Abstract
Strained silicon waveguides have been proposed to break the silicon centrosymmetry, which inhibits second-order nonlinearities. Even if electro-optic effect and second harmonic generation (SHG) were measured, the published results presented plenty of ambiguities due to the concurrence of different effects affecting the process. In this work, the origin of SHG in a silicon waveguide strained by a silicon nitride cladding is investigated in detail. From the measured SHG efficiencies, an effective second-order nonlinear susceptibility of ~0.5 pmV−1 is extracted. To evidence the role of strain, SHG is studied under an external mechanical load, demonstrating no significant dependence on the applied stress. On the contrary, a 254 nm ultraviolet (UV) exposure of the strained silicon waveguide suppresses completely the SHG signal. Since UV irradiation is known to passivate charged defects accumulated in the silicon nitride cladding, this measurement evidences the crucial role of charged centers. In fact, charged defects cause an electric field in the waveguide that via the third order silicon nonlinearity induces the SHG. This conclusion is supported by numerical simulations, which accurately model the experimental results.
Highlights
Integrating optical components on the silicon platform is appealing due to its compatibility with the complementary metal-oxide-semiconductor (CMOS) technology
We reported a study of the second harmonic generation (SHG) process in strained silicon waveguide with a silicon nitride (SiN) cladding
Studying SHG under external load, we demonstrated no significant dependence of the SHG efficiency value on the applied stress
Summary
We consider the maximum load applied by the screw, showing an increase of about 50% of the average strain inside the waveguide. Note that the simulated phase-matching wavelength shown in Fig. 4(d) is slightly different from the experimental one shown in Figure 3(b) (2293.8 nm against 2288 nm at ∆H = 0 μm) This is due to local variations of the waveguide geometry, as already discussed previously. Considering φ = 0°, they are Γxxy,xxy = −4 × 10−16 m2V-1 and Γxxy,yyy = −5.1 × 10−16 m2V-1 By using these values, the χs(tr2a)in,xxy map inside the waveguide is calculated, and it is shown in Fig. 5 referred to the cases ∆H = 0 μm and ∆H = 50 μm. In the experiment no significant variation of the SHG efficiency is observed
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