.02 from the mean value.

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Strong second harmonic generation (SHG) in silicon nitride has been extensively studied—among others, in terms of laser-induced SHG enhancement in Si3N4 waveguides. This enhancement has been ascribed to the all-optical poling induced by the coherent photogalvanic effect. Yet, an analogous process for Si3N4 thin films has not been reported. Our article reports on the observation of laser-induced threefold SHG enhancement in Si3N4 thin films. The observed enhancement has many features similar to all-optical poling, such as highly nonlinear power dependence, cumulative effect, or connection to the Si3N4–Si interface. However, identical experiments for low-oxygen silicon oxynitride thin films lead to complex behavior, including laser-induced SHG reduction. Following a thorough experimental study, including the effects of repetition rate or pulse length, the observed results were ascribed to heat-induced SHG variation. In addition to revealing a new mechanism of laser-induced SHG variation, our results also provide a means to identify this mechanism.