Studies Of Electromagnetically Induced Absorption

Quantum coherence and interference has become one of the important forward subjects in laser physics and quantum optics. Recent studies have shown that the quantum coherent population trapping, electromagnetically induced transparency, index enhancement without absorption, etc. These new effects have great potential for the control of the coherence properties of a medium, the production of high frequency lasers, the information storage and high precision measurements.In this paper we have done two work, One is we, in the dressed states representation, reveal that the constructive interference of atomic coherence on absorption leads to electromagnetically induced absorption, which is in sharp contrast to the case in which destructive interference of atomic coherence gives rise to electromagnetically induced transparency and the other is we reveal that coupling field linewidth inhibits electromagnetically induced absorption, by employing a four-level system in which three transitions are in N configuration and the middle transition serves as a probe transition.First, we study electromagnetically induced absorption.This significant feature of the system model is that a four-level system in which three-dipole transitions are in N configuration In this system, the middle transition serves as a probe transition and two other transitions serve as couple transitions. In the dressed states representation the probe transition is split into four transitions that is coupled to each other. The coupling originates from spontaneousemission of the probe transition and the spontaneous transfer of excited state coherence to ground state coherence. The former leads to couplings between all transitions with the same sign and to absorption reduction. The latter results in couplings, of which two terms have positive sign and with another two have negative sign. It is due to the couplings with positive and negative signs that significantly modify the contribution of dressed coherence to absorption, i.e., changes absorption inhibition to absorption enhancement. So we can say destructive interference of atomic coherence leads to electromagnetically induced absorption, constructive coherence of atomic coherence leads to electromagnetically induced transparency. This four-level-N-configuration atomic system is easily accessible, what's more, it has testified in the experiment.Next we consider the effect of the coupling field linewidth to electromagnetically induced absorption, by employing a four-level system in which three transitions are in N configuration and the middle transition serves as a probe transition, we find that coupling field linewidth inhibits electromagnetically induced absorption. This result is not in contradiction to the case of electromagnetically induced transparency where coupling field linewidth gives rise to or increases absorption, and linewidth is still suppresses the coherence of system.