A Knob for Changing Light Propagation from Subluminal to Superluminal

We show how the application of a coupling field connecting the two lower metastable states of a lambda-system can produce a variety of new results on the propagation of a weak electromagnetic pulse. In principle the light propagation can be changed from subluminal to superluminal. The negative group index results from the regions of anomalous dispersion and gain in susceptibility.Comment: 6 pages,5 figures, typed in RevTeX, accepted in Phys. Rev.

Gain Components in Autler-Townes Doublet from Quantum Interferences in Decay Channels

We consider non-degenerate pump-probe spectroscopy of V-systems under conditions such that interference among decay channels is important. We demonstrate how this interference can result in new gain features instead of the usual absorption features. We relate this gain to the existence of a new vacuum induced quasi-trapped-state. We further show how this also results in large refractive index with low absorption.Comment: Total 8 pages, 6 figures, submitted to Physical Review

Quantum interferences and the question of thermodynamic equilibrium

We derive from first principles the dynamical equations for the interaction between a heat bath and a multilevel atom with some near degenerate states. Such dynamical equations exhibit atomic coherence terms which arise from the interference of transition amplitudes. We address the question whether such equations lead to a steady state that is consistent with the thermodynamic equilibrium. We show that coherence affects the dynamics of the system, but the equilibrium conditions are still characterized by Boltzmann factors. We also show how an asymmetric treatment of spontaneous and stimulated processes could lead to a steady state which is at variance with the principles of thermodynamic equilibrium. We show that such a steady state can be realized by pumping with broadband laser fields. Finally, we show that coherences in the dynamical equations can be probed via the spectrum of fluorescence

Gain from cross talk among optical transitions

We analyze the probe absorption spectra of a strongly driven Λ system with arbitrary spacing between the two lower levels. Under the condition that the spacing between the two lower levels is of the order of a strong-pump-field Rabi frequency, important interference effects are observed due to same-field coupling with both the transitions. We report the possibility of a significant gain from the cross talk among optical transitions, and a strong dependence of gain and absorption on the relative polarizations of the pump and probe beams. We present an analysis based on dressed states to explain our numerical results

Effects of spontaneously generated coherence on the pump-probe response of a Λ system

Near-degenerate lower levels in a Λ system have an additional coherence term due to interaction with the vacuum of the radiation field. We report the effects of this spontaneously generated coherence on the formation of a trapped state in the presence of two coherent fields of arbitrary intensity. We show that such coherence preserves both electromagnetically induced transparency and coherent population trapping (CPT) phenomena. However, it changes the time scales associated with the formation of the CPT state, and brings about quantitative changes in the line profiles. We present a clear analytical explanation for our numerical results. We also report the dependence of line shapes on the relative phase between the two applied fields