Emission And Absorption Of Multi-level Atoms Driven By External Fields And In Photonic Crystals

Excited atom may decay to the ground state through stimulated radiation or spontaneous emission. Stimulated radiation can generate laser, while spontaneous emission is the origin of quantum noise. One of the fundamental issues in quantum optics is finding effective methods to suppresses or control spontaneous emission. There are three main approaches to change the spontaneous emission of atom:(1) by quantum measurement; (2) by applying driven field or quantum interference effect between different transition channels of multi-level atom; (3) by changing the density modes of the reservoir atom coupled to. In this paper, we study the effects of driven fields on the dynamics behaviors of the atomic populations and the emission and absorption-dispersion spectra of a multi-level atom embedded in photonic crystals.In chapter 2, the effects of an applied low frequency field on the dynamics of a two-level atom interacting with a single-mode field are investigated. It is shown that the time evolution of the atomic population is mainly controlled by the coupling constants and the frequency of the low frequency field, which leads to a low frequency modulation function for the time evolution of the upper state population. The amplitude of the modulation function becomes larger as the coupling constants increase. The frequency of the modulation function is proportional to the frequency of the low frequency field, and decreases with the coupling constant increasing.In chapter 3, the spontaneous emission of a four-level atom driven by two lasers with equal frequencies is investigated, and the dynamics behaviors of the atomic populations are studied for various phase differences. It is shown that with the relative phase changes, the spontaneous emission can be enhanced, or suppressed, or even totally cancelled.In chapter 4, the spontaneous emission spectra of a double V-type four-level atom in a double-band photonic crystals is investigated. The double V-type atom is, respectively, coupled by the free vacuum modes and the photonic band gap modes. There are three types of dark lines in the spectra: the first type originates from the quantum interference, the second type originates from the singularities of the density of states at the isotropic photonic band edges, and the third type originates from the quantum interference in the vacuum modes and the zero of the density of states in the band-gap of photonic crystals. The causes leading to these three types of dark lines are investigated by varying the detuning of the atomic transitions from the band age, by introducing smoothing parameter to eliminate the singularity of the density of states at the isotropic photonic band edges, and by introducing defect modes in the band gap of the photonic crystals.In chapter 5, the absorption-dispersion spectra of a six-level atom embedded in double-band photonic crystals are investigated. It is shown that if there is no defect mode in the photonic band gap, there are three types of transparency windows appear in the absorption-dispersion spectra. If a defect mode is introduced into the photonic band gap, we found some additional transparency windows in the absorption-dispersion spectra. One type of them appears as long as the defect mode exists, but the others appear only when the quantum interference occurs. The transparency windows can be changed by varying the parameters of the defect mode.