A team of German physicists has taken a step closer to the realization of an atomic de Broglie microscope. Exploiting the wave-particle duality of quantum theory, they have produced a highly focused and intense beam of helium-4 atoms.
Traditionally, imaging microscopes have used only electrons or photons, as either particle can be focused easily. However, imaging with helium atoms is an attractive alternative: at thermal energies (5-100 meV), chemically inert helium-4 atoms are non-penetrating and would not damage the subject of the imaging process. This is obviously useful in the imaging of biological samples. Also, the de Broglie wavelength of such atoms is only a few tenths of a nanometre, giving much better resolution than optical or electron microscopes for comparable energies of incident particles.
The problem is focusing a helium beam. Ground-state helium-4 atoms have low polarizability and no spin, so standard techniques cannot be used. However, wave-particle duality means that helium-4 atomics are diffracted by the slits of a grating just like waves. The effect is to focus the atoms into a narrow band - the central maximum of the diffraction pattern.
The researchers used a custom-made Fresnel zone plate - a grating of 100 nm slits in concentric rings. They saw a focused spot of less than 2 m m in diameter and of an intensity 1000 times as high as that achieved by any previous experiment.