Enhanced Nonlinearity Via Quantum Coherence At Low Intensities In Multi-level Atomic System

The electromagnetically induced transparency (EIT) effect induced by quantum interference in the interaction of atoms with light is one of important fields in quantum optics. Under the condition of EIT, the atomic medium exhibits unique optical properties: high transmission, steep dispersion and enhanced nonlinearity. Due to these unique properties, the EIT effect has been found applications in light speed reduction, optical quantum storage, generation of nonclassical light, realization of the entanglement between light and atoms. The EIT effect has attracted a great deal of attention and been studied extensively.The dissertation introduces the basic concepts on EIT effect and briefly reviews the history of research on EIT. Then it presents our main works which include light speed reduction, light storage, polarization rotation of a linearly polarized light and nonlinear effects in multi-level atomic medium.1) The light speed reduction and light storage in EIT medium are studied. Using the linewidth-narrowed laser, the group velocity of optical pulse is slowed down to 8000m/s in Rb cell with buffer gas. On the basis of light speed reduction, we experimentally realized the light storage and release by using dynamics process of EIT. The storage time of light pulse can be up to 100μs. By controllably turning on the retrieve control pulses at either 795 or 780nm to read the stored optical pulses in a four lever double A-type system, and further separating spatially these readout pulses through a grating, we realized a controllable releasing for stored light information into two separate photonic channels.2) The phenomenon of polarization rotation of a weak, linearly-polarized optical field is observed in a multi-level EIT system in rubidium atoms. By choosing circularly-polarized coupling beam to interact with atoms, the symmetry in number of EIT subsystems seen by the left- and right-circularly-polarized components of the weak probe beam can be broken, which makes the refrective indices of left- and right-circularly-polarized components of the probe beam different and leads to the polarization rotation of the probe beam. In the experiment, a large polarization rotation angle (up to 45 degrees) has been achieved with a coupling beam power of only 15mW. In addition, a new method to measure the polarization rotation angle is proposed, which can eliminate the influence of absorption on the measurement of polarization rotation angle and improve the measurement precision.3) By choosing left circularly-polarized probe, left circularly-polarized coupling beam and right circularly-polarized trigger beam to interact with the energy levels of D1 line in ⁸⁷Rb atoms, a tripod system is formed. We observed the two EIT windows induced by coupling and trigger beams respectively, and studied the enhanced EIT signal due to the interaction between the two EIT windows in the four-level tripod system. The two EIT dips produced by a strong coupling beam and a weak trigger beam are observed in the absorption spectrum of the probe field when the frequency detuning of trigger beam is different from that of the coupling beam. When the frequency detuning of trigger beam is near or equal to that of the coupling beam, the total depth of the EIT dip created by coupling beam clearly becomes larger. We have made numerical calculation for the enhanced EIT signal through semiclassical theory, these results are agreement with experimental results.4) The cross-Kerr nonlinear effect between two weak beams based on double EIT is investigated in a tripod system. We have observed the simultaneous EIT windows for probe and trigger fields (double EIT), and measured the cross-phase modulation (XPM) between the two fields using Mach-Zehnder interferometer. The experimental results show that the XPM coefficient of trigger beam is larger than 2×10^-5cm²/W when the accompanying transmissions of probe and trigger beams are higher than 60%.The characterized works among the above are as follows:â… . We realized a controllable releasing of stored optical pulses into two spatial optical channels. In a four level double A-type system, the light information stored in the EIT medium is released into optical pulse at two different wavelength by turning on the retrieve control pulses at either 795 or 780nm. These readout pulses are further separated spatially through a grating.â…¡. The phenomenon of optically induced polarization rotation is observed in an asymmetry EIT system. In the system, both of the two orthogonal polarized components of the probe field form EIT with coupling field, so the circular dichroism of the probe field is small. In the experiment, a large polarization rotation angle (up to 45 degrees) has been achieved with a coupling beam power of only 15mW. We have made detailed theoretical analyses and numerical calculation for the polarization rotation, which show that the asymmetry in the number of EIT subsystems for the two circularly probe components is the dominant mechanism to cause the polarization rotation of the linearly polarized probe beam.â…¢. A four-level tripod system is formed in the D1 line in ⁸⁷Rb atoms by choosing proper circularly-polarized probe, coupling and trigger beams. We observed the two EIT windows induced by coupling and trigger beams respectively, and studied the enhanced EIT signal due to the interaction between the two EIT windows in the four-level tripod system.â…£. In a four-level tripod system, the simultaneous EIT windows for probe and trigger fields (double EIT) are observed. The cross-phase modulation (XPM) between the probe and trigger fields are measured by using a Mach-Zehnder interferometer. The obvious XPMs are observed at low intensities, the XPM coefficient is larger than 2×10^-5cm²/W. The Mach-Zehnder interferometer is formed by using two beam displacing polarizers, and the optical path differences between the signal and reference beams are insensitive to the vibration of mirrors. So the measurements for cross-Kerr phase shifts are precision.