Controlling The Atomic Spontaneous Emission By External Driving Fields And Photonic Crystals

The atomic spontaneous emission is that an atom transits from its excited state spontaneously to its lower energy state to emit a photon without external disturbation. It is one of the most important research areas in quantum optics. The spontaneous emission light resulting from different atoms is arbitrary at frequency, phase and directions of polarization and propagation, so that the emitted light is incoherent. Therefore, how to effectively control the atomic spontaneous emission becomes an active research topic in quantum optics. It is well known that spontaneous emission depends not only on the properties of the excited atomic system but also on the nature of the surrounding environment. There are two main ways to control atomic spontaneous emission. One is to put the atoms into different environments, such as in frequency-dependent reservoirs, in microwave cavities, or near the edges of photonic band gaps. The density of state (DOS) in photonic crystal is shown to be significantly different from that of vacuum field. Its special DOS can influence and even change the atomic quantum interference effect. It also provides potential applications in high precision spectroscopy, lasing without inversion, quantum computation and quantum information etc. An alternative way to control atomic spontaneous emission is to couple atoms with external coherent fields.In this paper, we investgate the spontaneous emission of a four-level atom system driven by a microwave field and the spontaneous emission properties of atoms embedded in photonic crystals.In chapter 2, we investigate theoretically the features of the spontaneous emission spectra of a four-level atomic system driven by a coherent probe field, a coupling laser field and a microwave field. The two upper levels are coupled by a microwave field in a lower frequency. The time evolution of the populations is calculated. It is shown that the spectra behavior, such as spectral-line narrowing, spectral-line enhancement, spectral-line suppression, and total cancellation of fluorescence quenching, can be easily controlled by changing the phase and the amplitude of the microwave field.In chapter 3, we calculate the spontaneous emission spectra of a four-level atom embedded in a photonic crystal and driven by two coherent fields. It contains one upper level and three lower levels, the excited level is respectively coupled to two lower levels by two coherent fields, and coupled to the third level by the modified reservoir. The influence of the detuning of the external driving fields on the spontaneous spectra is investigated. It is shown that when the transition frequency of the atom is located outside the gap, the dark lines are found for some particular prepared initial states. However, when the transition frequency of the atom is located inside the gap, the atomic spontaneous emission can be suppressed strongly. Some interesting phenomena, such as spectral-line narrowing, spectral-line enhancement, spectral-line suppression and spontaneous emission quenching, are found when we adjust appropriately the external driving fields.In chapter 4, the spontaneous emission spectra of a three-level atom embedded in defect photonic crystals are investigated. The atomic transition from one upper level to the lower is coupled by the vacuum modes, while another upper level is coupled by the defect photonic band gap modes. It is shown that if a defect mode is introduced into the photonic band gap, we found some additional dark-lines in the spontaneous emission spectra, but in the absence of the driving field, the spontaneous emission spectra present Lorentzian. Additionally, the properties of the atomic spontaneous emission can be changed by varying the parameters of the defect mode.