  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 (\c{hi}(2)) effects, a body of experimental evidence
accumulated in the last decade demonstrates that when a strain gradient is
present, a significant \c{hi}(2) and Pockels coefficient can be observed. In
this work we connect a strain-gradient-induced \c{hi}(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) re-orientation
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 in not in the Si itself, but in the material
providing the strain: the silicon nitride cladding.
