The control of the optical properties of a medium by atomic coherence and quantum interference effects is an important area of research, not only for its fundamental aspects in the understanding of radiation and matter interaction, but also for its potential applications in the design of optical devices.; Atomic coherence can emerge in a medium through the coupling of a strong coherent field to an atomic transition. This coupling affects the optical properties of the medium in a very interesting ways. For example, it may yield light amplification without population inversion. Also, it can modify the traditional dispersion of an optical medium in such a way that maximum variations of the refractive index occur around a spectral region of vanishing absorption.; In the first part of this work we investigate some fundamental aspects of this coherent dynamics. We investigate how this non-inversion gain is affected by the presence of atomic motion. The coherent interaction between atom and radiation can lead to the interference between optical paths of decay. We report on an atomic setting where the interference structures depend upon the phase of the coherent field, an new effect which is absent in other resonance fluorescent phenomena.; As a potential application, we investigate a novel approach for the modulation of optical signal. This approach consists of injecting atomic coherence in the medium in order to tailor its optical properties so it is a more suitable medium for modulation.
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