Second-Order Optical Nonlinearity in Silicon Waveguides: Inhomogeneous Stress and Interfaces

The lack of a dipolar second-order susceptibility χ(2) in silicon due to the centrosymmetry of its diamond lattice usually inhibits efficient second-order nonlinear optical processes in the silicon bulk. Recently, the deposition of stressed silicon nitride layers and the corresponding inhomogeneous strain in silicon lead to the demonstration of second harmonic generation and electro-optic modulation in strained silicon waveguides. However, the respective impact of the stress/strain gradient and the involved interfaces is not clear. Here, the influence of the stress and the stressing silicon nitride layer using second harmonic generation measurements in transmission is investigated. The results show that the enhancement of the second-order nonlinearity arises from a constructive superposition of stress-induced and interface-related effects. Particularly, the stress gradient in silicon breaks the symmetry of the crystal lattice, while positive fixed charges at the silicon/silicon nitride interface are responsible for a pronounced electric-field-induced-second harmonic (EFISH) contribution. These results demonstrate the impact of external factors for the creation of an effective χ(2) in materials and open new perspectives for the use of second-order nonlinear optical processes in silicon photonics.