Bond orbital description of the strain induced second order optical susceptibility in silicon
We develop a theoretical model, relying on the well established sp3 bond-orbital theory, to describe the strain-induced $χ^{(2)}$ in tetrahedrally coordinated centrosymmetric covalent crystals, like silicon. With this approach we are able to describe every component of the $χ^{(2)}$ tensor in terms of a linear combination of strain gradients and only two parameters $α$ and $β$ which can be estimated theoretically. The resulting formula can be applied to the simulation of the strain distribution of a practical strained silicon device, providing an extraordinary tool for optimization of its optical nonlinear effects. By doing that, we were able not only to confirm the main valid claims known about $χ^{(2)}$ in strained silicon, but also estimate the order of magnitude of the $χ^{(2)}$ generated in that device.