Study Of Optical Effects Induced By Vacuum Induced Coherence In Driven Atomic Systems

In this thesis, we mainly discuss some optical effects induced by the vacuum induced coherence in driven atomic systems and the slow-light effect in medium.Firstly, we introduce the master-equation theory and the dressed-state method. The master equation depict the time evolvement of the density operator, the Bloch equation derived from the master equation determines the evolvements of the density matrix elements and their mutual relations and the analysis based on this is one of the basic methods in studying the interaction of atom and light. In the dressed-state method the atom and the external field are regarded as a global system, the eigenstates of this system are identified by the Hamiltonian of the atom, external field and their mutual interaction. The processes of absorption and emission are viewed as occurring between different dressed states.In the first section of Chapter 2, we calculate the absorption spectrum of a degenerate V-type atom driven by a coherent field an a dc-field, and discuss the influences of the vacuum induced coherence on the optical properties of the system. The results show for the different intensities of the driven field, the vacuum induced coherence can lead to some effects such as ultra-narrow absorption spectrum and quasi-population trapping. In the second part of Section 1, we introduce the calculation of the absorption spectra by employing the harmonic expansion method when the Bloch equation include explicit temporal dependent terms , this method is also applicable for other similar questions. In Section 2, we calculate the fluorescence spectra of one of the two allowed spontaneous transitions in a∧-type system driven by two coherent fields. The results demonstrate that the system will evolve to dark state when the vacuum induced coherence is ignored, and complete inhibition of the fluorescence occurs. When the strength of the vacuum induced coherence is maximal, the steady-state populations and the values of coherence are related with the initial values of the density matrix elements, which decide a zero or a non-zero fluorescence spectrum.The phenomenon of light pulse propagating with a velocity much less than the velocity of light in vacuum attract wide interests for its theoretical and practical values. In the third part of the article we introduce the concept of the group velocity of the pulse propagation and the relation between it and the refraction property of the medium. We indicate the key point of obtaining the slow-light propagation is obtaining normal dispersion relations with large values of the slope of the index of refraction in the medium. Based on this, we study the dispersion property of a V-type atom in the two-mode squeezed bath under the EIT condition, and find the critical condition for the light pulse from superluminal to subluminal propagation.