We investigate the spontaneous-emission properties of a two-level atom embedded in a three-dimensinal anaisotropic photonic crystal. In addition to the modified density of states, the atom is driven by a coherent intense lowfrequency field (LFF), which creates additional multihoton decay channels with the exchange of two low-frequency photons and one spontaneous photon during an atomic transition, Due to the ow frequency of the appliced field, the various transition pathways may interfere with each other and thus give rise to a modifeed system dynamics. We find that even if all the atomic(bare and induced) transition frequencies are in the conducting band of the photonic crystal, there still mya exist a photon-atom bound state in coexistence with propagating modes. the system also allows us to generate narrow linesin the spontaneous-emission spectrum. This spectrum is a funcion of the distance of the observer form the atom due to the band gap in the photonic crystal. The system properties depend on three characteristic frequencies, which are influenced by quantum interfernce effects. Thus these results can b attributed to a combination of interference and band-gap effects.

The decay and radiation of an atom in a photonic crystal with an incomplete gap was investigated. If the transtition frequency is close to a band edge, the Weisskopf-Wigner approximation is not applicable and the decay is nonexponential. The decay could be greatly enhanced or decreased by the self-emitted field.

The interactions between a two-level atom and a coherent field with a time-varying frequency have been investigated. The two typical cases-the frequency of the field varying with time in the forms of sine and rectangle-have been considered. It is found that the collapse-revival phenomenta of the atomic population inveersion are defomed as th frequencychangfing with tiem in sine form and the rectangular frequency modulation can lead to some new collapses and new revivals in the evolution of the atomic population inversion.

The steady-state populalions and Ihe absorption spectrum of a V-type theree-level atom driven by a strong coherent lield lind a weak slochastic one simultaneously ale ibvestigated. The populaltion inversion in bare-sale and dressed-state bases Can happen significantly due to the effect of tile stochastic field. A relation between tile level gap of both atomic exciled slates and the Rabi frequency is found to decide in which excited state the population is lalger than in another one for the same frequency detuning δ between the coherent and the stochastic fields. The line profiles of the absorption spectrumare strognly dependent on the frquency detuing δ, Different lineshapes such as a sharp emission peak (gain). an absorption peak or a dispersion-like peak can be exhibited al the line centre for differentδ. Asymmetrical spectral features and line narrowing can be reached by adjusling δ and the Rabi frequency. Plobe gain in sidebands can occur due to the unbalanced populalion distribuitn in dressed states.

We have studied the time evolutiort of the phase operators of the radiation field in the system contain-ing a V-type three-level atom interaeting with the bimodal field by means of the phase formalism given by Pegg and Borneo [J. Me.d. Opt. 36, 7 (1998); Phys: Rev. A 39, 1665 (1989)]. That the atomic coherent trapping will happen in an appropriate initial state of the system has also been verified.