Light typically pushes particles forward thanks to radiation pressure. But it can also pull them backward. A gradient in laser intensity, for instance, can produce optical forces large enough to move the particles—either upstream or down—toward a beam’s focus, where they remain trapped, a phenomenon known as optical tweezing (see Physics Today, December 1997, page 17). In 2006 Philip Marston of Washington State University realized that even a nonspreading beam could pull particles backward and do so over a far broader range than is possible with optical tweezers. As if caught in a tractor beam à la Star Trek, the particles may be reeled all the way back to the source of the beam, at least in principle.
Marston presented his theory in the context of acoustic waves,1 but it applies equally to photons.2,3 His trick was to use a Bessel beam, which has...
