# Recent Progress in On-Chip Erbium-Based Light Sources
## 3 Gain Medium Based on Different Host Materials
### Er Silicate  
For Er-doped gain mediums, the maximum material gains are limited by the solubility of Er\({}^{3+}\) in the host materials. In contrast, Er\({}^{3+}\) exists as compound cation in Er silicate. For this reason, the concentration of Er\({}^{3+}\) is not limited by the solid solubility and can reach up to 10\({}^{22}\) cm\({}^{-3}\), which is two orders higher than other Er doped gain mediums [50]. Moreover, by introducing yttrium (Y) or ytterbium (Yb) into Er silicate, the concentration quenching effect caused by high Er\({}^{3+}\) concentration can be effectively suppressed and the absorption efficiency of pump light can be improved, thus the optical gain of Er silicate can  
Figure 2: (**a**) The cross-section of the fabricated Si\({}_{3}\)N\({}_{4}\) slot waveguide. (**b**) The cross-section of the Al\({}_{2}\)O\({}_{3}\):Er-Si\({}_{3}\)N\({}_{4}\) hybrid slot waveguide. (**c**) The TE and (**d**) TM optical mode distribution at 1533 nm in the hybrid slot waveguides [40]. be future improved [51; 52; 53; 54; 55]. In 2004, [PERSON] et al., first fabricated Er silicate compounds (Er\({}_{2}\)SiO\({}_{5}\)) by a wet chemical method using ErCl\({}_{3}\) deposited on Si substrates and the photoluminescence (PL) was observed at approximately 1.53 um at room temperature [56]. Then, [PERSON]'s group fabricated Er silicate by radio frequency magnetron sputtering and oxygen annealing is proved to remove defects efficiently and the PL intensity can be strongly enhanced [57; 58]. [PERSON]'s research group demonstrated that the introduction of bismuth can effectively reduce the annealing temperature required for Er silicate crystallization [59]. In 2017, [PERSON] et al., demonstrated a giant net material gain of waveguide amplifier, which is benefits from the high material quality and high Er\({}^{3+}\) concentration of the single-crystal Er chloride silicate nanowire [39]. The experimental measurement is shown in Figure 3a. The backward pumping scheme was adopted, which can improve the utilization of the pump light and enhance the signal gain. The silicate nanowire was end-butt coupled to the taper fibers, as shown in Figure 3b,c and the coupling efficiency is on the order of 10-15%. Finally, a net material gain over 100 dB-cm\({}^{-1}\) at 1530 nm was achieved. Although over 100 dB-cm\({}^{-1}\) material gain has been realized, the fabrication of Er chloride silicate nanowire is complicated, the cross-section is small and coupling efficiency is low, thus it is hard to integrate with other on-chip devices. To solve this problem, [PERSON]'s research group have done a lot of work to achieve on-chip waveguide amplifiers and lasers based on Er silicate gain medium [59; 60; 61; 62; 63; 64; 65]. For the reason that Er silicate is hard to etch, some hybrid waveguide structures combining different materials are adopted to avoid etch Er silicate and the performance of the devices are optimized. For strip-loaded waveguide amplifier, as shown in Figure 4a, the SiO\({}_{2}\) film was deposited on the Er/Yb silicate film and etched into strip waveguide structure [60]. The optical mode of the signal light is shown in the inset and it can be well confined in the gain medium. Although the propagation loss is large, a 5.5 dB signal enhancement was observed when the pump power is 372 mW. For hybrid waveguide amplifier, as shown in Figure 4b, the Si\({}_{3}\)N\({}_{4}\) was etched into strip waveguide structure and then Er/Yb silicate was deposited on it [61]. Although the confinement factor of the signal light in the gain medium is small, the propagation loss could be greatly reduced due to the high quality of the Si\({}_{3}\)N\({}_{4}\) waveguide. Finally, up to 5.5 dB/cm and 10.1 dB/cm signal enhancement was achieved when the annealing temperature was 750 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C, respectively, which proved that the PL characters of Er silicate films is higher when the annealing temperature is higher. Figure 3: Experimental layout of the signal enhancement measurement.