1000 dB/cm gain in Yb³⁺-doped double tungstates

Rare-earth ions are impurities providing low gain, reaching ~10 dB/cm, because electronic transitions within their 4f subshell are parity forbidden, dictating low transition probabilities and cross-sections. Here we exploit the extreme inversion densities attainable in rare-earth-ion-doped microstructures in a host material, potassium double tungstate, that provides enhanced transition cross-sections and dopant concentrations, thereby demonstrating a gain of 935 dB/cm in channel- waveguide and 1028 dB/cm in thin-film geometry, comparable to the best values reported for semiconductor waveguide amplifiers. Further improvement seems feasible with larger dopant concentrations.