Studies Of Absorption And Dispersion Spectra Of The Systems Excited By The Polychromatic Field

Recently, atomic coherence or quantum interference effect has been the important work in quantum optics and laser physics. In this paper, we study deeply the modification of absorption and dispersion properties of a medium by atomic coherence via using optical semi-classical theory, and certain important results are obtained. Compared to the case of monochromatic excitation, polychromatic excitation has more important applications in resonance fluorescence and optics nonlinearity. Considering the fact that multiple switching from normal to anomalous dispersion at multiple frequency regimes cannot be replaced by a single switching in the applications such as quantum computation and quantum information, etc, we explore mainly the absorption and dispersion properties of system excited by the polychromatic field.We firstly adopt the harmonic expansion method of density matrix element and respectively investigate the nonlinear response of the V-type three-level system driven by the bichromatic field and trichromatic field to the monochromatic probe field with arbitrary intensity. When the probe field is very strong, the absorption and dispersion spectra evolve from the well resolved multiple narrow peaks into powered broadened broad structures. This method has wide application and can be extended to the systems with more atomic levels and more field components.Using the present numerical method, we also study theoretically the behalf of double switching from normal to anomalous dispersion of N-type four-level atomic system via trichromatic phase manipulation of electromagnetically induced transparency. The result demonstrates that the absorption and disper-sion spectra of system depends crucially on the sum of the relative phases of the sideband components to that of the central component. Normal dispersion with negligible absorption or anomalous dispersion with small gain can be achieved at multiple frequency regimes by varying the sum of the relative phases. When the sum phase is changed from 0 to π, double switching from normal to anomalous dispersion at two different frequency regimes is found. In addition, so long as we fix the sum of two relative phases, the absorption and dispersion spectra keep unchanged their own features no matter how we vary the respective relative phases.