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Journal of Physics B Atomic Molecular and Optical Physics

An all-optical velocity filter and beam splitter for generating cold molecular beams: a proposal and simulation

Xingjia Li, Runqin Liu, Jianping Yin

Published Nov 13, 2015• 1 citation
Publisher

Abstract

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.

DOI
10.1088/0953-4075/48/24/245305

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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 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.
\nA beam-splitter for cold atoms is realised with two dipole guides which intersect. Each guide is created by an intense laser beam produced by a Nd:YAG laser. The cold atom cloud, introduced in one guide via the gravity field, is split into two parts when it passes through the intersection region of the guides. The laser frequency is so far detuned from the rubidium atomic resonance that guiding and splitting are obtained without any losses. The coupling in the guide intersection is strong enough to realise an output angle of 7° and an efficiency of 40%. With the beam-splitter a macroscopic separation of the atomic packets larger than one millimetre is demonstrated at a distance of about 5 mm after the intersection. Furthermore the split atomic clouds are guided in each arm of the beam-splitter. Details about the guides (realisation and performance) and an application using a concave atomic mirror are also given in order to describe the beam-splitter and its use.\n
Summary form only given. The widespread use of magneto-optical traps (MOT) and the development of BEC over the last 10 years have led to the field of atom optics. This field requires the use of equivalent elements to optical mirrors, lenses and beam splitters. The generation of a large deviation atomic beam splitter is one of the most important goals, as it allows the construction of atom interferometers. Optical methods for achieving these elements are enticing, as the use of laser beams would make it easier to alter parameters of your guides within a vacuum chamber. Recent work has demonstrated the use of optical beams to guide and split atoms from a cold atom source. Here we present novel experimental studies of the optical guiding (in both the red- and blue-detuned regime) of a low-velocity intense source (LVIS) of atoms emanating from a rubidium MOT.
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.
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.