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Physical Review A

Positronium cooling in porous silica measured via Doppler spectroscopy

D. B. Cassidy, P. Crivelli, T. H. Hisakado, Laszlo Liszkay, V. E. Meligne, P. Pérez, Harry W. K. Tom, Allen Paine Mills

Published Jan 29, 2010• 143 citations
Publisher

Abstract

We have measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the linewidth of the Ps ${1}^{3}S$-${2}^{3}P$ transition. We find that the deeper in the target film that positrons are implanted the colder is the emitted Ps, an effect we attribute to cooling via collisions in the pores as the atoms diffuse back to the film surface. We observed a lower limit to the mean Ps kinetic energy associated with motion in the direction of the laser, ${E}_{x}=42\ifmmode\pm\else\textpm\fi{}3$ meV, that is consistent with conversion of the confinement energy of Ps in the 2.7-nm-diameter pores to kinetic energy in vacuum. An implication is that a porous sample would need to be composed of pores greater than around 10 nm in diameter in order to produce thermal Ps in vacuum with temperatures of less than 100 K. By performing Doppler spectroscopy on intense pulses of Ps we have experimentally demonstrated the production of many excited-state Ps atoms simultaneously, which could have numerous applications, including laser cooling and fundamental spectroscopic studies of Ps and the production of antihydrogen.

DOI
10.1103/physreva.81.012715

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Methods and techniques in the production, manipulation, and accumulation of positrons in a `Surko-type' buffer-gas trap are presented. The pulsed output of the trap is optimised for laser excitation of positronium (Ps). The techniques of positron time compression and single-shot positron annihilation lifetime spectroscopy (SSPALS) are discussed in detail and used throughout the study. A small array or Ps converter materials have been characterised at various temperatures. Ps cooling as a function of positron implantation is presented and discussed. Direct laser irradiation of the samples, and the effect on the Ps yield has also been studied. In some cases, particularly at cryogenic temperatures, laser induced paramagnetic centres are formed and this has a detrimental effect on the Ps yield as they cause triplet Ps to convert to the shorter-lived singlet state. Whereas in single crystal semiconductors, the laser has an enhancing effect due to an exciton-like positron-electron surface state. Many of the Ps converter materials studied here are formed from porous structures. The long term confinement of Ps within isolated cavities has also been observed, where a lifetime measurement technique using an excitation laser is presented. General methods of Ps-laser spectroscopy are also presented and the applications of which are discussed including laser-enhanced time-of-flight spectroscopy (LEPTOF), and the production, and electrostatic guiding of highly excited Rydberg Ps. Using these techniques a crossed beam scattering experiment involving Rydberg Ps, electrons, and Argon ions was carried out. This work presents important considerations for experiments to create a laser-cooled Bose-Einstein condensate (BEC) of Ps in an engineered porous material at cryogenic temperatures.

