The blue light indicator in rubidium 5S–5P–5D cascade excitation

The cascade system has played an important role in contemporary research areas related to fields like Rydberg excitation, four wave mixing and non-classical light generation, etc. Depending on the specific objective, co or counter propagating pump–probe laser experimental geometry is followed. However, the stepwise excitation of atoms to states higher than the first excited state deals with increasingly much fewer number of atoms even compared to the population at first excited level. Hence, one needs a practical indicator to study the complex photon–atom interaction of the cascade system. Here, we experimentally analyze the case of rubidium 5S → 5P → 5D as a specimen of two-step excitation and highlight the efficacy of monitoring one branch, which emits ~420 nm, of associated cascade decay route 5D → 6P → 5S, as an effective monitor of the coherence in the system.