Investigation Of The Nonlinear And Quantum Effect In The Coherent Atomic Medium

The interaction between light and atoms is one of the most important fields in physics. Coherent light interacting with multi-level atoms can lead to quantum interfererice in the amplitudes of optical transitions. In this way the optical properties of a medium can be dramatically modified, leading to many interesting and important phenomena. As the most typical one, electromagnetically induced transparency (EIT) effect has attracted great attention and has been researched extensively.This dissertation reviews the development of the research works on EIT effect, as well as its application on nonlinear and quantum effects caused by the properties (reduced absorption, steep dispersion, and greatly enhanced nonlinear susceptibility) of EIT medium. The experimental and theoretical works we completed in EIT medium were mainly introduced in this dissertation, including light speed reduction, optical quantum storage and its two channel releasing, controllable optical induced polarization rotation, multi-dark-state resonances in cold multi-Zeeman-sublevel atoms, prepartion and determination of spin-polarized states in multi-Zeeman-sublevel atoms, The main accomplished works are as follows:1) Designing and constructing diode lasers feedback by confocal F-P cavity or grating respectively. High quality light sources are needed to complete the high quality experiments of interaction between light and atoms. As required by different experimental systems, we designed and constructed diode lasers feedback by confocal F-P cavity and grating respectively, with wavelength corresponding to the D1 or D2 line of rubidium atoms. With optical feedback from the confocal F-P cavity, the linewidth of diode laser was reduced from 10 MHz at free running to 45 kHz. The continuously frequency-tuning range of 3.4GHz was experimentally realized with the injection-current scanning following. This kind of diode laser was mostly used in the experiments, which were sensitive to laser linewidth. Using optical feedback from grating with Littrow configuration, the linewidth of laser diode can be reduced to 4MHz. Its continuously frequency-tuning range can achieve 9GHz with the injection-current scanning following. This kind of diode laser was mostly used in the experiments, which were not sensitive to the laser linewidth, but needed widely frequency-tuning range.2) The effects of experimental parameters on EIT. Taking example for A-type three-level atomic system, the theoretical model of EIT effect was given using semi-classical theory. The absorption and dispersion properties of EIT medium were also analyzed. We experimentally researched the relationship between EIT linewidth and experimental parameters for several laser linewidths in Rb vapor cells with and without buffer gas. When the coupling beam is weak, it is experimentally demonstrated that the EIT linewidth is proportional to the Rabi frequency of the coupling laser beam; reducing the laser linewidths of both coupling and probe laser can narrow the EIT linewidth; as the linewidths of the lasers approaches the dephasing rateγ31 for the two ground states, decreasingγ31 will obviously reduce the EIT linewidth. These conclusions are very useful for better understanding the effects of experimental parameters on atomic coherent process; better understanding the physical substance of EIT effect; and better applying the EIT effect.3) Realizing the light speed reduction (reduced to 8000m/s) and optical quantum storage in EIT medium. We have observed the reduction of group velocity of light via EIT effect and measured the dependence of pulse delay on the one-photon detuning in three-level A-type atomic system. The results show that due to the effect of Doppler broadening the light speed reduction is significant in a range of±600MHz one-photon detuning. This conclusion provides theoretical and experimental references for controlling the group velocity of light by means of one-photon detuning. On the basis of light speed reduction, we experimentally realized the optical quantum storage and release through turning on and turning off the coupling light smoothly. The storage time of light pulse in EIT medium can be up to about 100μs. By controllably turning on the retrieve control pulses at 795 or 780 nm to read the stored optical pulses in a four-level double A-type atomic system, we can obtain the released probe pulse at 795 or 780 nm, respectively. These readout pulses can be further separated spatially and directed into different optical propagation channels through a grating. Such controlled release of stored optical pulses may extend the capabilities of the quantum information storage technique, and can have application in multi-channel all-optical switching, all-optical routing.4) Realizing the optical induced polarization rotation via asymmetry in EIT. By choosing a properly polarized coupling field and transition energy levels, the symmetry of the atomic medium to the propagation