•Mar 28, 2018•0 citations
We have measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the linewidth of the Ps 13S{1}^{3}S13S-23P{2}^{3}P23P transition. We find that the deeper in the target film that positrons are implanted the colder is the emitted Ps, an effect we attribute to cooling via collisions in the pores as the atoms diffuse back to the film surface. We observed a lower limit to the mean Ps kinetic energy associated with motion in the direction of the laser, Ex=42\ifmmode±\else\textpm\fi3{E}_{x}=42\ifmmode\pm\else\textpm\fi{}3Ex​=42\ifmmode±\else\textpm\fi3 meV, that is consistent with conversion of the confinement energy of Ps in the 2.7-nm-diameter pores to kinetic energy in vacuum. An implication is that a porous sample would need to be composed of pores greater than around 10 nm in diameter in order to produce thermal Ps in vacuum with temperatures of less than 100 K. By performing Doppler spectroscopy on intense pulses of Ps we have experimentally demonstrated the production of many excited-state Ps atoms simultaneously, which could have numerous applications, including laser cooling and fundamental spectroscopic studies of Ps and the production of antihydrogen.
Laser cooling of positronium (Ps) is an important technique to advance the frontier of science by, for instance, realizing the first Bose-Einstein condensation of antimatters. It was proposed that performing laser cooling on Ps trapped in a cryogenic porous material would be more efficient, but a recent study (Cooper, et al., Phys. Rev. B 97, 205302 (2018)) reported that Ps in the excited (2P) state inside nano pores of silica had a large decay rate into γ-rays, and the resonant spectrum of the transition was significantly broadened, both of which are critically problematic for the proposed cooling scheme. A possible cause of these unexplained results was proposed to be residual impurities which trapped Ps in the vicinity of the pore surface and then enhanced interactions between Ps and the material. This work reports fabrication and inspection of a new porous silica cavity without such impurities, and construction of an experimental system to induce the Lyman-α transition of Ps trapped in pores of the cavity.
We report experiments in which the line shape of the Lyman-alpha ($1S\mathrm{\text{\ensuremath{-}}}2P$) transition was measured for positronium (Ps) atoms both inside and outside a porous silica target. The energy interval $\ensuremath{\Delta}E$ for confined atoms was observed to be larger than that of free Ps by $1.26\ifmmode\pm\else\textpm\fi{}0.06\text{ }\text{ }\mathrm{meV}$. A configuration interaction calculation yields results that are consistent with our $\ensuremath{\sim}5\text{ }\text{ }\mathrm{nm}$ sample, and suggests that $\ensuremath{\Delta}E$ decreases dramatically for larger cavity diameters. The linewidth of the transition, $(0.066\ifmmode\pm\else\textpm\fi{}0.004)\text{ }\text{ }\mathrm{nm}$ (FWHM), is about half of what one would expect for free Ps at room temperature due to the Dicke line narrowing effect of confinement. Such measurements can be used to determine void sizes in porous films and Ps dynamics therein, and elimination of the Doppler spread of atoms in a porous film could be useful for the efficient excitation of a Ps gas.
Abstract The quest for making a triplet positronium (Ps) Bose–Einstein condensate confined in a micron-sized cavity in a material such as porous silica faces at least three interrelated problems: (1) About 10710^7107 10 7 spin polarized Ps atoms must be injected into a small cavity within a porous solid material without vaporizing it. (2) It is known that Ps atoms confined in 30–100 nm diameter cavities in porous silica do not remain in the gas phase, but become stuck to the cavity walls at room temperature (Cooper et al., Phys. Rev. B 97:205302, 2018). (3) Cooling a gas of Ps atoms to cryogenic temperatures by energy exchange with the walls would be a very slow process (Saito and Hyodo, in: Surko, Gianturco (eds) New Directions in Antimatter Chemistry and Physics, Springer Dordrecht, Netherlands, 2001) because of the relatively low collision rate with the walls and the large mismatch between the masses of the Ps and the wall atoms. A possible solution of these difficulties is presented, based on cooling the implanted positrons in an isotopically pure diamond single crystal target, subsequent saturating of the wall Ps coverage so that a substantial portion of the Ps will be in the gaseous state, and thermalizing the gas-phase Ps via collisions with the low effective mass wall Ps. A design process for the target material is outlined as well, including preliminary results in porous cavity fabrication using focused ion beam milling. Graphical abstract
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.
Abstract We report recent progress toward achieving positronium (Ps) Bose–Einstein condensation (BEC) by combining thermalization in nanoporous materials with ultraviolet (UV) laser Doppler cooling. Experiments were performed using silica aerogels held at two distinct thermal conditions—approximately room temperature and cryogenic temperature down to 8 K (measured at the sample-holder)—to study the laser-induced annihilation of Ps atoms excited to the 2P state. Resonant excitation at 243 nm significantly enhanced gamma-ray emission, with the annihilation signal reduced by approximately 50 % at low temperature compared to room temperature. Increasing the delay between Ps formation and laser excitation further reduced the annihilation probability. These observations confirm that Ps thermalization affects 2P-Ps annihilation. The results support a model of surface-induced 2P-state decay and suggest that excited-state Ps serves as a sensitive probe of surface interactions in nanoporous media. This constitutes an important step toward implementing laser cooling of Ps confined in solids—an essential prerequisite for realizing Ps-BEC. Ongoing efforts focus on systematic evaluation of pore structure, surface chemistry, and temperature effects to optimize Ps dynamics toward condensation.
Methods and techniques in the production, manipulation, and accumulation of positrons in a `Surko-type' buffer-gas trap are presented. The pulsed output of the trap is optimised for laser excitation of positronium (Ps). The techniques of positron time compression and single-shot positron annihilation lifetime spectroscopy (SSPALS) are discussed in detail and used throughout the study. A small array or Ps converter materials have been characterised at various temperatures. Ps cooling as a function of positron implantation is presented and discussed. Direct laser irradiation of the samples, and the effect on the Ps yield has also been studied. In some cases, particularly at cryogenic temperatures, laser induced paramagnetic centres are formed and this has a detrimental effect on the Ps yield as they cause triplet Ps to convert to the shorter-lived singlet state. Whereas in single crystal semiconductors, the laser has an enhancing effect due to an exciton-like positron-electron surface state. Many of the Ps converter materials studied here are formed from porous structures. The long term confinement of Ps within isolated cavities has also been observed, where a lifetime measurement technique using an excitation laser is presented. General methods of Ps-laser spectroscopy are also presented and the applications of which are discussed including laser-enhanced time-of-flight spectroscopy (LEPTOF), and the production, and electrostatic guiding of highly excited Rydberg Ps. Using these techniques a crossed beam scattering experiment involving Rydberg Ps, electrons, and Argon ions was carried out. This work presents important considerations for experiments to create a laser-cooled Bose-Einstein condensate (BEC) of Ps in an engineered porous material at cryogenic temperatures.