By means of adiabatically eliminationg the upper level or levels of a A-configuration atom coupling to a two-mode quantized field, we have studied the influencesofac Stark shifts on the atomic coherent population trapping in two important atom-field coupling systems via nondegenerated Raman two-photon processes.The states of the field that trap the atom in its two nondegenerate lower levels are obtained, and the important roles of ac Stark shifts and properties of the atomic upper level or levels in the states of the trapping field in two Raman-coupling systems are also analyzed.

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.

We investigate the effect of the dipole-dipole interaction on the dynamic properties and the atomic coherent trapping of a two-identical atom system in the presence of a laser field by means of quantum electrodynamics theory, The influence of the dipole-dipole interaction on the collapses and revivats of the atomic population is revealed. The conditions of the atomic eoherent trapping are given. A way to measure optically the coupling strength between the atoms is pointed out. Wc also show that the cffect of the dipole-dipole interaction may be screened duc to the coherent field in certain atomic superposition states.

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.