Second order nonlinearity in Si by inhomogeneous strain and electric fields

ABSTRACT The lack of a dipolar second order susceptibility (F (2) ) in silicon due to its centro-symmetric diamond lattice usually inhibits efficient second order nonlinear optical processes in the silicon bulk. Depositing stressed silicon nitride layers or growing a thermal oxide layer introduces an inhomogeneous st rain into the silicon lattice and breaks the centro-symmetry of its crystal structure thereby creating a F (2) . This causes enhanced second harmonic generation and was observed in reflection and transmission measurements for wavelengths in th e infrared. However strain is not the only means to break the structures symmetry. Fixed charges at the silicon nitride/silicon interface cause a high electric field close to the silico n interface which causes electric-field-induced-second-harmonic (EFISH) contributions too. The combination of both effects leads to F (2) values which are estimated to be of the order as classic F materials like KDP or LiNiO 3 . This paves the way for the exploitation of other second order nonlinear processes in the area of silicon photonics and is an example how fundamental optical properties of materials can be altered by strain. Keywords: susceptibility, second harmonic generation, inhomogenenous strain, silicon, nonlinear optics