Study Of Spontaneous Emission Property In Multi-level Atoms Under External Field Control

The inhibition or elimination of spontaneous emission and the study on concentrating it to useful forms are of significance in solving the problems such as enhancing luminous efficiency of photonic device and reducing emission noise. That is why control of spontaneous emission has been an active topic in photonics. Introducing external fields in multi-level atomic system provides rich coupling scheme, thus increases freedom degree of controlling parameters and occurrence of multi-channel interference phenomena. Therefore, effective control of decay properties can be achieved. Furthermore, the study of decay properties of field-driven atom can predict new phenomena or rules to guide the experimental work, which is of great meaning from both theoretical and experimental point of view. In this dissertation, we chose several multi-level atomic systems to investigate the effects of driving fields on the atomic spontaneous emission properties.Firstly, we study the spontaneous emission properties of a three-level V-type atom embedded in photonic crystals. Using Laplace transform and final value theorem, the analytical expression of spontaneous emission spectrum is presented and the atom's steady state behavior of spontaneous emission for different relative position of upper band-gap edge to the upper levels is investigated. Moreover, the effect of initial phase difference between the two excited states of the V-type atom on the quantum interference between the two transitions thus on its emission properties is also investigated.The steady-state distribution of upper-level populations and properties of resonance fluorescence of a double-driven V-type four-level atom are studied. The steady-state upper-level populations are caculated using density matrix method. It is found that quantum interference can lead to splitting of resonances in the population distribution curves. In order to understand the essence of quantum interference effect in the atomic system, symmetric state theory is used to analyze these phenomena. Using quantum regression theory, the fluorescence spectrum is obtained and dressed state theory is invoked to get a physical explanation of these results. A microwave coupling field is introduced in a four-level atomic system and the decay properties of the system are studied. It is shown that a few interesting spectral phenomena due to microwave generated coherence (MGC) such as spectral narrowing, partial cancellation, and spectral-line enhancement can be achieved in the spectra. Since no rigorous conditions are required, the expected phenomena may be more easily to observe in experiment. For the case that the atom is embedded in photonic crystal, combined effect of different relative locations between the upper band-edge and the two upper levels and the phase of microwave coupling field is discussed. It is shown that variation of phase of microwave field has effect not only on the shape of spectral-lines but also on the amounts of atomic emission.The multi-coherent control of atomic spontaneous emission is studied. For an inverted Y-type atom driven by four coherent fields, the analytical expression of the spontaneous emission spectrum is obtained using Laplace method. It is shown that quantum interference effects such as spectral-line narrowing, spectral-line enhancement, spectral-line suppression, partial cancellation and fluorescence quenching can be observed in the spectra by varying the Rabi frequency and phase of the driving fields or the initial population distribution. The spectral phenomena are analyzed using numerical method as well as dressed state theory. More generally, the inverted Y-type atom with two decay channels is studied. A wide variety of spectral behavior such as evolution from a single spectral line to four spectral line of different width, the symmetry change of the spectral lines and the variation of the number and position of dark-lines can be obtained by tuning the Rabi frequency of driving fields, the phase of the microwave field and the initial state of the atom. Moreover, the relation between the emission behaviors of two decay channels is analyzed.