Investigation On The Enhanced Nonlinear Effect Via EIT In The Multi-level Cold Atoms

The coherent interaction between light and atoms is one of the most important fields in physics. Coherent field interacting with the multi-level atoms can lead to quantum interference, which could make significative physical phenomenon. As the typical one, the electromagnetically induced transparency effect has attracted great attention for the unique optical properties, such as the high transmission, steep dispersion, and so on.Earlier studies of atomic physics are carried out in warm atomic media, which will result in the Doppler-broadened effect and decoherent effect by the collision. Using the technology of laser cooling and trapping neutral atoms, the cold atomic media has to be the research focus of quantum optics and atomic physics.This dissertation constructs a magnetically-optical-trap (MOT) systems for cooling and trapping the ⁸⁷Rb atoms, observes the Multi-dark-state resonances and determination of spin-polarized states in cold multi-Zeeman sublevel atoms, study the interacting dark states with enhanced nonlinearity in an ideal four-level tripod atomic system, establishes the system of high optical density Rb magneto-optical trap, and observes the optical pulse storage and release in the cold cloud.1) The theory for cooling and trapping the cold atoms is introduced briefly, and we construct a magneto-optical trap (MOT) for cooling and trapping the Rb atoms. The number of cold atoms is measured by detecting the power of fluorescence emitted from the cloud, and we obtain the density of the cloud by measuring the size of the cloud. By analyzing the absorption spectrum observed in the short-distance time-of-flight (TOF) method, we measure the temperature of the cold cloud in the MOT. The results show that the number of the cold atoms is about 10⁹, the density is about 10¹¹cm^-3, and the effective temperature is about 200μk. Such cold cloud can be used as an EIT medium to do the experiments about quantum coherent. 2) Observation of Multi-dark-state resonances and preparation and determination of spin-polarized states in cold multi-Zeeman sublevel atoms. For the different transition strengths between different Zeeman sublevels, using only one weak probe beam and one strong coupling beam, one can observed the MDSRs signal, which consisted of two EIT systems with different transparency width and one absorption system. By injecting the pumping beam with proper polarization and direction, one can preparation the population in one desired Zeeman sublevels with high purity. The population distributions can be determined by using a weak, circularly-polarized probe beam. This new technique can be used in researching quantum information process and quantum optics in the cold cloud.3) Interacting dark states with enhanced nonlinearity in an ideal four level tripod system. We propose and experimentally demonstrate an ideal four-level tripod system with a 95% prepared initial atomic state by properly choosing the polarization of probe, coupling and trigger beam. Two dark states formed and interact strongly with enhanced XPM nonlinearly effect, when both of the coupling beam and trigger beam on resonance. And one can measure the XPM phase shift of probe beam by using the M-Z interferometer. Increasing the intensity of the trigger beam, the crossed Kerr nonlinear coefficient will be enhanced. The experiment results show that the largest crossed Kerr nonlinear coefficient is 7.2×10^-5cm²/W, the crossed phase shift is 1×10^-2 rad, such results is a large step toward the conditional quantum phase gate.4) Realization of high optical density Rb magneto-optical Trap. For the optical density of cold atom cloud is lower, and can not meet the need of the next experiment, we have experiment demonstrated an Rb MOT with a high optical density. With 2.2-cm-diameter cooling laser beams, we achieved an optical density of～11 and～2.6×10¹⁰ trapped Rb atoms for an intensity of～6.6 mW/cm² per beam. The temperature of the cold atoms is～250μK. Furthermore, by ramping the magnetic field gradient to about 20 G/cm, the atomic cloud in MOT was compressed and the optical density was up to 16.5) In the atomic media with high optical density, we observed the optical pulse storage and releases in the Rb atoms, and measure the amplitude of the released signal dependence on the experimental parameters.The characterized works among the above are as follows:â… . Based on the different transition strength between Zeeman sublevels, we experimentally observed the Multi-dark-state resonances signal for the first time. And this signal only can be observed in the cold atoms. By choosing appropriately polarized coupling and pumping laser beams, we realized a simple method to prepare the atomic population in a needed Zeeman level of cold atoms. A directed all-optical method was proposed to measure the spin-polarized state for the first time.â…¡. We have experimentally demonstrated interacting dark states in an isolated four-level tripod system with well-prepared initial atomic state in cold ⁸⁷Rb D1 line. This is the first experimental demonstration of a true four-level tripod system with well-prepared atomic state in the multi-Zeeman sublevel systems. Enhanced EIT and XPM nonlinearity were both experimentally measured in such state-prepared atomic system.â…¢. By enlarging the diameter of cooling beam, and amplifying the output power of cooling laser by injecting the TA systems, we realized a Rb magneto-optical-trap system with high optical density with OD=11, and measure the optical density and the number depending on the intensity of cooling beam. After compressing the MOT, we can further increase the optical density to 16.