Neutral atom and molecule focusing using a Fresnel zone plate

Focusing of neutral atoms and molecules has several potential applications. The very first microscopy images using helium as an imaging probe were published earlier this year. Another possible application is to study the diffusion of atoms and molecules through materials with high spatial resolution by stepping a porous or permeable sample across the focused beam. With this application in mind, the authors present the best resolution transmission images hitherto achieved with helium atoms (less than 2 mu m) of a thin carbon film with 2 mu m holes. Furthermore, they present the first experiment using a Fresnel zone plate to focus neutral molecules. They used a beam of deuterium (D-2) which was focused down to 15.2 +/- 0.5 mu m. D-2 was chosen because it fits in mass to the geometry of our system, which is optimized for helium. However, the method can be extended to hydrogen (H-2) or other molecules by using a suitably adapted zone plate. In both cases the focus was limited by chromatic aberrations, caused by the velocity spread of the beams. Finally, they present calculations exploring the resolution limits for focusing of molecular beams using Fresnel zone plates. The calculations show that Fresnel focusing down to 170 nm full width at half maximum is possible with presently available techniques.