17 Jun 2002
Doughnut shaped light beams can support and manipulate metallic particles that are large enough to see.
Miles Padgett and colleagues at the University of Glasgow in the UK have developed optical tools that can manipulate micrometre scale opaque objects, such as metal particles. Padgett believes that his group is the first to trap metal particles this size in three dimensions.
Conventional optical tweezers work due to the refraction of light passing through the transparent particles. Padgett says that opaque particles present additional challenges because refraction is no longer involved.
"Instead it is the reflection that is important. We use a Laguerre-Gaussian beam pointing upwards to support a metal particle against the pull of gravity. The Laguerre-Gaussian beam is a bright ring with a hole in the middle. The metal particle sits in this hole, supported by light 'bouncing' off the bottom." This beam is important as it has angular momentum so allows objects to be rotated.
Applications of optical tweezers and spanners are growing around the world and include manipulating single parasites, measuring the elastic properties of DNA strands and cutting plant cells.