Simultaneously Subluminal And Superluminal Light Propagation In Degenerate Two-level Atomic Systems

The group velocity of light is the propagation velocity of wave envelope i.e. the practical propagation velocity of light wave. The subluminal and superluminal propagation of light (slow light and fast light) now can be obtained with the development of science and technology. The applications of slow light include the optical delay line, quantum memory, quantum switching, radio frequency photon, quantum computer and all-optical information process et al. Moreover, fast and slow light have applications in enhancing the sensitivity of an interferometer and gyroscope. The change of group velocity is due to the dispersion variation of medium. Sommerfeld and Brillouin pointed that normal dispersion associated with transparent will lead to the slow group velocity and the abnormal dispersion associated with absorption will lead to fast group velocity which is greater than the velocity c in vacuum and this phenomenon does not contradictive with the theory of relativity. Up to date, fast and slow light have obtained in various materials such as the semiconductor, the dye liquor, the solid at room temperature, optical fiber and coherent atomic vapor. Atomic media play a major role in the study of fast and slow light effects, it can offer a flexible platform for experiments through the ability to readily control and vary dispersion characteristics determined the group velocity of light pulses, which based on the linear and nonlinear interaction between field and atomic. Atomic coherence is an important physical process in quantum optics, which can produce many interesting phenomena. For example, in the three-level system, the number of coherent population trapping and electromagnetically induced transparency can be obtained via two-photon process between two ground states. For three-level system, the transform from transparence to absorption can be obtained by changing the coupling field from the traveling wave to standing wave. Simultaneously, the probe light is reflected from the atomic medium. This phenomenon can be called electromagnetically induced grating, and have wide applications in optical switches and four-wave mixing.In this thesis, we take the transition of Fg=4←�'Fe=3of133Cs atoms Dl line for its higher radiation efficiency. We investigate the dispersion and absorption characteristics of continuous probe field based on the degenerate two-level system of at55℃via four-wave mixing process. Further, we investigate theoretically and experimentally the modulation of the group velocity of probe pulse. In experiment, the standing-wave field is composing of two beams which are called forward coupling field and backward coupling field. The probe field and the forward coupling field inject into the atomic medium in the same direction without angle of two beams (the strength of coupling field is far greater than that of the probe field). We observe the characteristics of probe by only changing the power of backward coupling field from zero to the value of forward coupling field. We observe the characteristics of continuous probe field changes from electromagnetic induced transparency (EIT) into the electromagnetically induced absorption (EIA). Simultaneously, the reflected field is always the absorbed gain due to the effect of four-wave mixing. When the probe field is a Gaussian pulse, we observe the transmitted probe pulse changes from subluminal to superluminal propagation. The corresponding light velocity changes from0.00056c to-0.00046c. The signal field is always superluminal propagation and the maximum advanced light velocity is-0.00027c. Finally, the reason of experiment is analyzed theoretically. The relative research is useful for the future optical storage.