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arXiv:1311.0635 · Optics Letters

One-dimensional ultracold medium of extreme optical depth

Frank Blatt, Thomas Halfmann, Thorsten Peters

Published Jan 16, 2014• 47 citations
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Abstract

We report on the preparation of a one-dimensional ultracold medium in a hollow-core photonic crystal fiber, reaching an effective optical depth of 1000(150). We achieved this extreme optical depth by transferring atoms from a magneto-optical trap into a far-detuned optical dipole trap inside the hollow-core fiber, yielding up to 2.5(3)×10(5) atoms inside the core with a loading efficiency of 2.5(6)%. The preparation of an ultracold medium of such huge optical depth paves the way toward new applications in quantum optics and nonlinear optics.

DOI
10.1364/ol.39.000446

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arXiv:1107.4746•Physical Review A•Nov 7, 2011•13 citations
We report on the preparation of a one-dimensional ultracold medium in a hollow-core photonic crystal fiber, reaching an effective optical depth of 1000(150). We achieved this extreme optical depth by transferring atoms from a magneto-optical trap into a far-detuned optical dipole trap inside the hollow-core fiber, yielding up to 2.5(3)×10(5) atoms inside the core with a loading efficiency of 2.5(6)%. The preparation of an ultracold medium of such huge optical depth paves the way toward new applications in quantum optics and nonlinear optics.
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.
We describe the loading of laser-cooled rubidium atoms into a single-mode hollow-core photonic-crystal fiber. Inside the fiber, the atoms are confined by a far-detuned optical trap and probed by a weak resonant beam. We describe different loading methods and compare their trade-offs in terms of implementation complexity and atom-loading efficiency. The most efficient procedure results in loading of  30~30 30,000 rubidium atoms, which creates a medium with an optical depth of  180~180 180 inside the fiber. Compared to our earlier study [1] this represents a sixfold increase in the maximum achieved optical depth in this system.
We demonstrate the first guiding of cold atoms through a 88 mm long piece of photonic band gap fiber. The guiding potential is created by a far-off resonance dipole trap propagating inside the fiber with a hollow core of 12 μm. We load the fiber from a dark spot 85 Rb magneto-optical trap and observe a peak flux of more than 10 5 atoms s −1 at a velocity of 1.5 m s −1 . With an additional reservoir optical dipole trap, a constant atomic flux of 1.5 × 10 4 atoms s −1 is sustained for more than 150 ms. These results open up interesting possibilities to study nonlinear light–matter interaction in a nearly one-dimensional geometry and pave the way for guided matter wave interferometry.
We present a thorough experimental investigation of the loading process of laser-cooled atoms from a magneto-optical trap into an optical dipole trap located inside a hollow-core photonic bandgap fiber, followed by propagation of the atoms therein. This, e.g., serves to identify limits to the loading efficiency and thus optical depth which is a key parameter for applications in quantum information technology. Although only limited access in 1D is available to probe atoms inside such a fiber, we demonstrate that a detailed spatially resolved characterization of the loading and trapping process along the fiber axis is possible by appropriate modification of probing techniques combined with theoretical analysis. Specifically, we demonstrate the loading of up to $2.1\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms with a transfer efficiency of 2.12.1%2.1 during the course of 50 ms and a peak loading rate of $4.7\ifmmode\times\else\texttimes\fi{}{10}^{3}$ atoms ${\mathrm{ms}}^{\ensuremath{-}1}$ resulting in a peak atomic number density on the order of ${10}^{12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Furthermore, we determine the evolution of the spatial density (profile) and ensemble temperature as it approaches its steady-state value of $T=1400\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$, as well as loss rates, axial velocity and acceleration. The spatial resolution along the fiber axis reaches a few millimeters, which is much smaller than the typical fiber length in experiments. We compare our results to other fiber-based as well as free-space optical dipole traps and discuss the potential for further improvements.
We theoretically investigate the process of coupling cold atoms into the core of a hollow-core photonic-crystal optical fiber using a blue-detuned Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam to couple the atoms, the blue-detuned hollow beam can confine cold atoms to the darkest regions of the beam, thereby minimizing shifts in the internal states and making the guide highly robust to heating effects. This single optical beam is used as both a funnel and a guide to maximize the number of atoms into the fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical trap (MOT) above a vertically oriented optical fiber. We observe a gravito-optical trapping effect for atoms with high orbital momentum around the trap axis, which prevents atoms from coupling to the fiber: these atoms lack the kinetic energy to escape the potential and are thus trapped in the laser funnel indefinitely. We find that by reducing the dipolar force to the point at which the trapping effect just vanishes, it is possible to optimize the coupling of atoms into the fiber. Our simulations predict that by using a low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a 20-$\ensuremath{\mu}$m-radius core hollow fiber, it is possible to couple 11% of the atoms from a MOT 9 mm away from the fiber. When the MOT is positioned farther away, coupling efficiencies over 50% can be achieved with larger core fibers.