In order to understand the origin of second order nonlinear effect, we studied second harmonic generation (SHG) in strained silicon waveguides with a Si₃N₄ stressing layer. By optical pumping a strained waveguide, we found that the second order nonlinear coefficient χ⁽²⁾ is 0.5 pm/V [1].

This value is different from what other researchers found. Few reports indicated that different stressing layers induce different second order nonlinearities. Therefore, we decided to perform measurements under a controlled strain which is applied via a plastic deformation of the waveguide in a dedicated set-up. The results of a measurement where the strain in the waveguide is increased is shown in Fig. 1(a): a shift of the peak is observed but not an increase of the SHG. The shift is explained by the variation of the refractive index of the waveguide due to the photoeleastic effect. What is here surprising is the lack of the generation efficiency increase. Thus we look for another candidate for the not-negligible χ⁽²⁾ . It is well known that during the deposition of silicon nitride on silicon, there is the formation of charged interface states due to the dangling bonds at the interface. These charged states cause a DC field in the waveguide that interacts with the third order nonlinearity χ⁽³⁾ of silicon and causes a dressed χ⁽²⁾ = 3χ ⁽³⁾E_DC . It was shown that UV exposition neutralizes the interface state which removes the DC field in the waveguide. SHG measurements after 23 hours UV exposure are shown in Fig. 1(b). It is observed that after the UV exposition no more SHG peak is present. Together with the independence on the applied strain, this demonstrates that the SHG is not due to the strain induced by the stressing Si₃N₄ layer, but it is caused by the charges at the interface of the waveguide via the induced electric field.

[1] C. Castellan, A. Trenti, C. Vecchi, A. Marchesini, M. Mancinelli, M. Ghulinyan, G. Pucker and L. Pavesi "On the origin of second harmonic generation in silicon waveguides with silicon nitride cladding", Scientific Reports volume 9, Article number: 1088 (2019)