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Quantum Electronics and Laser Science Conference

An optical micro-linear accelerator for molecules and atoms

P. F. Barker, M. Shneyder

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. Acceleration of molecules within a time varying electric field produced by an optical traveling wave has been proposed as a means to accelerate neutral particles to high velocity. This technique is attractive because extremely large Stark forces can be produced by the high electric field gradients that can be created within an optical traveling wave. The electrodeless electric field gradient produced by focused laser beams can be orders of magnitude greater than electrostatic gradients, allowing acceleration of polar as well as polarizable molecules and atoms. This concept has already been used to accelerate ultracold atoms in a very weak optical periodic potentials, or lattices, and acceleration up to the velocities in the m/s velocity was observed. We report on the study of acceleration of polarizable particles to velocities in excess of 10 km/s range in an accelerated optical lattice, using high intensity pulsed fields.

DOI
10.1109/qels.2001.962101

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Summary form only given. Acceleration of molecules within a time varying electric field produced by an optical traveling wave has been proposed as a means to accelerate neutral particles to high velocity. This technique is attractive because extremely large Stark forces can be produced by the high electric field gradients that can be created within an optical traveling wave. The electrodeless electric field gradient produced by focused laser beams can be orders of magnitude greater than electrostatic gradients, allowing acceleration of polar as well as polarizable molecules and atoms. This concept has already been used to accelerate ultracold atoms in a very weak optical periodic potentials, or lattices, and acceleration up to the velocities in the m/s velocity was observed. We report on the study of acceleration of polarizable particles to velocities in excess of 10 km/s range in an accelerated optical lattice, using high intensity pulsed fields.
Analysis of the trapped and untrapped motion of particles, combined with direct numerical simulation of the 1D non-stationary Boltzmann equation, demonstrates that atoms and molecules initially at room temperature can be accelerated to velocities in the 10 to 100 km/s range over distances of 100's microns using an optical lattice. The quantity, final velocity, and velocity/energy spread of the accelerated distribution controlled by tailoring the fluence, duration and frequency chirp of the laser beam that make the lattice, indicating the potential for a compact, versatile, and configurable hyper-thermal accelerator/decelerator. Using similar lattice potentials, differential transport of atomic and molecular species with different polarizability/mass ratios leads towards the possibility of gas mixture separation in short pulses or in hollow fibers. A bulk drift can be induced in a gas by the lattice forces, even when the mean kinetic energy is much greater that the maximum dipole potential of the optical field. In this process the transfer of energy from the slowly traveling optical lattice to the gas is analogous to Landau damping of a plasma wave by charged particles.
This work presents the proposed schemes for decelerating cold molecules created in pulsed supersonic expansions using pulsed optical lattices. The first scheme relies on the application of a decelerating optical lattice with a dipole-potential well depth of approximately 1 K, created by rapidly chirping medium-intensity fields in the 10 W/cm/sup 2/ range. This scheme is an optical analog of the Start decelerator that has been successfully used to slow a range of polar molecules. A significant fraction (about 10%) of the very heavy I/sub 2 /molecules in an Ar buffer gas is shown to be slowed by using this method over sub-microsecond time scales. In the second technique, an optical lattice with larger well depth in the 100 K range travelling at half the supersonic beam velocity is used to trap a significant fraction of the cold, high-velocity molecules. This technique is developed based on the property of periodic motion of the trapped molecules in the optical lattice; the molecules reverse their initial (relative) velocities in the lattice reference frame after half a period. Using this method the cold molecules can be transferred from high speed to zero velocity on nanosecond time scales. As an example, 33% of a CO molecule beam (1 K) with a velocity of 230 m/s is slowed utilising optical intensities of less than 10/sup 12/ W/cm/sup 2/.
The creation of directed fluxes of positronium atoms using the optical dipole force from short, pulsed travelling optical lattices has been studied. By exploiting the favourable polarizability-to-mass ratio of positronium it has been found that accelerations in excess of 10 14 g are possible. Simulations have been performed for several cases in which lattice beams capture a substantial fraction of a cloud of ground state ortho-positronium atoms, assumed to be characterised by an initial temperature of 300 K. We show that, using conventional laser sources, bunches of positronium atoms can be accelerated, well within the ortho-positronium vacuum lifetime of 142 ns, to a wide variety of kinetic energies in the eV to 100s of eV range and with well-defined energy spreads. The final kinetic energy of the positronium can be tuned using the laser intensity, the pulse duration or the rate of change of the frequency between the two beams which form the lattice.
A method of slowing, accelerating, cooling, and bunching molecules and neutral atoms using time-varying electric-field gradients is demonstrated with cesium atoms in a fountain. The effects are measured and found to be in agreement with our calculations. Time-varying electric-field-gradient slowing and cooling is applicable to atoms that have large dipole polarizabilities, including atoms that are not amenable to laser slowing and cooling, to Rydberg atoms, and to molecules, especially polar molecules with large electric dipole moments. The possible applications of this method include slowing and cooling thermal beams of atoms and molecules, launching cold atoms from a trap into a fountain, and measuring atomic dipole polarizabilities.
The deceleration of molecules from cold molecular beams using electric and magnetic fields has become an important means for producing stationary, cold dipolar or paramagnetic molecular gases that can be trapped and potentially cooled to ultra-cold temperatures in the μK range. We report on a general scheme for the creation of cold molecules of essentially any type by deceleration of a cold molecular beam using intense optical fields. Deceleration of benzene molecules to zero velocity is achieved by utilizing a single half oscillation of the center-of-mass motion of the molecules within a moving optical lattice. The lattice traveling at half the speed of the molecular beam is created by the interference of two near-counter-propagating fields at different frequencies. We show that by rapidly switching on the optical lattice for approximately the time required for a half oscillation, a bunch of approximately 10 5 benzene molecules can be rapidly decelerated and brought to rest in the laboratory frame.