Quantum interference in a Λ system with a close lying excited level: probe absorption versus amplification

Abstract

ABSTRACTPump–probe spectroscopy of a four-level system (FLS) consisting of a Λ system with an additional close lying excited level is investigated. The level configuration gives rise to two simultaneously excited Λ resonances driven by the same pump and probe laser beams. Two such four-level configurations, (FLS1) and (FLS2) respectively in the D1 and D2 transitions of 87Rb, are used as model systems to study theoretically the probe absorption spectrum. It is observed that the signature of the quantum interference is dependent on the nature of the FLS; FLS1 exhibits probe amplification as against probe absorption in FLS2. This contrasting behaviour is further corroborated by analytical results, which highlight the importance of transition dipole moments, atom–field interaction parameters and incoherent decays associated with the FLS. The discussion is further augmented using quantum jump formalism, which provides useful insight into the underlying mechanism responsible for probe amplification versus absorption.