Centrosymmetry inhibits second order nonlinearities in centrosymmetric crystals, like silicon. However, the application of a stressing layer on top of a silicon waveguide has shown the capability to introduce second order nonlinearities [1]. We investigated the Second Harmonic Generation (SHG) process in a strained silicon waveguide engineered to achieve the multimodal phase-matching. As it is described in [2] and [3], we determined a second order nonlinear coefficient of about 0.3 pm/V. Nevertheless, the origin this nonlinearity is still under debate. In fact, it can be due not only to a breaking of the silicon centrosymmetry caused by strain, but it can also have other origins, related for example to the field induced by trapped charges or to the generation in the silicon nitride cladding [4].

In order to remove these doubts, SHG experiments are planned to be repeated using a sample holder equipped by a screw, as it is shown in Fig. 1. In this way, a tunable mechanical strain is introduced in the waveguide. Studying the generation efficiency as a function of the applied load will provide a clear proof of the role that strain has on second order nonlinearities. Our idea is to use a FEM software, through which determining the strain level reached into the waveguide.

In order to validate our ability to FEM simulations to describe strain inside a waveguide, we analyzed the role of strain in a set of silicon racetrack resonators, where loading was applied in a controlled way [5]. We performed FEM simulations taking into account both mechanical deformation of the device (which affects the resonator perimeter and the waveguide cross-section) and the strain-induced refractive index variation (due to the photoelastic effect), providing good agreement with experimental results.

Fig.1: Screw-equipped sample holder used to apply a stress to the sample.

[1] M. Borghi, et al., “Nonlinear silicon photonics,” Journal of Optics, vol. 19(9), pp. 093002, 2017.

[2] A. Trenti, et al., "Towards MIR SPDC generation in strained silicon waveguides." Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC, 2017 Conference on). IEEE, 2017.

[3] C. Castellan, et al., “From SHG to mid-infrared SPDC generation in strained silicon waveguides,” Quantum Photonic Devices, International Society for Optics and Photonics, Vol. 10358, 2017.

[4] C. Schriever, et al., "Second‐Order Optical Nonlinearity in Silicon Waveguides: Inhomogeneous Stress and Interfaces," Advanced Optical Materials, vol. 3.1, pp. 129-136, 2015.

[5] C. Castellan, et al. “Tuning the strain-induced resonance shift in silicon racetrack resonators by their orientation,” Optics Express, Vol. 26.4, pp. 4204-4218, 2018.