应变Si-SiN结构二阶非线性的成因
On the Origin of the Second-Order Nonlinearity in Strained Si-SiN Structures
摘要：
基于硅或其相关化合物（如氮化物和氧化物）的高效低损耗电光和非线性元件的开发有望通过消除对非CMOS兼容材料的需求来显著增强硅光子学。虽然体硅是中心对称的，因此没有显示出二阶（\c{hi}（2））效应，但在过去十年中积累的大量实验证据表明，当存在应变梯度时，可以观察到显著的\c{hi}（2）和普克尔斯系数。在这项工作中，我们将应变梯度诱导的\c{hi}（2）与另一种应变梯度诱导的现象，挠曲电效应联系起来。我们发现，即使存在极强的应变梯度，像Si这样的非极性材料的变化程度也不可能解释所观察到的chi^（2）现象的数量级。同时，在SiN这样的极性材料中，每个键都有很大的非线性极化率，所以当反转对称性被应变梯度破坏时，键的小（几度）重新取向就能产生实验观察到的chi^（2）量级。因此，我们认为应变Si结构中的非线性和电光效应的起源不在于Si本身，而在于提供应变的材料：氮化硅包层。
英文摘要：
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.
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