Towards MIR SPDC generation in strained silicon waveguides

Silicon has a centrosymmetric crystalline structure that leads to a null second order nonlinear χ(2) coefficient. On the other hand, χ(2) effects are highly sought in Si to develop low power and fast electro-optic modulators. Recently, it has been reported that by stressing a Si waveguide with a silicon nitride over-layer, a sizeable χ(2) is induced. In [1] Second Harmonic Generation (SHG) measurements were reported in large area strained Si waveguides. However, the SHG data were affected by different contributions caused by the interfaces, the free carrier induced internal electric fields, the inhomogeneous strain and the uncontrolled modal structure of the waveguides. Here, we present SHG in Silicon On Insulator (SOI) strained Si waveguides, designed by a nonlinear propagation model. The design maximizes the SHG efficiency by a detailed simulation of the waveguide modal structure to achieve proper phase matching: neff(λp) = neff(λp/2), (neff is the effective refractive index and λp is the pump wavelength). Since the refractive index is dispersive, this condition is never satisfied for the same optical mode. However, the phase matching condition can be satisfied by different optical modes (modal phase matching)...