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Optics Letters

Four-beam laser trap of neutral atoms

Fujio Shimizu, Kazuko Shimizu, Hiroshi Takuma

Published Mar 1, 1991• 84 citations
Publisher

Abstract

An efficient spontaneous-force laser trap of neutral atoms without standing waves is demonstrated by using the Ne 1s(5) metastable state. The metastable Ne beam is decelerated by a laser by using Zeeman tuning and is trapped in a trap consisting of a quadrupole magnetic field and four laserbeams in a tetrahedral configuration. The dynamics of atomic motion from the deceleration stage to the trap is discussed.

DOI
10.1364/ol.16.000339

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An efficient spontaneous-force laser trap of neutral atoms without standing waves is demonstrated by using the Ne 1s(5) metastable state. The metastable Ne beam is decelerated by a laser by using Zeeman tuning and is trapped in a trap consisting of a quadrupole magnetic field and four laserbeams in a tetrahedral configuration. The dynamics of atomic motion from the deceleration stage to the trap is discussed.
A neon atomic beam in the metastable state 1s5{s}_{5}s5​ has been decelerated and trapped by the spontaneous light force of a resonant laser in a quadrupole magnetic field. Trapping has been achieved on all isotopes 20Ne^{20}\mathrm{Ne}20Ne, 21Ne^{21}\mathrm{Ne}21Ne, and 22Ne^{22}\mathrm{Ne}22Ne using a sample with natural abundance. For 20Ne^{20}\mathrm{Ne}20Ne, approximately 4\ifmmode\times\else\texttimes\fi{}107{10}^{7}107 atoms are trapped.
The high velocity of free atoms associated with the thermal motion, together with the velocity distribution of atoms has imposed the ultimate limitation on the precision of ultrahigh resolution spectroscopy. A sample consisting of low velocity atoms would provide a substantial improvement in spectroscopy resolution. To overcome the problem of thermal motion, atomic physicists have pursued two goals; first, the reduction of the thermal motion (cooling); and second, the confinement of the atoms by means of electromagnetic fields (trapping). Cooling carried sufficiently far, eliminates the motional problems, whereas trapping allows for long observation times. In this work the laser cooling and trapping of an argon atomic beam will be discussed. The experiments involve a time-of-flight spectroscopy on metastable argon atoms. Laser deceleration or cooling of atoms is achieved by counter propagating a photon against an atomic beam of metastable atoms. The solution to the Doppler shift problem is achieved using spatially varying magnetic field along the beam path to Zeeman shift the atomic resonance frequency so as to keep the atoms in resonance with a fixed frequency cooling laser. For trapping experiments a Magnetooptical trap (MOT) will be used. The MOT is formed by three pairs of counter-propagating laser beams with mutual opposite circular polarization and a frequency tuned slightly below the center of the atomic resonance and superimposed on a magnetic quadrupole field.
A thermal atomic Na beam is opposed by a resonantly tuned laser beam. The radiation pressure decelerates and compresses the thermal velocity distribution, bringing the average velocity to zero and reducing the width from ~1000 m/s to a few tens of m/s or less.1 The density of stopped atoms is > 106/cm3 within a bandwidth of a few m/ s. These densities and velocities are suitable for trapping by a number of proposed optical and magnetic traps. Using these stopped atoms, we have demonstrated the first electromagnetic trapping of neutral atoms. The trap, a magnetic quadrupole formed by two separated, coaxial, opposed current loops, confines the atoms by virtue of the force exerted on the Na magnetic dipole moment by the inhomogeneous magnetic field. The time constant for exponential decay of the atomic population in the trap is 0.83(7) s and is limited mainly by collisions with fast background gas atoms. The maximum velocity of atoms contained in the trap is 3.5 m/s, corresponding to a maximum energy of 17 mK.
We propose several ideas for neutral atom traps which utilize spontaneous light forces produced by static laser beams. Although the “optical Earnshaw theorem”1 would seem to prohibit such traps, the theorem does not apply if the proportionality constant between force and light intensity is made to vary with position. We suggest possible ways to accomplish this: Zeeman tuning the atom’s resonant frequency by applying a changing magnetic field; varying the quantization axis of the system via a magnetic field cork-screw; utilizing optical pumping effects which arise from matrix element imbalances between hyperfine sublevels in certain atoms. These traps are true in the sense that they provide a potential energy minimum in three dimensions, as well as dissipative cooling. Compared with gradient force light traps, these traps are many times deeper, a million times larger, and can be operated cw.
Cold deuterium atoms in a supersonic beam have been decelerated from an initial velocity of 475475475 m/s to zero velocity in the laboratory frame using a 242424-stage Zeeman decelerator. The atoms have been loaded in a magnetic quadrupole trap at a temperature of  100~100 100 mK and an initial density of  10~10 106{}^{6}6 cm${}^{\ensuremath{-}3}$. Efficient deceleration was achieved by pulsing the magnetic fields in the decelerator solenoids using irregular sequences of phase angles. Trap loading was optimized by monitoring and suppressing the observed reflection of the atoms by the field gradient of the back solenoid of the trap.
The problem of creating well-collimated beams of atoms escaping from a trap is studied. This problem is of high importance for the realization of atom lasers. Nonadiabatic dynamics of neutral atoms in nonuniform magnetic fields, typical of quadrupole magnetic traps, has been considered. The main result of this report is that an unusual semiconfining regime of motion is found, when atoms are confined from one side of an axis but are not confined from another side. This regime can be achieved by means of only magnetic fields. The formed atomic beam can be forwarded in arbitrary direction.