If you're looking for more technical details you'll find a link to the recently published paper titled, Dexterous manipulation of microparticles using Bessel-function acoustic pressure fields at the source.

ENGADGET

The other two methods rely on specially shaped laser beams - instead of a beam whose intensity peaks at its centre and tails off gradually, the team is investigating two alternatives: solenoid beams and Bessel beams.

BBC: Nasa examines 'tractor beams' for sample gathering

The intensity peaks within a solenoid beam are found in a spiral around the line of the beam itself, while a Bessel beam's intensity rises and falls in peaks and troughs at higher distances from the beam's line.

BBC: Nasa examines 'tractor beams' for sample gathering

Solenoid beams have already proven their "tractor beam" abilities in laboratory tests published in the journal Optics Express, but the pulling power of Bessel beams, presented on the preprint server Arxiv in February, remains to be proved experimentally.

BBC: Nasa examines 'tractor beams' for sample gathering