Large optical gain in Er-doped Gd2O3 single crystal thin films on silicon at cryogenic temperatures

Er-doped rare-earth oxide single crystal thin films, which can be directly grown on silicon substrate, are promising material candidates for high-gain on-chip optical amplifiers. We have previously grown high-quality Er-doped Gd2O3 (Er3+:Gd2O3) thin films on silicon-on-insulator substrates and demonstrated low-loss SiN strip-loaded waveguides based on these thin films. In this work, at cryogenic temperatures from liquid helium to nitrogen temperatures, we observed in the waveguide transmission spectra that the absorption dips of Er3+ ions in the telecommunication band evolved into peaks upon optical pumping, thus demonstrating an unambiguous optical gain. We estimate the that peak material gain of our thin films around 1536 nm is larger than 55 dB/cm for Er doping concentration of ~3×10^20 cm-3. These results indicate that single crystal Er3+:Gd2O3 thin films are very promising candidates for silicon-based on-chip active photonic devices.