Second, semiconductor materials (e.g., III-V, Ge) have been also introduced for silicon-plus photonics so that one can realize lasers, amplifier, photodetecors, electro-optical modulators. Note that there is usually a large lattice mismatch between these semiconductor materials (III-V, Ge) and silicon, one usually needs some special fabrication processes. For the combination of III-V and silicon, the most popular two technologies are the molecular bonding developed at University of California, Santa Barbara, and the adhesive bonding at Ghent University. In these ways, III-V/silicon lasers [
4], modulators, and photodetectors [
5] have been developed very successfully in the past decade. Meanwhile, people have also been making great efforts to directly grow different semiconductor materials on silicon by optimizing the processes. Fortunately, there are some great progresses reported recently. For Ge, big progress has been achieved for the process of the epitaxial growth on silicon in recent years [
15–
18], e.g., using a two-step Ge epitaxial deposition. With the techniques of high-quality Ge growth on silicon, many Ge-on-Si active devices have been demonstrated, including high-speed optical modulators [
16], metal-semiconductor-metal (MSM) photodetectors [
17], and PIN photodetectors [
18]. Low defect-density growth of GaAs [
19] and InP [
20] using selective-area growth on pre-patterned (001)-Si has been also demonstrated. By leveraging this technology, recently millimeter-long InP waveguide was directly grown on silicon, with superior material quality [
21]. More recently, graphene has attracted lots of attention as one of the most attractive material to be working with silicon photonics because of its unique optoelectronic properties [
22–
24]. With the hybrid platform of graphene-silicon, one can achieve compact optical modulators [
7,
25] and high-speed photodetectors [
8–
10]. The large nonlinearity coefficient of graphene also makes it very attractive for all-optical devices [
26–
28].