Microscopy (whether using neutral systems, such as photons or atoms, or charged systems, such as electrons) requires the ability to “optically” manipulate low energy particles, through a series of “lenses” that are used to focus a beam and scan it across a surface (except in the case of atomic force microscopy, where the divergently emitting source is moved closely across the surface and no beam optics are needed). Beam elements for neutral systems (neutrons, atoms) can take the form of diffraction gratings (zone plates in the case of He), or of fields that act on inner states of a composite object (laser intensity variations, electric field gradients). To date, the neutral beams that have been used to carry out microscopy have relied on massive atomic systems, whose interaction with the surface to be scanned can not be neglected; electron microscopy on the other hand has the drawback of the sensitivity of the probe to external electric and magnetic fields. Positronium combines the benefits of the lightweight electrons with those of a neutral atomic system, with the potential to thus probe surfaces with a completely novel sensitivity to surface electron density.
We propose to explore the feasibility of building beam optics for positronium. Because Ps is emitted (from a membrane) isotropically at a velocity corresponding to roughly 100 K, we focus here on the possibility of laser-cooling the emitted o-Ps atoms in the plane parallel to the surface of the membrane in which o-Ps is formed. Such laser-cooling in the transverse plane will reduce the transverse momentum, and thus the divergence, of the emitted o-Ps. This emittance reduction is the critical requirement for subsequent transport and further manipulation and focusing via recently established techniques.
The cooling transition that we will target is the 1S-2P transition (243 nm), as the lifetime of the 2P state of o-Ps is very short: 3ns. Given the short lifetime of ground state positronium (142 ns for o-Ps), very little time is available for interacting photons with a given o-Ps atom; for a 100 ns long laser pulse, of the order of 100 photon interactions should be feasible. Although this lies far below the number of photons required to cool a normal atom (about 10000 photons needed), simulations show that given the very low positronium mass, this should be sufficient to achieve transverse temperatures of the order of a few K, dramatically reducing the divergence of the o-Ps beam. The unconventional laser requirements (broadband, long pulse duration and 243nm wavelength) are nevertheless met by state-of-the- art Alexandrite lasers that we plan to buy thanks to this ATTRACT project.
Together with the already established technique of forming metastable or highly-excited positronium atoms, it will then be possible to further manipulate (with external electric gradients) these long-lived positronium atoms, and thus ultimately focus and scan them over a surface to be probed.