Observation of two visible Dicke-superradiant transitions in atomic europium (*)

Abstract
We report the first experimental evidence of visible superradiance for the europium lines at λ = 557.7 nm and λ = 545.3 nm. The superradiant character of these lines is checked by studying the variation of pulse heights, widths and delays versus atomic density. We show that the peculiar population inversion mechanism is essentially due to the existence of a strong superradiant infrared emission at λ = 1 759.6 nm