Atomic coherences of on-resonant and off-resonant two-photon absorptions in a ladder-type atomic system

We report the physical origins of the on-resonant and off-resonant two-photon absorption (TPA) in an open-ladder-type atomic system of the 5S(1/2)(F = 1)-5P(3/2)(F' = 0, 1, 2)-5D(5/2)(F '' = 1, 2, 3) transitions in Rb-87 atoms. When the on-resonant TPA, including electromagnetically induced transparency (EIT), was transformed into the off-resonant TPA according to the coupling laser frequency detuning, we clarified the dynamics of the atomic coherences by decomposing into the two-photon coherence (TC) and the crossover coherence (CC) terms mixed between one-photon coherence (OC) and TC terms. The physical origins of the two TPAs were completely different; the cause of the on-resonant TPA was the CC term, and that of the off-resonant TPA was the TC term. (C) 2014 Optical Society of America