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Journal of the Optical Society of America B

Laser guiding of cold molecules in a hollow photonic bandgap fiber

Runqin Liu, Qi Zhou, Yaling Yin, Jianping Yin

Published Apr 17, 2009• 3 citations
Publisher

Abstract

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.

DOI
10.1364/josab.26.001076

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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.
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 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 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.
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.
A THz optical modulator for a continuous-wave laser using a hollow-core photonic crystal fiber (HC-PCF) was proposed and theoretically analyzed. Lightwaves traveling through the HC-PCF is modulated by interactions with coherently driven Raman-active molecules in the core. The coherent molecular motion is excited by a pulse train having an interval between successive pulses shorter than the molecular dephasing time. In principle, a rotational transition of molecular hydrogen (S 0 (1)) at a pressure of 1 atm has a long enough dephasing time to maintain molecular coherence during a 1 GHz commercially available mode-locked pulse train. Optimization of the waveguide dispersion would enable phase-matching between the probe beam and generated sidebands during optical modulation. The proposed scheme would be achievable with a reasonable pump beam power and HC-PCF length, and with a feasible pressure of molecules in the core.
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.