Strain engineering has emerged as a powerful tool to control the properties of electronic and photonic structures. Strain has a direct impact on the mechanical properties and on the material band structures. In electronics, the use of stressor layers is nowadays a standard tool to enhance the transistor carrier mobility in CMOS technology. In photonics, the use of strained semiconductor layers was a very important innovation for emitters to reduce the threshold of semiconductor lasers. More recently, strain engineering has become a key element for silicon photonics.[1] Applying strain can modify the electronic band structures of silicon-based materials as well as the lattice symmetry properties. The centrosymmetry of the silicon lattice can be broken by depositing a stressor layer on top of silicon waveguides thus leading to strain gradient.[2] This feature opens new opportunities for second-order nonlinear processes in silicon-like second harmonic generation, frequency mixing or optical rectification.[3] Another very important field for strain engineering in silicon photonics is the development of a group IV semiconductor laser based on germanium.

Ge is an indirect-band gap material characterized by a small splitting between direct valley (Γ) and indirect valley (L)≈ 140 meV at room temperature. This splitting can be reduced by introducing tensile strain [4–7] so that for a biaxial tensile strain between 1.7% and 1.9%[5, 6] Ge becomes a directband-gap.[8, 9] Strain is thus the major ingredient to transform an indirect band gap semiconductor into a direct band semiconductor, ie, turning a poorly efficient semiconductor