The Optical Properties Of Atomic System Under The Background Of The Vacuum Radiation

Since the age of 70 atomic coherence was aroused great concern, as an important basic physical phenomenon in quantum optics. Vacuum induced coherence (VIC) is one of the atomic coherence effects. When the two levels of the atom are nearly degenerate and coupled by the same vacuum radiation field to the intermediate state, the system can create a atomic coherence, i.e., VIC. Due to the VIC effect, a series of changes of the atomic spectra are shown, especially the electromagnetically induced transparency (EIT). In recent years, many interesting physical phenomena influenced by the VIC, such as coherent population trapping, gain without inversion, non-absorption high refraction index, spontaneous emission cancellation and enhancement, resonance fluorescence, subluminal group velocity, are investigated. Based on EIT and VIC, this thesis proposes an atomic model and analyzes the spectra properties. We study the effects of vacuum-induced coherence on the single and two-photon absorption properties in a degenerate four-level ladder atom. And the effects of the squeezed vacuum (SV) on the transient optical response in a∧-type three-level atomic medium are investigated, too.The thesis is organized as follows: In the first chapter, the principles as well as development of EIT and VIC are introduced. In the second chapter, we first review three pictures of the interaction between the light and matter. Using semiclassical theory, then we derive the Hamiltonian, probability amplitude, as well as the density matrix equation both in the two-level and the three-level atomic system, respectively. Besides, EIT is explained by the dressed state, dark state and density matrix equation.In Chapter 3, a new four-level system which contains the∧-type and the∨-type structures is depicted. When the two middle levels are coupled with a ground and a excited levels by the same vacuum radiation fields, respectively, there appear two kinds of VIC:∧-type and∨-type VIC, due to the quantum interference effects between the spontaneous decay channels. The effects of the VIC on the electromagnetically induced single- and two-photon transparency are studied, respectively. The∧-type VIC enhances the two-photon absorption, but has no effect on the single photon absorption; the∨-type VIC suppresses the single and two-photon absorptions.In Chapter 4, we investigate the effects of SV field on the transient absorption and population distribution in a∧-type three-level system. By comparing the SV and VIC on the transient absorption properties of the atomic system, we find that he transient absorption (gain), the steady-state value, and the response time from the transient to the steady situation are remarkably dependent on the SV field. Time evolution of the population with different initial conditions is also studied. When the SV effect is excluded, the steady population distribution is related to the initial conditions, whether the VIC is included or not; when considering the effect of SV, the steady population distribution is not related to the initial conditions with or without VIC. Consequently, we can conclude that dependence of the steady population distribution on the initial conditions is mainly determined by the SV field. These are useful to reveal the impact of environment on the atomic system as well as modulate the optical properties of atomic system.