Investigations On Propagation Properties Of Slow Light In Doppler Broadened Media

The discovery of electromagnetically induced transparency (EIT) indicates a new era of optics based on active optical media. In contrast with passive optical media, a dramatic enhancement of optical responses can be found in such kind, due to the resonantly interaction between light and materials. However, in the old days, active optical media were merely used in the field of research based on short pulse, due to the rigorous restriction caused by violent absorption of light. Therefore, throughout the early stage of nonlinear optics, people had no choice but utilizing intensive laser source to make the nonlinear responses in passive optical media observed. The description of intensive-light nonlinear optics pre-sented the reputation of the subject in that period. At the very beginning of the existence of EIT, people had even estimated the greatest value of EIT being the realization of high efficient multi-wave mixing.The physical principle underlying EIT is the quantum destructive interfer-ence between excitation pathways induced by the quantum states of atoms co-herently prepared by an extra control field. With the effect of EIT, both the optical fields and atomic states are modified, which leads to a series of unique properties in an optically thick medium:(ⅰ) an once opaque medium becomes transparent, (ⅱ) a normal dispersion with sharp steepness is associated with the phenomenon of transparency, (ⅲ) a greatly enhanced nonlinear susceptibility is given rise in the same spectral region. The application of EIT spans over an amount of ar-eas, such as quantum nondemolition measurement, quantum memories, quan-tum teleportation, weak-light ultraslow solitons and so on. EIT shows its great vitalityDuring the past decade, many prominent properties of EIT had been investi-gated and confirmed theoretically and experimentally. Nowadays, the realization of EIT in a more applied system attracts more and more attention, and a series of accompanied investigation need to be carried out. Theoretical studies on EIT-cored atomic systems with Doppler broadening or located in a micro waveguide seem to be of great importance. A suitable method to adjust slow-light solitons in EIT systems is also desirable. In this dissertation, we have studied the topics above. Our work includes the following aspects:1. The linear and nonlinear propagations of light in a hot atomic system is studied, in which Doppler broadening can not be neglected. We make a deriva-tion from the Boltzmann equation with internal states to the optical Bloch equa-tions of hot atomic systems. We present a systematic theoretical study to deal with linear and nonlinear light propagations in the system, with incoherent pop-ulation exchange between two lower energy levels taken into account. Through a careful analysis of base state and linear excitation, we show that the EIT con-dition of the system is given by|πc|2γ31>> 2γ21ΔωD'2, whereπc is half the Rabi frequency of the control field,ΔωD is the Doppler width, andγjl is the decay rate of the coherence between states |j> and |l>. Under this condition, the effect of incoherent population exchange is insignificant, while dephasing dominates the decoherence of the system. This condition also ensures the validity of the weak nonlinear perturbation theory used in this work for solving the Maxwell-Bloch equations with inhomogeneous broadening. We then investigate the nonlinear propagation of the probe field and show that it is possible to form temporal op-tical solitons in the Doppler-broadened medium. Such solitons have ultraslow propagating velocity and can be generated in very low light power. The possibil-ity of realizing (1+1)-dimensional and (2+1)-dimensional spatial optical solitons in the adiabatic regime of the system is also discussed.2. The linear and nonlinear optical properties of an atomic system inside a micro waveguide is studied. we develop a theoretical treatment of the optical responses of a multi-particle atomic system inside the waveguide. The enhance-ment of effective intensity of light is reflected by a factor Q in the optical Bloch equations, based on which, a new EIT condition is proposed. A sensitivity of the small changes of the waveguide dimension is also showed. The study on the nonlinear properties indicates a great enhancement relative to the case in free space.3.The modified effect of slow-light solitons in a resonant three-level atomic system via EIT by utilizing a microwave field is studied. We derive a high-order nonlinear Schrodinger equation by using a perturbation method of multiple-scales, and calculate the modification of soliton velocity and frequency shift. We find that in the presence of the microwave field an obvious decrease of propagating velocity of soliton can be realized, which provides an effective method to slow down optical solitons in EIT systems. We also find that the down shift of os-cillating frequency of soliton in such system can be largely suppressed by the microwave field.The effect of EIT owns a great potential application foreground. The results presented here may be useful for developing the fundamental theory of nonlinear optics in a coherent medium, and guiding the realization of EIT-cored optical instrument.