On the origin of the second-order nonlinearity in strained Si–SiN structures

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 (χ (2) ) effects, a body of experimental evidence accumulated in the last decade demonstrates that when a strain gradient is present, a significant χ (2) and Pockels coefficient can be observed. In this work we connect a straingradient-induced χ (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 χ (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 χ (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.

Expanding the Silicon Photonics Portfolio With Silicon Nitride Photonic Integrated Circuits

“…These results were achieved by a strain gradient that was induced by means of a strained silicon nitride overlayer [99]. The precise mechanism for this phenomenon is still a matter of debate, but one recent report attributes the effect to a strain gradient in the silicon nitride rather than that in the silicon [100]. This may open the door to strained SiN-based Pockels modulators.…”

Silicon Photonics: silicon nitride versus silicon-on-insulator

“…In recent years strong electro-refraction has been reported in SOI waveguides in which a strain gradient was induced by means of a strained silicon nitride overlayer. The precise mechanism for this phenomenon is still a matter of debate, but one recent report attributes the effect to a strain gradient in the silicon nitride rather than that in the silicon [24]. This may open the door to strained SiN-based Pockels-modulators.…”

Optical second harmonic generation in a ferromagnetic liquid crystal

“…Many such investigations are focused on silicon (Si) and related materials, because of their importance in photonics applications. [48][49][50] Also in crystalline media it is rather difficult to resolve different sources that contribute to the observed SHG signal. 51 In the ferromagnetic nematic LC medium, investigated in our experiments, we tried to distinguish between the EFISHG and other contributions to the SHG signal by inducing LC reorientation one time with external electric field and another time with external magnetic field.…”