of two orthogonal polarization components of a weak linearly-polarized probe field can be broken, which leads to a coherently controlled rotation of the probe field polarization. Proposing a method measuring the polarization rotation, which can eliminate the effect of absorption on polarization rotation. Because both the two orthogonal polarization components of the probe field form EIT with coupling field, we can get dramatic optical polarization rotation (with small circular dichroism) at near resonant frequency with relative low coupling power. A rotation angle of 45°can be realized when the coupling power is 15 mW. Polarization control can be achieved by coupling power, probe frequency detuning, and temperature. This scheme of optically induced polarization rotation can be used to realize some optical quantum devices, such as all-optical switch, atomic wave-plate et al.5) Multi-dark-state resonances in cold multi-Zeeman-sublevel atoms. Constructing a magneto-optical trap (MOT) for cooling and trapping the ⁸⁷Rb atoms. The number of cold atoms was measured by detecting the fluorescence emitted from cold atoms. Measuring the temperature of cold atomic cloud confined in the MOT by analyzing the absorption spectrum observed in the short-distance time-of-flight (TOF) method. The results show that the number of cold atoms is about 109, the density is about 10¹¹ /cm³ and the effective temperature of cold atomic cloud is about 200μK. A six-channel signal generator, based on the Labview 6.0 software and PCI-6713 DAQ card, was established to automatically control the cold atomic system. We present our experimental and theoretical studies of multi-dark-state resonances (MDSRs) generated in a unique cold rubidium atomic, system with only one coupling laser beam. Such MDSRs are caused by different transition strengths of the strong coupling beam connecting different Zeeman sublevels, and only can be observed in cold atoms. Controlling the transparency windows in such electromagnetically induced transparency system can have potential applications in multi-wavelength optical communication and quantum information processing.6) Preparation and determination of spin-polarized states in cold Multi-Zeeman-sublevel Atoms. Different methods for preparing and determining the spin-polarized states of atoms are reviewed. We point out that most of these techniques can not be used for cold atoms trapped in the MOT which is important for the experimental researches of quantum optics and quantum information processing. We demonstrate a simple, all-optical technique to prepare and determine the desired internal quantum states in multi-Zeeman-sublevel atoms. By choosing appropriate coupling and pumping laser beams, atoms can be easily prepared in a desired Zeeman sublevel with high purity. The population distributions or state purities of such prepared atomic states can be determined by using a weak, circularly-polarized probe beam due to differences in transition strengths among different Zeeman sublevels. This new technique will have potential impacts for quantum optics and quantum information processing in cold multi-level atomic systems.The characterized works among the above are as following:â… . In the work of light speed reduction in hot EIT medium, the pulse delay in EIT medium as a function of one-photon detuning of system is researched theoretically and experimentally.â…¡. Releasing the stored optical quantum information into two photonic channels under EIT condition was realized for the first time.â…¢. We propose and experimentally demonstrate that the optically induced polarization rotation can be realized in an asymmetry EIT system caused by a properly polarized coupling field for the first time. In this system, both of the two orthogonal polarization components of the probe field form EIT with coupling field, we can realize large polarization rotation (with small circular dichroism) with relative low coupling power. In the experiment, the rotation angle of 45°can be realized when the coupling power is 15 mW. The polarization rotation angle can be easily controlled through probe detuning, coupling power, and temperature. We proposed a new method measuring the polarization rotation caused by the changes of refractivity for the first time, which can eliminate the effect of absorption on polarization rotation using two pair of detector.â…£. The multi-dark-state resonances were experimentally observed in cold multi-Zeeman-sublevel atoms for the first time, which was caused by different transition strengths between different Zeeman sublevels, and only can be observed in cold atoms. Using semi-classic theory, we numerically fitted the experimental results.â…¤. By choosing appropriately polarized coupling and pumping laser beams, we realized the all-optical method to prepare the spin-polarized states of cold atoms, which can prepare cold atoms in any desired Zeeman sublevel with high purity. A directly all-optical method was proposed to measure the spin-polarized states for the first time.