Dynamics of Nonlinear Optical Losses in Silicon-Rich Nitride Nano-Waveguides

Free carrier absorption (FCA) is established to be the cause of nonlinear losses in plasma-enhanced chemical vapor deposition (PECVD) silicon-rich nitride (SRN) waveguides. To validate this hypothesis, a photo-induced current is measured in SRN thin films with refractive indices varying between 2.5 and 3.15 when a C-band laser light is illuminating the SRN films at various powers, indicating the generation of free carriers. Furthermore, nonlinear loss dynamics is, for the first time, measured and characterized in detail in SRN waveguides by utilizing high peak power C-band complex shape optical pulses for estimation of free carrier generation (FCG) and free carrier recombination (FCR) lifetimes and their dynamics. Both FCG and FCR are found to decrease with an increase in the refractive index of SRN, and, specifically, the FCR lifetimes are found (92 ± 7) ns, (39 ± 3) ns, and (31 ± 2) ns for the SRN indices of 2.7, 3, and 3.15, respectively. Lastly, nonlinear losses in high refractive index SRN waveguides are demonstrated to be minimized and altogether avoided when the pulse duration reduced below the free carrier generation lifetime, thus providing a way of taking a full advantage of the large inherent SRN nonlinear properties.