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ICTON Mediterranean Winter Conference (ICTON-MW)

Fast and slow light in thin gas cells

S. Ahmadi, Ghazal Rejali

Published Dec 1, 2009• 0 citations
Publisher

Abstract

We theoretically propose a strategy to use a travelling monochromatic wave in a simple two level atomic system to tune the group velocity over a wide range. Both subliminal and superluminal group velocity are demonstrated from the interaction of a light wave with atoms in the case where the cell length is much less than the light beam diameter. The tunable control of the light group velocity for the negative (superluminal light) to positive (subluminal light) are achieved in different physical conditions. In fact, we control the group velocity by tuning the relaxation parameters such as, the collision between atoms, the incident intensity and the beam diameter. This allows us to switch from subluminality to superluminality and back in a sub-Doppler system.

DOI
10.1109/ictonmw.2009.5385538

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We theoretically propose a strategy to use a travelling monochromatic wave in a simple two level atomic system to tune the group velocity over a wide range. Both subliminal and superluminal group velocity are demonstrated from the interaction of a light wave with atoms in the case where the cell length is much less than the light beam diameter. The tunable control of the light group velocity for the negative (superluminal light) to positive (subluminal light) are achieved in different physical conditions. In fact, we control the group velocity by tuning the relaxation parameters such as, the collision between atoms, the incident intensity and the beam diameter. This allows us to switch from subluminality to superluminality and back in a sub-Doppler system.
In this paper we demonstrate that the interaction of a traveling monochromatic light wave with atoms of a dilute gas in a cell whose length is much less than the diameter of the light beam can slow down the velocity of light. This slowing down is achieved by the specific transit relaxation of particles between walls of the cell (along the wave vector). For a two-level atomic system the group index is in the order of 103. The results show that such a slow down could be achieved by a single traveling light wave in a gas medium.
A four-level atomic system interacting with two strong drive fields and a weak probe field has been studied. We demonstrate that the group velocity of probe field can be controlled to switch from subluminal to superluminal or even negative values around the probe resonance without any significant absorption. The basic idea is to create a gain with slow light on the probe beam resonance with one drive field while the other drive creates a dip in the gain profile of probe leading to superluminal and negative group velocities. A simple switching from slow light to negative group velocity light can be obtained by changing the relative intensities of drive fields. We also describe a control to tune the position of slow light frequency continuously simply by adjusting the relative intensities of both the driving fields.
This work continues a theoretical investigation of the capabilities of the well-known method based on using a monochromatic probe light beam in combination with optical pumping of atoms (molecules) of a rarefied-gas medium by a broadband radiation in a thin cell the diameter of which is much larger than its internal thickness. In contrast to calculations carried out in the previous publications on this method of spectroscopy, here, we consider the case of arbitrary values of pump intensity and thickness of a cylindrical gas cell. Thus, all the possible mechanisms and specificities of velocity selection of atoms in optically excited levels caused by transit-time relaxation of such atoms in gas cells of this kind are analyzed. Within the framework of this approach, sub-Doppler absorption resonances of the probe light beam corresponding to quantum transitions from the upper level excited by optical pumping are investigated. The obtained results can be used in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization to established narrow spectral resonances.
We demonstrated superluminal light propagation with a negative group velocity of -5.7 m/s in a photorefractive Bi 12SiO20 crystal by using the dispersive phase coupling effect in a nondegenerate two-wave mixing process. To the best of our knowledge, this is the first experimental demonstration of superluminal light propagation at room temperature in solids by using a classical wave mixing technique. In addition, we showed the tunability of the group velocity of light between the negative (superluminal light) and the positive (subluminal light) by simply tuning the experimental conditions such as the frequency of the coupling beam, the incident intensity, and the externally applied electric fields.