Optical nonlinearities and ultrafast absorption dynamics in structurally tunable a:SiNx thin films

Abstract

Amorphous non-stoichiometric silicon nitride (a:SiN_x) has a wide and diverse range of optical and structural characteristic features based on the stoichiometric variation of silicon/nitrogen content. Such unique features are widely utilized in photonic and optoelectronic coatings and device concepts. Here we provide a comprehensive study of various silicon rich a:SiN_x thin films (R = [N]/[Si] from 0.22 to 1.16) from structural, linear and nonlinear optical properties and ultrafast absorption dynamics using diverse experimental approaches. While X-ray photoelectron spectroscopy provides a comprehensive understanding of Si/N chemical compositional variation, the linear optical studies confirm the stoichiometric dependency. Ultrafast pump–probe (transient absorption) spectroscopy was utilised to analyse the excited state dynamics, which shows a significant correlation with the optical bandgap variation due to the stoichiometric composition. The extensive study of femtosecond laser-based third-order nonlinear investigations with varied laser intensities and excitation wavelengths provide in-depth information related to nonlinear absorption and bandgap/ molecular polarisation dependency. The studies also witnessed a strong correlation of third-order nonlinearities with linear refractive index and optical bandgap. The significant structural dependent linear and nonlinear optical features and the understanding of excited state dynamics could be highly useful for many new ideas for photonic and optoelectronic novel device concepts.