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arXiv:quant-ph/9904031 · Physical Review Letters

Ultraslow Group Velocity and Enhanced Nonlinear Optical Effects in a Coherently Driven Hot Atomic Gas

Michael M. Kash, Vladimir A. Sautenkov, Alexander S. Zibrov, Leo W. Hollberg, George R. Welch, Mikhail D. Lukin, Yuri V. Rostovtsev, Edward S. Fry, Marlan O. Scully

Published Jun 28, 1999• 1,271 citations
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Abstract

We report the observation of small group velocities of order 90 m/s and large group delays of greater than 0.26 ms, in an optically dense hot rubidium gas ( $\ensuremath{\approx}360\mathrm{K}$). Media of this kind yield strong nonlinear interactions between very weak optical fields and very sharp spectral features. The result is in agreement with previous studies on nonlinear spectroscopy of dense coherent media.

DOI
10.1103/physrevlett.82.5229

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Physical Review Letters•Dec 20, 2001•888 citations
We report the observation of small group velocities of order 90 m/s and large group delays of greater than 0.26 ms, in an optically dense hot rubidium gas ( \ensuremath≈360K\ensuremath{\approx}360\mathrm{K}\ensuremath≈360K). Media of this kind yield strong nonlinear interactions between very weak optical fields and very sharp spectral features. The result is in agreement with previous studies on nonlinear spectroscopy of dense coherent media.
Summary form only given. Electromagnetically induced transparency (EIT) has been the subject of many experimental and theoretical studies over recent years. Most recently, substantial experimental and theoretical work in this area has been focused on the use of EIT to give ultraslow group velocities for light in both ultracold atomic gas and hot atomic vapors. Much of this interest is based on the fact that slowing the group velocity of a laser pulse down to the speed of sound in the same material can produce strong coupling between acoustic waves and the electromagnetic field. It has been suggested that a giant nonlinearity obtained by this method might be utilized for efficient multi-wave mixing and quantum nondemolition measurements. Slow light might also allow a very efficient nonlinear interaction between laser pulses of extremely low (down to a single photon) energies. Here, we report the first direct observation of an ultraslow group velocity of light down to 45 m/s in a solid, well below the speed of sound in the medium. This group velocity is fundamentally limited only by the inhomogeneous broadening of the ground state transition. At low coupling beam intensities, the fraction of centers that are pumped into the "dark state" increases with coupling laser power, so that the observed group delay is increasing with coupling beam intensity. At higher intensities, power broadening reduces the sharpness of the dispersive feature and the observed group delay decreases.
Coherent nonlinear effects such as Electromagnetically Induced Transparency\n(EIT), Coherent Population Trapping (CPT), and Slow light are studied in thermal\nRb vapor by both continuous and pulsed laser fields. This work primarily includes\nthree parts: (I) mode-locked rubidium laser and its applications (II) enhanced coupling\nbetween optical and sound waves in the forward direction via ultra-slow light\n(III) optical steering via ultra-slow light in rubidium vapor.\nIn part(I), I describe the construction and study of a mode-locked rubidium laser\noperating at the Rb D1 line using an active mode-locking technique inside the laser\ncavity. The mode-locked laser field is used to observe coherent effects in a dense\nrubidium gas.\nIn part(II), I experimentally demonstrate enhanced acoustic-optic coupling that\noccurs when the velocity of sound is close to the group velocity of light. Dragging of\nthe light by effective motion of the gas in a Rb cell is the origin of enhanced coupling.\nGood agreement between theory and experiment is found.\nIn part(III), I experimentally demonstrate optical beam deflection in coherently\ndriven rubidium vapor due to the steep refraction index profile in the region of EIT.
We experimentally study the group time delay for a light pulse propagating through hot 87Rb^{87}\mathrm{Rb}87Rb vapor in the presence of a strong coupling field in a $\ensuremath{\Lambda}$ configuration. We demonstrate that the ultraslow pulse propagation is transformed into superluminal propagation as the one-photon detuning of the light increases due to the change in the transmission resonance line shape. Negative group velocity as low as $\ensuremath{-}c∕{10}^{6}=\ensuremath{-}80\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$ is recorded. We also find that the advance time in the regime of the superluminal propagation grows linearly with increasing laser field power.
Properties of coherent media in the regime of \nelectromagnetically induced transparency (EIT) \nare studied. \nA study of the shape and width of the EIT resonance is presented \nfor coherent media with buffer gas. \nObservation of an absorption-like resonance for large one-photon detunings in a medium with \nbuffer gas and its properties are shown. \nThe regime of ``slow'' and ``fast'' group velocities are studied. \nObservation \nof narrow resonances with a phase broadened probe field is presented, \nand possible application of this regime are outlined.