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.

Spontaneous emission of an atom with two close upper levels arid one lower level in a multilayer dielectric medium, and quantum interference between transitions form the upper leves to the lower one, are considered. Since the medium is inhomogeneous along the direction normal to tbe interfaces of the dielectric layers, the vancuum had an anisotropic spatial structure, and quantum interference between the two decay channels can appear even if the two dipole matrix elemeats are ortbogonal to each other. If the atom is pLaced in a three-layer dielectric plate cavity, a strong quantum interference interference similar to that for an atom with two nearly parallel dipole moments in free space arises due to the influence of anisotropic of anisotropic vacuum. Putting the atom in a three-layer dielectric waveguide and a one-dimensional photonic crystal, there appears a strong quantum interference similar to that for an atom with nearly antiparallel dipole moments.

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.

In this paper, we have investigated the time evolution of |he phase operator of the radiation field in the Jaynes-Cummings model with and without tfie rotating-wave approximation, making use of the phase formalism of Barnett and Pegg [J. Mod. Opt. 56, 7 (1989); Phys. Rev. A 39, 1665 (1989)]. We verify by an analytical method that the atomic Rabi oscillation leads to phase dissipation of the radiation field, and also that the effect of the virtual-photon field can exhibit quantum fluctuations in the atomfield-coupling system and lead to a Frequency shift of the radiation field.