Giant optical gain in a rare-earth-ion-doped microstructure

Semiconductor optical waveguide amplifiers deliver high gain per unit length (up to ∼1000 dBcm−1) [1,2], enabling light amplification over short distances in photonic integrated circuits [3]. In contrast, rare-earth ions are regarded as impurities providing low gain (up to ∼10 dBcm−1) [4–7], because electronic transitions within their 4f subshell are parity forbidden, dictating low transition probabilities and cross-sections. Nevertheless, devices such as fiber amplifiers and solid-state lasers profit from accordingly long excited-state lifetimes–hence increased excitation densities–in rare-earth-ion-doped materials, combined with large device lengths. Here we exploit the extreme inversion densities attainable in rare-earth-ion-doped microstructures in a host material, potassium double tungstate [8], that provides enhanced transition cross-sections and dopant concentrations [9,10], thereby demonstrating a gain of 935 dBcm−1 in channel-waveguide and 1028 dBcm−1 in thin-film geometry, comparable to the best values reported for semiconductor waveguide amplifiers. Further improvement seems feasible with larger dopant concentrations. This gain is sufficient to compensate propagation losses in plasmonic nanostructures [11,12], making specific rare-earth-ion-doped materials highly interesting for future nanophotonic devices.