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New Journal of Physics

Directed fluxes of positronium using pulsed travelling optical lattices

P. F. Barker, M. Charlton

Published Apr 4, 2012• 11 citations
Publisher

Abstract

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.

DOI
10.1088/1367-2630/14/4/045005

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New Journal of Physics•Jun 15, 2021•4 citations
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.
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.
We numerically study laser deceleration, cooling and trapping of host fast ortho-positronium (Ps) bunches as produced in silica microchannel targets using positron bunches from a Surko trap. While Doppler laser cooling routinely allows to cool neutral atom species below milli-Kelvin kinetic temperatures, the fast ground state annihilation and the high recoil shift to line-width ratio in Ps pose new challenging limitations on timing and the required laser power. Earlier theoretical work and first experiments already exhibit viable prospects for radiation pressure cooling to create unprecedented cold and dense Ps ensembles. Our extended numerical studies confirm very good prospects to simultaneously decelerate, cool and optically trap Ps atoms in a standard Doppler cooling geometry on the 13S1^3\mathrm{S}13S--23P2^3\mathrm{P}23P transition. Including the full Zeeman state manifolds and adding additional transverse lasers on the 23P2^3\mathrm{P}23P--33D3^3\mathrm{D}33D transition improves timing and final temperature to allow for optical trapping at the end. The required laser powers and geometry parameters to implement effective slowing and trapping within a time frame, where the majority of atoms is not annihilated, is in reach of current technology of mJ\mathrm{mJ}mJ energy pulses of approximately 100 ns100\ \mathrm{ns}100 ns duration. Future extensions to collectively enhance cooling and trapping in optical cavities or hollow-core fibres should finally allow fast preparation of ultra-cold Ps systems as future basis of superradiant lasing and Bose--Einstein condensates.
The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.
Positronium in the 2(3)S metastable state exhibits a low electrical polarizability and a long lifetime (1140 ns), making it a promising candidate for interferometry experiments with a neutral matter-antimatter system. In the present work, 2(3)S positronium is produced, in the absence of an electric field, via spontaneous radiative decay from the 3(3)P level populated with a 205-nm UV laser pulse. Thanks to the short temporal length of the pulse, 1.5 ns full width at half maximum, different velocity populations of a positronium cloud emitted from a nanochanneled positron-positronium converter were selected by delaying the excitation pulse with respect to the production instant. 2(3)S positronium atoms with velocity tuned between 7 x 10(4) ms(-1) and 10 x 10(4) ms(-1) were thus produced. Depending on the selected velocity, a 2(3)S production efficiency ranging from similar to 0.8% to similar to 1.7%, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the 3(3)P-2(3)S spontaneous decay of (9.7 +/- 2.7)%. The present velocity selection technique could allow one to produce an almost monochromatic beam of similar to 1 x 10(3) 2(3)S atoms with a velocity spread of <10(4) ms(-1) and an angular divergence of similar to 50 mrad.
Positronium in the 23S2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}23S metastable state exhibits a low electrical polarizability and a long lifetime (1140 ns), making it a promising candidate for interferometry experiments with a neutral matter-antimatter system. In the present work, 23S2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}23S positronium is produced, in the absence of an electric field, via spontaneous radiative decay from the 33P3^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{P}33P level populated with a 205-nm UV laser pulse. Thanks to the short temporal length of the pulse, 1.5 ns full width at half maximum, different velocity populations of a positronium cloud emitted from a nanochanneled positron-positronium converter were selected by delaying the excitation pulse with respect to the production instant. 23S2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}23S positronium atoms with velocity tuned between $7\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{ms}}^{\ensuremath{-}1}$ and $10\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{ms}}^{\ensuremath{-}1}$ were thus produced. Depending on the selected velocity, a 23S2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}23S production efficiency ranging from $\ensuremath{\sim}0.8%$ to $\ensuremath{\sim}1.7%$, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the $3^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{P}\ensuremath{-}2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}$ spontaneous decay of $(9.7\ifmmode\pm\else\textpm\fi{}2.7)%$. The present velocity selection technique could allow one to produce an almost monochromatic beam of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{4pt}{0ex}}2^{\phantom{\rule{0.16em}{0ex}}3}\mathrm{S}$ atoms with a velocity spread of $<{10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{ms}}^{\ensuremath{-}1}$ and an angular divergence of $\ensuremath{\sim}50$ mrad.
Abstract The production of beams of antihydrogen atoms using the dipole force provided by a travelling optical lattice to accelerate a sample of the anti-atoms held in a magnetic gradient atom trap is investigated. By considering current and near-future antihydrogen trapping capabilities we find that useful fluxes of the anti-atoms can be achieved with directional properties that can be manipulated using laser parameters such as pulse duration and frequency chirp rate. Applications of the beams are briefly discussed.