Due to its transparency in the infrared and its high refractive index, silicon represents a formidable passive material for 2D photonic applications in the near and mid IR. However if nonlinear optical processes are required, silicon has severe drawbacks. The centrosymmetry of the silicon lattice inhibits an efficient bulk dipolar susceptibility of second order (χ⁽²⁾) .

Newly published research has now described the generation of a second order nonlinear susceptibility in silicon by the introduction of an inhomogeneous strain. Stressed oxide and nitride surface layers were applied to silicon waveguides leading to a deformation of the underlying silicon lattice and the breaking of its centrosymmetry, so that a dipolar second order nonlinear susceptibility was created in the silicon waveguides.

This second order susceptibility was demonstrated using second harmonic generation (SHG) measurements  in the waveguides and showing  the SHG-increase with rising stress gradient. However this stress/strain gradient is not alone responsible for the observed SHG signal. The nitride covered waveguides show a substantial offset in SHG intensity compared to the oxide covered waveguide that the researchers reponsible argue arises from a strong fixed positive electric charge density at the Si/SiN-interface. This charge creates an inversion and space charge layer in the silicon waveguide and introduces substantial electric fields into the silicon. Combined with the classic third order susceptibility of silicon this field  leads to the process of “electric field induced second harmonic generation” (EFISH) and contributes to the observed SHG efficiency.

With this, the work demonstrates how a man-made second order susceptibility  can be introduced into silicon using external straining cover layers and fixed charges near the interface. This knowledge will allow for the creation of an even larger second order susceptibility in silicon in the future when both effects are deliberately combined. Fulfilling additionally the  phase matching conditions in the silicon waveguides promises to lead to efficient second order nonlinear processes in silicon, which has the potential to introduce nonlinear processes like sum- and difference frequency generation to the area of silicon photonics.