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Chinese Physics B

Laser guiding of cold molecules in a hollow optical fiber and continuous-wave cold molecular beam generation

Runqin Liu, Yaling Yin, Jianping Yin

Published Mar 1, 2012• 4 citations
Publisher

Abstract

A novel scheme for guiding arbitrary buffer-gas cooled neutral molecules in a hollow optical fiber (HOF) using a red-detuned HE 11 mode is proposed and analysed theoretically. We give the electromagnetic field distribution of the HE 11 mode in the HOF and calculate the optical potential of an I 2 molecule, and study the molecule guiding mechanism using a classical Monte Carlo simulation. Using a 6 kW input laser, an S-shape HOF with a 2 cm curvature radius for both bends, and an input molecular beam with a transverse temperature of 0.5 K and longitudinal temperature of 5 K, we obtain a guiding efficiency of ∼0.126% for the scheme, and the transverse and longitudinal temperatures of the guided molecular beam are 1.9 mK and 0.5 K, respectively.

DOI
10.1088/1674-1056/21/3/033302

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A novel scheme for guiding arbitrary buffer-gas cooled neutral molecules in a hollow optical fiber (HOF) using a red-detuned HE 11 mode is proposed and analysed theoretically. We give the electromagnetic field distribution of the HE 11 mode in the HOF and calculate the optical potential of an I 2 molecule, and study the molecule guiding mechanism using a classical Monte Carlo simulation. Using a 6 kW input laser, an S-shape HOF with a 2 cm curvature radius for both bends, and an input molecular beam with a transverse temperature of 0.5 K and longitudinal temperature of 5 K, we obtain a guiding efficiency of ∼0.126% for the scheme, and the transverse and longitudinal temperatures of the guided molecular beam are 1.9 mK and 0.5 K, respectively.
We propose a promising scheme to guide buffer-gas cooled arbitrary neutral molecules in a hollow-core photonic bandgap (HC-PBG) fiber by using a red-detuned Gaussian mode and calculate the optical coupling efficiency of the Gaussian mode in the HC-PBG and its optical potential for I2 molecules. Also, we calculate both the straight guiding efficiency and bend guiding one by using a classical model and the dynamic process of the laser guiding of cold molecules in the HC-PBG by Monte Carlo simulation, by which we obtain the transverse and longitudinal velocity distributions of the output guided molecular beam. Our study shows that when the input laser power is 2 kW, the straight guiding efficiency is ~24.6% and when the input laser power is 200 W and R=0.2 cm, the bent guiding efficiency is ~1.2%. We also find from the simulated results that our laser bent guiding scheme is a simple and desirable method to generate a CW cold arbitrary molecular beam.
We propose a novel scheme to generate a cw cold molecular beam by optically guided buffer-gas-cooled molecules around an S-shaped integrated fiber bundle. First we calculate the two-color evanescent field around the fiber bundle and its optical potential for I2 molecules. Then we estimate the guiding efficiency of the output molecular beam and its transverse and longitudinal temperatures based on a classical model. Finally, we study the dynamic process of the molecular guiding by using a classical Monte Carlo simulation, by which we obtain some simulated results consistent with the theoretically calculated results. Our study shows that when the power of the input CO2 laser is 10 W and the curvature radius of the fiber bundle is 0.2 cm, a cw cold molecular beam with a longitudinal and transverse temperature of 57 mK and 0.29 mK can be generated by our scheme. It is clear that such a cold molecular beam has some important applications in molecular optics and other fields.
We propose and analyze theoretically a scheme for atomic guiding and cooling in a dark hollow laser beam generated from the collimated output beam of a LP01{\mathrm{LP}}_{01}LP01​ mode in a micrometer-sized hollow optical fiber. In the scheme, cold atoms from a magneto-optical trap are loaded into the blue-detuned dark hollow beam, move down in the dark hollow beam, and experience transverse Sisyphus cooling induced by the dark hollow beam and a weak repumping beam. In the longitudinal direction, the guided atoms experience heating from the gravity field and cooling from the upward-propagating dark hollow beam and repumping beam. We estimate the transverse two-dimensional equilibrium temperature, the final longitudinal mean velocity, and the total loss of the guided atoms. Our calculations show that a transverse equilibrium temperature of $\ensuremath{\sim}1.2\ensuremath{\mu}\mathrm{K},$ a final longitudinal mean velocity of 0--4.41 m/s, and a guiding efficiency of 65--95 % may be obtained.
In this paper, we propose a new scheme to guide cold atoms (or cold molecules) using a bundle of four single-mode optical fibers with a sub-micrometer size, and calculate the evanescent-wave field in the hollow region of four-fiber bundle and its optical potential for 85Rb atoms. Our study shows that such a blue-detuned hollow evanescent-wave field can also be used to realize the laser waveguide of cold atoms (or cold molecules), and, compared with the traditional atomic guiding scheme using a hollow optical fiber, our scheme is not only simple and cheap, but also easier to realize the single-mode waveguide of cold atomic matter-wave with high efficiency.
We propose a new scheme to guide cold atoms (or molecules) using a blue-detuned TE(01) doughnut mode in a hollow metallic waveguide (HMW), and analyze the electromagnetic field distributions of various modes in the HMW. We calculate the optical potentials of the TE(01) doughnut mode for three-level atoms using dressed-atom approach, and find that the optical potential of the TE(01) mode is high enough to guide cold atoms released from a standard magneto-optical trap. Our study shows that when the input laser power is 0.5W and its detuning is 3GHz, the guiding efficiency of cold atoms in the straight HMW with a hollow radius of 15 microm can reach 98%, and this guiding efficiency will be almost unchanged with the change of curvature radius R of the bent HMW as R > 2cm, which is a desirable scheme to do some atom-optics experiments or realize a computer-controlled atom lithography with an arbitrary pattern. We also analyze the losses of the guided atoms in the HMW due to the spontaneous emission and background thermal collisions and briefly discuss some potential applications of our guiding scheme in atom and molecule optics.
We present a general theory of atom guiding in a blue-detuned hollow laser beam. Using the dressed-atom approach, we obtain the mean dipole gradient force, the radiation pressure force, and the momentum diffusion coefficients for three-level Λ-type cold atoms. Using Monte Carlo simulation, we calculate the guiding efficiencies and the final velocity distributions of atoms for various conditions. We find that the guiding efficiency depends not only on the intensity and detuning of the guiding hollow beam but also on the atom-guiding direction with respect to the propagation direction of the hollow laser beam. Comparing our analyses with recent experimental results, we find that they are mutually consistent. The results that we present can also be applied to atom guiding by hollow optical fibers.