Second order optical nonlinearities induced in silicon waveguides by the application of an electric field are evidenced by measuring second harmonic generation (SHG) in the mid infrared. The electric field is produced by lateral p-i-n junctions which are periodically disposed across the waveguide to reach a quasi-phase matching condition. Here, we report on the modeling of the experimental results by using stochastic variations of waveguide and junction geometries which are compatible with the fabrication technique. These variations lead to a broad band multiple peaked spectrum of the SHG efficiency around the nominal phase matched wavelength. Agreement between experiments and simulations is found.
Second order nonlinearities are inhibited in centrosymmetric crystals, like silicon. However, in the last ten years many attempts have been carried out to induce second order nonlinear susceptibility applying a stressing layer of silicon nitride on the top of a silicon waveguide. Succesful experiments showed both Second Harmonic Generation (SHG) or electro-optic modulation in strained silicon waveguide. In order to develop new devices, a full comprehension of the origins of such a nonlinearity is needed. In fact, a lot of estimations of the second order nonlinear coefficient have been given, all different from each other and, in some cases, even contradictory.
In this work, we perform SHG in multimodal phase-matched silicon waveguides. We propose a way to individuate the origin of the nonlinearity, discriminating among the break of the centrosymmetry, the presence of charged states at the interfaces between silicon and silicon nitride and the overlap of the optical mode with the silicon nitride. We estimated a value of the second order nonlinear coefficient of 0.5 pm/V, demonstrating that it results from the coupling of the silicon third order nonlinear coefficient with the electric field induced by the presence of the trapped charges at the core/cladding interface.
We also show preliminary results on SHG in strained silicon microring resonators. Our results open the door to interesting applications, going from broad frequency conversion, to generation of quantum states of light, up to the generation of octave spanning frequency comb based on second order nonlinearities.