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Physical Review A

Parametric amplification in an electromagnetically-induced-transparency medium

Ken-ichi Harada, Kenji Mori, Junji Okuma, Nobuhito Hayashi, Masaharu Mitsunaga

Published Jul 8, 2008• 19 citations
Publisher

Abstract

In an ordinary electromagnetically-induced-transparency (EIT) experiment using a hot sodium vapor, we have observed a probe gain up to 32 in the presence of a coupling beam in an otherwise totally opaque (transmissivity $\ensuremath{\sim}{10}^{\ensuremath{-}5}$) situation, increasing the transmissivity by 6 orders of magnitude. This gain is attributable to parametric amplification (PA), accompanying the Stokes (or anti-Stokes) wave with similar output intensity, and is critically dependent upon the atomic density and the coupling beam power. Experimental and theoretical analyses have revealed that, while the EIT picture is valid for relatively low atomic densities, the PA picture becomes dominant for high atomic densities and high coupling powers. We have also found that the parametric coupling between the probe and Stokes waves can be nullified by the proper choice of the coupling detuning frequency, due to destructive interference of two elementary processes.

DOI
10.1103/physreva.78.013809

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In an ordinary electromagnetically-induced-transparency (EIT) experiment using a hot sodium vapor, we have observed a probe gain up to 32 in the presence of a coupling beam in an otherwise totally opaque (transmissivity \ensuremath∼10\ensuremath−5\ensuremath{\sim}{10}^{\ensuremath{-}5}\ensuremath∼10\ensuremath−5) situation, increasing the transmissivity by 6 orders of magnitude. This gain is attributable to parametric amplification (PA), accompanying the Stokes (or anti-Stokes) wave with similar output intensity, and is critically dependent upon the atomic density and the coupling beam power. Experimental and theoretical analyses have revealed that, while the EIT picture is valid for relatively low atomic densities, the PA picture becomes dominant for high atomic densities and high coupling powers. We have also found that the parametric coupling between the probe and Stokes waves can be nullified by the proper choice of the coupling detuning frequency, due to destructive interference of two elementary processes.
We have theoretically and experimentally investigated coherent pump-probe spectra for the 3S1/23{S}_{1/2}3S1/2​-3P1/23{P}_{1/2}3P1/2​ D1D1D1 transition of sodium atomic vapor. Probe transmission spectra in the presence of a coupling beam exhibit dramatic changes depending on experimental conditions. In the weak-excitation, low-atomic-density limit, the spectra are mainly characterized by electromagnetically induced transparency (EIT) and saturated absorption, but for the strong-excitation, high-density case, parametric amplification (PA) is dominant, featuring high probe gain and Stokes-wave generation. We have developed a theory that can explain these two seemingly totally different phenomena (EIT and PA) within the same theoretical framework by manipulating a few experimentally controllable parameters, and have successfully reproduced the observed spectra. Other than the main spectral features, many other interesting physical processes have been predicted and observed.
We claim that the concept of electromagnetically induced transparency is only valid for low atomic densities, and for high densities stimulated Raman scattering is totally dominant, where the incident probe wave is not transmitted but is converted to the Stokes wave. This behavior can be explained by the large difference in linear absorption coefficients between the probe and the Stokes waves. We have verified this claim by the experiments using sodium atomic vapor samples with and without a buffer gas. Theoretical analyses using the Liouville-Maxwell equations and numerical simulations also supported our observations.
Transmissivity of up to 200% has been observed in an absorption imaging study of electromagnetically induced transparency in cold sodium atoms, when a focused coupling beam is applied near the edge of the atom cloud. The signal behavior strongly depends on the frequency difference between the probe beam and the coupling beam. Such high transmissivity can be attributed to the strong focusing effect of the probe beam, when it propagates in a medium with spatially inhomogeneous refractive index induced by the coupling beam. A detailed numerical analysis based upon the Maxwell propagation equations is presented.
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.