Abstract

The development of efficient low-loss electro-optic and nonlinear components based on silicon or its related compounds, such as nitrides and oxides, is expected to dramatically enhance silicon photonics by eliminating the need for non-CMOS-compatible materials. While bulk Si is centrosymmetric and thus displays no second-order (${\chi }^{(2)}$) effects, a body of experimental evidence accumulated in the last decade demonstrates that when a strain gradient is present, a significant ${\chi }^{(2)}$ and Pockels coefficient can be observed. In this work we connect a strain-gradient-induced ${\chi }^{(2)}$ with another strain-gradient-induced phenomenon, the flexoelectric effect. We show that even in the presence of an extremely strong strain gradient, the degree by which a nonpolar material like Si can be altered cannot possibly explain the order of magnitude of observed ${\chi }^{(2)}$ phenomena. At the same time, in a polar material like SiN, each bond has a large nonlinear polarizability, so when the inversion symmetry is broken by a strain gradient, a small (few degrees) reorientation of bonds can engender ${\chi }^{(2)}$ of the magnitude observed experimentally. It is our view therefore that the origin of the nonlinear and electro-optic effects in strained Si structures lies not in the Si itself but in the material providing the strain: the silicon nitride cladding.

© 2015 Optical Society of America

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