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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Acceleration, deceleration, and separation of molecular ensembles in optical lattices

Mikhail N. Shneider, P. F. Barker

Published Sep 20, 2004• 8 citations
Publisher

Abstract

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.

DOI
10.1117/12.548827

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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.
We investigate the dynamics of untrapped molecules within a far-off resonant accelerating optical lattice. Our analysis shows that untrapped molecules can be temporarily transported by the lattice, and those that are transported for the longest time reach a unique, well-defined critical velocity that depends on the mass to polarizability ratio of the molecular species. We show that this species-dependent critical velocity leads to a velocity dispersion for different species within a gas mixture. Our numerical simulations show that the velocity distribution of a multicomponent gas evolves to form well-separated peaks in velocity space for each species. We propose a time-of-flight analysis technique that transforms the velocity dispersion to a temporal separation of different species, even for small differences in the mass to polarizability ratio. Separation utilizing this concept is demonstrated for atmospheric species and isotopes of nitrogen. Finally, we present an extension of this concept for both temporal and angular dispersion.
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.
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/.
We study the transfer of energy from an optical potential to atomic and molecular gases and demonstrate that this process is analogous to collisionless Landau damping of electrostatic potentials in plasmas and gravitational potentials observed on astrophysical scales. We show that a signficant fraction of the light attenuation within a cavity can be attributed to this mechanism when the cavity is filled with a gas at high density. The resulting motion of particles created by optical Landau damping can be used to induce transport when a periodic potential produced by two counterpropagating high-intensity pulsed optical fields is used. Bulk drift of the gas also appears feasible even when the mean kinetic energy is much greater that the maximum optical potential.
We describe the deceleration of nitric oxide, benzene and xenon atoms in a molecular beam using one-dimensional pulsed optical lattices created by fields with intensities in the 1012 W cm−2 range. We show that for the same pulse duration and lattice intensity the velocity of the molecules can be controlled by tailoring the lattice velocity. By utilizing the time-dependent oscillatory motion of the molecules within the lattice, we demonstrate the deceleration of nitric oxide from an initial velocity of 400 m s−1 to a final velocity of 290 m s−1 in a single 5.8 ns pulse. Using higher intensities, we measure the deceleration of benzene molecules from 380 m s−1 to 191 m s−1, representing a 75% reduction in the kinetic energy within the lattice over the same duration.
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.