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Journal of the Optical Society of America B

Generation of a continuous-wave cold molecular beam by using an optical velocity filter

Runqin Liu, Jianping Yin

Published Apr 25, 2011• 1 citation
Publisher

Abstract

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.

DOI
10.1364/josab.28.001252

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Physical Review A•Jul 1, 1998•107 citations
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.
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.
Abstract In order to generate one or two cold molecular beams that have neither a permanent electric dipole moment nor a magnetic dipole one, a controllable scheme to form an all-optical velocity filter and molecular beam splitter by using two red-detuned, crossing and cavity-enhanced guiding laser beams is proposed, and both the dynamic velocity filtering and beam splitting processes of the guided cold I 2 molecular beam are studied by using the three-dimensional Monte Carlo method. Our study shows that by adjusting the laser power difference between the two guiding laser beams from −124 W to 124 W, a splitting ratio of the two-arm output molecular beams from about 10.3% to 89.7% can be obtained. Also, by adjusting the intersection angle between the two standing wave cavities from 80° to 10°, an adjustable splitting ratio from about 1.6% to 98.4% can be obtained. If the intersection angle between the guided oblique beam and the guided straight beam is set to 80°, a cold I 2 molecular beam with a full-width at half-maximum longitudinal velocity of 1 m s −1 and a longitudinal temperature of 8 mK can be generated by using an all-optical velocity filter.
We generate a continuous-wave (cw) cold methyl cyanide (CH(3)CN) beam by using an L-shaped bent quadrupole electrostatic guide (i.e., by a low-pass energy or velocity filtering), and use a photo-ionized time-of-flight mass-spectrometer method to experimentally measure and study the dependences of the longitudinal and transverse temperatures of the guided CH(3)CN beam and its guiding efficiency on the guiding voltage. We find a new scaling law: the longitudinal and transverse temperatures (T(z),T(rho)) of the guided CH(3)CN beam are proportional to the guiding voltage (T(z),T(rho) proportional to V(guide)), and further verify another scaling law: the molecular guiding efficiency eta is proportional to the square of the guiding voltage (eta proportional to V2(guide)). We also obtain some simulated results consistent with our experimental ones. We also measure the divergent angle of the output molecular beam and study its dependence on the guiding voltage. Our study shows that when the guiding voltage is V(guide) = +/-1 kV, a cw cold CH(3)CN beam with a longitudinal temperature of approximately 500 mK and a transverse one of approximately 40 mK can be generated by our L-shaped electrostatic guide. The divergent angle of the output CH(3)CN beam is about 16.4 degrees as V(guide) = +/-4 kV. It is clear that such a resulting cold molecular beam has some important applications in the fields of cold molecular physics, physical chemistry and chemical physics, etc.
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 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.