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Research in Optical Sciences

Nonlinear Process in Atomic Coherent System

Junxiang Zhang

Published Jan 1, 2012• 0 citations
Publisher

Abstract

The quantum coherence of Electromagnetically Induced Transparency (EIT), and Electromagnetically Induced Absorption (EIA) attracted a lot attention in nonlinear process, generation of squeezing, and long-distance communication. EIT and EIA are two opposite coherence effects, EIT can result in efficient nonlinear effects due to the transparency, and EIA may prohibit atoms from generation of nonlinearity due to its absorption, however, the nonlinear in EIA system with standing wave coupling can still be observed, and was interpreted as Electromagnetically Induced Absorption Grating (EIG). We experimentally investigated the coherence in hot atoms, and observed high efficiency of emission in EIA system in both of the cases that Bragg condition is satisfied or can’t be fulfilled. Based on our calculation, the reflection is explained as the result of Four-Wave Mixing, and is enhanced by the quantum coherence and the compensation of phase mismatch from anomalous dispersion of EIA.

DOI
10.1364/hilas.2012.jt2a.24

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The quantum coherence of Electromagnetically Induced Transparency (EIT), and Electromagnetically Induced Absorption (EIA) attracted a lot attention in nonlinear process, generation of squeezing, and long-distance communication. EIT and EIA are two opposite coherence effects, EIT can result in efficient nonlinear effects due to the transparency, and EIA may prohibit atoms from generation of nonlinearity due to its absorption, however, the nonlinear in EIA system with standing wave coupling can still be observed, and was interpreted as Electromagnetically Induced Absorption Grating (EIG). We experimentally investigated the coherence in hot atoms, and observed high efficiency of emission in EIA system in both of the cases that Bragg condition is satisfied or can’t be fulfilled. Based on our calculation, the reflection is explained as the result of Four-Wave Mixing, and is enhanced by the quantum coherence and the compensation of phase mismatch from anomalous dispersion of EIA.
Summary form only given. It is well-known that nonlinear interference effects in the resonant atom-light interaction can lead to interesting and important phenomena, such as electromagnetically induced transparency (EIT) of atomic medium, coherent population trapping, lasing without inversion, and others. Common to all these phenomena is the appearance of light-induced coherence between atomic levels, which are not coupled by dipole transitions. Recently Akulshin and co-workers (1998-9) have observed subnatural-width resonances in the absorption on the D/sub 2/ line of rubidium vapor under excitation by two copropagating optical waves with variable frequency offset. It is remarkable that, apart from expected EIT-resonances with negative sign, authors detected positive resonances termed as electromagnetically induced absorption (EIA). More recently, similar effects in a (3+5)-state system have been described as a result of constructive interference of the dipole excitation channels. We propose a simple theoretical model for EIA - a four-state N-atom. Remember that three-state /spl Lambda/- and V-systems give the EIT-resonances only.
We have measured electromagnetically induced transparency (EIT) and four wave mixing (FWM) signals in a three level atomic system to enhance nonlinearity for a weak field due to quantum coherence with standing fields. We were able to observe quite different EIT signals according to a traveling or standing coupling field. (1) We obtained EIT and FWM signals depending on detunings, polarization, (2) and power of the coupling field. Our results will be analyzed theoretically in viewpoint of a photonic band gap model.
In a Doppler-broadened ladder-type Rubidium atomic system [Formula: see text]–[Formula: see text]–[Formula: see text], three-photon electromagnetically induced absorption (EIA) and transparency (EIT) in two schemes of two coupling lasers with different propagation directions are comprehensively studied. Two coupling fields are entangled tightly with each other and induce constructive interference such that we observe EIA as two coupling fields are counter-propagating. Two coupling fields are independent and cause destructive interference such that only enhanced EIT occurs when two coupling fields are co-propagating, although all of the Doppler-broadened atoms satisfy the condition for three-photon resonance. The experimental results are coincided with the numerically calculated spectra using the dressed perturbation method. The mechanisms of EIT and EIA could also be understood by the dressed-state pictures.
A nonlinear optical four-wave mixing scheme is presented and analysed for the generation of coherent light in a nearly four-level ladder-type atomic system in the context of electromagnetically induced transparency (EIT). We find that EIT can suppress nonlinear photon absorption and the peak of the generated mixing field is located at the centre of the transparency window where the loss is minimal, though there is a dip in the centre. Such a nonlinear optical process can also be used for generating coherent short-wavelength radiation.
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.
A thermal vapor of three-level atoms irradiated by two copropagating laser beams of suitable frequencies exhibits the phenomenon of coherent population trapping, leading to an electromagnetically induced transparency (EIT). When a mirror reflects the beams back onto themselves one finds that depending on the position along the resulting standing wave the fluorescence intensity from the sample (cesium in our case) decreases (EIT) or even increases [electromagnetically induced absorption (EIA)] with a period on the centimeter scale. An intuitive picture is based on the interference of coherent dark states but the explanation of the EIA effect requires consideration of Doppler effects in the thermal vapor. This allows for a quantitative comparison between experimental and calculated results.