Among all the silicon photonics related components, coherent sources and amplifiers are of the most challenging tasks due to the lack of Si-compatible high gain materials. One important approach to integrate gain materials on Si is to introduce rare earth ions as impurities into Si [
1]. Erbium (Er) ions, whose main features of emission are relatively insensitive to the host environment, have been intensively studied as dopants in different materials to activate optical transitions at telecommunication wavelengths. Erbium doped fiber amplifier [
2] (EDFA) and erbium doped fiber laser [
3] (EDFL) have achieved great success with sizes on the meter scale. To achieve device miniaturization for integrated on-chip photonic applications, Er-doped waveguide structures were proposed. Er-doped waveguide amplifiers [
4–
6] (EDWAs) with 5.3 dB/cm internal optical gain and Er-doped waveguide lasers [
7] (EDWLs) have been demonstrated. These achievements opened the era of active Er-based devices with sizes on the centimeter scale. However, further size reduction into millimeter range or even smaller is needed for integrated Si photonics. Given that the maximal Er density in Er-doped materials is around 10
20 cm
-3, the highest internal net gain is only a few dB/cm [
8]. The concentration of Er ions cannot be increased beyond a certain limit in Er-doped materials. Otherwise other detrimental phenomena occur, rendering the excited Er ions optically inactive, or worse, even decreasing optical gain.