Germanium is an indirect gap semiconductor, with a bandgap of 0.66 eV. However it also has a relative minimum in the conduction band at the Γ Γ point, with a direct bandgap of 0.8 eV, which corresponds to a wavelength of 1550 nm. QCSE in Ge/SiGe quantum wells can therefore be used to modulate light at 1.55 μ μ m , which is crucial for silicon photonics applications as 1.55 μ μ m is the optical fiber `s transparency window and the most extensively employed wavelength for telecommunications. By fine tuning material parameters such as quantum well depth, biaxial strain and silicon content in the well, it is also possible to tailor the optical band gap of the Ge/SiGe quantum well system to modulate at 1310 nm, which also corresponds to a transparency window for optical fibers. Electro-optic modulation by QCSE using Ge/SiGe quantum wells has been demonstrated up to 23 GHz with energies per bit as low as 108 fJ. and integrated in a waveguide configuration on a SiGe waveguide