Laser Induced Quantum Coherence In Atomic Medium

When an atomic medium is driven by laser fields, the induced atomic coherence and quantum interference may modify the dispersive and absorptive properties of the atomic medium and leads to a variety of interesting phenomena for fundamental studies and practical applications, such as coherent population trapping (CPT), electromagnetically induced transparency (EIT), lasing without population inversion (LWI), subluminal or superluminal light propagation, light storage, coherent nonlinear optics at low light levels and quantum information processing. This thesis presents the experimental and theoretical studies of laser induced quantum coherence effects in rubidium atoms, and the whole work can be summarized to the following three parts:1. We have set up a new magneto-optical trap (MOT) apparatus with the vacuum system, optical design and magnetic coils being optimized, the number of the trapped cold atoms is about 4×108, the diameter of the cold atom cloud is about 3mm. The time sequence of the experimental process is controlled by computer programming. This new MOT apparatus is very compact and stable. It's a good experimental platform for the study of laser induced quantum coherence effects in cold atoms.2. We studied the cross coupling effect via two or three transitions coupled simultaneously by one strong laser field. We experimentally demonstrated that this multi-coupling could cause multi-window EIT in a cascade system. We proposed applying cross talk among optical transitions to enhance the third Kerr nonlinearity in a four-levelΛsystem, and we found this system could be used to form slow dark optical solitons.3. Sub-Doppler spectral resolution has been studied in Doppler-broadened multi-level 87Rb atoms coherently coupled by two strong laser fields. Narrow spectral features of absorption or gain were observed in the center or sides of the Doppler-broadened absorption profile. Analytical and numerical calculations based on a four-level N type model were presented to explain the experimental results. We found this four-level N-type system could obtain large third nonlinear index of refraction with vanishing linear and nonlinear absorption in cold atoms.4. We studied the bichromatic electromagnetically induced transparency in our new MOT system. The dispersive curve of bichromatic EIT was measured by frequency-modulation spectroscopy. Based on the coupling of the bichromatic field, we observed a large four-wave mixing signal in cold atoms under non-resonance condition, the measured maximum conversion efficiency was about 18%.