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Journal of the Chinese Chemical Society

Laser Induced Atomic Coherence and Interference in Cold Atoms

Min Yan, Edward G. Rickey, Yifu Zhu

Published Jun 1, 2001• 1 citation
Publisher

Abstract

Abstract We report experimental studies of electro magnetically induced transparency and enhancement of third‐order optical nonlinearities in A‐type atomic systems realized in 87Rb atoms cooled and confined in a magneto‐optical trap. We observed complete transparency of a weak probe laser in the A type Rb system with a moderate coupling field. With a third pump laser tuned to a fourth atomic state, the atomic system exhibits the interference‐enhanced third‐order susceptibility and only absorbs two photons, but not one photon. Our experiment demonstrates large nonlinear absorption of the weak probe laser, from which greatly enhanced Kerr nonlinearity in the Rb system is inferred.

DOI
10.1002/jccs.200100047

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Abstract We report experimental studies of electro magnetically induced transparency and enhancement of third‐order optical nonlinearities in A‐type atomic systems realized in 87Rb atoms cooled and confined in a magneto‐optical trap. We observed complete transparency of a weak probe laser in the A type Rb system with a moderate coupling field. With a third pump laser tuned to a fourth atomic state, the atomic system exhibits the interference‐enhanced third‐order susceptibility and only absorbs two photons, but not one photon. Our experiment demonstrates large nonlinear absorption of the weak probe laser, from which greatly enhanced Kerr nonlinearity in the Rb system is inferred.
We report an experimental observation of third-order nonlinear absorption by quantum interference in (87)Rb atoms cooled and confined in a magneto-optic trap. A coupling laser creates electromagnetically induced transparency (EIT) in a multilevel Rb system in which the third-order nonlinear absorption is enhanced by constructive quantum interference while the linear absorption is inhibited by destructive interference. Our experiment demonstrates the EIT system proposed by Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)], which absorbs two photons but not one photon in the dressed-state picture.
Summary form only given. A coupling laser can create electromagnetically induced transparency (EIT) in an absorbing medium such that linear absorption of a weak probe laser at the line center is suppressed by quantum interference. The quantum interference may be modified by adding a pump laser coupled to a separate state, which leads to enhanced third-order nonlinearities in the atomic system. Large nonlinear probe absorption at the EIT center is controlled by the pump laser and may be used to realize absorptive photon switching as proposed by Harris and Yamamoto. We report an experimental study of enhanced third-order nonlinear absorption and absorptive photon switching in /sup 87/Rb atoms cooled and confined in a magneto-optical trap.
We report experimental studies of electromagnetically induced transparency in Λ-type and ladder-type atomic systems realized in 87Rb atoms cooled and confined in a magneto-optical trap. Complete transparency is observed in the Λ-type system with a moderate coupling field. Comparison between the two systems reveals that at the line center of a weak probe transition, destructive interference occurs for the Λ-type system, whereas constructive interference occurs for the ladder-type system. We discuss conditions of complete transparency in the Λ-type system that contains hyperfine magnetic sublevels. Our experimental measurements for the two systems agree with theoretical calculations based on simple three-level Λ-type and ladder-type models.
Atomic coherence and interference in an atomic medium exhibiting electromagnetically induced transparency may lead to enhancement or suppression of nonlinear susceptibilities. Absorptive photon switching has been observed by constructive quantum interference, which is based on the enhanced third-order, nonlinear absorption in a four-level system. In a different four-level system, suppression of the two-photon absorption by destructive quantum interference has been observed. Experiments were carried out on 87Rb atoms cooled and confined in a magneto-optical trap and the experimental results agree with theoretical calculations of simple four-level model systems.
This dissertation reports experimental studies of nonlinear optical effects manifested by electromagnetically induced transparency (EIT) in cold Rb atoms. The cold Rb atoms are confined in a magneto-optic trap (MOT) obtained with the standard laser cooling and trapping technique. Because of the near zero Doppler shift and a high phase density, the cold Rb sample is well suited for studies of atomic coherence and interference and related applications, and the experiments can be compared quantitatively with theoretical calculations. It is shown that with EIT induced in the multi-level Rb system by laser fields, the linear absorption is suppressed and the nonlinear susceptibility is enhanced, which enables studies of nonlinear optics in the cold atoms with slow photons and at low light intensities. Three independent experiments are described and the experimental results are presented. First, an experimental method that can produce simultaneously co-propagating slow and fast light pulses is discussed and the experimental demonstration is reported. Second, it is shown that in a three-level Rb system coupled by multi-color laser fields, the multi-channel two-photon Raman transitions can be manipulated by the relative phase and frequency of a control laser field. Third, a scheme for all-optical switching near single photon levels is developed. The scheme is based on the phase-dependent multi-photon interference in a coherently coupled four-level system. The phase dependent multi-photon interference is observed and switching of a single light pulse by a control pulse containing ∼20 photons is demonstrated. These experimental studies reveal new phenomena manifested by quantum coherence and interference in cold atoms, contribute to the advancement of fundamental quantum optics and nonlinear optics at ultra-low light intensities, and may lead to the development of new techniques to control quantum states of atoms and photons, which will be useful for applications in quantum measurements and quantum photonic devices.
Summary form only given. Nonlinear optical properties of an atomic medium can be modified by coherence and quantum interference that utilizes electromagnetically induced transparency (EIT). Because of the vanishing linear absorption and large enhancement of nonlinearities in an EIT medium, nonlinear optics may be studied at low light levels. We report studies of light absorption and parametric amplification based on the EIT enhanced third-order nonlinearity in a four-level atomic system. We have observed the enhanced nonlinear absorption when two coupling lasers and one probe laser are present. We are in the process of preparing the experiments for the double EIT demonstration and observation of the efficient parametric amplification in the four-level system. Because the experimental system is essentially free of Doppler broadening, quantitative comparisons between the experimental measurements and the theoretical calculations can readily be achieved.