# Recent Progress in On-Chip Erbium-Based Light Sources
## 3 Gain Medium Based on Different Host Materials  
### Er-Doped Al\({}_{2}\)O\({}_{3}\)  
Among the host materials, Al\({}_{2}\)O\({}_{3}\) is of particular interest due to its numerous advantages. The transmission loss of Al\({}_{2}\)O\({}_{3}\) is low over a wide wavelength range (1-6 \(\upmu\)m). In addition, the binding sites of Al\({}_{2}\)O\({}_{3}\) and Er\({}^{3+}\) are well matched, which can increase the solubility of Er\({}^{3+}\) and reduce the possibility of clustering. Therefore, the doping concentration of Er\({}^{3+}\) in Al\({}_{2}\)O\({}_{3}\) is approximately 2-3 orders of magnitude higher than that in EDFA, which is beneficial to achieve high gain in a compact device size. In 1993, [PERSON] et al. demonstrated the first successful incorporation of optically active Er in Al\({}_{2}\)O\({}_{3}\) films by ion implantation [43]. Since then, several approaches have been taken to improve photoluminescence performance of Al\({}_{2}\)O\({}_{3}\):Er [44]. In 2014, [PERSON] research group deposited Al\({}_{2}\)O\({}_{3}\):Er layers by co-sputtering in an oxygen-rich environment [45]. The background loss and dopant concentration in the Al\({}_{2}\)O\({}_{3}\):Er film were <0.1 dB/cm and 0.9 \(\times\) 10\({}^{20}\) cm\({}^{-3}\), respectively. Based on this gain medium, a DFB hybrid waveguide laser was designed. The Si\({}_{3}\)N\({}_{4}\) film was patterned to form DFB grating structure for the reason that Al\({}_{2}\)O\({}_{3}\) is hard to etch. The confinement factor in the gain medium is 83% and the pump-signal overlap factor is 90%. Therefore, the pump and signal light can be well confined in the gain medium, which is beneficial to improve the conversion efficiency of the on-chip waveguide amplifier. A maximum output power of 75 mW was experimentally demonstrated without any thermal damage. Since then, the research group designed several on-chip waveguide lasers with different structures  
Figure 1: The energy-level model of Er\({}^{3+}\). and an improvement of 1.8 times in the lasing efficiency was achieved by optimizing the waveguide laser structure [46; 47; 48]. In order to further improve the luminescence properties of the Al\({}_{2}\)O\({}_{3}\):Er thin film and reduce the propagation loss, Sun's research group prepared Al\({}_{2}\)O\({}_{3}\):Er thin films by ALD process in 2019 [40]. Si\({}_{3}\)N\({}_{4}\) was etched to form the slot waveguide structure so that the optical field can be better confined in the intermediate gain medium, as shown in Figure 2a. Al\({}_{2}\)O\({}_{3}\):Er thin films were deposited through layer-by-layer sequence and repeated multiple times on Si\({}_{3}\)N\({}_{4}\) slot waveguides, as shown in Figure 2b. The optical mode of TE and TM in the slot waveguide are shown in Figure 2c,d, respectively, which illustrates that the optical modes are well confined in the gain medium inside the slot waveguide. Finally, up to ~20.1 \(\pm\) 7.31 dB/cm net modal gain and at least ~52.4 \(\pm\) 13.8 dB/cm net material gain was experimentally observed in the sub-mm long hybrid Al\({}_{2}\)O\({}_{3}\):Er-Si\({}_{3}\)N\({}_{4}\) slot waveguides. Although the hybrid Al\({}_{2}\)O\({}_{3}\):Er-Si\({}_{3}\)N\({}_{4}\) waveguide structures have made great progress and achieve high on-chip optical gain, it is hard to couple and integrate with other on-chip optoelectronic devices. In order to solve this problem and further confine the optical mode in the gain medium, [PERSON] et al. achieved high-gain on-chip waveguide amplifiers via double-layer monolithic integration technology [49]. The Al\({}_{2}\)O\({}_{3}\):Er and the Si\({}_{3}\)N\({}_{4}\) waveguides are located in two individual layers which are separated by a thin SiO\({}_{2}\) film. Thus, the structure parameters of the Al\({}_{2}\)O\({}_{3}\):Er and the Si\({}_{3}\)N\({}_{4}\) waveguides can be optimized, respectively, and the pump and signal light can be better confined in the gain medium, which is conductive to improve the conversion efficiency of pump light.