Group Velocity Manipulation Of Multi-pulse Beams Based On Atomic Coherence

The atomic coherence effects can change the law of interaction between the light and matter via coupling different atomic energy level by coherent light fields, and result in the emergence of new physical phenomenon of characteristics of absorption, dispersion and refractive index of lights. Moreover, the coherent manipulation of the group velocity of pulse light has important application prospect in optical information storage, all-optical delay line, and laser radar and high-sensitivity interferometer. So, we focus on atomic coherence effects such as the electromagnetically induced transparency, electromagnetically induced absorption and four-wave mixing, investigate experimentally the manipulation of the group velocity of multi pulse beams based on these atomic coherence effects. The paper includes mainly the following four parts:The first chapter is a summarization for the manipulation technologies of the group velocity of pulse light beams. The research progresses of change of the group velocity based on the electromagnetically induced transparency, electromagnetically induced absorption, coherent population oscillation, four-wave mixing, stimulated Brillouin scattering, stimulated Raman scattering and structural dispersion in photonic crystal are given. Finally, the applications based on fast and slow light technologies are introduced.In the second chapter, we investigate the generation and manipulation of the transmitted and reflected fast light in degenerate two-level system. In the degenerate two-level system of cesium atom, the dispersion characteristics of the reflected signal and transmitted probe lights are investigated based on the coherent reflection of standing-wave. The group velocity of the injected probe and reflected signal pulses can be controlled simultaneously by changing the intensity of standing-wave coupling field. In experiment, the group velocity of the transmitted probe pulse can transform from subluminal i.e. slow light to superluminal i.e. fast light propagation by changing the power of backward coupling field from zero to the value of forward coupling field. Simultaneously, the reflected signal pulse is always the fast light propagation result from the resonance absorption. We show the simultaneous superluminal light propagation of the transmitted probe and reflected signal pulses at the perfect standing-wave. Moreover, we simulate numerically the experimental results by the absorption and dispersion theory of susceptibility.In the third chapter, we investigate experimentally the gain slow light propagation of two pulses based on the four-wave mixing effects. Firstly, in resonant four-wave mixing and suitable level structure, the injected probe light is enhanced and a new conjugate light is generated. In pulse mode, we investigate experimentally the characteristics of gain slow light propagation of the probe and conjugate pulses. Finally, in far off-resonant four-wave mixing and suitable level structure, the influences of one-photon Raman detuning, the system temperature, and pump power on the gain of the probe and conjugate lights are studied. The influence of the two-photon detuning on the gain and delay time of the probe and conjugate pulses are also obtained. The maximal fractional delay for the probe and conjugate pulses is respectively 2.07 and 1.83. We also investigate the influence of the pulse width on group velocity delay by using different width pulses.In the fourth chapter, we observation experimentally the gain slow propagation of four pulses based on four-wave mixing. In the double-lambda four-wave mixing system, the gain slow light characteristics of the forward probe and conjugate pulses are investigated. Furthermore, four four-wave mixing processes can coexist by improving experimental scheme. If one probe pulse is injected, the probe pulse can show slow light propagation, and three new slow light pulses will generate. We can obtain four slow light pulses in a single atomic system, in which the delay time of backward pulses is larger than that of forward pulses. The group velocity manipulation of multi pulse beams has important application prospect in multi-channel information processing.The innovative works are:â… . This experiment research on the simultaneous group velocity manipulation of the injected probe and reflected signal pulses by using the coherent reflection of standing-wave in degenerate two-level system of cesium atom. The simultaneous fast light propagation of two pulses is obtained at perfect standing-wave. The experimental results are explained theoretically by the dispersion characteristics of susceptibility.â…¡. Based on the double-lambda four-wave mixing effects, we investigate experimentally the continuous manipulation of the group velocity of the injected probe and generated conjugate pulses in resonant and far off-resonant level structure.â…¢. Four slow light pulses are obtained simultaneously in a single atomic system by injecting one probe pulse when four four-wave mixing processes are coexist in suitable experimental scheme.