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Journal of Physics Conference Series

Nonlinear superluminality in cold atoms confined in hollow core photonic crystal fiber

Sh. Petrosyan, Yu. P. Malakyan

Published Mar 14, 2012• 0 citations
Publisher

Abstract

We propose a scheme for superluminal propagation of a linearly polarized smooth light pulse in cold atoms trapped inside a hollow-core fiber with intensity-dependent negative group velocity. The tight confinement of the atoms inside the fiber core prevents atom-wall collisions, while the Doppler width is negligible as compared to atomic decay rate. We show that with initially prepared Zeeman coherence, the absorption of light is smaller enough to be ignored, while due to nonlinear dependence on laser intensity more intense parts of the pulse travel slower leading to the formation of the extremely sharp trailing edge and to the lengthening of the leading edge of the pulse. Such a behavior is contrast to the case of nonlinear propagation of the pulse in a medium with normal dispersion where a shock wave is generated. It is highly important that with 10 4 atoms inside the fiber the large nonlinearities in the medium refractive index can be achieved at extremely low light levels.

DOI
10.1088/1742-6596/350/1/012030

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We propose a scheme for superluminal propagation of a linearly polarized smooth light pulse in cold atoms trapped inside a hollow-core fiber with intensity-dependent negative group velocity. The tight confinement of the atoms inside the fiber core prevents atom-wall collisions, while the Doppler width is negligible as compared to atomic decay rate. We show that with initially prepared Zeeman coherence, the absorption of light is smaller enough to be ignored, while due to nonlinear dependence on laser intensity more intense parts of the pulse travel slower leading to the formation of the extremely sharp trailing edge and to the lengthening of the leading edge of the pulse. Such a behavior is contrast to the case of nonlinear propagation of the pulse in a medium with normal dispersion where a shock wave is generated. It is highly important that with 10 4 atoms inside the fiber the large nonlinearities in the medium refractive index can be achieved at extremely low light levels.
Cold atoms confined inside a hollow core photonic crystal fiber with core diameter of a few photon wavelengths are a promising medium for studying nonlinear optical interactions at extremely low light levels. The high electric field intensity per photon, large optical depths, and interaction lengths not limited by diffraction are some of the unique features of this system. We describe recent progress in our experiment that uses a combination of magnetic trapping and a red-detuned optical dipole trap to transfer cold rubidium atoms into the hollow-core fiber. We present data on transfer efficiencies as well as preliminary experiments towards nonlinear optics with few photon pulses.
Cold atoms confined inside a hollow-core photonic-crystal fiber with core diameters of a few photon wavelengths are a promising medium for studying nonlinear optical interactions at extremely low light levels. The high electric field intensity per photon and interaction lengths not limited by diffraction are some of the unique features of this system. Here, we present the results of our first nonlinear optics experiments in this system including a demonstration of an all-optical switch that is activated at energies corresponding to few hundred optical photons per pulse.
We investigate a light-pulse propagation with negative group velocity in a nonlinear coherent medium. We show that the necessary conditions for nonlinear superluminal effects to be observable are realized in a three-level $\ensuremath{\Lambda}$ system interacting with a linearly polarized laser field in the presence of a static magnetic field. The initially prepared Zeeman coherence cancels the resonant absorption of the medium almost completely, but preserves its dispersion anomalous and very high. In this medium the light pulse propagates superluminally in the sense that the peak of the transmitted pulse exits the medium before the peak of the incident pulse enters. At the same time the pulse group velocity is intensity dependent, which leads to the self-steepening of the pulse trailing edge due to the fact that the more intense parts of the pulse travel slower. This is in contrast with the shock wave generation in a nonlinear medium with normal dispersion. The predicted effect can be easily observed in the well-known schemes employed in the studies of nonlinear magneto-optical rotation. The upper bound of sample length is found from the criterion that the pulse self-steepening and group-advance time are observable without pulse distortion caused by the group-velocity dispersion.
Our goal is to study quantum optical phenomena in a strong nonlinear optical system. For this purpose, we create a hollow-core photonic-crystal (HCPC) fiber-based platform that confines photons and atomic ensembles together. This enables strong light-matter interactions which are crucial for implementing optical nonlinearities at single-photon levels [1]. We use an optical dipole trap at cesium's magic wavelength to guide laser cooled cesium atoms into and inside the fiber core. This approach will allow us to perform experiments with the dipole trap continuously on as opposed to the previously reported fiber-loading systems [2,3] in which the dipole trap was rapidly amplitude-modulated to avoid the effect from the spatially dependent AC Stark shift. In this system, we perform experiments on electromagnetically induced transparency (EIT) and demonstrate slow light and all optical switch. We also aim to observe large cross-phase modulation (XPM) using a three-level ladder scheme [4,5]. Lastly, the approaches to integrate a cavity into HCPCFs to increase light-matter interactions, thus the optical nonlinearities, will be described.