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Physical Review A

Focusing ground-state xenon in a pulsed optical field

R. C. Fulton, Alexis I. Bishop, P. F. Barker

Published Apr 13, 2005• 7 citations
Publisher

Abstract

We demonstrate a general scheme for manipulating atomic species by focusing ground state xenon atoms using large quasielectrostatic potentials created by high intensity $({10}^{12}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2})$, nanosecond, pulsed optical fields. The atomic density in the focal region was mapped out with a resolution of $2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ using resonantly enhanced multiphoton ionization of neutral atoms in the vicinity of the focus. Measurements of the transverse xenon velocity induced by the optical field, and the focal length of the atom-optical lens, are in good agreement with a quasielectrostatic model based on the static polarizablity of xenon.

DOI
10.1103/physreva.71.043404

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We demonstrate a general scheme for manipulating atomic species by focusing ground state xenon atoms using large quasielectrostatic potentials created by high intensity (1012W∕cm2)({10}^{12}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2})(1012W∕cm2), nanosecond, pulsed optical fields. The atomic density in the focal region was mapped out with a resolution of 2\ensuremathμm2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}2\ensuremathμm using resonantly enhanced multiphoton ionization of neutral atoms in the vicinity of the focus. Measurements of the transverse xenon velocity induced by the optical field, and the focal length of the atom-optical lens, are in good agreement with a quasielectrostatic model based on the static polarizablity of xenon.
The ultra-short optical pulses can be used for the confinement of neutral particles by an additional optical longitudinal force, connected with the Poynting vector and the influence of the magnetic field on their polarizability. The Poynting vector is connected also to the intensity of the electromagnetic field. On other hand, the intensity of a laser pulse can be manipulated by an optical lens, and thus, indirectly, the optical force can be increased or decreased or its direction to be changed. This in turn enables a unique possibility for the compression of neutral atoms, molecules, and particles into the focus of the lens. Here, we reported on an experimental study of neutral particle confinement in the focus of femtosecond laser pulses and further confirmed by a theoretical investigation. The findings demonstrate an innovative approach to the compression and confinement of neutral particles within the focal plane of a lens and their further manipulation. Feasible applications could be in the field of cold nuclear fusion and cooling of light-neutral atoms and molecules.
A near-resonant standing laser light field is used to focus a diverging beam of sodium atoms onto the atomic-beam axis. The investigations presented here distinguish between channeling of atoms with trajectories close to the axis and imaging of the atomic source. For laser detunings close to resonance, acceleration of the atoms in the transverse direction occurs. The amount of acceleration changes as either the intensity or the detuning of the standing wave is varied, thereby altering the position at which the atoms are imaged. This results in a variable focal-length lens for atoms. The experimental results are in good agreement with a theoretical model based on a continued-fraction solution for the standing-wave light forces.
A single-transverse-mode QQQ-switched Nd-glass laser, which can operate over a number of longitudinal modes variable from 1 to 100, has been used to investigate laser temporal-coherence effects on the 11-photon ionization of xenon atoms. When the laser pulse has an over-all bandwidth of 4 ${\mathrm{cm}}^{\ensuremath{-}1}$, the number of ions formed is enhanced by ${10}^{6.9\ifmmode\pm\else\textpm\fi{}0.3}$ over that produced by a monomode laser pulse with the same average laser intensity. The observed enhancement in the number of ions due to the change in temporal coherence of the pulse is in good agreement with recent theoretical prediction.
The optical forces on neutral two-level atoms by the subcycle pulsed focused vector field are investigated by numerically solving the density-matrix equations without the rotating-wave approximation. The phenomena of focusing or defocusing of the neutral atom are analyzed. For the transverse optical force, an acceleration of an atom that is 10 orders of magnitude higher than the Earth's gravitational acceleration ggg can be obtained. The optical force can change from a focusing force to a defocusing force depending on the initial state of the atom. The optical potential may be used to trap the neutral two-level atoms in the experiment of cold atoms.
We show that light force lenses can be used to focus a thermal beam of neutral atoms with extremely high resolution (\ensuremath{\sim}20 nm) and high contrast (\ensuremath{\sim}10:1), finally making such atom-optical lenses viable for proposed applications. Our (cylindrical) lenses for sodium atoms are created by the dipole force within each period of a standing wave. We determine the spatial distribution of the focused atoms (with nanometer resolution) by allowing the atoms to deposit on a substrate and scanning with a UHV scanning tunneling microscope. We find that the requirements for optimum focusing are very large, positive detuning; short focal lengths; and a well-collimated atomic beam. These dependences can be thoroughly understood from a simple theoretical model. \textcopyright{}1996 The American Physical Society.
The focusing of atoms after interacting with both far-detuned and resonant standing wave fields in the thin-lens regime is considered. Exact quantum expressions for the Fourier components of the density (that include all spherical aberration) are used to study the focusing numerically. The following lens parameters and density profiles are calculated as functions of the pulsed field area $\ensuremath{\theta}:$ the position of the focal plane, peak atomic density, atomic density pattern at the focus, focal spot size, depth of focus, and background density. The lens parameters are compared to asymptotic, analytical results derived from a scalar diffraction theory for which spherical aberration is small but non-negligible $(\ensuremath{\theta}\ensuremath{\gg}1).$ Within the diffraction theory analytical expressions show that the focused atoms in the far-detuned case have an approximately constant background density $0.5(1\ensuremath{-}0.635{\ensuremath{\theta}}^{\ensuremath{-}1/2})$ while the peak density behaves as $3.83{\ensuremath{\theta}}^{1/2},$ the focal distance as ${\ensuremath{\theta}}^{\ensuremath{-}1}(1+1.27{\ensuremath{\theta}}^{\ensuremath{-}1/2})$ ${L}_{T}/2\ensuremath{\pi},$ the depth of focus as ${0.304L}_{T}{\ensuremath{\theta}}^{\ensuremath{-}3/2},$ and the focal spot size $0.0592\ensuremath{\lambda}{\ensuremath{\theta}}^{\ensuremath{-}3/4},$ where LT{L}_{T}LT​ is the Talbot distance and \ensuremath{\lambda} is the wavelength of the light. Focusing by the resonant standing wave field leads to a new effect, a Rabi-like oscillation of the atom density. For the far-detuned lens, chromatic aberration is studied quantitatively with the exact Fourier results. Similarly, the degradation of the focus that results from angular divergence in beams or thermal velocity distributions in traps is studied quantitatively with the exact Fourier method and understood analytically using the asymptotic results. Overall, we show that strong thin-lens focusing is possible with modest laser powers and with currently achievable atomic beam characteristics.