Diffraction-free beams in inhomogeneous media

An axicon lens is flat on one side, conical on the other. After passing through the flat surface, a beam of light with a Gaussian profile emerges from the conical surface with a profile that looks approximately like a zeroth-order Bessel function. Bessel beams interest physicists because of a remarkable property: Thanks to the way the beams’ energy and phase components are spread in space, the beams barely diffract—even around certain obstacles. Now, Florian Fahrbach, Philipp Simon, and Alexander Rohrbach of the University of Freiburg in Germany have demonstrated that

Bessel beams can propagate without significant diffraction through inhomogeneous media. The accompanying figure shows the intensity of fluorescence emitted by a transparent fluorescent gel when three kinds of beam passed through from left to right: a sheet-shaped beam from a cylindrical lens, a Gaussian beam scanned from top to bottom, and a Bessel beam scanned from top to bottom. The different patterns arose from the beams’ interactions with hundreds of 2-µm-diameter glass beads that the Freiburg researchers had dispersed throughout the gel. As the light sheet and Gaussian beam encountered the beads, their scattered components interfered constructively, leading to bright streaks and therefore to what would amount to image artifacts. The Bessel beam had far less streaking. The researchers obtained similar results with a sample of fresh human skin, whose densely packed cells, like those of other biological tissues, are micron sized. Bessel beams could therefore form the basis of a new kind of biological microscopy. (F. O. Fahrbach, P. Simon, A. Rohrbach, Nat. Photon. , in press, doi:10.1038/nphoton.2010.204.)